The term respiration has two relatively distinct meanings in biology. First, respiration is the process by which an organism takes oxygen into its body and then releases carbon dioxide from its body. In this respect, respiration can be regarded as roughly equivalent to "breathing." In some cases, this meaning of the term is extended to mean the transfer of the oxygen from the lungs to the bloodstream and, eventually, into cells. On the other hand, it may refer to the release of carbon dioxide from cells into the bloodstream and, thence, to the lungs.
Aerobic respiration: Respiration that requires the presence of oxygen.
Anaerobic respiration: Respiration that does not require the presence of oxygen.
ATP (adenosine triphosphate): High-energy molecule that cells use to drive energy-requiring processes such as biosynthesis (the production of chemical compounds), growth, and movement.
Capillaries: Very thin blood vessels that join veins to arteries.
Diffusion: Random movement of molecules that leads to a net movement of molecules from a region of high concentration to a region of low concentration.
Fermentation: A chemical reaction by which carbohydrates, such as sugar, are converted into ethyl alcohol.
Gill: An organ used by some animals for breathing consisting of many specialized tissues with infoldings. It allows the animal to absorb oxygen dissolved in water and expel carbon dioxide to the water.
Glucose: also known as blood sugar, a simple sugar broken down in cells to produce energy.
Glycolysis: A series of chemical reactions that takes place in cells by which glucose is converted into pyruvate.
Hemoglobin: Blood protein that can bind with oxygen.
Lactic acid: Similar to lactate, a chemical compound formed in cells from pyruvate in the absence of oxygen.
Pyruvate: The simpler compound glucose is broken down into during the process of glycolysis.
Trachea: A tube used for breathing.
Second, respiration also refers to the chemical reactions that take place within cells by which food is "burned" and converted into carbon dioxide and water. In this respect, respiration is the reverse of photosynthesis, the chemical change that takes place in plants by which carbon dioxide and water are converted into complex organic compounds. To distinguish from the first meaning of respiration, this "burning" of foods is also referred to as aerobic respiration.
All animals have some mechanism for removing oxygen from the air and transmitting it into their bloodstreams. The same mechanism is used to expel carbon dioxide from the bloodstream into the surrounding environment. In many cases, a special organ is used, such as lungs, trachea, or gills. In the simplest of animals, oxygen and carbon dioxide are exchanged directly between the organism's bloodstream and the surrounding environment. Following are some of the mechanisms that animals have evolved to solve this problem.
Direct diffusion. In direct diffusion, oxygen passes from the environment through cells on the animal's surface and then into individual cells inside. Sponges, jellyfish, and terrestrial flatworms use this primitive method of respiration. These animals do not have special respiratory organs. Microbes, fungi, and plants all obtain the oxygen they use for cellular respiration by direct diffusion through their surfaces.
Diffusion into blood. In diffusion into the blood, oxygen passes through a moist layer of cells on the body surface. From there, it passes through capillary walls and into the blood stream. Once oxygen is in the blood, it moves throughout the body to different tissues and cells. This method also does not rely upon special respiratory organs and is thus quite primitive. However, it is somewhat more advanced than direct diffusion. Annelids (segmented worms) and amphibians use this method of respiration.
Tracheae. In tracheal respiration, air moves through openings in the body surface called spiracles. It then passes into special breathing tubes called tracheae (singular, trachea) that extend into the body. The tracheae divide into many small branches that are in contact with muscles and organs. In small insects, air moves into the tracheae simply by molecular motion. In large insects, body movements assist tracheal air movement. Insects and terrestrial arthropods (organisms with external skeletons) use this method of respiration.
Gills. Fish and other aquatic animals use gills for respiration. Gills are specialized tissues with many infoldings. Each gill is covered by a thin layer of cells and filled with blood capillaries. These capillaries take up oxygen dissolved in water and expel carbon dioxide dissolved in blood.
Lungs. Lungs are special organs in the body cavity composed of many small chambers filled with blood capillaries. After air enters the lungs, oxygen diffuses into the blood stream through the walls of these capillaries. It then moves from the lung capillaries to the different muscles and organs of the body. Humans and other mammals have lungs in which air moves in and out through the same pathway. In contrast, birds have more specialized lungs that use a mechanism called crosscurrent exchange. Crosscurrent exchange allows air to flow in one direction only, making for more efficient oxygen exchange.
In direct diffusion and tracheal systems, oxygen and carbon dioxide move back and forth directly between cells and the surrounding environment. In other systems, some mechanism is needed to carry these gases between cells and the outside environment. In animals with lungs or gills, oxygen is absorbed by the bloodstream, converted into an unstable (easily broken down) chemical compound, and then carried to cells. When the compound reaches a cell, it breaks down and releases the oxygen. The oxygen then passes into the cell.
