Describe the light reactions and the Calvin cycle, and explain how a floating-leaf-disk assay measures photosynthetic rate.
Photosynthesis takes place inside the chloroplast, which has two key compartments: stacks of membrane sacs called thylakoids (bundled into stacks called grana), where chlorophyll and other pigments are embedded, and the stroma, the fluid surrounding the thylakoids. In the light reactions, which take place in the thylakoid membranes, chlorophyll absorbs light energy, most strongly in the blue and red portions of the visible spectrum -- it reflects green light rather than absorbing it, which is exactly why healthy leaves look green to us. That absorbed energy splits water molecules apart, releasing oxygen gas as a byproduct (the same oxygen gas that makes up roughly a fifth of the atmosphere) and using the captured energy to produce ATP and a second energy-carrying molecule, NADPH.
The Calvin cycle then takes place in the stroma, using the ATP and NADPH produced by the light reactions, along with carbon dioxide pulled from the air, to build glucose in a process called carbon fixation. Although the Calvin cycle does not use light directly, it depends entirely on a fresh supply of ATP and NADPH from the light reactions, so in a living plant it effectively only runs during daylight. Combining both stages gives the overall equation from last week: 6CO2 + 6H2O + light energy -> C6H12O6 + 6O2.
Mastery looks like: They can describe what happens in the light reactions and the Calvin cycle, name where each occurs in the chloroplast, and explain how the floating-leaf-disk assay measures photosynthetic rate.
Common stumbles: Thinking the Calvin cycle literally requires light directly, rather than requiring the products of the light reactions; forgetting that the oxygen released comes from splitting water, not from carbon dioxide.