Chapter 11: Photosynthesis in Higher Plants (Plant Physiology)

Not with certainty. C₃ and C₄ plants can look alike externally; the real differences are in leaf anatomy (Kranz anatomy) and biochemistry (the first product of CO₂ fixation). Some clues help: C₄ plants are mostly tropical grasses and plants of hot, dry, sunny places (maize, sorghum, sugarcane), grow fast and stay green under high light and heat, but these are only hints; a leaf section or a biochemical test is needed.

The leaf anatomy seen in a vertical section. A C₄ leaf shows Kranz anatomy: large bundle sheath cells arranged like a wreath around the vascular bundles, with many chloroplasts, thick walls impervious to gases and no intercellular spaces, surrounded by mesophyll cells. A C₃ leaf lacks this: its bundle sheath cells are small with few or no chloroplasts, and the mesophyll (palisade and spongy) does the fixation.

(i) The C₄ cycle in mesophyll cells acts as a CO₂ pump: PEPcase fixes CO₂ efficiently even at low concentration, and C₄ acids release it in the bundle sheath, building a high CO₂ concentration at RuBisCO. (ii) So RuBisCO works as a carboxylase and photorespiration is absent: no fixed carbon is wasted. (iii) C₄ plants tolerate high temperature and high light, and can keep stomata partly closed (saving water) while still fixing CO₂. Hence their rate of net CO₂ fixation and biomass is high.

CO₂ and O₂ compete for RuBisCO's active site; the relative concentration decides which binds. In C₄ plants RuBisCO is only in the bundle sheath cells, where breakdown of C₄ acids (malic or aspartic acid) from the mesophyll releases CO₂, raising its intracellular concentration far above O₂; the thick, gas-impervious walls keep it there. So RuBisCO mostly binds CO₂ and acts as a carboxylase, minimising oxygenation (photorespiration).

No. Chlorophyll a is the chief pigment: the reaction centres P680 and P700 are chl a molecules, which lose excited electrons to start the light reaction. Chl b only absorbs light and passes the energy on; without chl a no photochemical reaction can occur. Plants have accessory pigments (chl b, xanthophylls, carotenoids) because they absorb wavelengths that chl a does not, transfer this energy to chl a (so a wider range of light is used) and protect chl a from photo-oxidation.

Chlorophyll is continuously broken down and needs light for its synthesis. In the dark, chlorophyll breaks down and is not replaced, so the green colour fades and the yellow carotenoids and xanthophylls become visible. The carotenoids (and xanthophylls) are more stable than chlorophyll.

Leaves in the shade are usually darker green. To capture enough of the weak light they have more chlorophyll (and larger grana and antennae) per unit area. Leaves in strong sunlight have less chlorophyll, as bright light can break down chlorophyll (photo-oxidation), and more protective carotenoids.

(a) Light is limiting in region A (the rising part, up to B), where the rate increases with light intensity. (b) In region A the limiting factor is light intensity itself; the rate depends directly on it. (c) C is the plateau: light is no longer limiting; the rate is limited by other factors (CO₂ concentration, temperature, chlorophyll content). D is the light intensity at which saturation is reached (the rate reaches its maximum E).

(a) C₃: acceptor RuBP (5C), first product PGA (3C), only RuBisCO, Calvin cycle in mesophyll, photorespiration high, optimum 20–25 °C, CO₂ saturation beyond 450 µL L⁻¹ (wheat, rice). C₄: acceptor PEP (3C) by PEPcase, first product OAA (4C), Calvin cycle in bundle sheath, photorespiration negligible, optimum 30–40 °C, saturation about 360 µL L⁻¹, higher productivity (maize, sugarcane). (b) Non-cyclic: PS II and PS I in series; electrons from water end in NADPH; products ATP, NADPH and O₂; grana lamellae. Cyclic: only PS I; electrons return to PS I; only ATP; no water splitting or O₂; stroma lamellae or light beyond 680 nm. (c) C₃ leaf: mesophyll of palisade and spongy cells; bundle sheath cells small, with few chloroplasts; no Kranz anatomy. C₄ leaf: Kranz anatomy: large bundle sheath cells with many chloroplasts, thick gas-impervious walls, no intercellular spaces, surrounded by radially arranged mesophyll cells.