respiration

The two phases of the process of respiration. Gas exchange occurs in the alveoli, tiny air tubes in the lungs.

Process that occurs inside cells in which carbohydrate, particularly glucose, is broken down to release energy that the cell can use. This energy is used for many different processes, but in all of them energy transfer occurs. The processes range from muscle contraction to the manufacture of protein for new cells. Respiration is a key feature of life and is carried out by all living cells. There are two kinds of respiration in organisms – aerobic and anaerobic respiration. Aerobic respiration is a complex process of chemical reactions in which oxygen is used to break down glucose into carbon dioxide and water. This releases energy in the form of energy-carrying molecules (ATP). Respiration sometimes occurs without oxygen, and this is called anaerobic respiration. In this case, glucose is only partially broken down, and the end products are energy and either lactic acid or ethanol (alcohol) and carbon dioxide; this process is termed fermentation.

The word ‘respiration’ should not be used to refer to the air movements in the air passageways. These air movements are called ventilation (breathing).

The starting and finishing points of aerobic respiration would be the same if glucose was burned. During burning the energy that is released is all in the form of heat. In aerobic respiration, however, the energy is released in a controlled way and less is released as heat. Most of the released energy is used to drive various processes in the cell, such as growth or movement.

In humans, anaerobic respiration can only carry on for a short time. The muscles producing lactic acid will stop working as it builds up. However, many micro-organisms can respire anaerobically for long periods of time or all the time. Yeast respires aerobically if oxygen is present, but, if there is no oxygen, it respires anaerobically. In anaerobic respiration it produces alcohol.

External and internal respiration The exchange of oxygen and carbon dioxide between body tissues and the environment is termed ‘external respiration’, or ventilation. In air-breathing vertebrates gas exchange takes place in the alveoli of the lungs, aided by the muscular movements of breathing. Respiration at the cellular level is termed internal respiration, and in all higher organisms occurs in the mitochondria. This takes place in two stages: the first stage, which does not require oxygen, is a form of anaerobic respiration; the second stage is the main energy-producing stage and does require oxygen. This is termed the Krebs cycle. In some bacteria the oxidant is the nitrate or sulphate ion.

ATP production Many of the metabolic processes taking place inside cells are dependent upon the use of enzymes. Respiration – the release of energy within cells – is a complex series of reactions which employs about 70 different enzymes that act as catalysts. The energy is released at several stages during this process, about three-quarters of it in the form of heat. Heat energy that cannot be used by the cell is lost, but the non-heat energy released is stored by the cell as a readily available substance called adenosine triphosphate (ATP).

In aerobic respiration – with the use of oxygen – the glucose molecules are broken down completely, releasing all the usable energy and producing carbon dioxide and water as waste, or by-products, as the following equation shows:

glucose + oxygen → carbon dioxide + water + energy

In anaerobic respiration, however, the glucose molecules are only partly broken down so that only part of the energy is released, and instead of carbon dioxide and water, the by-products are either carbon dioxide and ethanol (alcohol), or lactic acid:

glucose → ethanol + carbon dioxide + energy

glucose → lactic acid + energy

Thus, whereas in aerobic respiration one molecule of glucose can produce 38 molecules of ATP, in anaerobic respiration only two molecules of ATP are produced for each molecule of glucose.

Glucose supplies As these equations show, the essential substance for energy production is glucose. Plants are able to produce glucose for themselves through photosynthesis, but animals are unable to do this and need to obtain their glucose second-hand from the carbohydrates produced by plants. Hence, in this respect, plants are producers and animals are consumers.

Oxygen supplies The oxygen in the equations is obtained in different ways. Plants and animals obtain their oxygen directly from their surroundings. The part of the organism through which oxygen enters the body is called the ‘respiratory surface’. The by-product of the equation, carbon dioxide, is expelled from the organism also at the respiratory surface. This process is termed gas exchange.

Transport systems in unicellular organisms In single-cell organisms, such as amoebas that live in water, the respiratory surface is the cell membrane. Since oxygen dissolves in water, there is normally oxygen available in the surrounding water. The process of respiration inside the cell results in a lower concentration of oxygen in the cell than outside, and so oxygen diffuses into the cell across the cell membrane, while carbon dioxide diffuses out.

