Fundamentals of Plant Physiology [1 ed.] 9781605357904, 1605357901

Fundamentals of Plant Physiology is a distillation of the most important principles and empirical findings of plant phys

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Table of contents :
Cover
Brief Contents
Table of Contents
Preface
1 Plant and Cell Architecture
1-1 Plant Life Processes: Unifying Principles 1-1
1-2 Plant Classification and Life Cycles
Box 1.1 Evolutionary Relationships among Plants
Plant life cycles alternate between diploid and haploid generations
1-3 Overview of Plant Structure
Plant cells are surrounded by rigid cell walls
Primary and secondary cell walls differ in their components
The cellulose microfibrils and matrix polymers are synthesized via different mechanisms
Plasmodesmata allow the free movement of molecules between cells
New cells originate in dividing tissues called meristems
Box 1.2 The Secondary Plant Body
1-4 Plant Cell Types
Dermal tissue covers the surfaces of plants
Ground tissue forms the bodies of plants
Vascular tissue fom1s transport networks between different parts of the plant
1-5 Plant Cell Organelles
Biological membranes are bilayers that contain proteins
1-6 The Nucleus
Gene expression involves both transcription and translation
Posttranslational regulation determines the life span of proteins
1-7 The Endomembrane System
The endoplasmic reticulum is a network of internal membranes
Vacuoles have diverse functions in plant cells
Oil bodies are lipid-storing organelles
Microbodies play specialized metabolic roles in leaves and seeds
1-8 Independently Dividing Semiautonomous Organelles
Proplastids mature into specialized plastids in different plant tissues
Chloroplast and mitochondrial division are independent of nuclear division
1-9 The Plant Cytoskeleton
The plant cytoskeleton consists of microtubules and microfilaments
Actin, tubulin, and their polymers are in constant flux in the living cell
Microtubule protofilaments first assemble into flat sheets before curling into cylinders
Cytoskeletal motor proteins mediate cytoplasmic streaming and directed organelle movement
1-10 Cell Cycle Regulation
Each phase of the cell cycle has a specific set of biochemical and cellular activities
The cell cycle is regulated by cyclins and cyclin- dependent kinases
Mitosis and cytokinesis involve both microtubules and the endomembrane system
Summary
2 Water and Plant Cells
2-1 Water in Plant Life
2-2 The Structure and Properties of Water
Water is a polar molecule that forms hydrogen bonds
Water is an excellent solvent
Water has distinctive thermal properties relative to its size
Water molecules are highly cohesive
Water has a high tensile strength
2-3 Diffusion and Osmosis
Diffusion is the net movenent of molecules by random thermal agitation
Diffusion is most effective over short distances
Osmosis describes the net movement of water across a selectively permeable barrier
2-4 Water Potential
The chemical potential of water represents the free-energy status of water
Three major factors contribute to cell water potential
Water potentials can be ,measured
2-5 Water Potential of Plant Cells
Water enters the cell along a water potential gradient
Water can also leave the cell in response to a water potential gradient
Water potential and its components vary with growth conditions and location within the plant
2-6 Cell Wall and Membrane Properties
Small changes in plant cell volume cause large changes in turgor pressure
The rate at which cells gain or lose water is influenced by plasma membrane hydraulic conductivity
Aquaporins facilitate the movement of water across plasma membranes
2-7 Plant Water Status
Physiological processes are affected by plant water status
Solute accumulation helps cells maintain turgor and volume
Summary
3 Water Balance of Plants
3-1 Water in the Soil
A negative hydrostatic pressure in soil water lowers soil water potential
Water moves through the soil by bulk flow
3-2 Water Absorption by Roots
Water moves in the root via the apoplast, symplast, and transmembrane pathways
Solute accumulation in the xylem can generate "root pressure"
3-3 Water Transport through the Xylem
The xylem consists of two types of transport cells
Water moves through the xylem, by pressure-driven bulk flow
Water movement through the xylem requires a smaller pressure gradient than movement through living cells
What pressure difference is needed to lift water 100 meters to a treetop?
The cohesion-tension theory explains water transport in the xylem
Xylem transport of water in trees faces physical changes
Plants minimize the consequences of xylem cavitation
3-4 Water Movement from the Leaf to the Atmosphere
Leaves have a large hydraulic resistance
The driving force for transpiration is the difference in water vapor concentration
Water loss is also regulated by the pathway resistances
The boundary layer contributes to diffusion resistance
Stomata resistance is another major component of diffusional resistance
The cell walls of guard cells have specialized features
An increase in guard cell turgor pressure opens the stomata
3-5 Coupling Leaf Transpiration and Photosynthesis: Light-dependent Stomatal Opening
Stomatal opening is regulated by light
Stomatal opening is specifically regulated by blue light
3-6 Water-use efficiency
3-7 Overview: The Soil- Plant- Atmosphere Continuum
Summary
4 Mineral Nutrition
4-1 Essential Nutrients, Deficiencies, and Plant Disorders
Special techniques are used in nutritional studies
Nutrient solutions can sustain rapid plant growth
Mineral deficiencies disrupt plant metabolism and function
Analysis of plant tissues reveals mineral deficiencies
4-2 Treating Nutritional Deficiencies
Crop yields can be improved by the addition of fertilizers
Some mineral nutrients can be absorbed by leaves
4-3 Soil, Roots, and Microbes
Negatively charged soil particles affect the adsorption of mineral nutrients
Soil pH affects nutrient availability, soil microbes, and root growth
Excess mineral ions in the soil limit plant growth
Some plants develop extensive root systems
Root systems differ in form but are based on conmmon structures
Different areas of the root absorb different mineral ions
Nutrient availability influences root growth
Mycorrhizal symbioses facilitate nutrient uptake by roots
Nutrients move between mycorrhizal fungi and root cells
Summary
5 Assimilation of Inorganic Nutrients
5-1 Nitrogen in the Environment
Nitrogen passes through several forms in a biogeochemical cycle
Unassimilated ammonium or nitrate may be dangerous
5-2 Nitrate Assimilation
Many factors regulate nitrate reductase
Nitrite reductase converts nitrite to ammonium
Both roots and shoots assimilate nitrate
5-3 Ammonium Assimilation
Converting ammonium to amino acids requires two enzymes
Ammonium can be assimilated via an alternative pathway
Transamination reactions transfer nitrogen
Asparagine and glutamine link carbon and nitrogen metabolism
5-4 Amino Acid Biosynthesis
5-5 Biological Nitrogen Fixation
Free-living and symbiotic bacteria fix nitrogen
Nitrogen fixation requires microanaerobic or anaerobic conditions
Symbiotic nitrogen fixation occurs in specialized structures
Establishing symbiosis requires an exchange of signals
Nod factors produced by bacteria act as signals for symbiosis
Nodule formation involves phytohormones
The nitrogenase enzyme complex fixes N2
Amides and ureides are the transported forms of nitrogen
5-6 Sulfur Assimilation
Sulfate is the form of sulfur transported into plants
Sulfate assimilation occurs mostly in leaves
Methionine is synthesized from cysteine
5-7 Phosphate Assimilation
5-8 Iron Assimilation
Roots modify the rhizosphere to acquire iron
Iron cations form complexes with carbon and phosphate
5-9 The Energetics of Nutrient Assimilation
Summary
6 Solute Transport
6-1 Passive and Active Transport
6-2 Transport of Ions across Membrane Barriers
Different diffusion rates for cations and anions produce diffusion potentials
How does membrane potential relate to ion distribution?
The Nernst equation distinguishes between active and passive transport
Proton transport is a major determinant of the membrane potential
6-3 Membrane Transport Processes
Channels enhance diffusion across membranes
Carriers bind and transport specific substances
Primary active transport requires energy
Secondary active transport uses stored energy
Kinetic analyses can elucidate transport mechanisms
6-4 Membrane Transport Proteins
The genes for many transporters have been identified
Transporters exist for diverse nitrogen-containing compounds
Cation transporters are diverse
Anion transporters have been identified
Transporters for metal and metalloid ions transport essential micronutrients
Aquaporins have diverse functions
Plasma membrane tt+ -ATPases are highly regulated P-type ATPases
The tonoplast H+ -ATPase drives solute accumulation in vacuoles
H+ -pyrophosphatases a !so pump protons at the tonoplast
6-5 Ion Transport in Stomatal Opening
Light stimulates ATPase activity and creates a stronger electrochemical gradient across the guard cell plasma membrane
Hyperpolarization of the guard cell plasma membrane leads to uptake of ions and water
6-6 Ion Transport in Roots
Solutes n1ove through both apoplast and symplast
Ions cross both symplast and apoplast
Xylem parenchyma cells participate in xylem loading
Summary
7 Photosynthesis: The Light Reactions
7-1 Photosynthesis in Higher Plants
7-2 General Concepts
Light has characteristics of both a particle and a wave
When molecules absorb or emit light, they change their electronic state
Photosynthetic pigments absorb the light that powers photosynthesis
7-3 Key Experiments in Understanding Photosynthesis
Action spectra relate light absorption to photosynthetic activity
Photosynthesis takes place in complexes containing light-harvesting antennas and photochemical reaction centers
The chemical reaction of photosynthesis is driven by light
Light drives the reduction of NADP+ and the formation of ATP
Oxygen-evolving organisms have two photosystems that operate in series
7-4 Organization of the Photosynthetic Apparatus
The chloroplast is the site of photosynthesis
Thylakoids contain integral membrane proteins
Photosystems I and II are spatially separated in the thylakoid membrane
7-5 Organization of Light-Absorbing Antenna Systems
Antenna systems contain chlorophyll and are membrane-associated
The antenna funnels energy to the reaction center
Many antenna pigment-protein complexes have a common structural motif
7-6 Mechanisms of Electron Transport
Electrons from chlorophyll travel through the carriers organized in the Z scheme
Energy is captured when an excited chlorophyll reduces an electron acceptor molecule
The reaction center chlorophylls of the two photosystems absorb at different wavelengths
The PSII reaction center is a multi-subunit pigment-protein complex
Water is oxidized to oxygen by PSII
Pheophytin and two qui nones accept electrons from PSII
Electron flow through the cytochrome b6f complex also transports protons
Plastoquinone and plastocyanin carry electrons between photosystem II and photosystem I
The PSI reaction center reduces NADp+
Cyclic electron flow generates ATP but no NADPH
Some herbicides block photosynthetic electron flow
7-7 Proton Transport and ATP Synthesis in the Chloroplast
Summary
8 Photosynthesis: The Carbon Reactions
8-1 The Calvin- Benson Cycle
The Calvin-Benson cycle has three phases: carboxylation, reduction, and regeneration
