Cellular respiration is a fundamental process where living organisms convert biochemical fuels, like sugars, into energy that cells can use. This process captures chemical energy from nutrients and transforms it into adenosine triphosphate (ATP), the “energy currency” of the cell. It releases waste products, such as carbon dioxide and water, depending on the type of respiration. Cellular respiration occurs in nearly all life forms to power their activities.
Organisms Performing Aerobic Respiration
Aerobic respiration is an efficient form of energy production that relies on oxygen. This process breaks down glucose completely into carbon dioxide and water, yielding 30 to 32 ATP molecules per glucose molecule.
Animals, including humans, mammals, birds, and insects, perform aerobic respiration, taking in oxygen through lungs or gills. Plants also depend on aerobic respiration to extract energy from the glucose they produce, obtaining oxygen through stomata.
Fungi, such as mushrooms and many types of yeast, are aerobic organisms, absorbing oxygen through their structures. A wide array of bacteria also engage in aerobic respiration, including obligate aerobes that strictly require oxygen to survive, like Mycobacterium tuberculosis and Pseudomonas.
Organisms Performing Anaerobic Respiration
Some organisms generate energy without oxygen through anaerobic respiration or fermentation. These processes are less efficient than aerobic respiration, producing about 2 ATP molecules per glucose molecule because glucose is only partially broken down.
Obligate anaerobes are organisms, primarily certain bacteria and archaea, that only survive in environments devoid of oxygen, as oxygen is toxic to them. Examples include some methanogens found in soil and digestive systems of ruminants, which use carbon dioxide to produce methane. Sulfate-reducing bacteria and archaea also use anaerobic respiration, reducing sulfate to hydrogen sulfide.
Facultative anaerobes, including many bacteria (Escherichia coli, Staphylococcus, and Listeria) and some eukaryotes like yeast (Saccharomyces cerevisiae), can switch between aerobic and anaerobic pathways based on oxygen availability. They prefer aerobic respiration for its higher energy output but can resort to fermentation when oxygen is scarce. Human muscle cells also temporarily switch to lactic acid fermentation during intense exercise when oxygen supply cannot meet energy demand, leading to lactic acid buildup.
The Essential Role of Cellular Respiration
The fundamental purpose of cellular respiration is to produce ATP. ATP, or adenosine triphosphate, serves as the universal energy currency for all living cells, capturing chemical energy from food molecules to power various cellular activities. This energy molecule is indispensable for maintaining life, fueling processes that allow organisms to grow, move, and reproduce. For instance, ATP drives muscle contraction, enables movement in animals, and powers active transport across cell membranes. It is also necessary for processes like DNA and RNA synthesis, ensuring a steady supply of this energy-rich molecule for sustaining life.