What is C4 and CAM plants? — Biology

Definition 14.12 University Biology — Year 1 · Chapter 14 — Photosynthesis and Autotrophy

C4 plants (maize, sugar cane, sorghum, many tropical grasses) seal the leak by concentrating $\mathrm{CO_2}$ : in the mesophyll cells an enzyme with no affinity for $\mathrm{O_2}$ (PEP carboxylase) fixes bicarbonate into a four-carbon acid, which is shuttled into the bundle-sheath cells around the veins and decarboxylated there, raising the $\mathrm{CO_2}$ around RuBisCO tenfold and suppressing oxygenation — at a cost of two extra ATP per $\mathrm{CO_2}$ . The two cell types form the Kranz (“wreath”) anatomy. CAM plants (cacti, pineapple, agaves) separate the same two steps in time rather than space: they open their stomata at night, store $\mathrm{CO_2}$ as malic acid in the vacuole, and release it to RuBisCO by day behind closed stomata, losing a tenth of the water a C3 plant loses per carbon fixed.

Kranz anatomy in a maize leaf: each vein is wreathed by large bundle-sheath cells packed with chloroplasts, themselves ringed by mesophyll cells. CO_2 is captured in the outer ring and delivered, concentrated, to RuBisCO in the inner one. — Kranz anatomy in a maize leaf: each vein is wreathed by large bundle-sheath cells packed with chloroplasts, themselves ringed by mesophyll cells. $\mathrm{CO_2}$ is captured in the outer ring and delivered, concentrated, to RuBisCO in the inner one.

Examples

Example 14.13 (Which plant where)

At $20\,{}^{\circ}\mathrm{C}$ in moist air a C3 wheat leaf and a C4 maize leaf fix carbon at similar rates, and the maize pays two ATP more per carbon. At $35\,{}^{\circ}\mathrm{C}$ in dry air the wheat loses a third of its fixation to photorespiration while the maize loses none: C4 grasses dominate hot open country, C3 plants the cool and shaded. A cactus fixes slowly by either standard but survives where the others would die of thirst.

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