Carbon fixation is the secondary, light-independent stage of photosynthesis, converting inorganic carbon dioxide (CO₂) into organic carbohydrates. This process represents the primary entry point of carbon into the global biosphere. While the initial photophysical reactions of photosynthesis harvest light energy to generate ATP and NADPH within the thylakoid membrane, the subsequent carbon-fixation pathways utilize these chemical equivalents within the stroma (or cytosol of prokaryotes) to drive the endergonic reduction of carbon.

1. The Carbon-Fixation Cycle (The Calvin-Benson-Bassham Cycle)

The Calvin-Benson-Bassham (CBB) cycle (also designated as the reductive pentose phosphate cycle or the C₃ cycle) is the universal metabolic engine of carbon assimilation in all oxygenic photosynthetic organisms — plants, algae, and cyanobacteria. Elucidated by Melvin Calvin and Andrew Benson at the University of California, Berkeley (a discovery for which Calvin was awarded the Nobel Prize in Chemistry in 1961), the cycle is physically compartmentalized within the stroma of eukaryotic chloroplasts.

The direct organic product of the CBB cycle is not glucose, but the three-carbon triose phosphate glyceraldehyde-3-phosphate (G3P). The de novo synthesis of one net molecule of G3P requires the systematic fixation of three molecules of CO₂. The cycle operates in three tightly coordinated, sequential phases: Carboxylation, Reduction, and Regeneration of the RuBP acceptor.

1.1 Phase I: Carboxylation and the Mechanics of RuBisCO

The carboxylation phase is the thermodynamic entry point where inorganic carbon is covalently attached to an organic molecule. In this phase, CO₂ is incorporated into the five-carbon ketose sugar ribulose-1,5-bisphosphate (RuBP). This reaction is catalyzed by the pacemaker enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO).

1.1.1 Structural Architecture of RuBisCO

In land plants, RuBisCO is a massive, hexadecameric (L₈S₈) multisubunit complex with a total molecular mass of approximately 550 kDa:

1. Eight Large Subunits (L, ∼55 kDa): these subunits contain the catalytic active sites. They are encoded by the chloroplast genome (chloroplast DNA) and synthesized by stromal ribosomes.
2. Eight Small Subunits (S, ∼13 kDa): these subunits perform essential regulatory and structural roles. They are encoded by the nuclear genome, synthesized as precursor proteins on cytosolic ribosomes, and imported post-translationally into the chloroplast.

1.1.2 Activation Mechanism of RuBisCO (Lysine-201 Carbamylation)

RuBisCO is a highly regulated enzyme that remains completely inactive until it undergoes a specific conformational activation sequence.

1. Lysine-201 Carbamylation: the active site contains a conserved lysine residue (Lys-201). For the enzyme to become active, a molecule of non-substrate CO₂ must react with the uncharged ε-amino group of Lys-201 to form a negatively charged carbamate adduct.
2. Magnesium Coordination: once the carbamate is formed, it coordinates a divalent magnesium ion (Mg²⁺). This metal center stabilizes the carbamate structure and creates the precise coordination environment required to bind the substrate RuBP and activate the catalytic water molecule.
3. The Role of Rubisco Activase: in vivo, inactive RuBisCO often binds RuBP tightly at its active site before Lys-201 can be carbamylated, blocking its own activation. Rubisco activase is an ATP-dependent chaperone protein that binds to the inactive RuBisCO–RuBP complex. Using the energy of ATP hydrolysis, it induces a conformational change in RuBisCO that releases the trapped RuBP, exposing Lys-201 so it can be carbamylated and activated by CO₂ and Mg²⁺.

1.1.3 The Catalytic Mechanism of Carboxylation

Once active, the RuBisCO–Mg²⁺ complex drives carboxylation through a series of coordinated steps.

1. Enediolate Formation: active RuBisCO abstracts a proton from the C-3 position of RuBP to generate a highly reactive, symmetric enediolate intermediate.
2. Electrophilic Addition: the enediolate intermediate attacks a molecule of substrate CO₂ at its C-2 position, creating an unstable, six-carbon β-ketoacid intermediate: 2-carboxy-3-keto-D-arabinitol 1,5-bisphosphate.
3. Hydration and Cleavage: a water molecule is added to this unstable intermediate, driving a nucleophilic cleavage between the C-2 and C-3 carbons. This yields two identical molecules of the three-carbon acid 3-phosphoglycerate (3-PGA).

