Eukaryotic Module 6

chloroplast

• located predominately in mesophyll cells of leaves

• chloroplasts of higher plants are generally lens-shaped

  • approximately 2–4 μm wide and 5–10 μm long, and typically numbering 20–40 per cell

  • their dimensions make chloroplasts giants among organelles (as large as a mammalian blood cell)

• chloroplasts arise by fission from preexisting chloroplasts or their nonpigmented precursors called proplastids

• outer covering of a chloroplast consists of an envelope composed of two membranes separated by a narrow space

• outer membrane of the envelope contains several different porins that exhibit selectivity toward solutes

• inner membrane of the envelope is highly impermeable, substances moving through this membrane do so only with aid of variety of transporters

• thylakoid membrane: internal membrane of the chloroplast organized into flattened membranous sacs

  • much of the photosynthetic machinery is part of the internal membrane: light-absorbing pigments, complex chain of electron carriers, ATP synthesizing apparatus

  • arranged in orderly stacks called grana

  • space inside a thylakoid sac is the lumen

  • space outside the thylakoid and within the chloroplast envelope is the stroma, which contain enzymes responsible for carbohydrate synthesis

  • contains chlorophyll molecules and protein complexes that compromise the energy-transducing machinery of the chloroplast

  • high protein content and unusual in having relatively little phospholipid

  • high percentage of galactose-containing glycolipids

    • both fatty acids of these lipids contain several double bonds, making the lipid bilayer of the membrane highly fluid

    • fluidity of the bilayer facilitates lateral diffusion of protein complexes through the membrane during photosynthesis

• stroma: contains small double stranded circularf DNA molecules and prokaryotic like ribosomes

  • DNA of a chloroplast is a relic of the genoome of an ancient bacteiral endosymbiont

mitochondria vs chloroplast

• outer membrane of mitochondrion and outer membrane of chloroplast contain several different porins

• matrix of mitochondria and stroma of chloroplast contain small double stranded circularf DNA molecules and prokaryotic like ribosomes

• both have a double membrane

• both generate ATP using electron transport chain and proton gradient

• mitochondria is found is almost all eukaryotic cells while chloroplasts only in plants and algae

• inner membrane of mitochondrion is folded inward to form cristae while chloroplast inner membrane is smooth and encloses a third internal thylakoid membrnae system

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evolution of earth’s life forms

1. the earliest life forms were heterotrophs that depended on an external source of organic compounds, which were formed abiotically.

2. the appearance of autotrophs, which manufactured their own organic nutrients from the simplest inorganic molecules, such as carbon dioxide and hydrogen sulfide, tremendously boosted the evolution of life

two types of autotrophs

• utilize the energy stored in inorganic compounds such as ammonia, hydrogen sulfide, or nitrites.

• utilize the radiant energy fromt he sun

  • needs more energy since the oxygen atom has much stronger affinity for its electrons

cyanobacterium

• cyanobacteria switched from using hydrogen sulfide to using water as their source of electrons, which required evolving a very strong oxidizing agent

• this process produced molecular oxygen as a waste product, changing Earth's atmosphere and life.

symbiotic evolution of chloroplasts

• An ancient O2-producing cyanobacterium took up residence inside a primitive eukaryotic host cell and gradually transformed into the chloroplast organelle

• Most genes from the cyanobacterium were transferred to the plant cell's nucleus; therefore, polypeptides in modern chloroplasts are encoded by both the nuclear and chloroplast genomes, and all modern chloroplasts descend from a single, ancient symbiotic relationship.

photosynthetic plants

• provides the most of the organic nutrients for the lives on the planet

leaf anatomy

chloroplasts of higher plants

• generally lens-shaped

• approximately 2 to 4 μm wide and 5 to 10 μm long

• numbering 20-40 per cell

membranes of chloroplast

• outer membrane

• inner membrane

• internal membrane

outer membrane

• similar to the outer membrane of an mitochondrian

• the outer membrane of a chloroplast envelope contains porins

  • porins are integral proteins that have a relatively large channel surrounded by a barrel of b strands.

