Photosynthesis: how plants make food
Biology · Key Stage 3 · The School
The reaction
Plants take in CARBON DIOXIDE through pores in their leaves and WATER through their roots. Using energy from light, captured by the green pigment CHLOROPHYLL, they build GLUCOSE and release OXYGEN. In words: carbon dioxide plus water, using light energy, gives glucose plus oxygen. The plant is not taking food in; it is manufacturing it.
Where it happens
Photosynthesis takes place in CHLOROPLASTS, structures packed into leaf cells and containing the chlorophyll. Leaves are shaped for the job: broad and flat for surface area, thin so gas reaches every cell, with veins delivering water and pores called stomata letting carbon dioxide in and oxygen out. Form follows function throughout.
Why it matters beyond plants
Almost every food chain begins with a photosynthesising organism, because that is where energy enters living systems from outside. Animals cannot make their own food and ultimately depend on organisms that can. The oxygen in the atmosphere is also a product of photosynthesis — it was released by living things over billions of years, not present from the start.
Cover part of a leaf on a living plant with black card and leave it in light for two days. Then boil the leaf in water to soften it, boil it in ethanol to remove the green, rinse, and add iodine. The uncovered part turns blue-black — starch is present. The covered part stays orange-brown — no starch. Same leaf, same plant, same water and air. Only light differed, and only the lit part made food.