The fixation of CO2 via carboxylation of ribulose 1,5-bisphosphate and the reduction of the product 3-phosphoglycerate yield triose phosphates
The regeneration of ribulose 1,5-bisphosphate ensures the continuous assimilation of CO2
An induction period precedes the steady state of photosynthetic CO2 assimilation
Many mechanis.ms regulate the Calvin-Benson cycle
Rubisco activase regulates the catalytic activity of Rubisco
Light regulates the Calvin-Benson cycle via the ferredoxin-thioredoxin system
Light-dependent ion movements modulate enzymes of the Calvin-Benson cycle
8-2 Photorespiration: The C2 Oxidative Photosynthetic Carbon Cycle
The oxygenation of ribulose 1,5-bisphosphate sets in motion the C2 oxidative photosynthetic carbon cycle
Photorespiration is linked to the photosynthetic electron transport chain
8-3 Inorganic Carbon-Concentrating Mechanisms
8-4 Inorganic Carbon-Concentrating Mechanisms: The C4 Carbon Cycle
Malate and aspartate are the primary carboxylation products of the C4 cycle
The C4 cycle assimilates CO2 by the concerted action of two different types of cells
Bundle sheath cells and mesophyll cells exhibit anatomica I and biochemical differences
The C4 cycle also concentrates CO2 in single cells
Light regulates the activity of key C4 enzymes
Photosynthetic assimilation of CO2 in C4 plants den1ands more transport processes than in C3 plants
In hot, dry climates, the C4 cycle reduces photorespiration
8-5 Inorganic Carbon-Concentrating Mechanisms: Crassulacean Acid Metabolism (CAM)
Different mechanisms regulate C4 PEPCase and CAM PEPCase
CAM is a versatile mechanism, sensitive to environmental stimuli
8-6 Accumulation and Partitioning of Photosynthates-Starch and Sucrose
Summary
9 Photosynthesis: Physiological and Ecological Considerations
9-1 The Effect of leaf Properties on Photosynthesis
Leaf anatomy and canopy structure maximize light absorption
Leaf angle and leaf movement can control light absorption
Leaves acclimate to sun and shade environments
9-2 Effects of light on Photosynthesis in the Intact leaf
Light-response curves reveal photosynthetic properties
Leaves must dissipate excess light energy
Absorption of too much light can lead to photoinhibition
9-3 Effects of Temperature on Photosynthesis in the Intact leaf
Leaves must dissipate vast quantities of heat
There is an optimal temperature for photosynthesis
Photosynthesis is sensitive to both high and low temperatures
Photosynthetic efficiency is temperature-sensitive
9-4 Effects of Carbon Dioxide on Photosynthesis in the Intact leaf
Atmospheric CO2 concentration keeps rising
CO2 diffusion to the chloroplast is essential to photosynthesis
CO2 imposes limitations on photosynthesis
How will photosynthesis and respiration change in the future under elevated CO2 conditions?
Summary
10 Translocation in the Phloem
10-1 Patterns of Translocation: Source to Sink
10-2 Pathways of Translocation
Sugar is translocated in phloem sieve elements
Mature sieve elements are living cells specialized for translocation
Large pores in cell walls are the prominent feature of sieve elements
Damaged sieve elements are sealed off
Companion cells aid the highly specialized sieve elements
10-3 Materials Translocated in the Phloem
Phloem sap can be collected and analyzed
Sugars are translocated in a nonreducing form
Other solutes are translocated in the phloem
10-4 Rates of Movement
10-5 The Pressure-Flow Model, a Passive Mechanism for Phloem Transport
An osmotically generated pressure gradient drives translocation in the pressure-flow model
Some predictions of pressure flow have been confirmed, while others require further experimentation
There is no bidirectional I transport in single sieve elements, and solutes and water move at the same velocity
The energy requirement for transport through the phloem pathway is small in herbaceous plants
Sieve plate pores appear to be open channels
Pressure gradients in the sieve elements may be modest; pressures in herbaceous plants and trees appear to be similar
10-6 Phloem Loading
Phloem loading can occur via the apoplast or symplast
Abundant data support the existence of apoplastic loading in some species
Sucrose uptake in the apoplastic pathway requires metabolic energy
Phloem loading in the apoplastic pathway involves a sucrose-H+ symporter
Phloen, loading is symplastic in some species
The polymer-trapping model explains symplastic loading in plants with intermediary-type companion cells
Phloem loading is passive in several tree species
10-7 Phloem Unloading and Sink-to-Source Transition
Phloem unloading and short-distance transport can occur via symplastic or apoplastic pathways
Transport into sink tissues requires metabolic energy
The transition of a leaf from sink to source is gradual
10-8 Photosynthate Distribution: Allocation and Partitioning
Allocation includes storage, utilization, and transport
Various sinks partition transport sugars
Source leaves regulate allocation
Sink tissues compete for avaiLable translocated photosynthate
Sink strength depends on sink size and activity
The source adjusts over the long term to changes in the source-to-sink ratio
10-9 Transport of Signaling Molecules
Turgor pressure and chemical signals coordinate source and sink activities
There is no bidirectional transport in single sieve elements, and solutes and water move at the same velocity
Plasmodesmata function in phloem signaling
Summary
11 Respiration and Lipid Metabolism
11-1 Overview of Plant Respiration
11-2 Glycolysis
Glycolysis metabolizes carbohydrates from several sources
The energy-conserving phase of glycolysis extracts usable energy
Plants have alternative glycolytic reactions
In the absence of oxygen, fermentation regenerates the NAD+ needed for glycolytic ATP production
11-3 The Oxidative Pentose Phosphate Pathway
The oxidative pentose phosphate pathway produces NADPH and biosynthetic intermediates
The oxidative pentose phosphate pathway is redox-regulated
11-4 The Tricarboxylic Acid Cycle
Mitochondria are semiautonomous organelles
Pyruvate enters the mitochondrion and is oxidized via the TCA cycle
The TCA cycle of plants has unique features
11-5 Mitochondrial Electron Transport and ATP Synthesis
The electron transport chain catalyzes a flow of electrons from NADH to O2
The electron transport chain has supplen,entary branches
ATP synthesis in the mitochondrion is coupled to electron transport
Transporters exchange substrates and products
Aerobic respiration yields about 60 molecules of ATP per molecule of sucrose
Plants have several mechanisms that lower the ATP yield
Short-term control of mitochondrial respiration occurs at different levels
Respiration is tightly coupled to other pathways
11-6 Respiration in Intact Plants and Tissues
Plants respire roughly half of the daily photosynthetic yield
Respiratory processes operate during photosynthesis
Different tissues and organs respire at different rates
Environmental factors alter respiration rates
11-7 Lipid Metabolism
Fats and oils store large amounts of energy
Triacylglycerols are stored in oil bodies
Polar glycerolipids are the main structural lipids in membranes
Membrane lipids are precursors of important signaling compounds
Storage lipids are converted into carbohydrates in germinating seeds, releasing stored energy
Summary
12 Signals and Signal Transduction
12-1 Temporal and Spatial Aspects of Signaling
12-2 Signal Perception and Amplification
Signals must be amplified intracellularly to regulate their target molecules
Ca2+ is the most ubiquitous second messenger in plants and other eukaryotes
Changes in the cytosolic or cell wall pH can serve as second messengers for hormonal and stress responses
Reactive oxygen species act as second messengers mediating both environmental and developmental signals
12-3 Hormones and Plant Development
Auxin was discovered in early studies of coleoptile bending during phototropism
Gibberellins promote stem growth and were discovered in relation to the "foolish seedling disease" of rice
Cytokinins were discovered as cell division-promoting factors in tissue culture experiments
Ethylene is a gaseous hormone that promotes fruit ripening and other developmental processes
Abscisic acid regulates seed maturation and stomatal closure in response to water stress
Brassinosteroids regulate floral sex determination, photomorphogenesis, and germination
Salicylic acid and jasmonates function in defense responses
Strigolactones suppress branching and promote rhizosphere interactions
12-4 Phytohormone Metabolism and Homeostasis
Indole-3-pyruvate is the primary intermediate in auxin biosynthesis
Gibberellins are synthesized by oxidation of the diterpene ent-kaurene
Cytokinins are adenine derivatives with isoprene side chains
Ethylene is synthesized from methionine via the intermediate ACC
Abscisic acid is synthesized from a carotenoid intermediate
Brassinosteroids are derived from the sterol campesterol
Strigolactones are synthesized from ᅫᄇ-carotene
12-5 Signal Transmission and Cell-Cell Communication
12-6 Hormonal Signaling Pathways
The cytokinin and ethylene signal transduction pathways are derived from the bacterial two-component regulatory system
Receptor-like kinases mediate brassinosteroid signaling
The core ABA signaling components include phosphatases and kinases
Plant hormone signaling pathways generally employ negative regulation
Protein degradation via ubiquitination plays a prominent role in hormone signaling
Plants have mechanisms for switching off or attenuating signaling responses
The cellular response output to a signal is often tissue-specific
Cross-regulation allows signal transduction pathways to be integrated
Summary
13 Signals from Sunlight
13-1 Plant Photoreceptors
Photoresponses are driven by light quality or spectral properties of the energy absorbed
Plant responses to light can be distinguished by the amount of light required
13-2 Phytochromes
Phytochrome is the primary photoreceptor for red and far-red light
Phytochrome can interconvert between Pr and Pfr forms
13-3 Phytochrome Responses
Phytochrome responses vary in lag time and escape time
Phytochrome responses fall into three n1ain categories based on the amount of light required
Phytochrome A mediates responses to continuous far-red light
Phytochrome regulates gene expression
13-4 Blue-Light Responses and Photoreceptors
Blue-light responses have characteristic kinetics and lag times
13-5 Cryptochromes
Blue-light irradiation of the cryptochrome FAD chromophore causes a conformational change
The nucleus is a primary site of cryptochrome action
Cryptochrome interacts with phytochrome
13-6 Phototropins
Phototropism requires changes in auxin mobilization
Phototropins regulate chloroplast movements
Stomatal opening is regulated by blue light, which activates the plasma membrane H+-ATPase
13-7 The Coaction of Phytochrome, Cryptochrome, and Phototropins
13-8 Responses to Ultraviolet Radiation
Summary
14 Embryogenesis
14-1 Overview of Embryogenesis
14-2 Comparative Embryology of Eudicots and Monocots
Morphological similarities and differences between eudicot and monocot embryos dictate their respective patterns of development
Apical-basal polarity is maintained in the embryo during organogenesis
Embryo development requires regulated communication between cells
Auxin signaling is essential for embryo development
Polar auxin transport is mediated by localized auxin efflux carriers
Auxin synthesis and polar transport regulate embryonic development
Radial patterning guides formation of tissue layers
The protoderm differentiates into the epidermis