• exhibit selective permeability towards various solutes

inner membrane

• highly impermeable

• substances moving through the inner membrane needs the aid of a variety of transporters

internal membrane

• arranged in stacks of disc-like thylakoids that are physically separate from the outside double membrane that forms the envelope.

• contains much of the photosynthetic machinery

  • light-absorbing pigments

  • complex chain of electron carriers

  • ATP-synthesizing apparatus

thylakoids

• flattened membranous sacs

• space inside a thylakoid sack is the lumen

• space outside the thylakoid and within the chloroplast envelope is the stroma

• thylakoids arranged in orderly stacks are called grana thylakoids

chloroplast structure

membranes + spaces in chloroplasts

thylakoid membranes

• high protein content, relatively little phospholipid

• contain high percentage of galactose-containing glycolipids

  • both fatty acids of these lipids contain several double bonds, making the bilayer highly fluid

chloroplast genome

• after taking residence in a proeukaryotic cell, most chloroplast genes were lost or transferred into the nuclear genome

• only small, double-stranded, circular DNA molecules are present in the chloroplast stroma

• depending on the organism, chloroplasts contain between 60 to 200 genes involved in either gene expression or photosynthesis

• vast majority of chloroplast proteins (200-3500) are encoded by nuclear DNA synthesized in the cytosol and imported into the chloroplast by a specialized transport machinery

• chloroplasts are reproduced by fission from preexisting chloroplasts or their nonpigmented precursors- proplastids

photosynthetic metabolism

• in 1931, van Niel proposed that oxygen released during photosynthesis of green plants comes from splitting water (H2O) molecules, not from carbon dioxide

• van Niel studied sulfur bacteria and green plants

van Niel’s general equation for photosynthesis

• CO2 + 2H2A ——> (CH2O) + A2

• arrow is light

• H2A is oxidized (loses hydrogen)

• CO2 is reduced (gains hydrogen to form carbohydrates)

photosynthetic equation for sulfur bacteria and its byproducts

• CO2 + H2S ———> (CH2O) + S2

• arrow is light

• byproduct is elemental sulfur (instead of oxygen like photosynthetic green plants)

• H2S is electron/hydrogen donor

photosynthetic equation for green plants

• CO2 + H2O —————> (CH2O) + O2

• arrow is light

• water (H2O) replaces H2S as the hydrogen donor

confirmation of van Niel’s hypothesis

• 1941- Samuel Ruben and Martin Kamen experiemented on suspensions of green algae using ¹⁸O replacing the common isotope ¹⁶O

• labeled C[¹⁸O₂] and unlabeled H₂O

• unlabeled CO₂ and labeled H₂[¹⁸O]

• the O2 released from the experiment was labeled, which confirmed van Niel’s hypothesis that light energy is used to split H₂O to form O₂.

photosynthesis in chloroplast vs aerobic respiration in mitochondrion

• photosynthesis in chloroplast is essentially the reverse of mitochondrial respiration

two series of reactions in photosynthesis

• light-dependent reactions: energy from sunlight is absorbed and stored as chemical energy in two biological molecules (ATP and NADPH)

• light-independent reactions: carbohydrates are synthesized from CO2 using the energy stored in ATP and NADPH molecules produced in light-dependent reactions

absorption of light

• photons: packets of energy light travels in

• energy cotent of a photon: E=hc/λ

  • h: Planck’s constant

  • c: speed of light in a vacuum

• step 1 (ground state): electron is at rest in its stable, lowest-energy orbital

• step 2 (excitation): when a photon of light hits the molecule, the electron absorbs the photon’s energy, jumping to a higher outer energy shell

• step 3 (relaxation): the high energy state is unstable and lasts for a fraction of a second (10^-9 s). the electron rids itself of the excess energy to return to the ground state

• once excited, the energy takes one of two paths to return to ground state

  • fluorescence pathway: chlorophyll will release this energy as heat and a lower-energy photon of light

  • electron acceptor pathway: the energy released is captured by primary electron acceptors in chloroplasts, which passes the energy in the ETC to photosynthesis

photosynthetic pigments

• organisms containing chlorophyll are primary light-absorbing photosynthetic plants

