Absorption

The process by which substances are taken into the tissues of organisms is called absorption. It is essential to functions such as digestion, circulation, and respiration.

During digestion, valuable nutrients are absorbed across the epithelial lining of the digestive tract. Absorption occurs largely in the small intestine, which has developed a large surface area for this purpose. The walls of the small intestine contain numerous finger-like projections called villi, which are in turn covered by countless microvilli. Different nutrients are absorbed across the gut epithelium in different ways.

The methods of absorption include active transport , facilitated diffusion , and passive diffusion . Active transport requires energy in the form of adenosine triphosphate (ATP ), as well as special carrier molecules that ferry nutrients, (their substrates), across the gut lining. Active transport is involved in the absorption of proteins, which have usually been processed into amino acids or other small peptides. Most ions are also absorbed through active transport, as are most carbohydrates.

Some carbohydrates, however, are absorbed in a process known as facilitated diffusion. Facilitated diffusion describes a situation in which special carrier molecules are necessary, but energy (ATP) is not. Fructose is an example of a carbohydrate that is absorbed through facilitated diffusion.

Other nutrients, such as lipids , are absorbed through passive diffusion. In passive diffusion, neither energy expenditure nor a special carrier molecule is required. Lipids interact with bile salts from the liver, combining with them to form structures known as micelles. Micelles are able to diffuse freely through cell membranes and so can pass directly across the gut lining. Water is another substance that diffuses passively across the gut walls.

The circulatory system transfers nutrients and other products throughout the body. Tissues absorb the products they need from tiny blood vessels called capillaries. Capillaries are characterized by very high surface areas and very low blood-flow rates, both of which facilitate absorption. The walls of capillaries are also very thin, consisting of only one or a few layers of flattened endothelial cells. Capillaries also possess small pores through which transport and absorption can occur.

The absorption of materials from the capillaries occurs in one of several ways. Lipid-soluble substances are able to diffuse directly across the cell membranes of capillary cells into the tissues. Water diffuses directly as well, although it makes use of special pores in the cell membranes of capillary cells. Exchange via diffusion is comparatively rapid.

The absorption of other nutrients from the blood requires transportation through the capillary walls inside special vesicles. This process is called transcytosis. The vesicles are membrane-bound and are believed to be constructed by a cellular organelle known as the Golgi apparatus. Vesicles shuttle products repeatedly between the inner and outer walls of capillary cells. Because capillary beds in the brain are characterized by fewer transport vesicles, many substances cannot be absorbed into brain tissue, and the absorption of those that can be is slowed. This is often referred to as the blood-brain barrier.

In the process of respiration, oxygen is absorbed by the integument , lungs , gills , or trachea from the air or water. As with the circulatory and digestive systems, large respiratory surface areas allow for efficient absorption.

Oxygen is absorbed from the environment by the red blood cells, or erythrocytes . Erythrocytes contain respiratory pigments , which bind oxygen and works to transport it to tissues. These specialized oxygen-binding molecules are called pigments because they are often brightly colored when carrying bound oxygen. Respiratory pigments have a high affinity for oxygen and are also able to dramatically increase the oxygen-carrying capacity of blood.

Hemoglobin is the respiratory pigment in vertebrate erythrocytes and is also common throughout the animal kingdom. Hemoglobin is a large molecule consisting of four polypeptide chains, each of which is capable of binding an oxygen molecule. The oxygen-binding part of the chain is called the heme group and includes an iron atom. Hemoglobin binds oxygen cooperatively, meaning that once it has bound a single oxygen molecule, it is more likely to bind additional oxygen molecules. Hemoglobin's oxygen affinity, or the degree to which oxygen binds to it, varies according to such external factors as pH. This plasticity (flexibility) of oxygen affinity allows hemoglobin simultaneously to bind oxygen in the oxygen-rich environment of the lungs and to release it in the oxygen-poor environments of the tissues.

Another respiratory pigment, myoglobin, is present in the muscles and is responsible for pulling oxygen molecules from the blood into the tissues. Myoglobin resembles hemoglobin but consists of only a single polypeptide chain.

see also Digestion; Transport.

Jennifer Yeh

Bibliography

Gould, James L., and William T. Keeton. Biological Science, 6th ed. New York: W. W. Norton, 1996.

Withers, Philip C. Comparative Animal Physiology. Fort Worth, TX: Saunders College Publishing, 1992.

absorption The transfer of energy from a photon to an atom or molecule. If the energy of the photon is just sufficient to raise an electron from one energy level to another, the result is an absorption line at a particular wavelength; this is called a bound–bound transition. Absorption at a wide range of wavelengths is called continuous absorption, and occurs when the absorbed photons have energies greater than the minimum needed to eject an electron completely; this is called a bound–free transition, or photoionization. Absorption may occur whenever radiation traverses a gas, for example the outer layers of a star, the Earth's atmosphere, a nebula, or interstellar space (see Interstellar Absorption).

absorption [Lat.,=sucking from], taking of molecules of one substance directly into another substance. It is contrasted with adsorption , in which the molecules adhere only to the surface of the second substance. Absorption may be either a physical or a chemical process, physical absorption involving such factors as solubility and vapor-pressure relationships and chemical absorption involving chemical reactions between the absorbed substance and the absorbing medium.

absorption Taking up chemically or physically of molecules of one substance into another.

This includes a gas taken in by a liquid and a liquid or gas absorbed by a solid. The process is often utilized commercially, such as the purification of natural gas by the absorption of hydrogen sulphide in aqueous ethanolamine. Absorption is also the term used when light and other electromagnetic waves lose some energy as they pass through a medium. See also adsorption

ab·sorp·tion / əbˈzôrpshən; -ˈsôrp-/ • n. 1. the process or action by which one thing absorbs or is absorbed by another. ∎ Physics the process or action by which neutrons are absorbed by the nucleus. 2. the fact or state of being engrossed in something. DERIVATIVES: ab·sorp·tive / -tiv/ adj.

absorption The amount of seismic energy lost during transmission, by conversion to heat. The absorption coefficient is the fractional loss of energy over a distance of one wavelength; hence higher-frequency signals are attenuated more readily than those of lower frequencies over the same path. Typical values for rocks range from 0.25 to 0.75 dB per wavelength.

absorption The movement of fluid or a dissolved substance across a plasma membrane. In many animals, for example, soluble food material is absorbed into cells lining the alimentary canal and thence into the blood. In plants, water and mineral salts are absorbed from the soil by the roots. See osmosis; transport protein.

absorption. Transition between a wall-sur-face or a pier and the springing of a vault or vault-ribs in Gothic architecture, where the vault seems to flow into (or be absorbed by) the wall or pier.

absorption (ăb-sorp-shŏn) n. the uptake of digested food from the intestine into the blood and lymphatic systems. See also assimilation, digestion.

absorption The uptake of substances, usually nutrients, water, or light, by cells or tissues.

absorption The uptake of substances, usually nutrients, water, or light, by cells or tissues.

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