Match Each Pathway to Where and When It Fixes CO2
Photosynthesis questions ask which pathway a plant uses, which photosystems a stage involves, and how pigments separate. CAM, cyclic and non-cyclic photophosphorylation, and pigment chromatography are the repeated themes.
Photosynthesis uses light energy to make food in green plant cells; transport tissues move water and sugar where they are needed.
Track what enters, leaves and moves inside
H₂O rises from roots in xylem · sugar travels from the leaf in phloem
CO₂ enters through stomata; water arrives from roots; oxygen leaves the leaf. Sugar moves from the source leaf to use or storage sites.
CAM plants fix CO2 at night
In Crassulacean acid metabolism (CAM) the stomata open at night, when PEP carboxylase combines CO2 with phosphoenolpyruvate to form oxaloacetate, which is stored as malic acid in the vacuole. In the day the stomata stay closed, the acid releases CO2 internally, and the Calvin cycle uses it. CAM saves water, so it is typical of succulents and plants of dry places such as Aloe, Crassula and Opuntia, not of palms, mangroves or submerged plants like Vallisneria. The Hatch-Slack pathway is C4, which separates the steps in space between mesophyll and bundle-sheath cells instead of in time.
Cyclic and non-cyclic photophosphorylation
Non-cyclic photophosphorylation uses both photosystems: PSII (P680) splits water and releases oxygen, and electrons pass to PSI (P700), giving ATP and NADPH. Cyclic photophosphorylation uses only PSI: the excited electron returns to the reaction centre, so ATP is made but no NADPH and no oxygen, and water is not needed because P680 and PSII are not involved.
Separating leaf pigments
Carotenes are purely hydrocarbons, the least polar, and run highest; xanthophylls (carotenols) come next, then chlorophyll a, and chlorophyll b, with its extra aldehyde group, is the most polar and stays closest to the origin. From the bottom upward the bands are therefore chlorophyll b, chlorophyll a, xanthophylls and carotenes.
OlympIQ exam lens
- ✓CAM: stomata open at night, PEP carboxylase makes oxaloacetate and malate, Calvin cycle by day with stomata closed
- ✓CAM plants are succulents of dry habitats; C4 is the Hatch-Slack pathway
- ✓Cyclic photophosphorylation: PSI only, ATP only, no NADPH, no O2, no water splitting
- ✓Non-cyclic: PSII and PSI, water split, O2, ATP and NADPH
- ✓Chromatography order from the origin: chlorophyll b, chlorophyll a, xanthophylls, carotenes
Do not assume the stomata of every plant open in the day. In CAM plants they open at night and close by day, so daytime water loss is tiny even in strong sunlight.
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