The central vascular cylinder is elaborated by cytokinin-regulated progressive cell divisions
14-3 Formation and Maintenance of Apical Meristems
Auxin and cytokinin contribute to the formation and maintenance of the RAM
SAM formation is also influenced by factors involved in auxin movement and responses
Cell proliferation in the SAM is regulated by cytokinin and gibberellin
Summary
15 Seed Dormancy, Germination, and Seedling Establishment
15-1 Seed Structure
Seed anaton,y varies widely among different plant groups
15-2 Seed Dormancy
There are two basic types of seed dormancy mechanisms: exogenous and endogenous
Non-dormant seeds can exhibit vivipary and precocious germination
The ABA:GA ratio is the primary determinant of seed dormancy
15-3 Release from Dormancy
Light is an important signal that breaks dormancy in small seeds
Some seeds require either chilling or after-ripening to break dormancy
Seed dormancy can be broken by various chemical compounds
15-4 Seed Germination
Germination and postgermination can be divided into three phases corresponding to the phases of water uptake
15-5 Mobilization of Stored Reserves
The cereal aleurone layer is a specialized digestive tissue surrounding the starchy endosperm
15-6 Seedling Establishment
The development of emerging seedlings is strongly influenced by light
Gibberellins and brassinosteroids both suppress photomorphogenesis in darkness
Hook opening is regulated by phytochrome, auxin, and ethylene
Vascular differentiation begins during seedling emergence
Growing roots have distinct zones
Ethylene and other hormones regulate root hair development
Lateral roots arise internally from the pericycle
15-7 Cell Expansion: Mechanisms and Hormonal Controls
The rigid primary cell wall must be loosened for cell expansion to occur
Microfibril orientation influences growth directionality of cells with diffuse growth
Acid-induced growth and cell wall yielding are mediated by expansins
Auxin promotes growth in stems and coleoptiles, while inhibiting growth in roots
The outer tissues of eudicot stems are the targets of auxin action
The o'linimum lag tin1e for auxin-induced elongation is 10 minutes
Auxin-induced proton extrusion loosens the cell wall
Ethylene affects microtubule orientation and induces lateral cell expansion
15-8 Tropisms: Growth in Response to Directional Stimuli
Auxin transport is polar and gravity-independent
The Cholodny-Went hypothesis is supported by auxin movements and auxin responses during gravitropic growth
Gravity perception is triggered by the sedimentation of amyloplasts
Gravity sensing may involve pH and calcium ions (Ca T) as second messengers
Phototropins are the tight receptors involved in phototropism
Phototropism is mediated by the lateral redistribution of auxin
Shoot phototropism occurs in a series of steps
Summary
16 Vegetative Growth and Senescence
16-1 The Shoot Apical Meristem
The shoot apical meristem has distinct zones and layers
16-2 Leaf Structure and Phyllotaxy
Auxin-dependent patterning of the shoot apex begins during embryogenesis
16-3 Differentiation of Epidermal Cell Types
A specialized epidermal lineage produces guard cells
16-4 Venation Patterns in Leaves
The primary leaf vein is initiated in the leaf primordium
Auxin canalization initiates development of the leaf trace
16-5 Shoot Branching and Architecture
Auxin, cytokinins, and strigolactones regulate axillary bud outgrowth
The initial signal for axillary bud growth may be an increase in sucrose availability to the bud
16-6 Shade Avoidance
Reducing shade avoidance responses can improve crop yields
16-7 Root System Architecture
Plants can modify their root system architecture to optimize water and nutrient uptake
Monocots and eudicots differ in their root system architecture
Root system architecture changes in response to phosphorus deficiencies
16-8 Plant Senescence
During leaf senescence, nutrients are remobilized from the source leaf to vegetative or reproductive sinks
The developmental age of a leaf may differ from its chronological age
Leaf senescence may be sequential, seasonal, or stress-induced
The earliest cellular changes during leaf senescence occur in the chloroplast
Reactive oxygen species serve as internal signaling agents in leaf senescence
Plant hormones interact in the regulation of leaf senescence
16-9 Leaf Abscission
The timing of leaf abscission is regulated by the interaction of ethylene and auxin
16-10 Whole Plant Senescence
Angiosperm life cycles may be annual, biennial, or perennial
Nutrient or hormonal redistribution may trigger senescence in monocarpic plants
Summary
17 Flowering and Fruit Development
17-1 Floral Evocation: Integrating Environmental Cues
17-2 The Shoot Apex and Phase Changes
Plant development has three phases
Juvenile tissues are produced first and are located at the base of the shoot
Phase changes can be influenced by nutrients, gibberellins, and epigenetic regulation
17-3 Photoperiodism: Monitoring Day Length
Plants can be classified according to their photoperiodic responses
Photoperiodism is one of many plant processes controlled by a circadian rhythm
Circadian rhythms exhibit characteristic features
Circadian rhythms adjust to different day-night cycles
The leaf is the site of perception of the photoperiodic signal
Plants monitor day length by measuring the length of the night
Night breaks can cancel the effect of the dark period
Photoperiodic timekeeping during the night depends on a circadian clock
A coincidence model links oscillating light sensitivity and photoperiodism
Phytochrome is the primary photoreceptor in photoperiodism
17-4 Vernalization: Promoting Flowering with Cold
17-5 Long-distance Signaling Involved in Flowering
Gibberellins and ethylene can induce flowering
17-6 Floral Meristems and Floral Organ Development
The SAM in Arabidopsis changes with development
The four different types of floral organs are initiated as separate whorls
Two major categories of genes regulate floral development
The ABC model partially explains the determination of floral organ identity
17-7 Pollen Development
17-8 Female Gametophyte Development in the Ovule
Functional megaspores undergo a series of free nuclear mitotic divisions followed by cellularization
17-9 Pollination and Double Fertilization in Flowering Plants
Two sperm cells are delivered to the female gametophyte by the pollen tube
Pollination begins with adhesion and hydration of a pollen grain on a compatible flower
Pollen tubes grow by tip growth
Double fertilization results in the formation of the zygote and the primary endosperm cell
17-10 Fruit Development and Ripening
Arabidopsis and tomato are model systems for the study of fruit development
Fleshy fruits undergo ripening
Ripening involves changes in the color of fruit
Fruit softening involves the coordinated action of many cell wall-degrading enzymes
Taste and flavor reflect changes in acids, sugars, and aroma compounds
The causal link between ethylene and ripening was demonstrated in transgenic and mutant tomatoes
Climacteric and non-climacteric fruits differ in their ethylene responses
Summary
18 Biotic Interactions
18-1 Beneficial Interactions between Plants and Microorganisms
Other types of rhizobacteria can increase nutrient availability, stimulate root branching, and protect against pathogens
18-2 Harmful Interactions of Pathogens and Herbivores with Plants
Mechanical barriers provide a first line of defense against insect pests and pathogens
Specialized plant metabolites can deter insect herbivores and pathogen infection
Plants store constitutive toxic compounds in specialized structures
Plants often store defensive chemicals as nontoxic water-soluble sugar conjugates in specialized vacuoles
18-3 Inducible Defense Responses to Insect Herbivores
Plants can recognize specific components of insect saliva
Phloem feeders activate defense signaling pathways similar to those activated by pathogen infections
Jasmonic acid activates defense responses against insect herbivores
Hormonal interactions contribute to plant-insect herbivore interactions
JA initiates the production of defense proteins that inhibit herbivore digestion
Herbivore damage induces systemic defenses
Long-distance electrical signaling occurs in response to insect herbivory
Herbivore-induced volatiles can repel herbivores and attract natural enemies
Herbivore-induced volatiles can serve as long-distance signals within and between plants
Insects have evolved mechanisms to defeat plant defenses
18-4 Plant Defenses against Pathogens
Microbial pathogens have evolved various strategies to invade host plants
Pathogens produce effector molecules that aid in the colonization of their plant host cells
Pathogen infection can give rise to molecular "danger signals" that are perceived by cell surface pattern recognition receptors (PRRs)
R proteins provide resistance to individual pathogens by recognizing strain-specific effectors
The hypersensitive response is a common defense against pathogens
A single encounter with a pathogen may increase resistance to future attacks
18-5 Plant Defenses against Other Organisms
Some plant parasitic nematodes form specific associations through the formation of distinct feeding structures
Plants compete with other plants by secreting allelopathic secondary metabolites into the soil
Some plants are biotrophic pathogens of other plants
Summary
19 Abiotic Stress
19-1 Defining Plant Stress
Physiological adjustment to a biotic stress involves trade-offs between vegetative and reproductive development
19-2 Acclimation versus Adaptation
19-3 Environmental Stressors
Water deficit decreases turgor pressure, increases ion toxicity, and inhibits photosynthesis
Salinity stress has both osmotic and cytotoxic effects
Temperature stress affects a broad spectrum of physiological processes
Flooding results in anaerobic stress to the root
Light stress can occur when shade-adapted or shade-acclimated plants are subjected to full sunlight
Heavy metal ions can both mimic essential mineral nutrients and generate ROS
Combinations of abiotic stresses can induce unique signaling and metabolic pathways
Sequential exposure to different a biotic stresses sometimes confers cross-protection
Plants use a variety of mechanisms to sense abiotic stress
19-4 Physiological Mechanisms That Protect Plants against Abiotic Stress
Plants can alter their morphology in response to abiotic stress
Metabolic shifts enable plants to cope with a variety of abiotic stresses
Heat shock proteins maintain protein integrity under stress conditions
Membrane lipid composition can adjust to changes in temperature and other abiotic stresses
Chloroplast genes respond to high-intensity light by sending stress signals to the nucleus
A self-propagating wave of ROS mediates systen1ic acquired acclimation
Abscisic acid and cytokinins are stress-response hormones that regulate drought responses
Plants adjust osmotically to drying soil by accumulating solutes
Epigenetic mechanisms and small RNAs provide additional protection against stress
Submerged organs develop aerenchyma tissue in response to hypoxia
Antioxidants and ROS-scavenging pathways protect cells from oxidative stress
Exclusion and internal tolerance mechanisms allow plants to cope with toxic metal and metalloid ions
Plants use cryoprotectant molecules and antifreeze proteins to prevent ice crystal formation
Summary
Glossary
Illustration Credits
Index
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Fundamentals of Plant Physiology [1 ed.]
 9781605357904, 1605357901