1.2 Phase II: The Reduction Phase

In the reduction phase, the six molecules of 3-phosphoglycerate are converted into six molecules of the high-energy triose phosphate glyceraldehyde-3-phosphate (G3P). This phase consumes both ATP and NADPH generated during the light-dependent thylakoid reactions.

1. Phosphorylation: each molecule of 3-PGA is phosphorylated at the expense of one ATP to form 1,3-bisphosphoglycerate (1,3-BPG). This reversible reaction is catalyzed by phosphoglycerate kinase.
2. Reduction: each molecule of 1,3-BPG is reduced by NADP-glyceraldehyde-3-phosphate dehydrogenase using one molecule of NADPH to yield glyceraldehyde-3-phosphate (G3P) and release an inorganic phosphate (P_i).

1.3 Phase III: Regeneration of RuBP (The Sugar Phosphate Shuffle)

For the carbon-fixation cycle to operate continuously, the five-carbon acceptor RuBP must be regenerated. Out of the six molecules of G3P (18 carbons) produced in Phase II, one molecule of G3P (3 carbons) exits the cycle as the net organic product, available for starch, sucrose, or amino acid synthesis, while five molecules of G3P (15 carbons) are routed into a complex series of sugar rearrangements to regenerate three molecules of RuBP (15 carbons).

This regeneration pathway, known as the sugar phosphate shuffle, utilizes a series of aldol condensations, dephosphorylations, and carbon-transfer reactions.

1.3.1 Step-by-Step Carbon Rearrangements

1. Isomerisation: two of the five G3P molecules are reversibly isomerized into dihydroxyacetone phosphate (DHAP) by triose phosphate isomerase.
2. First Aldolase Condensation: one G3P and one DHAP condense to form the six-carbon sugar fructose-1,6-bisphosphate (FBP), catalyzed by aldolase.
3. First Dephosphorylation: FBP is hydrolyzed by fructose-1,6-bisphosphatase to yield fructose-6-phosphate (F6P) and release an inorganic phosphate (P_i). This is an irreversible, regulatory step.
4. First Transketolase Transfer: transketolase (a TPP-dependent enzyme) transfers a two-carbon unit from F6P (6C) to a third G3P (3C). This yields a four-carbon sugar, erythrose-4-phosphate (E4P), and a five-carbon sugar, xylulose-5-phosphate (Xu5P).
5. Second Aldolase Condensation: the resulting E4P (4C) condenses with the second DHAP (3C) to form the seven-carbon sugar sedoheptulose-1,7-bisphosphate (SBP), catalyzed by aldolase.
6. Second Dephosphorylation: SBP is hydrolyzed by sedoheptulose-1,7-bisphosphatase to yield sedoheptulose-7-phosphate (S7P), releasing a second inorganic phosphate (P_i). This represents another irreversible, regulatory step.
7. Second Transketolase Transfer: transketolase transfers a two-carbon unit from S7P (7C) to the final G3P (3C). This yields two five-carbon sugars: ribose-5-phosphate (R5P) and a second xylulose-5-phosphate (Xu5P).
8. Isomerisation to Ru5P: the accumulated five-carbon intermediates are isomerized into ribulose-5-phosphate (Ru5P) — the two Xu5P molecules are converted to Ru5P by ribulose-5-phosphate 3-epimerase, and the single R5P molecule is converted to Ru5P by ribose-5-phosphate isomerase.
9. Phosphorylation: finally, the three molecules of Ru5P are phosphorylated by phosphoribulokinase using three molecules of ATP to regenerate three molecules of RuBP.

1.4 Net Stoichiometry and Energetics of Carbon Fixation

To synthesize one net three-carbon G3P molecule, the Calvin cycle requires three complete turns:

Because glucose is a six-carbon sugar, its net synthesis requires six rounds of CO₂ fixation (equivalent to two net G3P molecules), doubling the energetic requirement:

2. Light-Dependent Regulation of the Calvin Cycle

Although the Calvin cycle reactions do not consume photons directly, the pathway is strictly inactive in the dark. This prevents a wasteful, simultaneous “futile cycle” of carbon fixation and carbohydrate oxidation (glycolysis). The cycle is regulated in response to light through three main mechanisms.

2.1 Stromal pH and Magnesium Fluxes

During the light reactions, active proton pumping by the electron transport complexes translocates H⁺ from the stroma into the thylakoid lumen.

