All About Cellular Respiration

ATP production
The three processes of ATP production or celluar respiration include glycolysis, the tricarboxylic acid cycle, and oxidative phosphorylation. Encyclopaedia Britannica/UIG/Getty Images

We all need energy to function, and we get that energy from the foods we eat. Extracting those nutrients necessary to keep us going and then converting them into useable energy is the job of our cells. This complex yet efficient metabolic process, called cellular respiration, converts the energy derived from sugars, carbohydrates, fats, and proteins into adenosine triphosphate, or ATP, a high-energy molecule that drives processes like muscle contraction and nerve impulses. Cellular respiration occurs in both eukaryotic and prokaryotic cells, with most reactions taking place in the cytoplasm of prokaryotes and in the mitochondria of eukaryotes. 

There are three main stages of cellular respiration: glycolysis, the citric acid cycle, and electron transport/oxidative phosphorylation.

Sugar Rush

Glycolysis literally means "splitting sugars," and it is the 10-step process by which sugars are released for energy. Glycolysis occurs when glucose and oxygen are supplied to the cells by the bloodstream, and it takes place in the cell's cytoplasm. Glycolysis can also occur without oxygen, a process called anaerobic respiration, or fermentation. When glycolysis occurs without oxygen, cells make small amounts of ATP. Fermentation also produces lactic acid, which can build up in muscle tissue, causing soreness and a burning sensation.

Carbs, Proteins, and Fats

The Citric Acid Cycle, also known as the tricarboxylic acid cycle or the Krebs Cycle, begins after the two molecules of the three carbon sugar produced in glycolysis are converted to a slightly different compound (acetyl CoA). It is the process that allows us to use the energy found in carbohydratesproteins, and fats. Although the citric acid cycle does not use oxygen directly, it works only when oxygen is present. This cycle takes place in the matrix of cell mitochondria. Through a series of intermediate steps, several compounds capable of storing "high energy" electrons are produced along with two ATP molecules. These compounds, known as nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD), are reduced in the process. The reduced forms (NADH and FADH2) carry the "high energy" electrons to the next stage.

Aboard the Electron Transport Train

Electron transport and oxidative phosphorylation is the third and final step in aerobic cellular respiration. The electron transport chain is a series of protein complexes and electron carrier molecules found within the mitochondrial membrane in eukaryotic cells. Through a series of reactions, the "high energy" electrons generated in the citric acid cycle are passed to oxygen. In the process, a chemical and electrical gradient is formed across the inner mitochondrial membrane as hydrogen ions are pumped out of the mitochondrial matrix and into the inner membrane space. ATP is ultimately produced by oxidative phosphorylation—the process by which enzymes in the cell oxidize nutrients. The protein ATP synthase uses the energy produced by the electron transport chain for the phosphorylation (adding a phosphate group to a molecule) of ADP to ATP. Most ATP generation occurs during the electron transport chain and oxidative phosphorylation stage of cellular respiration.