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Fundamentals of

Plant Physiology Lincoln Taiz • Eduardo Zeiger Ian Max M0ller • Angus Murphy

Fundamentals of

Plant Physiology

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Fundamentals of

Plant Physiology Lincoln Taiz Professor Emeritus, University of California, Santa Cruz

Eduardo Zeiger Professor Emeritus, University of California, Los Angeles

Ian Max M0l ler Professor Emeritus, Aarhus University, Denmark

Angus Murphy Professor, University of Maryland

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Xylem conduits and their interconnections. (A) Structural comparison of tracheids and vessel elements. Tracheids are elongated, hollow, dead cells with highly lignified walls. The walls contain numerous pits-regions where secondary wall is absent but primary wall remains. The shapes of pits and the patterns of wall pitting vary with species and organ type. Tracheids are present in all vascular plants. Vessels consist of a stack of two or more vessel elements. Like tracheids, vessel elements are dead cells and are connected to one another by perforation plates-regions of the wall where pores or holes have developed. Vessels are connected to other vessels and to tracheids through pits. Vessels are found in most angiosperms and are lacking in most gymnosperms. (8) Tracheids (left) and vessels (right) form a series of parallel, interconnected pathways for water movement. (C) Scanning electron micrograph showing two vessels (running diagonally from lower left to upper right). Pits are visible on the side walls, as are the scalariform end walls between vessel elements. (C © Steve Gschmeissner/Science Source.) Figure 3.6

vessel elements Nonliving water-conducting cells with perforated end walls, found only in angiosperms and a small group of gymnosperms. perforation plate The perforated end wall of a vessel element in the xylem. vessel A stack of two or more vessel elements 1n the xylem.

movement of gas bubbles. Thus, pit membranes of both types play an important role in preventing the spread of gas bubbles, ca lled emboli, within the xylem. Vessel elements tend to be shorter and wider than tracheids and have perforated end walls that form a perforation plate at each end of the cell (See Figure 3.6A). Li ke tracheids, vessel e lements have pits on their lateral walls (see Figure 3.6C). Unlike in tracheids, the perforated end walls allow vessel elements to be stacked end to end to form a much longer conduit called a vessel (see Figure

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3.68). Vessels are multicellular conduits that vary in length both within and among species. Vessels range from a few centimeters in length to many meters. The vessel elements found at the extreme ends of a vessel lack perforations in their end walls and are connected to neighboring vessels via pits. Water moves through the xylem by pressure-driven bulk flow Pressure-driven bulk flow of water is responsible for long-distance transport of water in the xylem. It also accounts for much of the water flow through the soil and through the cell walls of plant tissues. In contrast to the diffusion of water across senupermeable membranes, pressure-driven bulk flow is independent of solute concentration gradients, as long as viscosity changes are negligible. If we consider bulk flow th rough a tube, the rate of flow depends on the radius (,) of the tube, the viscosity (,1) of the liquid, and the pressure gradient (~'¥/LU) that drives the flow. Jean Leonard Marie Poiseuille {1797-1869) was a French physician and physiologist, and the relation just described is given by one form of Poiseuille's equation:

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expressed in cubic meters per second (m 3 s-1). This equation tells us that pressure-driven bu lk flow is extremely sensitive to the radius of the tube. If the radius is doubled, the volume flow rate increases by a factor of16 (2°1). Vessel elements up to 500 µm in diameter, nearly an order of m.agnitude greater than the largest tracheid s, occur in the stemsofdimbingspecies. These large-diameter vessels permit vines to h·ansport large an, ounts of water despite the slenderness of their stems. Equation 3.2 describes water flow through a cylindrical tube and thus does not take into account the fact that xylem conduits are of fin ite length, such that water must cross many pit mem branes as it flows from the soil to the leaves. All else being equal, pit membranes should impede water Oow through single-celled (and t hus shorter) tracheids to a greater extent t han through multicellu lar (and thus longer) vessels. However, the pit mem branes of conifers are much more permeable to water than are those found in other plants, allowing conifers to grow into large trees despite producing onlytracheids. Water movement through the xylem requires a smaller pressure gradient than movement through living cells The xylem provides a pathway of low resistivity for water movement. Some numerical values will help you appreciate the extraordinary efficiency of the xylem.

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Chapter 3 We will calcu late the driving force required to move water through the xylen, at a typical velocity and compare it with the driving force t hat would be needed to move water through a pathway made up of living cells at the same rate. For the purposes of this comparison, we ,,vi II use a value of 4 m m s-1 for the xylem transport velocity and 40 µmas the vessel radius. This is a high velocity for such a narrow vessel, so it will tend to exaggerate the pressure gradient required to support water flow in the xylem. Using a version of Poiseuille's equation (see Equation 3.2), we can calculate the pressure gradient needed to move water at a velocity of 4 mn1 s-1 through an ideal tube with a uniform inner radius of 40 µm. The calculation g ives a va lue of 0.02 MPa m-1 . Of course, real xylem conduits have irregular inner wall surfaces, and water flow through perforation plates and pits adds resistance to water transport. Such deviations from the idea l increase the frictiona l drag: Measu rements show that t he actual resistance is greater by approximately a factor of 2. Let's now compare this value with the driving force that would be necessary to move water at the same velocity from cell to cell, crossing the plasma membrane each time. The driving force needed to move water through a layer of cells at 4 mn1 s- 1 is 2 x 10 8 MPa m- 1. This is ten orders of magnitude greater than the driving force needed to move water through our 40-µm-radius xylem vessel. Our calculation clearly shows t hat water flow t hrough the xylem is vastly more efficient than water flow across living cells. Nevertheless, the xylem can make a sign ificant contribution to the tota I resistance to water flow through the plant.

What pressure differe nce is needed to lift w ater 100 meters t o a treetop? \!\Tith the foregoing example in mind, let's see what pressure gradient is needed to move water up to the top of a very taU tree. The tallest trees in the world are the coas t redwoods (Sequoia se111pervire11s) of North America and the mountain ash (Eucalyptus rcgnans) of Australia. Individuals o f both species can exceed 100 m. If we think of the stem of a tree as a long pipe, we can estimate the pressure diffe rence that is needed to overcome the frictiona l drag of moving water from t he soil to the top of the tree by mu ltiply ing the pressure gradient needed to move the water by the height of the tree. The pressure gradients needed to move water through the xylem of very tall trees are on the order of 0.01 MPa m- 1, sma ller than in our previous exan,ple. If we multiply this pressure gradient by the height of the tree (0.01 MPa m -1 x 100 m), we fi nd that the total pressure difference needed to overcome the frictional resistance to water movement through the stem is equal to 1 MPa. In addition to frictional resistance, we must consider gravity. As described by Equation 2.4, for a height difference of 100 m, the difference in 'Pg is approximately 1 MPa. That is, '/1' is 1 MPa higher at the top of the tree than at the g round level. 8 So the other components of water potential must be 1 MPa more negative at the top of the tree to counter the effects of gravity. To allow t ranspiration to occur, the pressure gradien t due to gravity must be added to that required to cause water movement through the xylen1. Thus, we calculate that a pressure difference of roughly 2 MPa, from the base to the top branches, is needed to carry water up the talles t trees. Th e cohesion- tension theory explains w ate r tra nsport in the xyl em In theory, the pressure gradients needed to move water through the xylem cou ld res ult from the generation of positive pressures at the base of the plant or negative pressures at the top of the plant. We mentioned previously that some roots can develop positive hydrostatic pressure in their xylem. However, root pressure is typically less than 0.1 MPa and disappears with transpiration or when soils are

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Water Balance of Plants

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dry, so it is clearly inadequate to move water up a tall tree. Furthermore, because root pressure is generated by the accumulation of ions in the xylem, relying on this for transporting water would require a mechanism for deal ing with these solu tes once the water evaporates from the leaves. Instead, the water at the top of a tree develops a large tension {a negative hydrostatic pressure), and this tension pulls water through the xylem. This mechanism, fi rst proposed toward the end of t he n ineteenth century, is called the cohesiontension theory of sap ascent because it requires the cohesive properties of water to sustain large tensions in the xylen1 water colunms. One can readily den1onstrate xylem tension by puncturing intact xylem th rough a drop of ink on the surface of a stem from a transpiring plant. \!\Then the tension in the xylem is relieved, the ink is drawn instantly into the xylem, resulting in visible streaks along the stem . The xylem tensions needed to pull water from the soil develop in leaves as a consequence of transpiration. How does t he loss of water vapor through open stomata result in the flow of water from t he soil? When leaves open their stomata to obtain CO 2 for photosynthesis, water vapor diffuses out of the leaves. This causes water to evaporate from the surfa ce of cell wa lls inside the leaves. In tum, the loss of water from the cell walls causes the water poten tial in the walls to decrease (Figure 3.8). This creates a gradient in water poten tial that causes water to flow towa rd the sites of evaporation . O ne hypothesis for how a loss of water from cell walls results in a decrease in water potential is that as water evaporates, the surface of the remain ing water is

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Figure 3.8 The driving force for wate r movement through plants originates in leaves. One hypothesis fo r how t his occurs is that as water evaporates fro m t he surfaces of mesophyll cells, water withdraws farther into the interstices of the cell wall. Because cellulose is hydrophilic (contact angle = 0°), the force resulting from surface tension causes a negative pressure in the liquid phase. As the radius of curvature of the air- water interfaces decreases, the hydrostatic pressure becomes more negative, as calculated from Equation 3.1. (Micrograph from Gunning and Steer 1996.)

