Sunlight, sugar and air: how cells get their energy

Every cell needs energy to move, grow and repair itself. That energy comes from sugar, and almost all the sugar on Earth is made by plants, algae and some bacteria using sunlight.

Photosynthesis: making fuel

Inside chloroplasts, the green pigment chlorophyll captures light energy. The plant uses it to join carbon dioxide from the air and water from the soil into a sugar called glucose. Oxygen is left over and goes out into the air. Plants store extra glucose as starch, or use it to build cellulose, the tough material in cell walls. Leaves are thin and flat so light and gases can reach every cell.

Respiration: using fuel

Cellular respiration happens in the mitochondria of plant and animal cells, day and night. Glucose is broken down using oxygen, and chemical energy is released for the cell to use. Carbon dioxide and water are left over as waste. Respiration is not the same as breathing. Breathing moves air in and out of your lungs; respiration is what every one of your cells does with the oxygen.

Diffusion: the delivery service

Gases get into and out of cells by diffusion. Particles spread from where there are lots of them to where there are fewer, with no pumping needed. Oxygen made in a leaf spreads out through tiny holes called stomata; carbon dioxide spreads in. Diffusion is fast over short distances but slow over long ones, which is one reason cells stay so small.

In the 1770s Joseph Priestley found that a plant could 'restore' air that a candle had used up, and Jan Ingenhousz showed that plants only do this in light. Their experiments are cousins of the leaf-disc test you can do in class.