the light reactions
/ LITE ree-AK-shuhnz /
Think of a solar panel charging a battery. The panel does not power your phone directly; it first captures sunlight and stores it as charge, which you then spend later. The light reactions are the plant's solar-charging stage: they grab the energy of sunlight and stash it in chemical batteries that the next stage will use to build sugar.
The light reactions take place in stacked membranes inside the chloroplast. Pigments, chiefly the green chlorophyll, absorb photons of light, and the energy boosts electrons to a high-energy state. Those excited electrons travel down an electron transport chain very much like the one in respiration, pumping protons across a membrane to build a gradient. ATP synthase then uses that gradient to make ATP, exactly as in mitochondria. The electrons end up loaded onto a carrier called NADP+, making NADPH. To replace the electrons it keeps losing, the system splits water molecules apart, and the leftover oxygen is released into the air as a byproduct.
So the light reactions deliver two things: ATP and NADPH, the energy and the electron-carrying "batteries" the Calvin cycle needs. They also produce essentially all the free oxygen in our atmosphere; the oxygen you breathe is waste from splitting water during this stage, billions of times over. Notice the deep similarity to respiration: both run electron chains across a membrane and use the same ATP synthase machine, a striking case of life reusing one good design.
When sunlight hits a leaf, the light reactions split water to grab electrons, releasing the oxygen bubbles you can see rising from an underwater water plant on a bright day.
Light reactions = solar charging: they make ATP and NADPH and release oxygen from split water.
The oxygen released by photosynthesis comes from splitting water, not from the carbon dioxide. The light reactions need light directly; the Calvin cycle that follows does not.