Photosynthesis and cellular respiration are linked processes between plants and animals. Photosynthesis uses energy from sunlight to convert carbon dioxide and water into glucose and oxygen in plants. Animals then respire the glucose and oxygen to produce carbon dioxide, water, and energy to power their cells. Enzymes act as biological catalysts to accelerate reactions like photosynthesis and cellular respiration so that life can be sustained. They do this by lowering the activation energy of reactions through a cyclic process of binding to substrates. Cells also regulate enzyme activity through various mechanisms.