Photosynthesis: Light Reactions

Light Reactions of Photosynthesis

• Life on Earth depends on solar energy, which is harvested through photosynthesis
• Photosynthesis is the only biologically important process capable of capturing energy
• A large portion of the planets energy resources come from photosynthetic activity in ancient and recent times, such as fossil fuels
• Photosynthesis literally means "synthesis using light"
• Photosynthetic organisms synthesize complex carbon compounds using solar energy
• In higher plants, the mesophyll of the leaves contain the most active photosynthetic tissue
• Mesophyll cells are host to multiple chloroplasts
• Light energy is converted into chemical energy via two functional units known as photosystems, which are located in the chloroplasts.
• Absorbed light energy is used to transfer electrons through a series of electron donors and acceptors
• Electrons reduce NADP+ to NADPH and oxidize H2O to O2
• Light energy is used to generate a proton motive force (PMF) across the thylakoid membrane, which synthesizes ATP

General Concept of Photosynthesis

• Light has both particle and wave characteristics

Light Has Both Particle and Wave Characteristics

• Light has properties of both particles and waves
• Waves are characterized by wavelength
• A light wave is a transverse electromagnetic wave where electric and magnetic fields oscillate perpendicularly
• Sunlight is composed of photons of different frequencies
• Human eyes are only sensitive to a small range of frequencies within the visible-light region of the electromagnetic spectrum
• The absorption spectrum of chlorophyll-a shows what solar output is used by plants
• Absorption spectra provide the amount of light energy taken up by a molecule as a function of the wavelength of light
• Chlorophyll appears green because it absorbs light mainly in the red and blue parts of the spectrum
• Chlorophyll absorbs a photon in its ground state and transitions to an excited state
• Blue light excites chlorophyll to a higher energy state than red light, because photons have higher energy when wavelengths are shorter
• Chlorophyll is unstable in the higher excited state
• Chlorophyll rapidly dissipates energy as heat and enters the lowest excited state, where it can be stable for nanoseconds
• Energy capture must be rapid due to excited state instability

Available Energy Disposal

• Four alternative pathways exist for disposing of available energy in the lowest excited state of chlorophyll:
• Excited chlorophyll re-emits a photon and returns to its ground state, known as fluorescence
• Excited chlorophyll returns to its ground state by directly converting excitation energy into heat, without emission
• Excited chlorophyll participates in energy transfer, transferring its energy to another molecule
• Photochemistry occurs when energy from the excited state causes chemical reactions
• The photochemical reactions occurring during photosynthesis are among the fastest known chemical reactions
• Extreme speed enables photochemistry to compete with the three other possible excited state reactions

Photosynthetic Pigments Absorb the Light That Powers Photosynthesis

• Sunlight energy is first absorbed by plant pigments located in the chloroplast
• Chlorophylls and bacteriochlorophylls represent the typical pigments in photosynthetic organisms
• Chlorophylls a and b are found in green plants
• Chlorophylls c and d are found in some protists/cyanobacteria
• All chlorophylls have a complex ring structure chemically related to porphyrin groups in hemoglobin/cytochromes
• Carotenoids are linear molecules with multiple conjugated double bonds
• Absorption bands in the 400-500 nm region give carotenoids an orange color and are present in all photosynthetic organisms
• Carotenoids transfer light energy to chlorophyll for photosynthesis and are known as accessory pigments
• Carotenoids help protect from light damage
• Phycobilisomes are the primary light harvesting antennae for photosystem II in cyanobacteria and red algae
• Supramolecular complexes are composed of water-soluble phycobiliproteins with covalently attached tetrapyrroles, known as phycobilins
• Absorbed light energy is transferred by a rapid, radiation-less downhill energy transfer
• Energy goes from phycoerythrin/phycoerythrocyanin to C-phycocyanin, and then to allophycocyanin species, acting as final energy transmitters/reaction centers
• Action spectrums show the magnitude of a biological systems response to light as a function of wavelength
• Action spectrums for photosynthesis can be constructed from measurements of oxygen evolution at different wavelengths