• secondary pigments: secondary light collectors that draw excess energy away from excited chlorophyll and dissipate it as heat

• chlorophyll molecules are primary light-absorbing pigments while carotenoids are secondary/accessory pigments that support chlorophyll

  • when a plant absorbs too much sunlight, carotenoids deaw the excess energy away from excited chlorophyll molecules and dissipate it as heat, preventing damage to the cell

structure of chlorophyll

• porphyrin ring with conjugated double bonds that function in light absorption

• hydrophobic phytol chain that keeps chlorophyll embedded in photosynthetic membrane

structure of B-carotene

a linear system of conjugated double bonds

absorption spectrum for chlorophyll a, b and b-carotene

• chlorophylls absorb the strongest in the violet, blue, and red regions

• carotenoids absorb into the green region

action spectrum and absorption spectrum of chlorophyll a,b and b-carotene

• the red line (action spectrum) indicates the relative efficiency with which light of various wavelengths is able to promote photosynthesis in the leaves of a plant

• an action spectrum can be generated by measuring the O2 produced by leaves following exposure to various wavelengths

• green line is the combined absorption spectrum of all pigments

light-dependent reactions

• light absorbing reactions of photosynthesis occur in large pigment protein complexes called photosystems

chlorophyll molecules in a chloroplast

• not all chlorophyll molecules in a chloroplast are directly involved in conversion of light energy into chemical energy

• in 1932, Robert Emerson and William Arnold used green algae Chlorella and flashing light of extremely short duration at saturating intensity

  • one molecule of oxygen was released during a brief flash of light for every 2500 molecules of chlorophyll present

  • a minimum of 8 photons must be absorbed to produce 1 molecule of O2.

• the experiment shows that chlorophyll molecules work together rather than individually

• 300 molecules of chlorophyll is needed

photosynthetic unit

• hundreds of chlorophyll molecules act together as one photosynthetic unit

• reaction-center chlorophyll: transfers electrons to electron receptors

  • the only member of the unit that can do this

• the bulk of pigment molecules do not directly participate in the conversion of light energy to chemical energy, but are responsible for light absorption

light-harvesting antenna

• pigment molecules form the light light harvesting antenna

• absorbs photons of varying wavelength

• very rapidly transfers the energy (excitation energy) to the pigment molecule at the reaction center

light-harvesting antenna and reaction-center

• transfer of excitation energy from one pigment molecule to another is sensitive to the distance between molecules

• chlorophyll molecules of the antenna are kept in close proximity and proper orientation by noncovalent linkage to integral membrane polypeptides

  • less than 1.5 nm separation

• “rule” that operates among antenna pigments is that energy can only be passed to a pigment molecule that absorbs light of equal or longer wavelength/lower energy than that absorbed by the donor pigment

• the energy transferred to a chlorophyll of the reaction center absorbs light of longer wavelength than its neighbors

• once the energy is received by the reaction center, the electron excited by light absorption can be transferred to its acceptor

oxygen formation

• evolution of organisms capable of using H2O as an electron source was accompanied by major changes in the photosynthetic machinery

  • energetics of oxygenic photosynthesis is the reason for these changes

• O2-H2O couple standard redox potential: +0.82 V

• NADP+-NADPH couple standard redox potential: -0.32 V

• the difference between the redox potentials of these two couples provides measure of minimal energy that the system must absorb to remove an electron from H2O and pass it to NADP+ under standard conditions (1.14 V)

  • it is estimated that during actual operations in the chloroplast, more than 2 V of energy are untilized to carry out the reaction

• one mole of photons of 680 nm wavelength (red light) is equivalent to a change in redox potential of 1.8 V

  • it is theoretically possible for one photon red light to boost an electron to an energy level required to reduce NADP+ under standard conditions

• the process is accomplished, however, through the combined action of two sequestial light-absorbing reactions

photosystems

• large pigment protein complexes where light-absorbing reactions of photosynthesis occur

• two types of photosystems are required to catalyze the two light-absorbing reactions in oxygenic photosynthesis

