Every living thing is built from cells, and every cell must capture, store, and spend energy to stay alive. In this section we look at the structures that handle energy and the processes that move materials in and out of the cell.
Making and storing energy
Inside most cells are the mitochondria, structures that break down nutrients to release energy. That energy is captured in a molecule called ATP, which the cell uses like a rechargeable battery to power its work. The chemical reactions that build and break ATP are sped up by an enzyme, a special protein that makes a reaction happen faster without being used up.
How plants capture sunlight
Plant cells can make their own food through photosynthesis. Using sunlight, water, and carbon dioxide, they produce sugar and release the oxygen that most living things need to breathe.
Moving materials across the membrane
A cell is wrapped in a semipermeable membrane, a thin barrier that lets some substances pass through while blocking others. Small particles spread from crowded areas to less crowded ones through diffusion. Water moves in a related way through osmosis, crossing the membrane toward the side where dissolved substances are more concentrated.