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- In contrast, low-affinity transporters. Figure 6.12 shows the rate of sucrose uptake simple diffusion through open channels is ideally by soybean cotyledon protoplasts as a function of the externa l sucrose directly proportional to the concentration of the con centration . Uptake increases sharply with concentration a nd transported solute or, fo r an ion, to the diffe rence in begins to saturate at about 10 mM. At concentra tions above 10 mM, electrochemical potential across the membrane. uptake becomes lin ear and non saturable w ithin the concentration range tested. Inhibition of ATP synthesis with metabolic poisons blocks the saturable component, but not the linear one. The interpretation of the pattern shown in Figure 6.12 is that sucrose uptake at low concen trations is an energy-dependent, ca rrier-mediated process (H+-sucrose symport). At higher concentrations, sucrose enters the cells by di ffu sion down its concentration gradient and is therefore insensitive to metabolic poisons. Consistent with these data, both H+- sucrose symporters and sucrose facilitators (i.e., transport proteins that mediate transmembrane sucrose flux down its free-energy gradien t) have been iden ti fied at the molecula r level. ~nax

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Photosynthesis: Th e Li ght Reactions effective as any other photon in driving photosynthesis. However, the yield drops dramatically in the far-red region of chlorophyll absorption (greater than 680 nm). This drop cannot be caused by a decrease in chlorophyll absorption, because the guantum yield measures only light that has actually been absorbed . Thus, light with a wavelength greater than 680 nm is much less efficient than light of shorter wavelengths. Another puzzling experimental result was the enhancement effect, also discovered by En, erson. He n,easured the rate of photosynthesis separately with light of two different wavelengths and then used the two beams simultaneously. When red and far-red light were given together, the rate of photosynthesis was greater than the sum of the individual rates, a startling and surprising observation. These and others observations were eventually explained by experiments performed in the 1960s that led to the discovery that two photochemical complexes, now known as photosystems I and II (PSI and PSII), operate in series to carry out the early energy-storage reactions of photosynthesis. PSI preferentially absor bs far-red light of wavelengths g reater than 680 nm; PSII preferentially absorbs red light of 680 nm and is driven very poorly by far-red light. This wavelength dependence explains the enhancement effect and the red drop effect. Another difference bet ween the photosystems is that:

enhancement effect The synergistic (higher) effect of red and far-red light on the rate of photosynthesis, as compared with the sum of the rates when the two different wavelengths are delivered separately. photosystem I (PSI) A system of photoreactions that absorbs maximally far-red light (700 nm), oxidizes plastocyanin, and reduces ferredoxm. photosystem II (PSII) A system of photoreactions that absorbs maximally red light (680 nm). oxidizes water, and reduces plastoquinone. Operates very poorly under far-red light.

• PSI produces a strong reductant, capable of reducing NADP-, and a weak oxidant. • PSII produces a very strong oxidant, capable of oxidizing water, and a weaker reductant than t he one produced by PSI.

The reductant produced by PSII re-reduces the oxidant produced by PSI. These properties of the two photosystems are shown schematically in Figure 7.13. The scheme of photosynthes is depicted in Figure 7.13, cal led the Z (for zigzag) scheme, has become the basis for understanding 0 2-evolving (oxygenic) pho tosynthetic organisms. It accounts for the operation of two physically and chemically distinct photosystems (I and II), each wi th its own antenna Strong-reductant

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Low Figure 7.17 Funneling of excitation energy from the antenna system toward the reaction center. (A) The excited-state energy of pigments increases with distance from the reaction center; that is, pigments closer to the reaction center are lower in energy than those farther from the reaction center. This energy gradient ensures that exatation transfer toward the reaction center is energetically favorable and that excitation tra nsfer back out to the peripheral portions of the antenna 1s energetically unfavorable. (8) Some energy is lost as heat to the environment by this process, but under optimal conditions almost all the excitation energy absorbed in the antenna complexes can be delivered to the reaction center. The asterisks denote excited states.

As a result of this arrangement, when excitation is transferred, for exan1ple, from a chlorophyll b molecule absorbing maximally at 650 nm to a chlorophylJ a molecu le absorbing maxin1a lly at 670 nm, the d ifference in energy between these two excited chlorophylls is Jost to the environment as heat. For t he excitation to be transferred back to the chlorophyll b, the energy lost as heat would have to be resupplied. The probability of reverse transfer is therefore sn1a1ler sin1ply because thermal energy is not sufficient to n1ake u p the deficit between the lower-energy and higher-energy pigments. This effect gives the energy-trapping process a degree of directionality orirreversibil ity and makes the delivery of excitation to the reaction center more efficient. In essence, the system sacrifices some energy from each quantum so that nearly all of the quanta can be trapped by the reaction center.

light-harvesting complex II (LHCII} The most abundant antenna protein complex, associated primarily with photosystem II. chlorophyll alb antenna proteins Chlorophyll-containing proteins associated with one or the other of the two photosystems in eukaryotic organisms. Also known as light-harvesting complex proteins (LHC proteins).

Many antenna pigment- protein complexes have a common structural motif In all eukaryotic photosynthetic organisms that contain both ch lorophyll a and chlorophyll b, the most abundant antenna proteins are members of a large family of structurally related proteins. Some of these proteins are associated primarily with PSII and are called light-harvesting complex II (LHCII) proteins; others are associated with PSI and are called LHCI proteins. These an tenna complexes are also known as chlorophyll alb antenna proteins. The structure of one of the LHCII proteins has been determined (Figure 7 .18). The protein contains three a -helical regions and binds 14 chlorophyll a and b molecules, as welI as four carotenoids. The structure of the LHCT proteins is genera Uy similar to that of the LHCII proteins. All of these proteins have significant sequence similarity and are almost certainly descendants of a common ancestral protein.

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Photosynthesis: Th e Li ght Reactions

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(C) Lumenal side

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Figure 7.18 Structure of the trimeric LHCII antenna complex from higher plants. The antenna complex is a transmembrane pigment protein; each monomer contains three helical regions that cross the nonpolar part of the membrane. The trimeric complex 1s shown (A) from the stromal side, (8) from within the membrane, and (C) from the lumenal side. Gray, polypeptide; dark blue, Chi a; green, Chi b; dark orange, lutein; light orange, neoxanthin; yellow, violaxanthin; pink, lipids. (After Barros and Kuhlbrandt 2009.)

Light absorbed by carotenoids or ch lorophyll bin the LHC proteins is rapidly transferred to chlorophyll a and then to other antenna pigments that are in timately associated with the reaction center. The LHCil complex is also involved in regulatory processes, which we discuss later in the chapter.

Mechanisms of Electron Transport Some of the evidence that led to the idea of two photochem ical reactions operating in series was discussed earlier in this chapter. In this section we consider in detail the chemical reactions involved in electron transfer during photosynthesis. We discuss the excitation of chlorophyll by light and the reduction of the first electron acceptor, the flow of electrons through photosystems II and I, the oxidation of water as the primary source of electrons, and the reduction of the final electron acceptor (NADp+)_ The chemiosmotic mechanism that n1edia tes ATP synthesis is discussed in detail later in the chapter {see the section Proton Transport a11d ATP Synthesis in the Chloroplast). Electrons from chlorophyll travel through the carriers organized in the Z scheme Figure 7. 19 shows a current version of the Z schen1e, in which all the electron carriers known to function in electron flow from H 2O to NADp+ are arranged vertically at their midpoint redox potentials. Components known to react with each other are connected by arrows, so the Z scheme is really a synthesis of both kinetic and thermodynamic inforn, ation. The large vertical arrows represent the input of light energy into the system.

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1.5 ~ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Photosystem II Phot osyst em I Figure 7.19 Detailed Z scheme for 0 2 -evolving photosynthetic organisms. The redox carriers are placed at their midpoint redox potentials (at pH 7). (1) The vertical arrows represent photon absorption by the reaction center chlorophylls: P680 for photosystem II (PSII) and P700 f or photosystem I (PSI). The excited PSII reaction center chlorophyll, P680*. transfers an electron to pheophytin (Pheo). (2) On the oxidizing side of PSII (to the left of the arrow Joining P680 with P680*), P680 oxidized by pheophytin after light excitation is re-reduced by Y z• which has received electrons from oxidation of water. (3) On the reducing side of PSII (to the right of the arrow joining P680 with P680*), pheophytin t ransfers electrons t o the acceptors PQA and PQ 8,

whteh are plastoquinones. (4) The cytochrome b5' complex transfers electrons t o plastocyanin (PC), a soluble protein, which in turn reduces P7QQ+ (oxidized P700). (5) The acceptor of electrons from P700* (Ac,) is thought to be a chlorophyll, and the next acceptor (A1) is a quinone. A series of membrane-bound iron-sulfur proteins (FeSx, FeSA• and FeS8) transfers electrons to soluble ferredoxin (Fd). (6) The soluble flavoprot ein ferredoxinNAOP+ reductase (FNR) reduces NAOP+ to NAOPH, which is used in the Calvin-Benson cycle to reduce CO 2 (see Chapter 8). The dashed line indicates cyclic electron flow around PSI. (After Blankenship and Prince 1985.)