Transport systems in multicellular organisms In single-cell organisms the oxygen is quickly able to reach the centre of the organism, but in larger and more complex organisms, such as humans, which contain vast numbers of cells, this is not practicable. And so in these organisms, the blood system acts as a transport system carrying oxygen to all the body's cells and removing carbon dioxide.

Lungs An additional problem for larger organisms is the provision of sufficient oxygen to meet the energy demands of the body. Since the surface area to volume ratio decreases with increased body size, this problem is overcome in larger organisms by the provision of respiratory surfaces within the body – namely, the lungs. Each lung is filled with many tiny air sacs (alveoli) where the oxygen diffuses into the blood. Air, containing oxygen, enters the body either through the nose or mouth and passes into the trachea (windpipe). From there, it continues into the upper part of the body, the thorax, where the trachea divides into two branches, the right and left bronchi. Each bronchus goes to a lung and then branches out again into a number of smaller tubes called bronchioles. At the end of each bronchiole are the alveoli – the respiratory surfaces where gas exchange takes place. Air is moved in and out of the lungs by breathing. This process is helped by two sets of muscles: the intercostal muscles, which lie between the ribs, and the muscles in the diaphragm. As these two sets of muscles alternate between contraction and relaxation, air is pulled into the lungs (inspiration) and expelled (expiration). The lungs are covered and lined by membranes (pleura), that produce a fluid. This fluid lubricates the lungs and ensures that they do not rub against the rib cage or adhere to it as it is moved up and down by the breathing process. If the thorax becomes punctured, such as in an accident, allowing air to get between the pleural membranes, the lungs cannot work and collapse.

Oxygen debt The oxygen produced in the lungs is carried by the bloodstream to all cells in every part of the body, which need it in order to produce energy. The ATP produced by the mitochondria in muscles provides the power to enable them to work. Where a set of muscles is required to work extra hard, such as leg muscles in running, energy production has to be increased to match the demand. If that demand exceeds the supply from aerobic respiration, extra energy is produced by anaerobic respiration. When this happens glucose in the blood is broken down without combining with oxygen and, as demonstrated by the equation, this results in the production of lactic acid. Once the activity ceases and energy needs return to normal levels, the lactic acid that has been produced must be broken down by oxygen; consequently the increase in breathing rate and heartbeat has to be sustained until this process has been completed. This additional energy that has been acquired without oxygen is termed the oxygen debt – in other words, it has been ‘borrowed’ and has to be ‘repaid’.

Disorders The optimal operation of the respiratory system is dependent upon both an adequate supply of the basic materials, oxygen and glucose, and the efficient working of all the body organs, tissues, and metabolic processes involved. Change or disruption in any one aspect will affect the overall process, and consequently the well-being of the body. Change can come about as a result of an alteration in oxygen or sugar intake, or the degeneration of, or damage to, organs and tissues, either as a result of accident, ageing, or disease.

Cigarette smoking Cigarette smoking can create radical changes to the respiratory system that result not only in damage to the lungs, but also to the heart and blood vessels. The three main components of cigarette smoke are nicotine, tar, and carbon monoxide, all of which have adverse effects on the body. Nicotine is an addictive and poisonous substance which is absorbed into the bloodstream. The tar in cigarette smoke is absorbed by lung cells, especially those lining the bronchi and bronchioles which normally form a thin, protective layer. Tobacco tar causes the cells to divide and build up into a thicker layer, and this process of cell division may become a continuous one, developing into cancer. Carbon monoxide anaesthetizes the cilia in the trachea and bronchi so that they are no longer effective in their task of preventing dust and bacteria reaching the lungs. If bacteria colonize the mucus in the bronchi and bronchioles, this can result in infections and diseases such as emphysema and bronchitis. In addition, carbon monoxide is absorbed into the blood where it combines with haemoglobin inside the red cells. This reduces the amount of oxygen in the blood cells and reduces the ability of the body cells to absorb oxygen from the blood. Carbon monoxide also increases the amount of cholesterol deposited inside the arteries, thus narrowing the arteries and reducing their elasticity. All or any one of these effects can lead to atherosclerosis and coronary heart disease.