Light-Harvesting Antennas and Photochemical Reaction Centers

• Light absorption results as a cooperation between chlorophyll and carotenoid molecules
• Most pigments serve as an antenna complex
• Antenna complexes collect light and transfer energy to reaction centre complexes
• Chemical oxidation and reduction reactions occur in reaction center complexes and leads to long-term energy storage
• A single chlorophyll molecule absorbs few photons each second
• The system is kept active when a reaction center receives energy from many pigments at once
• Several hundred pigments are associated with each reaction center
• Each reaction center must operate four times to produce one molecule of oxygen - hence 2500 chlorophylls per O2.
• Reaction centers and most antenna complexes are integral components of the photosynthetic membrane
• These membranes are found within the chloroplast in eukaryotic photosynthetic organisms
• In photosynthetic prokaryotes, the site of photosynthesis is the plasma membrane or its derivatives

Oxygen-Evolving Organisms Have Two Photosystems

• The quantum yield of photochemistry is 1.0.
• ~10 photons are required to produce each O2 molecule; therefore, the overall max quantum yield of O2 production is 0.1
• Any photon absorbed by chlorophyll or other pigments drives photosynthesis equally
• Yield drops dramatically in the far-red region of chlorophyll absorption (>680nm)
• Emerson discovered the enhancement effect
• In 1960, it was discovered that two photochemical complexes, photosystem I and II (PSI and PSII) which operate in series to carry out early energy storage reactions of photosynthesis
• Photosystem I absorbs Far-red light.
• Photosystem II absorbs red light
• Photosystem I produces a strong reductant (reducing NADP+) and a weak oxidant.
• Photosystem II produces a strong oxidant (oxidizing water) and a weaker reductant than photosystem I

Organization of Photosynthetic Apparatus

• The chloroplast is the site of photosynthesis
• Photosynthesis takes place in the subcellular organelle (chloroplast) in photosynthetic eukaryotes
• The chloroplast contains internal membranes known as thylakoids, which are the site of light reactions
• Carbon reduction reactions are catalyzed by water-soluble enzymes and take place in the stroma, which can be found in the chloroplast
• Thylakoid membranes closely associated with each other are known as grana lamellae
• Exposed membranes lacking stacking are known as stroma lamellae
• Envelopes surround chloroplasts and consist of two separate lipid bilayer membranes
• Chloroplasts contain DNA, RNA, and ribosomes

Thylakoids Contain Integral Membrane Proteins

• Proteins essential to photosynthesis are embedded in thylakoid membranes
• Reaction centers, antenna pigment-protein complexes, and most electron carrier proteins are integral membrane proteins
• Thylakoid membrane proteins oriented toward the stromal side of membrane, and toward the interior space of thylakoid (lumen)
• Chlorophylls and accessory light-gathering pigments always form pigment-protein complexes
• Antenna and reaction center chlorophylls are organized to optimize energy transfer in antenna complexes and electron transfer in reaction centers

Spatially Separated Photosystem I and II in the Thylakoid Membrane

• PSII, antenna chlorophylls, and associated electron transport proteins are located predominantly in the grana lamellae.
• PSI reaction center, antenna pigments, electron transfer proteins, and ATP synthase (catalyzes ATP formation) are found in the stroma lamellae and at the grana lamellae edges
• Cytochrome b6f complex, which connects the two photosystems, is evenly distributed between stroma and granum lamellae
• Two photochemical events in O2-evolving photosynthesis are spatially separated
• Separation implies one or more electron carriers diffuses from the grana region to the stroma region where electrons are delivered to photosystem I
• PSII to PSI ratio is about 1.5:1, which can change based on light conditions

Organization of Light-Absorbing Antenna Systems

• Different classes of photosynthetic organisms have remarkably varied antenna systems
• Reaction centers are similar even in distantly related organisms
• The variety of antenna complexes reflects evolutionary adaptation to different environments