Photons excite the specialized chlorophyll of t he reaction centers {P680 for PSII; P700 for PSI), and an electron is ejected. The electron then passes through a series of electron carriers and eventually reduces P700 (for electrons from PSII) or NADp+ (for electrons from PSI). Much of the following discussion describes the journeys of these electrons and the natu re of their carriers. Almost all the chemical processes that make up the light reactions of photosynthesis are carried out by fou r major protein complexes: PSII, the cytochrome b6f complex, PSI, an d ATP synthase. These four integral membrane complexes are vectorially oriented in the thylakoid membrane to function as follows (Figure 7.20; also see Figure 7.16): • PSII oxidizes water to 0 2 in the thylak oid lu men and in the process releases protons into the lu1nen. The reduced product of photosystem ll is plastohydroquinone (PQHi). • Cytochrome b6f oxidizes PQH2 molecules tha t were reduced by PSII and delivers elect rons to PSI via the soluble copper protein plastocyanin. The oxidation of PQH2 is coupled to proton transfer in to the lumen from t he stroma, generating a proton motive force. • PSI reduces NADr+ to NADPH in the stroma by the action of ferredoxin (Fd) and the flavoprotein ferredoxin- NADp+ reductase (FNR).

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Transfer of electrons and protons in the thylakoid membrane is carried out vectorially by four protein complexes (see Figure 7 .16B for structures). Water 1s oxidized and protons are released in the lumen by PSII. PSI reduces NADp+ to NADPH in the stroma, via the action of f erredoxin (Fd) and the flavoprotein ferredoxin-NADP+ reductase (FNR). Protons are also transported into the lumen by the action of the cytochrome

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Energy is captured when an excited chlorophyll reduces an electron acceptor molecule As discussed earlier, the function of light is to excite a specialized chlorophyll in the reaction center, either by direct absorption or, more frequently, via energy transfer from an antenna pigment. This excitation process can be envisioned as the promotion of an electron from the highest-energy fill ed orbital of the chlorophyll to the lowest-energy unfi lled orbital (Figure 7.21). The electron in the upper orbital is only loosely bound to the chlorophyll and is easily lost if a molecule that can accept the electron is nearby. The first reaction that converts electron energy into chemical energy-that is, the primary photochemical event- is the transfer of an electron from t he excited state of a chlorophyll in the reaction center to an acceptor molecule. An equivalent way to view this process is that the absorbed photon causes an elect ron rearrangen1.ent in the reaction center chlorophyll, followed by an electron transfer process in which part of the energy in the ph oton is captured in the form of redox energy. In,mediately a fter the photoche1nical event, the reaction cen ter chlorophyll is in an oxidized s tate (electron deficient, or positively charged), and t he nearby electron acceptor molecule is reduced (electron rich, or negatively charged). The system is now at a critica l juncture. The lower-energy orbital of the positively charged oxidized reaction center chlorophyll shown in Figure 7.21 has a vacancy

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Chapter 7

and can accept an elect ron. If the acceptor molecule dona tes its elect ron back to the reaction center chlorophyll, the system will be returned to the state that exis ted before the light Poor Good excita tion, and all the absorbed energy will be converted Acceptor oxidizing Donor reducing orbital orbital agent agent into heat. This wasteful recombination process, however, does not appear to occur to any substantia l degree in functioning reacLight tion centers. Instead, the acceptor transfers its extra electron Donor to a secondary acceptor and so on down t he electron transport Acceptor Good Poor orbital orbital oxidizing reducing chain. The oxidized reaction center of the chlorophyll that ~ ~ agent agent had donated an electron is re-reduced by a secondary donor, Ground-state Excited-state wh ich in turn is reduced by a tertiary donor. In plants, the chlorophyll chlorophyll ultima te electron donor is H 20, and the ultimate electron acceptor is NADp+ (see Figure 7.19). Figure 7.21 Orbital occupation diagram for the ground and The essence of photosynthetic energy storage is t hus the excited states of reaction center chlorophyll. In the ground initial t ransfer of an electron from an excited chlorophyll to an state the molecule is a poor reducing agent (loses electrons from a low-energy orbital) and a poor oxidizing agent (accepts acceptor molecule, followed by a very rapid series of secondary electrons only into a high-energy orbital). In the excited state chen1ical reactions that separate the positive and negative the situation is markedly different, and an electron can be charges. These secondary reactions separate the charges to lost from the high-energy orbital, making the molecule an opposite sides of t he thylakoid membran e in approximately extremely powerful reducing agent. This is the reason for the 200 picoseconds (1 picosecond = 10- 12 s). extremely negative excited-state redox potential shown by W ith the charges thus separated, the reversal reaction P680* and P700* in Figure 7 .19. The excited state can also is ma ny orders of magnitude slower, and t he energy has act as a strong oxidant by accepting an electron into the lower-energy orbital, although this pathway is not significant m been captured. Each of the secondary electron transfers reaction centers. (After Blankenship and Prince 1985.) is accompanied by a loss of some energy, thus making the process effectively irreversible. The quantum y ield for the production of stable products in p urified reaction centers from photosynthetic bacteria has been measured as 1.0; that is, every photon produces stable products, and no reversal reactions occur. Measured quantum requiremen ts for 0 2 production in higher plants under optimal conditions Oow-intensity light) indicate that the va lues for the primary photochemical events are also very close to 1.0. The structure of th e reaction center appears to be extremely fine- tuned for maximum rates of productive reactions and minimum rates of energy-wasting reactions. Redox properties of ground and excited states of reaction center chlorophyll

i

*

bleaching The loss of chlorophyll's characteristic absorbance due to its conversion mto another structural state, often by oxidation.

P700 The chlorophyll of the photosystem I reaction center that absorbs maximally at 700 nm in its neutral state. The P stands for pigment. P680 The chlorophyll of the photosystem II reaction center that absorbs maximally at 680 nm in its ne utral state.

t-

Th e reaction center chlorophylls of the two photosystems absorb at different w avelengths As discussed earlier in the chapter, PSI and PSII have distinct absorp tion characteristics. P recise measurements of absorption maxima a re made possible by optical changes in the reaction center chlorophylls in the reduced and oxidized states. The reaction center chlorophyll is transiently in an oxidized state after losing an electron and before being re-reduced by its electron donor. In the oxidized state, chlorophylls lose their characteristic strong light absorbance in the red region of the spectru m; t hey become bleached. It is therefore possible to monitor the redox state of these chlorophylls by time-resolved optical absorbance measurements in which this bleaching is monitored directly. Using such techniques, it was found that t he reaction center chlorophyll of PSI absorbs maximally at 700 nm in its reduced state. Accordingly, t his chlorophyll is named P700 (the P stands for pigment). The analogous optical transient of PSII is at 680 nm, so its reaction center chlorophyll is known as P680. T he prima ry donor of PSI, P700, is also a dimer of chlorophyll a molecules. PSII a lso contains a dimer of chlorophylls, although the primary electron transfer event may not originate from these pigments. In the oxidized state, reaction center ch lorophylls contain an unpaired electron.

.

"Cl

Photosyn th esis: Th e Li ght Reactions Molecules with unpaired electrons can often be detected by a magnetic-resonance technique known as electron spin resonance (ESR) spectroscopy. ESR studies, along with the spectroscopic measurements already described, have led to the discovery of many intermediate electron carriers in the photosynthetic electron transport system.

The PSII reaction center is a multi-subunit pigment-protein complex PSII is contained in a multi-subunit protein supercomplex. In higher plants, the supercomplex has two con1plete reaction centers and some antenna complexes. The core of the reaction center consists of two membra ne proteins known as D1 and 02, as well as other proteins, as shown in Figure 7.22. The primary donor ch lorophyll, additiona l chlorophylls, carotenoids, pheophytins, and plasto quinones (two electron acceptors described below) are

201

electron spin resonance (ESR) spectroscopy A magnetic-resonance technique that detects unpaired electrons in molecules. Instrumental measurements that identify intermediate electron carriers in t he photosynthetic or respiratory electron transport system.

(A)

Nonheme Fe

STROMA

Figure 7.22 Structure of t he PSII reaction center from the cyanobacterium Thermosynechococcus elongatus, resolved at 0.35 nm. The structure includes the D1 (yellow) and D2 (orange) core reaction center proteins, the CP43 (green) and CP47 (red) antenna proteins, cytochromes b 559 and c550, the extrinsic 33-kDa protein PsbO (dark blue), and the pigme nts and other cofactors. (A) Side view parallel to t he membrane plane. (B) View from t he lumenal surface, perpendicular to the plane of the membrane. (C) Detail of the Mn-containing oxygen-evolving complex (OEC). (A and B from Ferreira et al. 2004; C from Umena et al. 2011.)

Two-fold axis

(B)

rwo-iold syrnrn_etr\l

---

3)(IS

Psbl

,,

,

His 332

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.

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202

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Chapter 7

pheophytin A chlorophyll in which the central magnesium atom has been replaced by two hydrogen atoms. plastohydroquinone (PQH 2) The fully reduced form of plastoquinone. cytochrome b6 f complex A large multi-subunit protein complex containing two b-type hemes, one c-type heme (cytochrome f), and a Rieske iron-sulfur protein. A relatively immobile complex distributed equally between the grana and the stroma regions of the thylakoid membranes. cyt ochrome f A subunit in the cytochrome b 6 f complex that plays a role in electron transport between photosystems I and II. Rieske iron- sulfur protein A protein subunit in the cytochrome b 6 f complex, in which two iron atoms are bridged by two sulfur atoms, with two histidine and two cysteine ligands.

N

bound to the membrane proteins D1 and D2. Other protei ns serve as antenna complexes or are involved in oxygen evolution. Some, such as cytochrome b55 9' have no known function but may be involved in a protective cycle around PSil.