1. pH Shift: this shifts the stromal pH from 7.0 (in the dark) to 8.0 (in the light). RuBisCO, fructose-1,6-bisphosphatase, and sedoheptulose-1,7-bisphosphatase have evolved to exhibit maximum catalytic activity at pH 8.0.
2. Magnesium Efflux: to maintain electrical neutrality across the thylakoid membrane, the influx of protons into the lumen is compensated by the efflux of Mg²⁺ into the stroma. This increase in stromal Mg²⁺ concentration directly promotes the carbamylation of RuBisCO Lys-201 and serves as an essential cofactor for the bisphosphatases.

2.2 The Nocturnal Inhibitor (CA1P)

In many plant species, RuBisCO is regulated in the dark by a natural sugar phosphate inhibitor called 2-carboxyarabinitol-1-phosphate (CA1P).

1. Mechanism: CA1P is synthesized in the dark and binds tightly to the carbamylated active site of RuBisCO, blocking substrate entry.
2. Reversal: when light returns, Rubisco activase uses ATP to displace CA1P from the active site, allowing the inhibitor to be enzymatically degraded.

2.3 The Ferredoxin–Thioredoxin System

Four key regulatory enzymes of the Calvin cycle — fructose-1,6-bisphosphatase, sedoheptulose-1,7-bisphosphatase, phosphoribulokinase, and glyceraldehyde-3-phosphate dehydrogenase — are regulated by light via covalent disulfide–sulfhydryl exchange.

1. In the Light: electrons from water travel through PSI to reduce ferredoxin. Reduced ferredoxin transfers these electrons to the small regulatory protein thioredoxin, a reaction catalyzed by ferredoxin-thioredoxin reductase. The reduced thioredoxin (containing active sulfhydryl, −SH, groups) then reduces critical, inhibitory disulfide bonds (S–S) within the target enzymes, converting them to their active, reduced state.
2. In the Dark: in the absence of electron flow, the sulfhydryl groups on the enzymes are spontaneously re-oxidized back to disulfide bonds by oxygen, rendering the enzymes inactive.

3. Starch and Sucrose Biosynthesis

The triose phosphates generated by the Calvin cycle serve as the starting material for the synthesis of the two primary carbohydrates in plants: starch (the immobile, chloroplastic storage form) and sucrose (the mobile, cytosolic transport form).

3.1 The Triose Phosphate Translocator (P_i Antiporter)

The partition of carbon between starch and sucrose synthesis is regulated by a specific transport protein embedded in the inner chloroplast membrane: the triose phosphate–orthophosphate translocator. This antiporter exports one molecule of triose phosphate (G3P or DHAP) into the cytosol in strict exchange for importing one molecule of inorganic orthophosphate (P_i).

3.2 Chloroplastic Starch Biosynthesis

Starch is synthesized inside the chloroplast stroma during the day.

1. Triose phosphates are converted to fructose-1,6-bisphosphate and then to fructose-6-phosphate (F6P).
2. F6P is isomerized to glucose-6-phosphate (G6P) and then converted to glucose-1-phosphate (G1P) by phosphoglucomutase.
3. G1P reacts with ATP to form ADP-glucose, a reaction catalyzed by ADP-glucose pyrophosphorylase. This represents the committed, rate-limiting step of starch synthesis.
4. Starch synthase transfers the glucose residue from ADP-glucose to the non-reducing end (carbon 4) of a growing amylose chain, forming α(1→4) glycosidic bonds.
5. A branching enzyme introduces α(1→6) glycosidic linkages to synthesize the branched starch polymer, amylopectin.

3.3 Cytosolic Sucrose Biosynthesis

Sucrose, a soluble disaccharide of glucose and fructose, is synthesized in the cytosol to be exported to sink tissues (such as roots and developing seeds).

1. Triose phosphates exported to the cytosol are converted to F6P, G6P, and G1P.
2. G1P reacts with UTP to form UDP-glucose, catalyzed by UDP-glucose pyrophosphorylase.
3. Sucrose phosphate synthase condenses UDP-glucose with fructose-6-phosphate to form sucrose-6-phosphate. This step is regulated by metabolite levels and covalent phosphorylation.
4. Finally, sucrose phosphate phosphatase hydrolyzes the phosphate group to yield free, transportable sucrose.

4. Photorespiration (The Glycolate / C₂ Cycle)

Although RuBisCO has a high affinity for CO₂, it is a bifunctional enzyme that can also use molecular oxygen (O₂) as a substrate. This oxygenase activity initiates a metabolic salvage pathway known as photorespiration (or the C₂ oxidative photosynthetic carbon cycle). This phenomenon, originally known as the Warburg effect, was recognized as the light-dependent release of CO₂ due to the oxygenase activity of RuBisCO.