Antenna Systems Contain Chlorophyll and are Membrane Associated

• Antenna System varies in size by organism
• Higher plants contain 200-300 chlorophylls per reaction center
• Algae and bacteria contain thousands of pigments per reaction center
• Antenna pigments are associated with proteins to form pigment-protein complexes
• Excitation energy moves from chlorophyll which absorbs light to the reaction center via fluorescence resonance energy transfer (FRET)
• Energy is transferred from one molecule to another by a nonradiative process
• Antenna pigments transfer 95-99% of photons absorbed, toward the reaction center where it can be used for photochemistry
• Energy transfer among antenna pigments are physical
• Electron transfer involves chemical (redox) reactions

Antenna Funnels Energy to Reaction Center

• Pigment sequence in antennae that funnel absorbed energy towards the reaction center have absorption maxima that are progressively shifted toward longer red wavelengths
• The energy of the excited state is lower nearer the reaction center
• Eukaryotic photosynthetic organisms containing chlorophyll a and b, have most abundant antenna proteins belonging to a large family of structurally related proteins
• Those associated with photosystem II are called light-harvesting complex II (LHCII) proteins
• Those associated with photosystem I are called LHCI proteins, also known as chlorophyll a/b antenna proteins.
• All have significant sequence similarity

Mechanisms of Electron Transport

• Electrons From Chlorophyll Travel Through The Carriers Organized In The "Z Scheme"
• Electron carriers function in electron flow from H2O to NADP+
• Components known to react with each other are connected by arrows, the Z scheme is a synthesis of kinetic and thermodynamic information
• Vertical arrows represent the input of light energy into the system
• Photons excite specialized chlorophyll of the reaction centers (P680 (PSII) and P700 (PSI) and an electron is ejected
• The electron passes carriers and reduces P700 (electrons from PSII) or NADP+ (electrons from PSI)
• Light reactions use 4 major protein complexes: photosystem II, the cytochrome b6f complex, photosystem I, and the ATP synthase
• These 4 integral membrane complexes are vectorially oriented in the thylakoid membrane
• Photosystem II oxidizes water to O2 in the thylakoid lumen, which releases photons to the lumen
• Cytochrome b6f oxidizes plastohidroquinone (PQH2) molecules (reduced by PSII) and delivers electrons to PSI.
• Oxidation of plastohidroquinone is coupled to proton transfer into the lumen from the stroma, which generates a proton motive force
• Photosystem I reduces NADP+ to NADPH in the stroma via ferredoxin (Fd) and the flavoprotein ferredoxin-NADP reductase (FNR)
• ATP synthase produces ATP as protons diffuse back through it from the lumen into the stroma

The Photosystem II

• PSI and PSII have distinct absorption characteristics
• The reaction center chlorophyll of photosystem I absorbs maximally at 700 nm (P700).
• The analogous optical transient of photosystem II is at 680 nm (P680)
• Photosystem II exists in a multisubunit protein supercomplex
• D1/D2 membrane proteins found in the reaction center core
• Additional chlorophylls, carotenoids, phaeophytins, and plastoquinones are bound to D1 and D2 membrane proteins
• Water is oxidized to oxygen by photosystem II
• Water molecules have four electrons removed, generating an O2 molecule and four hydrogen ions
• Protons are released to the thylakoid lumen and transferred to the stroma via ATP synthase
• Protons released during water oxidation contribute to electrochemical potential driving ATP formation
• Manganese (Mn) is an essential cofactor in the water-oxidizing process
• Oxidation of Mn ions/S states oxidation is linked to H2O oxidation and O2 generation
• Phaeophytin and two quinones accept electrons, in the electron acceptor complex, in photosystem II

Cytochrome b6f Complex

• Cytochrome b6f complex is a large multisubunit protein with several prosthetic groups
• It is equally distributed between grana and stroma regions of the membranes
• Precise mechanisms aren't fully understood
• The Q cycle accounts for most observations
• In this mechanism, plastohydroquinone (PQH2) is oxidized.
• One electron is passed along a linear electron transport chain towards photosystem I
• The other electron goes through a cyclic process that increases the number of protons pumped across the membrane
• In the linear transport chain, the oxidized Rieske protein (FeSR) accepts an electron from PQH2 and transfers it to cytochrome f
• Cytochrome f then transfers an electron to plastocyanin (PC), which in turn reduces oxidized P700 of PSI
• Plastocyanin (PC) is a small water soluble copper containing protein that transfers electrons between the cytochrome b6f complex and P700