Water is oxidized to oxygen by PSII Water is oxidized according to the following chemical reaction: 2 H 20

-+

0 2 + 4 H• + 4 e-

(7.8)

This equation indicates that fo ur electrons are removed from two water molecules, generati ng an oxygen molecule and four hydrogen ions. Water is a very stable molecule. Oxidation of water to form molecular oxygen is very difficult: The photosynthetic oxygen-evolving complex (0 EC) is the o nly known biochemical system that carries out this reaction, and is the source of almost all the oxygen in Earth's atmosphere. The protons produced by water oxidation are released into the lumen of the thylakoid, not directly into the stromal compartment (see Figure 7.20). They are released into the lumen because of the vectorial nature of t he membrane and the fact that the oxygen-evolving complex is localized near the interior surface of the thylakoid membrane (see Figure 7.22A). These protons are eventua lly transferred from the lun1en to the stroma by t ranslocation through the ATP synthase. In this way, the pro tons released during water oxidation contribute to the electrochemical potential driving ATP formation (see Figure 7.20). lt has been known for many years that manganese (Mn) is an essential cofactor in the water-oxidizingprocess (see 01apter 4), and a classic hypothesis in photosynthesis research postulates that Mn ions undergo a series of oxidations-known as S states and labeled S0, S1, S2, S3, and S4- that are perhaps linked to H 2Ooxidation and the generation of 0 2 . Thjs hypothesis has received strong support from a variety of experiments, most notably X-ray absorption and ESR studies, both of which detect the manganese ions directly. Analytical experiments indicate that four Mn ions are associated with each oxygen-evolving complex. Other experiments have shown that c1- and Ca 2• ions are essential for 0 2 evolution. The detailed chemical n1echanism of the oxidation of water to 0 2 is not yet well understood, but with structural information now available, rapid progress is being made in this area. One e lectron carrier, generally identified as Yz, functions between theorygen-evolving complex and P680 (see Figure 7.19). To fu nction in this region, Yz nee ds to have a very strong tendency to retain its electrons. This species has been identified as a radical formed from a tyrosine residue in the D1 protein of the PSII reaction center.

Pheophytin and two quinones accept electrons from PSII Spectral and ESR studies have revea led the structura l arrangement of the carriers in the electron acceptor complex. Pheophytin, a chlorophyll in w hich the central magnesium ion has been replaced by two hydrogen ions, acts as an early acceptor in PSII. The structural change gives pheophytin chemica l and spectra l properties that are slightly different from those of Mg-based chlorophylls. Pheophytin passes electrons to a complex of two plastoquinones in close proximity to an iron ion. The two plastoquinones, PQA and PQa, are bound to the reaction center a nd rece ive electrons fron1 pheophytin in a sequential fashion. Transfer of the two electrons to PQ5 reduces it to PQ/-, and the reduced PQl- takes two protons fron1 the strom a side of the medium, yielding a fu lly reduced plastohydroqu inone (PQH 2) (Figure 7 .23). The PQH2 then dissociates from the reaction center complex and enters the hydrocarbon portion of the membrane, where it in turn transfers its electrons to the cytochrome b6f complex. Unlike the large protein complexes of the thylakoid membrane, PQH 2 is a s mall, non polar molecule that d iffuses readily i n the nonpolar core of the membrane bilayer.

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Photosynthesis: Th e Li ght Reactions (A)

203

Figure 7.23 Structure and reactions of plastoquinones that operate in PSII. (A) The plastoquinone consists of a qurno1d head and a long nonpolar tail that anchors it in the membrane. (B) Redox reactions of plastoquinone. The fully oxidized plastoquinone (PQ), anionic plastosemiquinone (PQ• ), and reduced plastohydroquinone (PQH 2) forms are shown; R represents the side chain. 0

Plastoquinone (B)

o•

0

H3 C

R

+

e-

H3 C

H3 C

OH R

+

1 e-

2 H+

"":::

R

+

H3C 0

H3C

/7

H3C

OH

0-

Plastoquinone

Plastosemiquinone

(PQ)

(PQ• )

Plastohydroquinone (PQH 2)

Electron flow through the cytochrome b6 f complex also transports protons

The cytochrome b 6f complex is a large mu lti-subunit protein with severa l prosthetic groups (Figure 7. 24) . It contains two b-type hemes and one c-type heme (cytochrome f ). In c-type cytochromes the heme is covalently attached to the peptide; in b-type cytochromes the chemically sim ilar protohe1ne group is not covalently attached. In addition, the complex contains a Rieske iron-sulfur protein (named for the scientist who discovered it), in which two iron ions are bridged by two s ulfide ions. The functional roles of all these cofactors are reasonably well understood, as described below. However, the cytochrome b6f complex also

PQH2

e-

e-

--------------LUMEN

/

L

------------· ,.. -------------2H+

'-......[2Fe- 2S]

e-

, cluster

Hemet

Figure 7.24 Structure of the cytochrome b6f complex from cyanobactena. The diagram on the right shows the arrangement o f the proteins and cofactors in the complex. Cytochrome b 6 protein is shown in blue, cytochrome fprotein in red, Rieske iron-sulfur protein in yellow, and other

smaller subunits in green and purple. On the left, the proteins have been omitted to more clearly show the posit ions of the cofactors. [2 Fe-2S] cluster, part of the Rieske iron-sulfur protein; PC, plastocyanin; PQ, plastoquinone; PQH 2 , plastohydroqu1none. (After Kurisu et al. 2003 .)

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Chapter 7

Q cycle A mechanism for oxidation of plastohydroquinone (reduced plastoquinone, also called plastoquinoO in chloroplasts and of ubihydroquinone (reduced ubiquinone, also called ubiquinol) in mitochondria.

FeSR An iron- and sulfur-containing subunit of the cytochrome b5' complex, involved in electron and proton transfer.

contains additional cofactors-including an additional heme group (called heme c11), a chlorophyll, and a carotenoid- whose functions are yet to be resolved. The s tructures of the cytochrome b6 f complex and the related cytochrome bc1 complex in the mitochondrial electron transport chain (see Chapter 11) suggest a mechan ism for electron and proton flow. The precise way by which electrons and protons flow through the cytochrome b6 f c01nplex is not yet fully understood, but a mechan ism known as the Q cycle accounts for most of the observations. In t his mechanism, plastohydroguinone (also ca lled plastoguinol) (PQH2) is oxidized, and one of the two electrons is passed along a linear electron transport chain toward PSI, while the other electron goes through a cyclic process that increases t he number of protons pumped across the membrane (Figure 7 .25). In the linear electron transport chain, the oxidized Rieske protein (FeSR) accepts an electron from PQH2 and transfers it to cytochrome f (see Figure 7.25A). Cytochrome f then transfers an electron to the blue-colored copper protein plastocyani:n (PC), which in turn reduces oxidized P700 of PSI. In the cyclic part of the process (see Figure 7.25B), the plastosemiquinone (see Figure 7.23) tra nsfers its other electron to one of the b-type hemes, releasing both of its protons to the lumenal side of the membrane. The first b-type heme transfers its electron through the second b-type heme to an oxidized plastoguinone molecule, reducing it to the semiquinone form near the stron1al surface of the con1plex. Another similar sequence of electron flow (see Figure 7.25B) fully reduces the plastoquinone, which picks up protons from the stromal side of the membrane and is released from the b6f complex as plastohydroquinone. (A) First QH 2 oxidize d Cytochrome b6' complex

STROMA

PQ PQ•~...., Heme Cn •, Cyt b 1 - - - - - - - - - ~

Thylakoid membrane

Figure 7.25 Mechanism of electron and proton transfer in the cytochrome b/ complex. This complex contains two b-type cytochromes (Cyt b), a c-type cytochrome (Cyt c, historically called cytochrome f), a Rieske Fe-S protein (FeSR), and two qumone ox1dat1on- reductmn srtes. (A) The noncyclic or linear processes: A plastohydroquinone (PQH 2) molecule produced by the action of PSII (see Figures 7 .20, 7 .23) IS oxidized near the lumenal side of the complex, transferring its two electrons to t he Rieske Fe-S protein and one of the b-type cytochromes and simultaneously expelling two protons to the lumen. The electron transferred to FeSR is passed to cytochrome f (Cyt f) and then to plastocyanin (PC). which reduces P700 of PSI. The reduced b-type cytochrome transfers an electron to the other b-type cytochrome, which reduces a plastoquinone (PQ) to the plastosemiquinone (PQ•) state (see Figure 7.23). (8) The cyclic processes: A second PQH 2 is oxidized, with one electron going from FeSR to PC and finally to P700. The second electron goes through the two b-type cytochromes and reduces the plastosemiquinone to the plastohydroquinone, at the same time picking up two protons from the stroma. Overall, four protons are transported across the membrane for every two electrons delivered to P700.

~ PQH2

~

0

,,•··► Cyt b ...••••

PQ .,_-7 •..

, - - - - - - ---,r-1 •►

®

,:(

(t

,t. •:

FeSR ······~yt f \

...___ __ _ __.,_,,. 2 H+ LUMEN

(B) Second QH2 oxidized STRO MA Thylakoid membrane

2 H+

2 H+

LUMEN

PC

· ••••,,'

o ···;lastocyanin

.