• photosystem II (PSII): boosts electrons from an energy level below that of water to a midway point

• photosystem I (PSI): raises electrons from a midway point to an energy level well above that of NADP+

• PSII and PSI act in series

• although have different photochemical reactions, they are similar in protein composition and architecture in photosynthetic plants and bacteria

  • shared properties suggest the photosynthetic reaction centers have evolved from a common ancestral structure

reaction center of the two photosystems

• PSII reaction center: a chlorophyll dimer referred to as P680

  • p for pigment, 68p for the wavelength of light absorbed most strongly by this pair of chlorophyll

• PSI reaction center: chlorophyll dimer referred to as P700

• when sunlight strikes a thylakoid membrane, energy is absorbed by antenna pigments of both PSII and PSI and passed to the reaction centers of both photosystems

• electrons of both reaction-center pigments boosted to an outer orbital, and each electron is transferred to a primary electron acceptor

• after losing their electrons, the reaction centers become positively charged and the electron acceptors turn negatively charged

  • this separation of charge within the photosystems is the light reaction (conversion of light to energy) because positively charged centers act as attractants for electrons, and negatively charged acceptors act as electron donors

• separation of charge within each photosystem sets the stage for the flow of electrons along chain of specific carriers

structure of PSII

• consists of >20 different peptides most of which are embedded in the thylakoid membrane

• two proteins, D1 and D2, together bind the P680 reaction centerchlorophyll dimer and cofactors involved in electron transport

• separate pigment-protein complex light-harvesting complex II (LHC II) is situated outside the core of the photosystem

  • binds both chlorophylls and carotenoids

• uses absorbed light energy to generate proton gradient across the thylakid membrane

first step: flow of electrons from PSII to plasto-quinone

• first step in PSII activation is the absorption of light by an antenna pigment

• light energy from antenna pigments to PSII reaction center

• the excited reaction center P680^* transfers a single photo-excited electron to a closely associated chlorophyll-like pheophytin molecule

  • Pheo is the first electron transfer and it generates a separation of charge in PSII (P680 and Pheo)

• P680⁺ and Pheo^- are generated

step 2: pheo and plastoquinone

• pheo^- transfers its electrons to a molecule of plastoquinone (PQ_A)

• PQ_A is bound near the outer (stromal) side of the membrane

PQ_A (plastoquinone)

lipid soluble molecule similar in structure ot ubiquinone

step 3: PQ_A

• PQ_A transfers its electron to a second plastoquinone (PQ_B) to produced a semireduced form of the molecule, plastoquinol (PQ_B-) that remain firmly bound to the D1 protein

• with each of these transfers, the electron moves closer to the stromal side of the membrane

step 4: PQ_B^2-

• a second photon sends a second energized electron along the path from P680 to PQ_A to (PQ_B-), forming PQB^2-

step 5: PQH₂

• PQ_B^2- combines with two protons to form PQH₂

• The reduced PQH₂ molecule dissociates from D1 protein and diffuses into the lipid bilayer

step 6: PQ

• The displaced PQH₂ is replaced by a fully oxidized PQ molecule derived from a small “pool” of plastoquinone in the bilayer

flow of electrons from water to PSII

• all of these electrons come water and are released when water is split into protons and molecular oxygen

• the redox potential of the oxidized form of P680 (1.17 V) is sufficiently strong enough to pull tightly held (low-energy) electrons from water (+ 0.82)- thus splitting the water molecule

• photolysis: splitting of water during photosynthesis

• formation of one molecule of oxygen during photolysis is thought to require the simultaneous loss of four electrons from two molecules of water

  • yet, one PSII reaction can only generate one positive charge or oxidizing equivalent at a time

• solution to the problem: S-state hypothesis by Pierre Joliot and Bessel Kok, allowing the photosystem to accumulate the four oxidizing equivalents needed to oxidize water

  • there is a cluster of five metal atoms associated with the D1 protein of PSII at its luminal surface: four Mn atoms and one Ca atom

  • cluster is stabilized and protected by an oxygen-evolving complex that accumulates 4 oxidizing equivalents by transferring 4 electrons, one at a time, to P680⁺