Photosystem 1 Reaction Center Reduces NADP+

• PSI reaction center complex is a large multisubunit complex
• A core antenna of 100 chlorophylls are integral to the PSI reaction center
• The core antenna and P700 are bound to PsaA and PsaB proteins
• Electrons from PSI reaction center are transferred to ferredoxin (Fd)
• Fd is a small water-soluble iron-sulfur protein
• Membrane-associated flavoprotein ferredoxin-NADP-reductase (FNR) reduces NADP+ to NADPH
• Accomplishes noncyclic electron transport which begins wih oxidation of water
• Some cytochrome b6f complexes are found in the stroma region of the membrane, where photosystem I is located
• Under certain conditions, cyclic electron flow proceeds from the reducing side of photosystem I via plastohydroquinone and the b6f complex and back to P700
• Cyclic electron flow is coupled to proton pumping into the lumen, which is used for ATP synthesis but does not oxidize water or reduce NADP+

Proton Transport and ATP Synthesis in the Chloroplast

• Some captured light energy is used for light-dependent ATP synthesis (photophosphorylation)
• Photophosphorylation works via the chemiosmotic mechanism:
• Proposed by Peter Mitchell in the 1960s
• Chemiosmosis is a unifying aspect of membrane processes in life
• Ion concentration and electric-potential differences across membranes are free energy sources that can be utilized by the cell
• Electron flow is accompanied with proton flow from one side of the membrane to the other
• Direction of proton translocation is such that the stroma becomes more alkaline (fewer H+ ions) and the lumen becomes more acidic (more H+ ions) due to electron transport
• Mitchell proposed that the total energy available for ATP synthesis, is the proton motive force
• Proton motive forces are the sum of a proton chemical potential and a transmembrane electric potential
• A Transmembrane of one pH unit is equivalent to a membrane potential of 59 mV.
• ATP is synthesized by the enzyme complex known as ATP synthase, ATPase, and CF0-CF1
• This enzyme consists of two parts:
• A hydrophobic membrane-bound portion called CF0
• A portion that sticks out into the stroma called CF1
• The internal stalk and probable much of the CF0 of the enzyme rotate during catalysis
• The enzyme is actually a tiny molecular motor
• The stoichiometry of protons translocated to ATP formed is 14/3, or 4.67
• Three molecules of ATP are synthesized for each rotation of the enzyme

Carbon Reactions of the Photosynthesis Process

• Solar radiant energy (ca. 3 x 1021 Joules/year) is converted via endergonic reactions in plants into carbohydrates (ca. 2 x 1011 tonnes of carbon/year)
• Transforming sunlight energy into various forms of chemical energy is one of the oldest biochemical reactions on Earth
• A billion years ago, heterotrophic cells used primary endosymbiosis with a cyanobacterium to convert sunlight into chemical energy
• Original endosymbiosis has diversified into an enormous variety of organelles
• The transition from endosymbiont to organelle loses unnecessary functions in host cell and gains metabolic pathways
• Chloroplasts are where both light and carbon reactions of photosynthesis occur
• ATP and NADPH from light reactions are moved from thylakoid membranes to fluid phase (stroma)
• Then they drive reduction of atmospheric CO2 to carbohydrates and cell components via enzyme-catalyzed reduction of atmospheric CO2
• Those stroma-localized reactions depend on products of photochemical processes and also are regulated directly by light
• So they are referred to as carbon reactions of photosynthesis

The Calvin-Benson Cycle

• Atmospheric CO2 is incorporated into organic compounds appropriate for life via the Calvin-Benson cycle, which produces starch and sucrose.
• Two Major products are the reserve polysaccharide starch (accumulated transiently in chloroplasts), and sucrose (exported from leaves to other areas)
• The Calvin-Benson cycle is found in many prokaryotes and in all photosynthetic eukaryotes
• The cycle can be named reductive pentose phosphate cycle

