Photosynthesis is how plants, algae and some bacteria use light energy to build organic molecules. In a typical land plant, roots take up water, and carbon dioxide enters leaves through tiny pores called stomata. Light supplies energy, while carbon dioxide provides the carbon the plant builds into sugars. The sugars can feed growth or be stored as starch. Oxygen is released along the way. Think about a plant getting bigger over time: some of the new material you can hold in your hands began as carbon dioxide in the air.

Inside many leaf cells are chloroplasts, compartments that contain chlorophyll. In the membranes of flattened structures called thylakoids, pigments absorb light and transfer energy into a chain of reactions. This part of photosynthesis splits water and releases oxygen. It also produces ATP and NADPH, which carry energy and electrons to the next stage. Oxygen released by the plant therefore comes from water. This is an easy detail to miss if you only memorize the overall photosynthesis equation, which puts both water and carbon dioxide on the same side.

The Calvin cycle takes place in the fluid surrounding the thylakoids. An enzyme called Rubisco helps attach carbon dioxide to a molecule already in the cycle. Using ATP and NADPH from the light reactions, the cycle produces a small sugar phosphate called G3P. Some G3P can be used to build other carbohydrates, while the rest helps regenerate the starting molecule so the cycle can continue. Textbooks call these reactions light independent because they do not absorb light directly. They still depend on supplies made by the light reactions.

Use two similar seedlings of the same species in matching pots. Put one in brighter indirect light and the other in a dimmer spot, ideally where both experience similar temperatures. Start with equally moist potting mix and monitor moisture rather than assuming both pots dry at the same rate. Photograph both seedlings from the same angle every few days and compare leaf color and new leaves alongside height. A seedling can stretch in dim light, so greater height alone does not show better growth. This is a comparison of growth under different light conditions, not a direct measurement of photosynthesis.