Photosynthesis in Higher Plants Class 11 Notes

Photosynthesis as a Physicochemical Process

Photosynthesis occurs in chloroplasts, mainly in the mesophyll cells of leaves. It involves two coupled stages — the light reaction (in the thylakoid membranes) and the dark reaction / Calvin cycle (in the stroma). The overall equation was first proposed by Cornelius van Niel and later confirmed by Ruben and Kamen using the isotope ¹⁸O.

Light Reaction — Z-scheme

The light reaction takes place in the thylakoid membrane and involves two photosystems and an electron transport chain.

• PS II (P680): absorbs light at 680 nm, loses electrons, which pass through the electron transport chain (plastoquinone → cytochrome b6f → plastocyanin) to PS I. ATP is generated by proton gradient (chemiosmosis) as protons are pumped into the thylakoid lumen.
• PS I (P700): absorbs light at 700 nm, re-energises electrons, which pass through ferredoxin to NADP⁺ reductase, producing NADPH.
• Non-cyclic photophosphorylation: electrons flow from PS II → PS I and do not return to PS II. Both ATP and NADPH are produced. O₂ is released.
• Cyclic photophosphorylation: electrons from PS I cycle back through the cytochrome b6f complex to PS I via plastocyanin. Only ATP is produced (no NADPH, no O₂). This occurs when NADPH is in excess or when the Calvin cycle needs more ATP.

Dark Reaction — Calvin Cycle (C3 Pathway)

The Calvin cycle (Calvin–Benson cycle) takes place in the stroma of the chloroplast. It uses the ATP and NADPH from the light reaction to fix CO₂ into organic molecules. The cycle has three phases: carbon fixation, reduction, and regeneration of RuBP.

• Carbon fixation: CO₂ combines with ribulose-1,5-bisphosphate (RuBP, a 5-carbon sugar) catalysed by RuBisCO to form an unstable 6-carbon intermediate that splits into two molecules of 3-phosphoglycerate (3-PGA, a 3-carbon compound).
• Reduction: 3-PGA is phosphorylated by ATP and reduced by NADPH to glyceraldehyde-3-phosphate (G3P). One G3P exits the cycle per 3 CO₂ fixed — this is the net product.
• Regeneration of RuBP: the remaining G3P molecules are rearranged and phosphorylated by ATP to regenerate RuBP, allowing the cycle to continue.

It takes 6 turns of the Calvin cycle (fixing 6 CO₂) to produce one molecule of glucose (C₆H₁₂O₆). C3 plants (e.g. rice, wheat) use only the Calvin cycle for carbon fixation.

C4 Pathway and Kranz Anatomy

C4 plants (e.g. maize, sugarcane, sorghum) have evolved a carbon-concentrating mechanism to overcome photorespiration. They use two types of cells for carbon fixation:

• Mesophyll cells: CO₂ is fixed by PEP carboxylase (which has no oxygenase activity) into oxaloacetate (OAA, a 4-carbon compound) from PEP and CO₂. OAA is converted to malate or aspartate.
• Bundle sheath cells: the 4-carbon acid is transported to bundle sheath cells, where it is decarboxylated to release CO₂. The released CO₂ is then fixed by RuBisCO in the Calvin cycle. The high CO₂ concentration around RuBisCO suppresses photorespiration.
• Kranz anatomy: C4 leaves have a distinctive arrangement — bundle sheath cells surround the vascular bundles with thick walls, and mesophyll cells are arranged concentrically around them. This close packing ensures efficient CO₂ transfer.

Photorespiration and Its Drawbacks

Photorespiration is a wasteful process that occurs when RuBisCO acts as an oxygenase instead of a carboxylase — it fixes O₂ instead of CO₂ onto RuBP. This produces one molecule of 3-PGA and one molecule of 2-phosphoglycolate (a 2-carbon compound). The 2-phosphoglycolate must be salvaged through a pathway involving chloroplasts, peroxisomes and mitochondria (the C2 pathway), consuming ATP and releasing CO₂ without producing useful energy.

Factors Affecting Photosynthesis

• Light intensity: rate increases linearly at low intensities, then plateaus at the light saturation point. At very high intensities, rate may decline due to photooxidative damage.
• Light quality: chlorophyll absorbs mainly red (660–680 nm) and blue (430–450 nm) light. Green light is mostly reflected — hence leaves appear green.
• CO₂ concentration: increasing atmospheric CO₂ from ~0.04% to ~0.1% significantly boosts the rate in C3 plants. C4 plants are already saturated at ambient CO₂ because of their CO₂-concentrating mechanism.
• Temperature: photosynthesis has a Q₁₀ of ~2 up to the optimum (~25–35 °C for most C3 plants), beyond which the rate drops due to enzyme denaturation (RuBisCO, etc.) and increased photorespiration.
• Water: water stress causes stomatal closure, reducing CO₂ entry and hence limiting the rate.

Solved Examples

Example: How many ATP and NADPH molecules are consumed to produce one molecule of glucose in the Calvin cycle?

Solution: Producing one glucose (6 CO₂ fixed) requires 6 turns of the Calvin cycle. Each turn fixes 1 CO₂ and consumes 3 ATP + 2 NADPH. For 6 CO₂: 6 × 3 = 18 ATP and 6 × 2 = 12 NADPH are consumed. The 18 ATP come from both cyclic and non-cyclic photophosphorylation; the 12 NADPH come only from non-cyclic photophosphorylation.

Example: A C4 plant and a C3 plant are placed in an environment with high temperature, high light intensity and low CO₂ concentration. Which plant will photosynthesise more efficiently and why?

Solution: The C4 plant will be more efficient. Under these conditions (high O₂, low CO₂, high temperature), RuBisCO in C3 plants fixes O₂ instead of CO₂, leading to photorespiration and loss of fixed carbon. In C4 plants, PEP carboxylase initially fixes CO₂ into OAA in mesophyll cells; the CO₂ is then concentrated in bundle sheath cells where RuBisCO operates at high CO₂/O₂ ratio, virtually eliminating photorespiration.