  • a tyrosine residue on the D1 protein (Tyr_z) mediates the electron transfer between Mn and P680⁺

    • intermediate electron carrier

4 step cycle (S states)

• S₀ to S₄

• every flash of light pushes the system up by one state

• Once it hits S₄ it immediately releases oxygen and resets back to S₀

kinetics of O₂ release

• because you need 4 steps/flashes to release oxygen, the peaks in the graph happen every 4 flashes

• the first peak occurs after 3 flashes because dark-adapted chloroplasts primarily rest in the S₁ (already possessing 1 oxidizing equivalent) rather than S₀

  • Mn clusters are in the S₁ state

• aqs the number of flashes increases, the oscillations damp out due to desynchronization.

flow of electrons from PSII to PSI

• step one: two molecules of PQH2 are formed by accepting 4 electrons from PSII and combining with 4 protons from the stroma

• step two: PQH2 diffuses through the lipid bilayer and binds to a large multiprotein complex, cytochrome b6f

• step 3: electrons are transferred from cytochrome b6f to a mobile, water-soluble, copper-containing, peripheral protein, plastocyanin (PC)

• step 4: PC transfers electrons to P700⁺

flow of electrons from PSI to NADP⁺

• step 1: following energy absorption, P700^* transfers an electron to a separate monomeric chlorophyll a molecule (A₀)

• step 2: from A₀^- to a phylloquinone molecule (A₁)

• step 3: from A₁^- to an iron-sulfur cluster (F_x)

• step 4: from F_x^- to two iron-sulfur clusters (F_B and F_A)

• step 5: from F_B and F_A to a small water-soluble iron-sulfur protein, ferredoxin

• step 6: reduction of NADP⁺ by ferredoxin-NADP+ reductase

overview of photosynthetic electron transport

• 2H2O + 2NADP+ ——→ 1O2 + 2NADPH

• PSII handles water splitting and oxygen release

  • generates a strong oxidizing agent capable of producing O2 from water

• the ETC moves the electrons and builds proton gradient

  • protons are pumped from the stroma into the thylakoid lumen

• ATP synthase uses that proton gradient to make ATP

• PSI finishes the job by making NADPH

  • generates a strong reducing agent capable of producing NADPH from NADP+

• light reactions uses 8 photons to produce 3 things needed for the Calvin Cycle (O2, ATP, NADPH)

mitochondria vs chloroplast proton-motive force

• in mitochondria: proton motive force is the electrochemical potential

• in chloroplast: the proton motive force is largely due to a gradient of pH

  • gradient is across the thylakoid membrane

photophosphorylation

• making ATP using light

• two ways- noncyclic and cyclic

• ATP is synthesized in the thylakoid membrane

• NADPH is synthesized in the stroma of thylakoid membrane

noncyclic photophosphorylation

• linear electron flow

• PSII to PSI to NADP+

• as electrons move down the chain, they pump protons to build a proton motive force, which drives ATP synthase to make ATP alongside making NADPH and oxygen

• electrons do not cycle back

cyclic photophosphorylation

• sometimes the plant needs extra ATP without making extra NADPH so electrons from PSI loop back into electron transport chain to pump more protons and make ATP, but they do not end up at NADP+

• carried out by PSI independent of PSII

• after electron is passed to ferredoxin, it is passed back to the electron-deficient PSI to complete the cycle

• energy released from the flow of the electron is utilized to translocate protons by the cytochome b6f complex and proton graident is generate for ATP synthesis

• cyclic phosphorylation provides additional ATP for carbohydrate synthesis and other ATP-requiring activities in the chloroplast.

summary of light-dependent reactions

discovery of light-independent reactions

• “dark reactions”/ calvin cycle

• after world war II, melvin calvin and colleagues andrew benson and james bassham began to study enzymatic reactions by whch carbon dioxide was assimilated into the organic molecules of the cell

• used long-lived radioactive isotope of carbon (¹⁴C) and two dimensional paper chromatography to study green algae chlorella cell suspension

• algal suspension was drained into container of hot alcohol (stops enzymatic activity and extracts soluable molecules)

