Cellular Respiration: Energy in Living Things
Living things need a constant power consumption; the cells use it in the form of chemical energy. Cellular respiration, the process used by most animal and plant cells, is the degradation of biomolecules (glucose, lipids, proteins) to produce the necessary energy release, so the organism can fulfill its vital functions. By degradation of glucose (glycolysis), pyruvic acid is formed. This acid is split into carbon dioxide and water, generating 36 ATP molecules.
Cellular respiration is a part of metabolism, more precisely catabolism, in which the energy of different biomolecules is released in a controlled manner. During respiration, some of that energy is used to synthesize (manufacture) ATP, which in turn is used for maintenance and development of the body (anabolism). Cellular respiration is a process by which cells oxidize nutrients from food to release energy. As a result, the carbon in these nutrients is oxidized, i.e. becomes carbon dioxide, and is eliminated through the breath into the atmosphere.
To complete cellular respiration, the presence of oxygen is essential (aerobic respiration). Animals take it from the atmosphere through specialized organs (lungs, gills). Plants do it through a device called stomata, located in the leaves, which will be explained later. Breathing is done within 24 hours. The amount of oxygen that plants absorb from the atmosphere as a result of the breathing process is less than that given off when making photosynthesis, and carbon dioxide is also emitted less than the amount absorbed. At night, when the plants do not perform photosynthesis, the reverse is true.
While photosynthesis provides the carbohydrates needed for plant cell respiration, it is the process where the energy content of these carbohydrates is released in a controlled manner. In aerobic respiration, glucose degradation involves a series of reactions. However, the general chemical equation can be represented by the following formula, the reverse of photosynthesis:
C6H12O6 + 6 O2 —> 6CO2 + 6 H2O + ATP
Cellular Respiration and Mitochondria
Cellular respiration occurs within mitochondria, small organelles located in the cytoplasm of eukaryotic cells. These structures, oblong and flat, process oxygen and convert carbohydrates, fatty acids, and proteins from food into energy.
Types of Cellular Respiration
Cellular respiration can be divided into two types, depending on the presence of oxygen:
- Aerobic respiration: Uses O2 as the final acceptor of electrons detached from the organic substances. It is the most widespread form of breathing, characteristic of a group of bacteria and eukaryotes. That is why organisms that require oxygen are called aerobic.
- Anaerobic respiration: Oxygen is not involved, but other terminal electron acceptors are used, usually minerals. Anaerobic respiration is characteristic of some prokaryotes, in general inhabitants of soils and sediments, and of vital importance in biogeochemical cycles of elements. Organisms that do not require oxygen are called anaerobes.
Stomata and Gas Exchange in Plants
As mentioned above, plants perform gas exchange through the stomata. The stomata (Greek: “stoma” = mouth) are two large guard cells surrounded by accompanying cells that give rise to small pores in the leaves of plants. They are located on both sides of the leaf, but overall there are more stomata on the underside. The separation that occurs between the two cells regulates the total size of the pore.
Through the stomata, gas exchange with the environment occurs. Oxygen and carbon dioxide are exchanged with the atmosphere through these pores, allowing the processes of photosynthesis and plant respiration to develop. However, its opening also causes loss of water in vapor form, through a mechanism called transpiration. That is why the opening and closing of stomata is carefully regulated by environmental factors like light, the concentration of carbon dioxide, and water availability to plants. The stomata open when the light intensity increases and close when it falls.