Three Stages of Calvin-Benson Cycle

• In the 1950s, M. Calvin, A. Benson, and their colleagues investigated the Calvin-Benson cycle
• It proceeds in three stages:
• Carboxylation: of the CO2 acceptor molecule. The committed enzymatic step occurs to generate two molecules of a 3-carbon intermediate (3-phosphoglycerate).
• Reduction: of 3-phosphoglycerate.
• Regeneration: of the CO2 acceptor ribulose 1,5-bisphosphate.
• In the first step, three molecules of CO2 and three molecules of H2O react with three molecules of ribulose 1,5-bisphosphate, resulting in six molecules of 3-phosphoglycerate
• This reaction is catalyzed by the chloroplast enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase, (rubisco)
• The reduction stage of the Calvin-Benson cycle reduces the carbon of 3-phosphoglycerate coming from the carboxylation stage.
• Continuous atmospheric CO2 uptake requires constant regeneration of the CO2 acceptor ribulose 1,5-bisphosphate to prevent depletion of Calvin-Benson cycle intermediates 3CO2 + 3 ribulose 1,5-bisphosphate + 3H2O + 6NADPH + 6H+ +6ATP ---> 6 triose phosphates + 6NADP+ + 6ADP +6Pi
• Triose phosphates are formed in the carboxylation / reduction phases of the Calvin-Benson cycle.
• They expense energy (ATP) and store reducing equivalents (NADPH)
• Generated in thylakoid membranes of chloroplasts:
• Five triose phosphates are used in the regeneration phase that restores ribulose 1,5-bisphosphate, the CO2 acceptor.
• The sixth triose phosphate represents net synthesis from CO2
• It is a building block for other metabolic processes 5 triose phosphates + 3ATP ---> 3 ribulose 1,5-bisphosphate + 3ADP

Regulation of Calvin-Benson Cycle

• The ratio of ATP:NADPH required - 3:2
• The efficient use of energy in the Calvin-Benson cycle requires specific regulatory mechanisms
• The mechanisms ensure all intermediates in cycle are present at adequate concentrations in light and that the cycle is turned off in the dark
• Rubisco plays a critical role in the carbon cycle
• The catalytic rate is slow (1-12 CO2 fixations per second)
• Rubisco must be activated before acting as a catalyst
• CO2 molecule has a dual role
• Participates in the transformation of the enzyme from inactive to active form (modulation)
• Serves as the substrate for the carboxylase reaction (catalysis)
• Light controls the activity of four other enzymes of the Calvin-Benson cycle via the ferredoxin-thioredoxin system
• The system consists of ferredoxin, ferredoxin-thioredoxin reductase, and thioredoxin.
• Deactivation in dark occurs by reversing the reduction (activation) pathway
• Oxygen/reactive oxygen species transform reduced thioredoxin (-SH HS-) to oxidized state (-S-S-)
• Oxidized state converts the reduced target enzyme to the oxidized state and catalytic activity is lost
• Illumination couples the flow of protons from stroma into the thylakoid lumen with the release of Mg2+ from the intrathylakoid space to the stroma
• These decrease stromal concentration of H+ (pH increases from 7 to 8) and increase Mg2+ by 2-5mM
• Several Calvin-Benson cycle enzymes using Mg2+ for catalysis, are more active at pH 8 than pH 7 including rubisco, fructose-1,6-bisphosphatase, sedoheptulose-1,7-bisphosphatase, and phosphoribulokinase
• The light-mediated increase of Mg2+ and H+ enhances activity of key enzymes of the Calvin-Benson cycle

Rubisco

• Rubisco can catalyze both the carboxylation and oxygenation of ribulose 1,5-bisphosphate
• Carboxylation yields two molecules of 3-phosphoglycerate
• Oxygenation produces one molecule each of 3-phosphoglycerate and 2-phosphoglycolate
• Oxygenation initiates a coordinated network of enzymatic reactions compartmentalized in chloroplasts, leaf peroxisomes, and mitochondria.
• This process (photorespiration) causes the partial loss of CO2 fixed by the Calvin-Benson cycle
• Elevated levels of CO2 can create an increase in yield when grown in greenhouse.

