Standard content

Beyond the textbook: a full account of the ATP, NADPH, carbon and photons that photosynthesis spends, a point-by-point comparison of the C3 and C4 pathways, and the reading of limiting-factor graphs and classic experiments. Twelve harder problems and a full timed mock exam.

What's inside

1. Counting energy, electrons and carbon: Every molecule of glucose in the Calvin cycle needs 6 carbon dioxide, 18 ATP and 12 NADPH. The 12 NADPH need 24 electrons, which come from 12 split water molecules, and at the least 48 photons. A C4 plant spends 2 more ATP for each carbon dioxide. Cyclic flow makes up the ATP that the non-cyclic path alone cannot give.
2. C3, C4 and photorespiration compared: C3 and C4 plants differ in the first stable product, the first enzyme, the cell that holds the Calvin cycle, photorespiration, the temperature optimum and the response to carbon dioxide. A C4 plant spends more ATP, but it wins in hot, bright places. A C3 plant can do as well in cool, shady places. Photorespiration makes no sugar, no ATP and no NADPH.
3. Limiting-factor graphs and classic experiments: A graph with two curves shows which factor limits where. Two curves that coincide at low light show light limiting there. Curves that part at high light show a second factor, such as carbon dioxide, limiting. Classic experiments test one factor at a time: a half leaf in potassium hydroxide, a variegated leaf, a covered strip and bubbles counted at different lamp distances.