Photosynthesis
▶︎ Watch it animatedinteractive step-through · ~3 min · optionalPhotosynthesis is how a plant builds sugar out of air and light. It runs in two linked stages inside the chloroplast. In the light reactions, on the thylakoid membrane, absorbed light energy is used to split water — and it is that splitting of water, not the fixing of CO₂, that releases the O₂ you breathe. The same reactions load energy into two carriers, ATP and NADPH, which are the actual currency handed to the next stage.
That next stage is the Calvin cycle, in the stroma. It needs no light directly; instead it spends the ATP and NADPH from the light reactions to fix CO₂ — pulling carbon out of the air and assembling it into sugar. So the two stages divide the labor: the light reactions capture energy and free oxygen from water, and the Calvin cycle uses that captured energy to turn carbon dioxide into carbohydrate. Light in, water and CO₂ in; sugar and O₂ out.
Interactive · Photosynthesis
Run the two stages and trace where each atom goes. Watch water split on the thylakoid to release O₂ and charge ATP and NADPH, then follow that energy into the stroma, where the Calvin cycle fixes CO₂ into sugar.
Photosynthesis · Open the full sandbox →The mistakes here all confuse where things come from and where they go. One is thinking the released O₂ comes from the CO₂ a plant takes in, when in fact it comes from splitting water (U3-BIO12). Another is assuming the Calvin cycle needs light directly, or forgetting that it runs on the ATP and NADPH the light reactions supply (U3-BIO13). And the most stubborn of all is believing that plants only photosynthesize — that they don't respire. They do: plants run cellular respiration around the clock to spend the sugar they store (U3-BIO11, U3-BIO16). Every scenario in this topic asks you to track energy and matter through the chloroplast rather than trust the tidy summary equation.
The work
3 ways in · any order
Lesson
Photosynthesis
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Photosynthesis is two linked stages, and most errors come from blurring them. The lesson walks the ways students misread it: thinking the released oxygen comes from CO₂ rather than water, forgetting that the Calvin cycle runs on the ATP and NADPH the light reactions supply, and assuming plants only photosynthesize and never respire. It closes with a ten-scenario applet that asks you to track energy and matter through the chloroplast.
Diagnostic
10-item topic check
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Ten items on photosynthesis — that the released O₂ comes from splitting water, not from CO₂ (U3-BIO12); that the Calvin cycle fixes CO₂ using the ATP and NADPH the light reactions supply (U3-BIO13); and that plants respire too, around the clock, not only photosynthesize (U3-BIO11, U3-BIO16). Take it cold to surface which of these are still tangled, or after the lesson to confirm they hold.
Targeted Practice
Drill a single misconception
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Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears the misconception and moves you to the next.