Competing Reactions - Carboxylation and Oxygenation

• Being able to catalyze the oxygenation of ribulose 1,5-bisphosphate is a property of all rubiscos
• By specific chloroplast phosphatase, 2-phosphoglycolate formed in the chloroplast by oxygenation of ribulose 1,5-bisphosphate is rapidly hydrolyzed to glycolate
• Glycolate exits the chloroplast (specific transporter protein), diffuses to peroxisome
• Cooperations include the peroxisomes and mitochondria.
• Oxidation of glycolate is catalyzed by glycolate oxidase, producing H2O2/glyoxylate.
• Catalase breaks down the H2O2, releasing O2, while glyoxylate undergoes transamination with glutamate - Glycine.
• Glycine exits peroxisome and enters the mitochondrion
• Glycines converted to serine and CO2
• Serine diffuses to peroxisome, converted to glycerate
• Chloroplast re-entry. This is phosphorilated to yield 3-phosphoglycerate
• Factors determining C2 and Calvin-Benson balance:
• Inherent to the plant (kinetic properties of rubisco)
• Linked to the environment (temperature and the concentration of substrates, CO2 and O2).
• External Temperature Increase
• Modifies kinetic constants of rubisco, increasing rate of oxygenation more than rate of carboxylation
• Lowers concentrations of CO2, more than of O2 in solution, equilibrium with water. The increase in photorespiration limits efficiency of photosynthetic carbon assimilation under warm temperatures
• A progressive increase in temperature tilts the balance away from the Calvin-Benson cycle and toward the C2 oxidative photosynthetic cycle

Carbon Concentrating Mechanisms

• Reduction in CO2, and a rise in O2 triggered a series of adaptions to handle a certain environment - Promoted photorespiration in photosynthetic organisms, Adaptions include strategies for active uptake of CO2, from environment
• Accumulation of inorganic carbon near rubisco.

C4 Cycle

• C4 photosynthesis minimizes rubisco's oxygenase activity/carbon loss through the photorespiratory cycle
• It's a carbon-concentrating mechanism that land plants use to compensate for limitations related to the low atmospheric CO2
• The C4 photosynthetic carbon cycle (also known as the Hatch-Slack cycle/ C4 cycle) was elucidated by M.D. Hatch and C.R. Slack
• They found that malate and aspartate are the first stable detectable intermediates of photosynthesis in sugarcane leaves
• The metabolic pathway takes place in two morphologically distinct cell types (mesophyll and bundle sheath cells)
• The enzyme phosphoenolpyruvate carboxylase (PEPCase) catalyzes the primary carboxylation rather than rubisco
• A 4-carbon acid flows across the diffusion barrier to the vascular region, is decarboxylated, then CO2 is refixed by rubisco via the Calvin-Benson cycle

Plant Cells in C4 Cycle

• C4 cycle exists in leaves of plants whose vascular tissues are surrounded by two distinctive photosynthetic cell types
• Internal ring of bundle sheath cells wrapped with an outer ring of mesophyll cells.
• Chloroplasts in bundle sheath cells have large starch granules/unstacked thylakoid membranes in concentric arrangement
• Mesophyll cells contain randomly arranged chloroplasts with stacked thylakoids

Five Successive Stages of C4 Cycle (in anatomical context):

• HCO3- fixed by PEPCase in mesophyll cells
• 4-Carbon acids (malate, aspartate ) are transported to bundle sheath cells
• Of the 4-carbon acids undergo decarboxylation generating CO2
• This is reduced to carbohydrate via the Calvin-Benson cycle
• 3-carbon backbone (pyruvate /alanine) is transported back to the mesophyll cells
• And regenerating the HCO3- acceptor