• extracts of cells were placed as a spot on chromatographic paper and subjected to 2D chromatography

• photographs were developed by pressing against the chromatogram and kept in the dark to locate the radioactive compounds at the end of the procedure

• after photographic development, identification of the radiolabeled compounds on the autoradiogram was made by comparison to known standards

carbohydrate synthesis in C₃ plants

• if the incubation period after adding ¹⁴C was very short, one radioactive spot on the chromatogram predominated,

• compound that formed the spot was PGA (3-phosphoglycerate)

  • an intermediate of glycolysis

• because the first intermediate was three-carbon molecule, plants using this pathway to fix CO₂ are referred to as C₃ plants

light-indepedent reaction

• the initial acceptor is a five-carbon compound: ribulose 1,5-biphosphate (RuBP)

1. RuBP condenses with CO₂ to produce a six-carbon molecule

2. the 6-carbon molecule rapidly fragments into 2 molecules of PGA

• both reactions are carried out in the stroma by a large multisubunit enzyme, ribulose bisphosphate carboxylase (Rubisco)

• rubisco is a very low efficient enzyme (3 CO2/sec) therefore is present in large amount (1/2 of leaf proteins)

calvin cycle

• three main parts:

  • carboxylation of RuBP to form PGA

  • reduction of PGA to glyceraldehyde 3-phosphate (G3P or GAP)

  • regeneration of RuBP

summary of calvin cycle

• 3CO2 + 9ATP + 6NADPH ——> glyceraldehyde-3-P + 9 ADP + 8 Pi + 6 NADP+

• first step: RuBisCO attaches a CO2 molecule to a RuBP

regulation of calvin cycle

• regulation by light: light activated electron transfer is linked to pumping H+ into thylakoid disks

• pH in the stroma increases to about 8

• alkaline (basic) pH activates stromal calvin cycle enzymes: RuBP carboxylase, fructose-1,6-bisphosphatase & sedoheptulose bisphosphatase

• the light-activated H+ shift is countered by Mg2+ release from thylakoids to stroma

• RuBP carboxylase (in stroma) requires Mg2+ binding to carbamate at the active site

• reduced thioredoxin (the form more prevelant in the light) activates several calvin cycle enzymes, including fructose-1,6-bisphosphatase, sedoheptulose-1,7- bisphosphatase, and RuBP carboxylase activase, by reducing specific disulfides in those enzymes to thiols

redox control of the calvin cycle

• in light, ferredoxin is reduced, and a fraction of these electrons are transferred to the small protein thioredoxin, which reduces the disulfide groups of certain calvin cycle enzymes, maintaining them in an active state

• in dark, electron flow to thioredoxin ceases, the sulfhydryl groups of the regulated enzymes become oxidized to the disulfide state, and the enzymes are inactivated.

redox control of the calvin cycle

• picture for the dark

photorespiration (C₂ Cycle)

• light-dependent metabolic process in plants where enzyme rubisco fixes oxygen instead of carbon dioxide

  • which consumes energy and releases previously fixed carbon

  • wasteful pathway: instead of making two molecules of 3-PGA, it makes one molecule of 3-PGA and one toxic, 2-carbon molecule called phosphoglycolate. The plant has to spend energy (ATP and reducing power) to recycle this byproduct, releasing previously fixed CO2 in the process, reducing photosynthetic efficiency

• rubisco catalyzes two distinct reactions:

  • rubisco binds RuBP, which adopts the enediol form. The enediol form of RuBP can be attacked by either CO2 or O2. The predominant direction of the rubisco-catalyzed reaction is determined by the ratio of CO2/O2 available.

• rate of the reaction is determined by O2/CO2 ratio, temperature, and stomatal closure

  • temperature rises, RuBisCO’s affinity for CO2 drops

  • stomatal closure: when a plant is stressed by heat or lack of water, it closes its stomata (pores) to prevent water loss. This traps oxygen produced by the light reactions inside while blocking new CO2 from entering, causing the internal CO2/O2 ratio to drop and photorespiration to increase

• occurs in the chloroplast but needs cooperation from chloroplast, peroxisome, and mitochondria

  • step one: RuBisCO binds O2 to RuBP, producing 2-phosphoglycolate, which is quickly converted to glycolate

  • step two: glycolate travels to the peroxisome, where it is oxidized into glyoxylate and then converted into the amino acid glycine.

