Follow light energy, water, electrons, and carbon through the light reactions and Calvin cycle.
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Thylakoids are membrane sacs, often stacked into grana. The stroma is the fluid around them. Light reactions occur in thylakoid membranes; the Calvin cycle occurs in the stroma.
The thylakoid interior also provides a space for H+ accumulation. Structure creates the separate locations needed for the two linked sets of reactions.
A growing tree gains a large amount of dry mass. What is the main source of the carbon in that new organic material?
Carbon dioxide from the air.
Photosynthesis fixes CO₂ into organic molecules. Soil supplies water and minerals, but it is not the main source of a plant’s organic carbon.
Light reactions use light energy and water to produce ATP and NADPH. Oxygen is released when water is split. Chlorophyll helps capture the light.
Electrons move through carriers, and a proton gradient helps power ATP synthase. ATP and NADPH then supply energy and reducing power for the Calvin cycle.
During photosynthesis, does the released oxygen gas come from CO₂ or H₂O?
H₂O.
Water is split during the light reactions, supplying electrons and releasing oxygen. The carbon from CO₂ is used in organic molecules.
The Calvin cycle fixes CO₂, reduces carbon compounds using ATP and NADPH, and regenerates its CO₂ acceptor. Its direct sugar product is G3P, which can help make glucose and other molecules.
The cycle does not absorb light directly, but in normal conditions it depends on products of the light reactions. Calling it “dark reactions” can misleadingly suggest it runs only at night.
If a leaf stays in darkness long enough that its supplies of ATP and NADPH fall, why does the Calvin cycle slow?
It lacks the energy and reducing power normally supplied by the light reactions.
A process can depend on light indirectly. The Calvin cycle uses CO₂ as carbon input, but CO₂ alone cannot supply all the energy needed to build sugar.
Stomata allow gas exchange but also allow water to escape. In hot, dry conditions, closing them conserves water while reducing CO₂ entry. C4 and CAM plants use adaptations that help limit photorespiration.
C4 plants separate initial CO₂ fixation and the Calvin cycle between cell types. CAM plants initially fix CO₂ at night and use it during the day.
True or false: plants photosynthesize, so they do not need cellular respiration.
False. Plants also perform cellular respiration to make ATP from organic fuel.
Photosynthesis captures energy in organic molecules. Respiration makes that energy available for many cellular tasks, including in roots and during the night.