Photosynthesis in Higher Plants

Photosynthesis splits cleanly into two linked stages, one that captures light energy and one that uses that captured energy to build sugar, and the exam-relevant detail sits mostly in exactly where each stage happens and which specific molecules and enzymes carry it out.

The light reactions: capturing energy, splitting water

The light reactions occur in the thylakoid membranes of the chloroplast and require light directly. Chlorophyll and associated pigments absorb light energy and use it to split water molecules (photolysis), releasing oxygen as a by-product, while channelling the captured energy into producing two key energy-carrying molecules: ATP and NADPH, both of which are then exported to fuel the second stage.

The Calvin cycle: fixing carbon dioxide into sugar

The Calvin cycle (the "dark reaction," though it does not actually require darkness, only that it does not itself need light directly) occurs in the stroma, the fluid-filled region of the chloroplast surrounding the thylakoids, and uses the ATP and NADPH generated by the light reactions to convert carbon dioxide into sugar. In the standard C3 pathway, found in the majority of plants, the enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) fixes carbon dioxide directly, producing 3-phosphoglycerate (PGA), a three-carbon compound, as the first stable product, which gives the pathway its name.

Photorespiration: RuBisCO's costly wrong turn

RuBisCO is not a perfectly specific enzyme: alongside its intended carboxylase activity (binding carbon dioxide), it can also act as an oxygenase, binding oxygen instead, particularly under hot, dry conditions when a plant's stomata partly close to conserve water and internal carbon dioxide concentration drops relative to oxygen. When RuBisCO binds oxygen instead of carbon dioxide, the resulting reaction feeds into photorespiration, a wasteful pathway that consumes energy and, worse for the plant's own carbon economy, causes a net loss of previously fixed carbon dioxide, directly undermining the very process photosynthesis is meant to accomplish.

The C4 pathway and Kranz anatomy: an evolved workaround

Certain plants (many tropical grasses, maize and sugarcane among them) have evolved the C4 pathway specifically to minimise this photorespiration problem. In C4 plants, carbon dioxide is first captured in mesophyll cells not by RuBisCO at all, but by a different enzyme, PEP carboxylase (PEPC), using phosphoenolpyruvate (PEP) as the carbon dioxide acceptor; since PEP carboxylase has no oxygenase activity of its own, it is not vulnerable to the same wasteful side reaction RuBisCO is. This first fixation step produces oxaloacetate, a four-carbon compound, the reason the pathway is called "C4." That four-carbon compound is then shuttled inward to specialised bundle sheath cells surrounding the leaf's vascular bundles, where it releases its carbon dioxide again at high local concentration, specifically so that concentrated carbon dioxide can now feed the ordinary C3 Calvin cycle running inside those bundle sheath cells, with RuBisCO there kept so thoroughly saturated with carbon dioxide that it has little practical opportunity to bind oxygen instead. This division of labour between two concentric leaf cell layers, an outer ring of mesophyll cells and an inner ring of bundle sheath cells surrounding the vascular tissue, is called Kranz anatomy (from the German for "wreath," describing the visible ring-like arrangement), and it is this specific anatomical arrangement, not just the biochemical pathway alone, that makes C4 photosynthesis's photorespiration-suppressing trick work.

Quick revision points

• Light reactions: occur in the thylakoid membranes, split water (releasing oxygen), produce ATP and NADPH.
• Calvin cycle (dark reaction): occurs in the stroma, uses ATP and NADPH to fix carbon dioxide into sugar.
• C3 pathway: RuBisCO fixes CO2 directly, producing 3-phosphoglycerate (PGA), a 3-carbon compound, as the first product.
• Photorespiration: RuBisCO's oxygenase activity (binding O2 instead of CO2, favoured when stomata partly close under heat/drought stress) wastes energy and causes a net loss of fixed carbon dioxide.
• C4 pathway: PEP carboxylase (no oxygenase activity) fixes CO2 in mesophyll cells using PEP, producing oxaloacetate (4 carbons, hence "C4"); this is shuttled to bundle sheath cells, which release concentrated CO2 there to feed the ordinary C3 Calvin cycle while keeping RuBisCO saturated with CO2 rather than exposed to oxygen.
• Kranz anatomy: the ring-like mesophyll-cells-around-bundle-sheath-cells arrangement in C4 plant leaves that makes this two-compartment CO2-concentrating strategy work.