Photorespiration is triggered when the concentration of CO₂ is low and O₂ is high, a state commonly induced when plants close their stomata during hot, dry conditions to limit water loss.

Because 2-phosphoglycolate is a toxic metabolite that cannot be utilized in the Calvin cycle, cells process it through a metabolic loop that spans three distinct organelles: chloroplasts, peroxisomes, and mitochondria.

4.1 Step-by-Step Pathway of the C₂ Cycle

1. In the Chloroplast:
   • RuBP is oxygenated to yield one molecule of 3-PGA and one molecule of 2-phosphoglycolate.
   • The 2-phosphoglycolate is rapidly hydrolyzed by a specific stromal phosphoglycolate phosphatase to yield glycolate.
   • Glycolate is exported from the chloroplast via a membrane translocator.
2. In the Peroxisome:
   • Glycolate enters the peroxisome and is oxidized to glyoxylate by the flavin mononucleotide-dependent enzyme glycolate oxidase, transferring electrons to oxygen to form hydrogen peroxide (H₂O₂). The toxic peroxide is immediately degraded into water and oxygen by catalase.
   • Glyoxylate undergoes transamination by glutamate-glyoxylate aminotransferase to form the amino acid glycine.
   • Glycine is exported from the peroxisome.
3. In the Mitochondrion:
   • Two molecules of glycine enter the mitochondrial matrix.
   • They undergo a coordinated decarboxylation and deamination to synthesize one molecule of the three-carbon amino acid serine, a reaction catalyzed by the glycine decarboxylase complex (GDC) and serine hydroxymethyltransferase (SHMT).
   • This step releases a molecule of CO₂ (representing a loss of fixed carbon) and a molecule of ammonia (NH₃). The ammonia is rapidly recovered and reassimilated into glutamate in the chloroplast by glutamine synthetase and GOGAT to prevent nitrogen loss.
4. In the Peroxisome:
   • Serine is transported back into the peroxisome, where it undergoes transamination to form hydroxypyruvate.
   • Hydroxypyruvate is reduced to glycerate by an NADH-dependent hydroxypyruvate reductase (glycerate dehydrogenase).
5. In the Chloroplast:
   • Glycerate enters the chloroplast, where it is phosphorylated by glycerate kinase using one ATP to form 3-phosphoglycerate (3-PGA), which can re-enter the Calvin cycle.

4.2 Carbon Recovery Calculations

Through the C₂ cycle, two molecules of 2-phosphoglycolate (4 carbons) are processed to yield one molecule of 3-PGA (3 carbons), while one carbon is lost as CO₂. Thus, 75 percent of the carbon diverted into 2-phosphoglycolate is successfully recovered and returned to the Calvin cycle, while 25 percent is lost to the atmosphere.

4.3 The Energetic and Physical Cost of Photorespiration

Because it releases previously fixed carbon dioxide and consumes additional ATP and reducing equivalents (NADH/NADPH) without generating new carbohydrates, photorespiration is highly wasteful. The rate of photorespiration is driven by the physical properties of RuBisCO and is influenced by environmental conditions.

1. The Effect of Temperature: as the temperature rises, the solubility of both gases in water declines, but CO₂ solubility decreases much faster than O₂ solubility.
2. Structural Sensitivity: high temperatures induce structural changes in RuBisCO that lower its affinity for CO₂, favoring its oxygenase activity. Consequently, in warm climates, photorespiration can waste more than 25 percent of the net carbon fixed by C₃ plants.

5. The C₄ Pathway (The Hatch-Slack Cycle)

To overcome the energetic waste of photorespiration, some plant species have evolved a metabolic adaptation called the C₄ pathway (or Hatch-Slack cycle). Discovered by Marshall Davidson Hatch and Charles Roger Slack in Australia in 1966, this pathway functions to physically concentrate CO₂ at the site of carboxylation, keeping the local concentration of CO₂ high enough for RuBisCO to bind carbon dioxide rather than oxygen.

5.1 Spatial Separation and Kranz Anatomy

The C₄ pathway relies on spatial segregation of carbon capture and carbon fixation, requiring cooperation between two distinct cell layers.

1. Mesophyll Cells: form an outer layer. They contain chloroplasts with well-developed grana but lack RuBisCO. Instead, they express phosphoenolpyruvate carboxylase (PEPC) in their cytosol.
2. Bundle Sheath Cells: form a tightly packed ring around the vascular bundles. This concentric arrangement is known as Kranz anatomy (from the German word for “wreath”). Bundle sheath cells contain large chloroplasts that lack grana (agranal) and are rich in RuBisCO. Because they lack grana, these chloroplasts do not run Photosystem II, preventing light-driven oxygen evolution in the same space where RuBisCO operates.

