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.