Cellular respiration

Cells need a constant supply of energy to stay alive and do work. This energy is stored in a special molecule called adenosine triphosphate, or ATP.

Cellular respiration is the way cells get this energy. It is a set of reactions that move energy from nutrients to ATP. Most cells use sugar, amino acids, or fatty acids as fuel. They also use an electron acceptor to help the process work. Oxygen is the most common acceptor used by living things. When oxygen is used, the process is called aerobic respiration.

This energy-making process happens in several specific steps. First, a step called glycolysis occurs in the cell's fluid, or cytosol. Glycolysis literally means "sugar splitting." It breaks one glucose molecule into two smaller molecules called pyruvate. This step makes a small amount of ATP and two molecules of NADH. Next, the pyruvate moves into the mitochondria. If oxygen is present, the pyruvate enters the citric acid cycle, also known as the Krebs cycle.

This cycle breaks the molecules down further into carbon dioxide and water. It also creates more energy-carrying parts like NADH and FADH2.

The final and most important part is oxidative phosphorylation. This happens in the mitochondrial cristae, which are the folds inside the mitochondria. The cell uses an electron transport chain to move electrons through the membrane. This movement helps create a proton gradient, which is a buildup of protons.

The cell then uses an enzyme called ATP synthase to make ATP from ADP and a phosphate group. Oxygen sits at the end of the chain to catch the electrons. When oxygen joins with protons, it turns into water. This is a very efficient way to power a cell.

Scientists have studied how much energy these steps actually produce. Textbooks often say that one glucose molecule can make 38 ATP molecules. This includes 2 from glycolysis, 2 from the Krebs cycle, and about 34 from the electron transport system. However, the real number is usually a bit lower. Some energy is used just to move molecules like pyruvate and ADP into the mitochondria. Because of these costs and some leaky membranes, the actual yield is often between 29 and 30 ATP.

This shows how complex the work is inside a single cell.

You can think of cellular respiration like a slow and controlled fire. In a real fire, fuel burns all at once to release heat. Respiration is a combustion reaction, but it is much more controlled. Instead of a sudden burst, the cell releases energy in tiny, useful steps. This allows the cell to capture the energy in ATP rather than losing it. This process helps cells do everything from moving to building new parts. It is the fundamental way that life stays active and grows.