In the reverse process, carbon dioxide is released from a cell into the bloodstream. There the carbon dioxide is used to form another unstable chemical compound, which is carried by the bloodstream back to the gills or lungs. At the end of this journey, the compound breaks down and releases the carbon dioxide to the surrounding environment.
Various animals use different substances to form these unstable compounds. In humans, for example, the substance is a compound known as hemoglobin. In the lungs, hemoglobin reacts with oxygen to form oxyhemoglobin. Oxyhemoglobin travels through the bloodstream to cells, where it breaks down to form hemoglobin and oxygen. The oxygen then passes into cells.
On the return trip, hemoglobin combines with carbon dioxide to form carbaminohemoglobin. In this (and other) forms, carbon dioxide is returned to the surrounding environment.
Animals other than humans use compounds other than hemoglobin for the transport of oxygen and carbon dioxide. Certain kinds of annelids (earthworms, various marine worms, and leeches), for example, contain a green blood protein called chlorocruorin that functions in the same way that hemoglobin does in humans.
Whatever substance is used, the compound it forms with oxygen and carbon dioxide must be unstable, it must break down easily. This property is essential if the oxygen and carbon dioxide are to be released easily at the end of their journeys into and out of cells, lungs, and gills.
Cellular respiration. Cellular respiration is a process by which the simple sugar glucose is oxidized (combined with oxygen) to form the energy-rich compound adenosine triphosphate (ATP). Glucose is produced in cells by the breakdown of more complex carbohydrates, including starch, cellulose, and complex sugars such as sucrose (cane or beet sugar) and fructose (fruit sugar). ATP is the compound used by cells to carry out most of their ordinary functions, such as production of new cell parts and chemicals, movement of compounds through cells and the body as a whole, and growth.
The overall chemical change that occurs in cellular respiration can be represented by a fairly simple chemical equation:
6C 6 H 12 O 6 + 6 O 2 → 6 CO 2 + 6 H 2 O + 36 ATP
That equation says that six molecules of glucose (C 6 H 12 O 6 ) react with six molecules of oxygen (O 2 ) to form six molecules of carbon dioxide (CO 2 ), six molecules of water (H 2 O) and 36 molecules of ATP.
Cellular respiration is, however, a great deal more complicated that this equation would suggest. In fact, nearly two dozen separate chemical reactions are involved in the overall conversion of glucose to carbon dioxide, water, and ATP. Those two dozens reactions can be grouped together into three major cycles: glycolysis, the citric acid (or Krebs) cycle, and the electron transport chain.
In glycolysis, glucose is broken down into a simpler compound known as pyruvate. Pyruvate, in turn, is converted in the citric acid cycle to a variety of energy-rich compounds, such as ATP and NADH (nicotinamide adenine dinucleotide). Finally, all of these energy-rich compounds are converted in the electron transport chain to ATP.
Anaerobic respiration. As the equation above indicates, cellular respiration usually requires the presence of oxygen and is, therefore, often known as aerobic (or "using oxygen") respiration. Another form of respiration is possible, one that does not make use of oxygen. That form of respiration is known as anaerobic (or "without oxygen") respiration.
Anaerobic respiration begins, as does aerobic respiration, with glycolysis. In the next step, however, pyruvate is not passed onto the citric acid cycle. Instead, it undergoes one of two other chemical reactions. In the first of these reactions, the pyruvate is converted to ethyl alcohol in a process known as fermentation. Fermentation is a well-known chemical reaction by which grapes, barley, rice, and other grains are used to make wine, beer, and other alcoholic beverages.
The second anaerobic reaction occurs when cells are unable to obtain oxygen by methods they normally use. For example, a person who exercises vigorously may not be able to inhale oxygen fast enough to meet the needs of his or her cells. (Glucose is used up faster than oxygen is supplied to the cells.) In that case, cells switch over to anaerobic respiration. They convert glucose to pyruvate and then to another chemical known as lactate or lactic acid (two forms of the same compound). As lactic acid begins to build up in cells, it causes an irritation similar to placing vinegar (acetic acid) in an open wound.
Most cells are able to switch from aerobic to anaerobic respiration when necessary. But they are generally not able to continue producing energy by this process for very long.
Scientists believe that the first organisms to appear on Earth's surface were anaerobic organisms. Those organisms arose when Earth's atmosphere contained very little oxygen. They had to produce the energy they needed, therefore, by mechanisms that did not require oxygen. As the composition of Earth's atmosphere changed to include more oxygen, organisms evolved to adapt to that condition.