How is oxygen released in cellular respiration

Cellular respiration

Cellular respiration

Humans and animals cannot produce their own energy. They therefore absorb the energy-rich compounds from plant or animal food. This food is broken down in order to be able to use the energy released in this way. Plants also break down specially produced high-energy compounds. The energy released is used for metabolic reactions. This decomposition of high-energy organic substances into low-energy substances is referred to as cell respiration.

Cell respiration can be divided into three different processes:

The aim of all these processes is to produce the energy carrier ATP from an energy-rich compound. The high-energy connection is broken down into a low-energy connection. Energy-rich compounds are produced by plants themselves and taken in by humans or animals through food and broken down through digestion.


In glycolysis, the starting material for cellular respiration, a glucose molecule, is broken down into pyruvic acid (BTS). This process takes place in the cytoplasm of animal or plant cells. First, the glucose molecule (C6H12O6) is broken down into two C3 molecules, the 3-phosphoglyceraldehyde molecules, while consuming energy. These two molecules are then oxidized and donate electrons to the oxidizing agent NAD +, which turns it into the reducing agent NADH / H +. In addition, two molecules of ATP are created. Glycolysis produces two molecules of pyruvic acid, two molecules of NADH / H + and two molecules of ATP.

Citrate cycle

If an organism has oxygen available, this is used in the course of the citrate cycle to completely break down the pyruvic acid and in this way to gain as much energy as possible. The citrate cycle and the subsequent respiratory chain take place in the mitochondria of the cells. At the beginning, carbon dioxide is split off from the pyruvic acid molecules, whereby the carbon structure of the molecule is broken down. In the process, CO2 is released as a waste product and new reducing agents are created in the form of NADH / H +. The C2 molecules8 are then bound to C4 acceptor molecules and CO2 is split off twice more. ATP is also formed in the process. The acceptor molecules are then regenerated, with reducing agents again being produced in the form of NADH / H + and also in the form of FADH2. The balance from the citrate cycle is therefore 6 molecules of CO2, two molecules of ATP, 8 molecules of NADH / H + and 2 molecules of FADH2.

Respiratory chain

In the respiratory chain, or also end oxidation, all previously created reducing agents are oxidized with the help of oxygen (as an electron acceptor), whereby water is split off and ATP is created as an energy carrier. The reaction energy released during the reaction of hydrogen and oxygen to form water is gradually released so that the ATP molecules can be built up.

The reducing agents NADH / H + and FADH2 give off hydrogen (H) to so-called redox systems in the inner mitochondrial membrane, which are arranged next to each other. They either transfer hydrogen or pass electrons on. ATP is formed with each of these transfers. In the end, electrons are transferred to oxygen molecules, which in turn react with hydrogen ions to form water. The respiratory chain creates a total of 34 molecules of ATP.

In the overall balance of cell respiration, one molecule of glucose results in 38 ATP in the so-called aerobic breakdown. If an organism has no oxygen available, only glycolysis takes place, which is followed by anaerobic fermentation. This process of generating energy is much more inefficient, which is why most living things, if possible, use oxygen as an electron acceptor when generating energy.