Respiration

I

Introduction

Respiration, term with dual meaning in biology: first, the physical process by which living things (organisms) take in oxygen from the surrounding medium and emit waste carbon dioxide—in this sense it is variously known as physical respiration, breathing, ventilation or gas exchange; second, the chemical process by which fuel molecules such as sugars (see Sugar Metabolism) and fats are broken down within a cell to liberate energy for cellular life processes—in this sense it is also known as chemical, biochemical, or cellular respiration.

II

Cellular Respiration

Cellular respiration is similar in most organisms, from simple one-celled bacteria, amoebas, and diatoms to complex plants like trees, and large animals such as elephants, whales, and human beings (see also Metabolism; Photosynthesis). In most organisms apart from prokaryotes (chiefly bacteria and blue-green algae) cellular respiration takes place in the tiny sausage-shaped organelles within the cell called mitochondria.

In fact there are two versions of cellular respiration (both described in more detail in their separate articles, Aerobic Respiration, and Anaerobic Respiration). Aerobic respiration requires oxygen to break down high-energy fuel molecules—typically a simple sugar such as glucose (blood sugar)—through a series of chemical stages. The results are carbon dioxide, water, and energy in the form of molecules of ATP—the cell’s standard energy-carrier. The biochemical stages in the breakdown of glucose form a circular pathway or cycle known as the Krebs cycle or citric acid cycle.

Anaerobic respiration involves the breakdown of high-energy molecules, again typically glucose, but without the need for oxygen and therefore without the need for physical respiration (breathing). This version incorporates the chemical pathway termed glycolysis and results in lactic acid (see Fermentation). It occurs as a matter of course in certain bacteria and other microbes called obligate anaerobes. It is also used as a reserve or emergency form of obtaining energy in other organisms, usually when oxygen is in short supply.

III

Physical Respiration in Simple Organisms and Plants

The vital oxygen obtained by physical respiration makes up one-fifth of the air (see Atmosphere) around us and is also dissolved in water. Small organisms of the kingdoms Protoctista and Prokaryota have no specialized respiratory mechanisms. They rely on the simple seepage or diffusion of oxygen in and carbon dioxide out across the cell membrane. As oxygen is used up and carbon dioxide is produced as waste in cellular respiration so the concentration of oxygen in the organism is lower than in the surrounding air or water, and the concentration of carbon dioxide is higher. As a result, oxygen diffuses into the organism, and carbon dioxide diffuses out.

Physical respiration in plants and in some simple animals, such as sponges, is based on the same process of passive diffusion without the need for special respiratory parts.

IV

Respiratory Systems

Most animals have body systems specialized to obtain oxygen from the surroundings and convey it to all body parts, while usually getting rid of waste carbon dioxide at the same time. These are called respiratory systems. Typically, they link to a circulatory system. This is based on a circulating medium, usually a body fluid known as blood, which conveys the oxygen. Rather than carrying the oxygen in dissolved form, this fluid may contain substances specialized to attach to oxygen, termed respiratory pigments. The medium is kept flowing or circulating by some form of muscular pump, commonly called the heart.

External respiration is the interchange of the gases oxygen and carbon dioxide between the surroundings and the organism’s body, often called breathing. Internal respiration is the interchange of gases taking place inside the body, between the blood and tissues.

In simpler aquatic animals the respiratory gases, oxygen and carbon dioxide, diffuse in and out of tissues near the surface, and the medium carries them to and from more distant internal cells.

More complex animals have, as part of the respiratory system, specialized organs to increase the area of exposure between the circulating fluid to the external medium. These are often called gills in aquatic creatures and lungs in land animals.