5.2 The Five Stages of the C₄ Cycle

The Hatch-Slack pathway operates in five coordinated stages.

1. Carbonate Fixation in Mesophyll: atmospheric CO₂ entering the mesophyll is hydrated to bicarbonate (HCO₃⁻) by carbonic anhydrase. PEP carboxylase condenses HCO₃⁻ with the three-carbon substrate phosphoenolpyruvate (PEP) to form the four-carbon dicarboxylic acid oxaloacetate (OAA). PEPC has an extremely high affinity for HCO₃⁻ and is insensitive to oxygen, allowing it to fix carbon even at exceptionally low internal CO₂ concentrations.
2. Organic Acid Conversion: OAA is rapidly converted into other four-carbon organic acids — in some species OAA is reduced to malate by NADPH-dependent malate dehydrogenase within the chloroplast, while in other species OAA is transaminated to aspartate in the cytosol.
3. Transport to Bundle Sheath: these four-carbon acids are transported from the mesophyll into adjacent bundle sheath cells through connecting cytoplasmic channels (plasmodesmata).
4. Decarboxylation: inside the bundle sheath cells, the four-carbon acid is decarboxylated to release a high concentration of CO₂ and a three-carbon acid. This locally elevated CO₂ concentration saturates RuBisCO’s active sites, suppressing its oxygenase activity and photorespiration.
5. Regeneration of PEP: the three-carbon acid (pyruvate or alanine) is transported back to the mesophyll cell. There, pyruvate-phosphate dikinase (PPDK) uses ATP and inorganic phosphate to convert pyruvate back into PEP, allowing the cycle to restart.

5.3 The Three Decarboxylation Subtypes

Based on the specific decarboxylating enzymes used within the bundle sheath cells, C₄ plants are classified into three distinct physiological subtypes.

NADP-Malic Enzyme Type (e.g., Maize, Sugarcane)

1. Carboxylation: in mesophyll cytosol, PEP + HCO₃⁻ → OAA (via PEPC).
2. Reduction: in mesophyll chloroplasts, OAA + NADPH + H⁺ → Malate + NADP⁺ (via MDH).
3. Transport: malate diffuses to bundle sheath chloroplasts.
4. Decarboxylation: Malate + NADP⁺ → Pyruvate + CO₂ + NADPH + H⁺ (via chloroplastic NADP-malic enzyme).
5. Fixation: released CO₂ enters the Calvin cycle.
6. Regeneration: pyruvate is returned to the mesophyll, where PPDK converts it to PEP. Regenerating AMP back to ATP requires the hydrolysis of two high-energy phosphate bonds.

5.4 Energetic Cost of the C₄ Pump

The active transport of carbon from the mesophyll to the bundle sheath represents a major energetic cost. The conversion of pyruvate to PEP by PPDK converts ATP to AMP and pyrophosphate (PP_i); regenerating this AMP back to ATP requires the hydrolysis of 2 ATP equivalents.

6. The CAM Pathway (Crassulacean Acid Metabolism)

In extremely arid environments where water loss is the primary threat to survival, some succulent plants (such as pineapple, cacti, and jade plants) utilize a metabolic adaptation called Crassulacean Acid Metabolism (CAM).

Unlike C₄ plants, which separate carbon capture and fixation spatially between two cell types, CAM plants separate these processes temporally (in time) within a single mesophyll cell.

6.1 Night-Time Reactions (Stomata Open)

During the cool night, CAM plants open their stomata to capture CO₂ while limiting water loss through transpiration.

1. The entering CO₂ is hydrated to HCO₃⁻ and fixed by cytosolic PEP carboxylase to form oxaloacetate (OAA).
2. OAA is reduced to malate by cytosolic malate dehydrogenase using NADH.
3. This malate is transported across the tonoplast membrane and stored as malic acid inside the large central vacuole. Over the course of the night, the accumulation of vacuolar malic acid causes the cell’s pH to drop significantly.

6.2 Day-Time Reactions (Stomata Closed)

During the hot day, CAM plants keep their stomata tightly closed to prevent water loss.