Enzyme Compartmentalization & Carbon Fixation

• An inorganic carbon source is taken up by mesophyll cells
• The cycle ensures the Calvin-Benson cycle /bundle sheath of cells can be further fixed.
• Fixed and exported to pholem cells
• Requires operation in C4 cycle:
• Cooperation of the chlorophyll - containing cell types
• Concentration of CO2 results in suppression of ribulose 1,5 of biophosphate activity
• Chloroplast proteomes exhibit qualitatively and quantitatively similar proteomes in envelope, in both C3 and C4 plants
• translocators are abundant in the envelopes of C4 plants, more than in C3 plants
• abundance ensures fluxes of metabolic intermediates across the chloroplast envelope (C4 plants > C3 plants)
• Reduced water loss and photorespiration with C4 cycle
• Elevated temperature does affect photosynethic CO2 asssimilation in C3 plants b/c
• Solubility of CO2 decreases.
• Carboxylative campacity of rubricso, C4 plants feature overcome its effects:
• High affinity of PEPCase, for rubiscos substrate
• Saturated enzyme at reduced CO2 levels
• Oxygenace activity - Suppressed ( doesn't compete with original carboxylation)
• Allows high activity for tomato plants to reduce the tomato aperture at high temperatures and fix 2 at higher rates.

CAM: Crassulacean acid metabolism

• Variant of photosynthetic carbon fixation that concentrate CO2 at the site of rubisco named CAM (historically named crassulacean acid)
• Observed in succulent member of the Crassulaceae (Bryophyllum calycinum)
• Attain high biomass in habitats where precipitation is inadequate, evaporation great
• This metabolism is associated with anatomical features that reduce water loss:
• Thick cuticles
• Volume ratios -Large Veicues -Stomata with small apertures.
• Improves CAM performance (restricts CO2 during day)

CAM Plants

• CAM spatially close, temporally out of phase, compared to atmosphere capture C4acids + final incorporation + into C skeletons
• By nearly 12 hours (24hr light dark cycle)
• Night time fixes atmospherically
• Respiratory CO2 into oxaloacetate - forms via glycolytic breakdown stored carbohydrates
• Cytosolic NAD malate dehydrogenase converts converts oxaloacetate to malate
• Malate stored and transported to chloroplast
• Day: Stored malate decarboxylated, released CO2 available - Chloroplast for procession via Calvin Benson Cycle
• This and complementary 3 carbon, are converted to triose phosphates and to starch or sucrose in C plants
• Rate - Carbon change creates 24hr CAM cycle divide into four phases night time early morning / day afternoon.

Four Phases of CAM

• Phase1- Nocturnal - Stomata open - Leaves respiring Captured CO2 - stired ass malate vacuole
• Phase2 :PEPCcase dominate
• Phase3- Diurnal , Stomata close, leaves photosynthesizing
• Stired malate is decarboxylated
• Phase 4
• Rubisco/ advective Photorespiration is alleviated - Transient shifts, prepare 1/3 respectively

Rubisco Activity

• Increases Phase 2- decreases phase 4

PEPCcase

• increases phase 4 - decrease phase 2 versatility- Sensitve Stimulient
• Can open stormata. Close- minimize water with -

CO2 in CAM

• Because stormata closes the transfer from M not escapes and coverts carbo - Calvin Cycle
• Storma helps concern +

Partioning Starch/Sucrose

Eukaryotes

• assimilation from absorbed / site synt
• YIELDS sucrose/starch
• CYTOSOL AND CHLORO physically CYTS: Suc flows / heterotrophic - Duresnch gr - Suc source sink Source: sink sugar-
• darkness- stops As- Starts Degradtion of clhoro-Falls dra - Converted, LOW levels
• HIGH- Plant growth storage.

Limiting Factors in Photosynthesis

• Factors: direct environmental and indirect response
• Rate of photosynthesis at that time
• Function - leaves.
• Result from Photosyn
• Light is flux energy.
• Energy- Ratio light: less 5% ->

Maximizing Light Absortion

• TRANSParet visible light.
• Top -> Columar cells

Light Absortion With Leaf

• Angle move -> Control
• Tend /fulls Sun -> STEEP angles
• Angle LEAVES -> Horizontal
• Plants- contol solar
• Remains perpum - Rays

Acclimation Adaptations

• Growth process leaf -> Suit to environment
• Matuared -> abscissed
• Better
• Either shade light

Conditions Transfering

• Shadet: Die Shade leaves
• Centers Chlorply
• Thinner than Shade
• Shade -> 31 PHOTOS
• 21 SUN
• Enhance/better /energy shade

light /intact

• Mito/ Resp ->
• Light -> CO2 photo
• LIGHT COMPENSATE
• Point sun 102-m
• Linear photon
• 15 -