"Cl

Photosynthesis: Th e Li ght Reactions The overa II result of two turnovers of the complex is that two electrons are transferred to P700, two plastohydroguinones are oxidized to the plastoguinone form, and one oxidized plastoguinone is reduced to the plastohydroquinone form. In the process of oxidizing the plastohydroqu inones, four protons a re transferred from the stromal to the lumenal side of the membrane. By this n1echanisn1, electron flow connecting the acceptor side of the PSII reaction center to the donor side of the PSI reaction center also gives rise to an electrochemical potential across the n1embrane, due in part to H+ concentration differences on the two sides of the membrane. This electrochemical potential is used to power the synthesis of ATP. The cyclic electron flow through the cytochrome band plastoguinone increases t he number of protons pumped per electron beyond what could be achieved in a strictly linear seguence. Plastoquinone and plastocyanin carry electrons between photosystem II and photosystem I The location of the two photosystems at different sites on the thylakoid membranes (see Figure 7.16) reguires that at least one component is capable of moving along or within the n1embrane in order to deliver electrons produced by PSII to PSI. The cytochrome b6f complex is distributed egua Uy between the grana and the stron1a regions of the membranes, but its large size makes it unlikely that it is the mobile carrier. Instead, plastoquinone or plastocyanin or possibly both are thought to serve as mobile carriers to connect the two photosystems. Plastocyanin (PQ is a small (10.5 kDa), water-soluble, copper-containing protein that transfers electrons between the cytochron1e b6f con1plex and P700. This protein is found in the lumenal space (see Figure 7.25). Th e PSI reaction center reduces NADP• The PSI reaction center complex is a large multi-subunit complex (Figure 7 .26). Unlike in PSil, in which the antenna chlorophylls are associated with the reaction center but are present on separate pign1ent-proteins, a core antenna consisting of about 100 chlorophylls is an integral part of the PSI reaction center. The core antenna and P700 are bound to two proteins, PsaA and PsaB, with molecular masses in the range of 66 to 70 kDa. The PSI reaction center complex from pea contains four LHCI complexes in addition to the core structure similar to that found in cyanobacteria {see Figure 7.26). The tota I number of ch lorophyl I molecules in this con1plex is nearly 200. The core antenna pigments form a bowl surrounding the electron transfer cofac tors, which are in the center of the complex. In their reduced form, the electron carriers that function in the acceptor region of PSI are all extremely strong reducing agents. These reduced species are very unstable and thus difficult to identify. Evidence indicates that one of these early acceptors is a chlorophyll molecule, and another is a guinone species, phylloquinone, also known as vitamin K1. Additiona l electron acceptors indude a series of three membrane-associated iron- sulfur proteins, also known as Fe-S centers: FeSx, FeSA, and Fe S8 (see Figure 7.26). FeSx is part of the P700-binding protein; FeSA and FeS 8 reside on an 8- kDa protein that is part of the PSI reaction center con1plex. Electrons are transferred through FeS A and FeS 8 to ferred oxin (Fd), a small, water-soluble iron-sulfur protein (see Figures 7.19 and 7.26). The membrane-associated flavoprotein f erredoxin- NADP• reductase (FNR) reduces NADp+ to NADPH, thus completing the seguence of noncyclic elect ron transport that begins with the oxidation of water. Tn add ition to the reduction ofNAD-ri-, reduced ferredoxin produced by PSI has several other functions in the chloroplast, such as supplying reductant for nitrate reduction and regulating some of the carbon fixation enzymes (see Chapter 8).

205

plastocyan in (PC) A small (10.5 kDa), water-soluble, copper-containing protein that transfers electrons between the cytochrome b6 f complex and P700. This protein is found in the lumenal space. Fe-S centers Prosthetic groups consisting of inorganic iron and sulfur that are abundant in proteins in respiratory and photosynthetic electron transport. FeSx, FeSA' FeS 8 Membrane-bound iron-sulfur proteins that transfer electrons between photosystem I and ferredoxin. ferredoxin (Fd) A small, water-soluble iron-sulfur protein mvolved in electron transport in photosystem I. ferredoxin- NADP• re ductase (FNR) A membrane-assoaated flavoprotein that receives electrons from photosystem I and reduces NADp+ to NADPH.

N 0

u,

.

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206

N 0

Chapter 7

(A)

0)

(8)

Ferredoxin

STROMA

D

FeS cluster

Major protein PsaA

Major protein PsaB

L I

FeSx

,A,

Phylloquinone

A(_-+l'--l- Chlorophyl l J

HG

molecule

Minor protein PsaN N Light

Chlorophyll molecule

Protein ribbon for LHCI complex

Figure 7.26

Structure of PSI. (A) Structural m odel of the PSI react ion center from higher plants. Components of the PSI reaction center are organized around two major core proteins, PsaA and PsaB. Minor proteins PsaC to PsaN are labeled C to N. Electrons are transferred from plastocyanin (PC) to P700 (see Figures 7 .19 and 7 .20) and then to a chlorophyll molecule (Ao), to phylloquinone (A 1), to t he Fe-S centers FeSx, FeSA, and FeS8 , and finally to the soluble iron- sulfur protein ferredoxin (Fd). (B) Structure of the PSI reaction center complex from pea at 0.44 nm resolution, including the LHCI antenna complexes. This is viewed from the stromal side of t he membrane. (A after Buchanan et al. 2000; B after Nelson and Ben-Shem 2004.)

Cyclic electron flow generates ATP but no NADPH Some of the cytochrome b6f complexes are fou nd in the s troma region of the n1embrane, where PSI is located. Under certain cond itions, cyclic electron flow is known to occur from the reducing side of PSI via plastohydroquinone and the b6f complex and back to P700. This cyclic electron flow is coupled to pro ton pumping into the lumen, which can be used for ATP synthesis but does not oxidize water or reduce NADP" (see Figure 7.16B). Cyclic electron flow is especially important as an ATP source in the bundle sheath chloroplasts of some plants that carry out C4 carbon fixation (see Chapter 8). The molecular mechanism of cyclic electron flow is not well understood.

cyclic electron flow In photosystem I, the flow of electrons from the electron acceptors through the cytochrome b 6 f complex and back to P700, coupled to proton pumping into the lumen. This electron flow energizes ATP synt hesis but does not oxidize water or reduce NADp+_

Some herbicides block photosynthetic electron flow The use of herbicides to kill unwanted plants is widespread in modern agriculture. Many different classes of herbicides have been developed. Some act by blocking amino acid, carotenoid, or lipid biosynthesis or by disrupting cell division. Other herbicides, such as dichlorophenyldimethylurea (DCMU, also known as diuron) and paraquat, block photosynthetic electron flow (Figure 7.27). DCMU blocks electron flow at the qui none acceptors of PSII, by con1peting for the binding site of plastoquinone that is normally occupied by PQ8 . Paraquat accepts electrons from the early acceptors of PSI and then reacts with oxygen to form superoxide, 0 2•, a reactive oxygen s pecies that is very damaging to chloroplast components.

.

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Photosynthesis: Th e Li ght Reactions

207

-N+O-QN+-

CH 3

CH 3

2

er

Paraquat (methyl viologen) DCMU (diuron) (3.4-di ch loroph enyl di methylurea) (B)

Paraquat

" ::

f"NAD":J

!"(

NADPH

P680

Chemical structure and mechanism of action of two important herbicides. (A) Chemical structure of dichlorophenyldimethylurea (DCMU) and methyl viologen (paraquat, a cloride salt), two herbicides that block photosynthetic electron flow. DCMU is also known as diuron. (B) Sites of action of the two herbicides. DCMU blocks electron flow at the plastoquinone acceptors of PSII by competing for the binding site of plastoquinone. Paraquat acts by accepting electrons from the early acceptors of PSI.

Figure 7.27

Proton Transport and ATP Synthesis in the Chloroplast In the preceding sections we learned how captured light energy is used to reduce NAO? to NADPH. Another fraction of the captured light energy is used for light-dependent ATP synthesis, which is known as photophosphorylation. This process was discovered by Daniel Arnon and his coworkers in the 1950s. Under norma l cellular conditions, photophosphorylation requires electron flow, a lthough under some conditions e lectron flow and photophosphorylation can take place independently of each other. Electron flow without accompanying phosphorylation is said to be uncoupled . Tt is now widely accepted that photophosphorylation \Vorks via the chemiosmotic mechanism. This mechanism was first proposed in the 1960s by Peter Mitchell. The same general mechanism drives phosphorylation during aerobic respiration in bacteria and mitochondria (see Chapter 11), as well as the transfer of many ions and metabolites across membranes {see Chapter 6). Chemiosmosis appears to be a unifying aspect of membrane processes in all forms of life. In Chapter 6 we discussed the role of ATPases in chemiosmosis and ion transport at the cell's plasma membrane. The ATP used by the plasma rnembrane ATPase is synthesized by photophosphorylation in the chloroplast and oxidative phosphorylation in the mitochondrion. Here we are concerned w ith chemiosmosis and transmembrane proton concentration differences used to make ATP in the chloroplast. The basic principle of chemiosmosis is that ion concentration differences and electrical potential differences across membranes are sources of free energy

The formation of ATP from ADP and inorganic phosphate (Pi), catalyzed by the CFoF,ATP synthase and using light energy stored in the proton gradient across the thylakoid membrane. photophosphorylation

A process by which coupled reactions are separated in such a way that the free energy released by one reaction is not available to drive the other reaction. uncoupling

N 0 -.J

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208

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Chapter 7

co that can be used by the cell. As described by the second law of thermodynamics, any nonuniform distribution of matter or energy represents a source of energy. Differences in chemical potential of any molecular species whose concentrations are not the same on opposite sides of a membrane provide such a source of energy. The asyn1me tric nat ure of the photosynthetic n1embrane and the fact that proton flow from one side of the membrane to the other accomp anies electron flow were discussed earlier. The direction of proton translocation is such that the stroma becomes more alkaline (fewer H+ ions) and the lumen becomes more acidic (more H+ ions) as a result of electron transport (see Figures 7.20 and 7.25). Some of the early evidence supporting a chen1iosmotic n1echanism of photosynthetic ATP formation was provided by an elegant experiment carried out by Andre Jagendorf and coworkers (Figure 7.28). They suspended chloroplast thyla koids in a pH 4 buffer, and the buffer diffused across the membrane, causing the interior, as well as the exterior, of the thylakoid to equilibrate at this acidic pH. They then rapidly transferred t he t hylakoids to a pH 8 buffer, thereby creating a pH difference of four units across the thylakoid me1nbrane, with the inside acidic relative to the outside. They found that large an,ounts of ATP were formed from ADP and P; by this process, with no light input or electron transport. This result supports the predictions of the chemiosmotic hypothesis, described in the paragraphs that follow. Mitchell proposed that the total energy available for ATP synthesis, which he ca lled the proton motive force (6.p), is the sum of a proton chemical potential and a transmembrane electrica l potential. These two components of the proton motive force from the outside of the membrane to the inside are given by the following equation:

chemical potential The free energy associated with a substance that is available to perform work. proton motive force (PMF) The e nergetic effect of the electrochemical H+ gradient across a membrane, expressed in units of electrical potential.

(7.9) where ti£ is the transmembrane electrical potential, and pH1 - pH0 (or 6 pH) is the pH difference across the membrane. The constant of proportiona lity (at

(~

I

~