1. Malic acid is released from the vacuole back into the cytosol.
2. The malate is decarboxylated by NADP-malic enzyme (or PEP carboxykinase) to release a high concentration of CO₂ inside the cell.
3. This CO₂ is fixed by RuBisCO in the chloroplast via the Calvin cycle, driven by the ATP and NADPH generated by the light reactions.
4. The remaining three-carbon pyruvate is converted into starch via gluconeogenesis, which serves as the source of PEP for the next night’s carbon capture.

7. Comparative Ecophysiology of C₃, C₄, and CAM Plants

The structural and biochemical differences between these pathways result in distinct physiological properties and ecological distributions.

8. Light and CO₂ Compensation Points

The balance between photosynthetic carbon gain and respiratory carbon loss is defined by two threshold parameters.

8.1 The Light Compensation Point

The light compensation point is the specific light intensity at which the rate of photosynthetic CO₂ uptake exactly equals the rate of respiratory CO₂ release (including cellular respiration and photorespiration). At this point, the net gas exchange of the plant is zero: Photosynthesis (P) = Respiration (R).

1. Sun Plants: adapted to grow in open, high-light environments. They maintain high levels of RuBisCO and photosynthetic machinery, resulting in high rates of respiration. Consequently, sun plants have high light compensation points.
2. Shade Plants: adapted to grow in forest understories. They minimize energetic costs by maintaining lower levels of RuBisCO and thin leaves, resulting in very low respiration rates. This allows them to achieve positive net photosynthesis at very low light intensities, giving them low light compensation points.

8.2 The CO₂ Compensation Point

The CO₂ compensation point is the concentration of atmospheric CO₂ at which the rate of carbon fixation by photosynthesis exactly balances the rate of carbon loss via respiration and photorespiration.

1. In C₃ Plants: because RuBisCO must compete with oxygen, C₃ plants require a relatively high concentration of CO₂ to balance photorespiration. At 25°C, their CO₂ compensation point lies between 30 and 70 ppm.
2. In C₄ Plants: because PEP carboxylase is highly efficient and insensitive to oxygen, C₄ plants can maintain active carbon fixation at extremely low internal carbon levels. Their CO₂ compensation point is exceptionally low, lying between 0 and 10 ppm.

9. Comparative Adaptations: Sun vs. Shade Plants

To survive in their respective light environments, sun and shade plants have evolved distinct biochemical and structural traits.

1. Ratio of Photosystem II to Photosystem I: shade plants exhibit a significantly higher ratio of PSII to PSI (typically 3:1) compared to sun plants (typically 2:1). This enrichment of PSII allows shade plants to absorb more of the longer, far-red wavelengths of light that filter through the canopy.
2. Chlorophyll b to Chlorophyll a Ratio: shade plants contain a higher ratio of chlorophyll b to chlorophyll a. Chlorophyll b has a shifted absorption spectrum that excels at capturing green and blue-green wavelengths, which are abundant in the shade of forest understories.
3. Enzymatic Allocation: sun plants allocate significantly more resources to carbon-fixation enzymes, maintaining high levels of RuBisCO and high rates of respiration. Shade plants express low levels of RuBisCO, reducing their overall protein and maintenance costs.

10. Quantum Yield Dynamics: Temperature and CO₂ Influences

Quantum yield is defined as the number of molecules of CO₂ fixed (or molecular oxygen evolved) per photon of light absorbed:

The quantum yields of C₃ and C₄ plants respond differently to temperature and gas concentrations due to the presence or absence of photorespiration.

10.1 The Influence of Temperature

1. In C₃ Plants: as the leaf temperature rises, the oxygenase activity of RuBisCO increases relative to its carboxylase activity, accelerating photorespiration. Because photorespiration consumes ATP and NADPH while releasing fixed carbon, the energy cost per net CO₂ fixed increases. Consequently, the quantum yield of C₃ plants decreases steadily with increasing temperature.
2. In C₄ Plants: because the C₄ pump concentrates CO₂ around RuBisCO, photorespiration is suppressed at all temperatures. Although the C₄ pump has a high energetic cost, this cost remains constant. Thus, the quantum yield of C₄ plants is independent of temperature, remaining constant across a wide temperature range.

10.2 The Influence of CO₂ Concentration

1. In C₃ Plants: supplying adequate light and elevating the concentration of atmospheric CO₂ suppresses the oxygenase activity of RuBisCO. This reduces photorespiration and increases the quantum yield of C₃ plants.
2. In C₄ Plants: because their internal carbon-concentrating mechanism is already fully saturated, C₄ plants do not benefit from increases in atmospheric CO₂. Their quantum yield remains unchanged.