Living organisms must continually capture, store, and release energy to maintain their activities. Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy stored in glucose. The overall equation summarizes the transformation: \[ 6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2. \] This reaction not only feeds nearly all life on Earth but also releases the oxygen that animals breathe.
Photosynthesis occurs in two linked stages inside chloroplasts. During the light-dependent reactions, chlorophyll within the thylakoid membranes captures sunlight, which is used to split water molecules. This produces ATP, NADPH, and oxygen as byproducts. These energy carriers are then passed to the Calvin cycle, also called the light-independent reactions, which takes place in the stroma surrounding the thylakoids. Here, ATP and NADPH power the fixation of carbon dioxide into glucose through a series of enzyme-driven steps.
The glucose produced by photosynthesis is then broken down by cells through cellular respiration to release its stored energy in the form of ATP. The overall equation is essentially the reverse of photosynthesis: \[ C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP}. \] Cellular respiration begins with glycolysis in the cytoplasm, where one glucose molecule is split into two pyruvate molecules, yielding a small amount of ATP and NADH. Pyruvate is then transported into the mitochondrial matrix, where the Krebs cycle, also called the citric acid cycle, further oxidizes it, releasing carbon dioxide and producing additional NADH, FADH\(_2\), and ATP. Finally, the electron transport chain, embedded in the inner mitochondrial membrane, uses electrons from NADH and FADH\(_2\) to pump protons across the membrane. The resulting gradient drives ATP synthase to produce large amounts of ATP through oxidative phosphorylation.