Definition
Cellular respiration, also known as aerobic (from ancient Greek “aer” – air) cellular respiration, describes in humans the breakdown of nutrients such as glucose or fatty acids with the consumption of oxygen (O2) for energy production, which is necessary for the survival of the cells. During this process the nutrients are oxidized, i.e. they give off electrons, while oxygen is reduced, which means that it takes up electrons. The end products that are formed from the oxygen and nutrients are carbon dioxide (CO2) and water (H2O).
Function and tasks of cellular respiration
All processes in the human body require energy. Physical movement, brain function, the beating of the heart, the production of saliva or hair and even digestion require energy to take place. In addition, the body needs oxygen to survive.
Here, cellular respiration is of particular importance. With the help of this and the gas oxygen, the body is able to burn energy-rich substances and produce the energy it needs. Oxygen itself does not provide us with energy, but it is needed to carry out the chemical combustion processes in the body and is therefore essential for our survival.
The body knows many different types of energy sources:
- Glucose (sugar) is the main energy source and basic building block as well as the end product split from all starchy foods
- Fatty acids and glycerin are the end products of fat cleavage and can also be used in energy production
- The last group of energy sources are the amino acids, which are the product of protein splitting. After a certain transformation in the body, these can then also be used in cell respiration and thus for energy production
The energy source most used by the human body is glucose. There is a chain of reactions which, with the consumption of oxygen, ultimately lead to the products CO2 and H2O.
This process includes glycolysis, i.e. the splitting of glucose and the transfer of the product pyruvate via the intermediate step of acetyl-CoA into the citrate cycle (synonym: citric acid cycle or also cancer cycle). This cycle also includes the cleavage products of other nutrients such as amino acids or fatty acids. The process in which the fatty acids are “crushed” so that they can also flow into the citrate cycle is called beta-oxidation.
The citrate cycle is thus a kind of supply point where all energy sources can be supplied to the energy metabolism. The cycle takes place in the mitochondria, the “energy power stations” of human cells. During all these processes, energy in the form of ATP is partly consumed, but already produced, as is the case in glycolysis, for example.
In addition, other intermediate energy stores (e.g. NADH, FADH2) are predominantly created, which only fulfill their function as intermediate energy stores during energy production. These intermediate storage molecules then flow into the last step of cell respiration, namely the step of oxidative phosphorylation or also called respiratory chain. This is the step that all processes have been working towards so far.
The respiratory chain, which also takes place in the mitochondria, again consists of several steps, in which the energy-rich intermediate storage molecules are used to produce the all-purpose energy carrier ATP. In total, the degradation of one glucose molecule results in a total of 32 ATP molecules. The respiratory chain contains various protein complexes, which play a very interesting role here.
They function as pumps which, when the intermediate storage molecules are consumed, pump protons (H+ ions) into the cavity of the mitochondrial double membrane, so that there is a high concentration of protons. This causes a concentration gradient between the intermembrane space and the mitochondrial matrix. With the help of this gradient, a protein molecule is ultimately formed, which functions similarly to a kind of water turbine. Driven by this gradient of protons, the protein synthesizes an ATP molecule from an ADP and a phosphate group.
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