  • step three: two molecules of glycine are brought into the mitochondria, and are combined to form serine, which releases a molecule of CO2 and ammonia

  • step 4: serine travels back to the peroxisome to be converted to glycerate and glycerate enters the chloroplast, where an ATP molecule phosphorylates it into 3-PGA, allowing it to finally rejoin the normal calvin cycle

C4 plants

• In 1965, using [14C]O2, Hugo Kortschak found that in sugarcane the first compound appeared contained 4-carbon skeletons (predominately oxaloacetate and malate). These 4-carbon compounds resulted from combination of CO2 with phosphoenolpyruvate (PEP), which is the first of the C4 pathway

• C4 plants, such as sugarcane, corn, and sorghum, live in a dry and hot environment.

• they are adapted to close their stomata to prevent water loss, which also decreases the CO2 concentration in the leaves

• the enzyme responsible for linkage of CO2 and PEP (PEP carboxylase) can fix CO2 even when the leaf stomata are almost entirely closed and the CO2 level in the cell is very low

C4 metabolism

1. CO2 enters the mesophyll cell and is bonded to phosphoenolpyruvate (3C) to form oxaloacetate (4C) by enzyme PEP carboxylase

2. the oxaloacetate is converted into malate (4C) and shuttled into the chloroplast of the bundle sheath cell through plasmodesmata.

3. the malate is decarboxylated to form pyruvate (3C) and CO2

4. the CO2 can be used in the Calvin cycle in the bundle sheath cell.

5. the pyruvate is cycled back to the mesophyll cell, where it can be used to regenerate phophoenolpyruvate

• C4 plants are able to generate high CO2/O2 ratios in the local Rubisco environment, thereby favoring the process of CO2 fixation over that of photorespiration

comparison of C3 and C4 plants structures

comparison of C3 and C4 metabolism

• first carbon product:

  • C3: 3 carbon compound 3-phosphoglycerate

  • C4: 4 carbon compound oxaloacetate

• enzyme for initial CO2 fixation:

  • C3: rubisco

  • C4: PEP carboxylase

• anatomy/cell structure:

  • C3: no specialized bundle-sheath cell separation; standard chloroplast arrangement

  • C4: bundle-sheath cells surround leaf veins

• separation of reactions:

  • C3: carbon fixation and the Calvin cycle happen in the same cells

  • initial fixation in mesophyll cells; Calvin cycle in bundle-sheath cells

• photorespiration

  • C3: high (Rubisco binds O2 in hot/dry conditions, wasting energy)

  • C4: Minimized/Suppressed (PEP carboxylase does not bind O2 and CO2 is concentrated around Rubisco)

• water/thermal efficiency:

  • C3: lower efficiency in high heat; loses more water through open stomata

  • C4: high efficiency; saves water and thrives in warm, bright conditions

• best environment:

  • C3: cool, wet, or moderate climates

  • C4: hot, sunny, and arid/tropical regions

CAM plants

• many desert plants, such as cacti, have another biochemical adaptation that allows then to survive in very hot, dry habitats.

• these are also C4 plants but instead of segregating the C4 and C3 pathways in different parts of the leaf, they separate them in time instead.

• CAM stands for Crassulacean Acid Metabolism because it was first studied in members of plant family Crassulaceae

night vs morning processes in CAM plants

night:

1. CAM plants take in CO2 through their open stomata (they tend to have reduced numbers of them).

2. the CO2 joins with PEP to form the 4-carbon oxaloacetic acid.

3. this is converted to 4-carbon malic acid that accumulates during the night in the central vacuole of the cells.

morning:

1. the stomata close (thus conserving moisture as well as reducing the inward diffusion of oxygen).

2. the accumulated malic acid leaves the vacuole and is broken down to release CO2.

3. the CO2 is taken up into the calvin (C3) cycle