Feedback can occur at the cellular level and on a larger-scale. For example, when the energy-producing units of a cell-the mitochondria-begin converting glucose into ATP, the ATP feeds back to inhibit the enzymes that act on glucose. In this way, a cell makes only enough energy to serve its needs and saves the rest of the glucose for later. This type of feedback is called negative feedback because the end-product of the process inhibits the process from continuing any further. This is the most common type of control mechanism involved in homeostasis.
When faced with changes in the external environment, an animal can respond in two ways: conformity or regulation. Regulation is the typical form of homeostasis, as described above. Conformity, however is another method of ensuring the prevention of stochastic changes in internal environment, and is where environmental fluctuations induce internal body changes that simply parallel the external conditions. Such animals are called conformers, and may be unable to maintain homeostasis for one or more internal conditions, such as tissue oxygenation or fluid salinity. Echinoderms, such as the starfish, Asterias, for example, are osmoconformers whose internal body fluids quickly come to equilibrium with the seawater that surrounds them. They can move position to areas of higher or lower salinity to alter their internal conditions, in effect, changing the external conditions themselves.
This method of environmental control appears less efficient in comparison to the homeostatic processes of other organisms however, as the internal changes are much more erratic, and the organisms has much less control because changes take longer to respond to and the internal conditions can be less finely tuned.
In general, any one variable is controlled by a number of different processes, for example the corresponding method of salinity regulation in most mammals involves the removal of salts from the blood via the kidneys, on a longer term scale, but also the use of selectively permeable membranes in living cells. The….system, the main component, of which is the kidney, controls the pH (by removal of acid waste through the urine), the salinity of the fluids in the body, and also controls to some extent, the reabsorption of glucose and other useful molecules. The kidney contains many thousands of nephrons, each of which is comprised of a Bowman’s capsule, a proximal convoluted tubule, a loop of Henle, and a distal convoluted tubule leading to a collecting duct. The function of the Bowman’s capsule is to absorb excess water and solutes from the blood and the other features primarily reabsorb any of this which is useful to the body e.g. glucose. In this way, more or less of the useful substances can be reabsorbed according to the concentration already present in the body.
As part of the kidney’s role, it controls the water content of the blood via a negative feedback system, where the receptor cells are cell in the hypothalamus and the effector cells are cells in the pituitary gland and the walls of the distal convoluted tubule. The amount of water in the blood is constantly being monitored by cells called osmoreceptors, within the hypothalamus. Loss of water by osmosis from these cells causes a reduction in volume, which triggers stimulation of nerve cells in the hypothalamus. These nerve cells secrete a hormone called antidiuretic hormone, or ADH. The ADH effectively adds and removes water-permeable channels to the walls of the collecting duct so as to regulate the amount of water than can be reabsorbed by the tissue surrounding the collecting duct, so conserving water. This therefore helps to regulate the water content present in the body.
The kidney also regulates the pH of the body in combination with the lungs. Two factors combine to control pH in mammals: excretion of CO2 via the lungs and excretion of acid via the kidneys (as described above). As a result, we can see that the lungs are a type of homeostatic organ, and like the other organs, have adapted to produce the best possible chances of being able to efficiently expel CO2, such adaptations including alveoli, which give a large surface area for CO2 molecules to diffuse across from the capillaries, and moist surface to allow gases to dissolve in and diffuse across quickly.
The prime example of a homeostatic organ in animals is however the pancreas, which controls the blood glucose levels hormonally. The pancreas contains large groups of cells called Islets of Langerhans, which are divided into α cells and β cells. α cells secrete glucagon and β cells secrete insulin. Carbohydrate is transported though the human bloodstream in the form of glucose, in solution in the blood plasma. For storage, glucose can be converted into the polysaccharide glycogen. In healthy humans, each 100cm3 of blood normally contains between 80 and 120 mg of glucose. If blood glucose levels drop below this level, then cells may run short of glucose for respiration, and may not be able to form enough energy. This is especially important for cells that can only respire glucose, such as brain cells. Very high blood glucose levels can also disrupt normal cell behaviour. After a meal containing carbohydrate, glucose from the digested food is absorbed from the small intestine and passes into the blood. As this blood flows through the pancreas, the α and β cells detect the raised glucose levels. The α cells respond by stopping the secretion of glucagon, while the β cells respond by secreting insulin into the blood plasma. The insulin is carried to all parts of the body in the blood.
Insulin primarily affects the liver and muscles by causing an increased absorption of glucose from the blood into the cells, an increase in the rate of use of glucose in respiration, and an increase in the rate at which glucose is converted into the storage polysaccharide, glycogen. All of these processes, take glucose out of the blood, so lowering the blood glucose levels towards the set point.
A drop in blood glucose concentration is detected by the α and β cells in the pancreas. The α cells respond by secreting glucagon and the β cells respond by stopping the secretion of insulin. The lack of insulin puts a stop to the increased uptake and usage of glucose by liver and muscle cells, although uptake still occurs at a more ‘normal’ rate. The presence of glucagon affects the activities of the liver cells, because muscle cells are not responsive to glucagon because they do not contain the specific receptors required. The effects on the liver cells include the breakdown of glycogen to glucose, the use of fatty acids instead of glucose as the main fuel in respiration, and the production of glucose from other compounds, such as fats.
As a result, the liver releases glucose into the blood, raising the blood glucose levels. Blood glucose levels, never remain constant, even in the healthiest person, because of the time delay between a change in the blood glucose level and the onset of the actions to correct it. This time delay results in oscillations about the set point.
Homeostasis also occurs on a much smaller scale in the prokaryotae kingdom, and in individual eukaryotic cells in multicellular organisms. Bacteria can maintain relatively constant ion concentrations in their cytoplasm by the use of their plasma membrane which is highly selective and can thus regulate what exits and enters the cell. Molecules which are hydrophobic can pass straight through the non-polar lipid bilayer by diffusion along a concentration gradient, thus ensuring the equilibrium of ions of the intracellular and extracellular environments. Polar, or hydrophilic molecules must pass through carrier proteins in the cell membrane which often act via active transport, requiring energy from ATP. In both this cases, the membrane controls the relative ion concentration in the cell cytoplasm.
The homeostasis of water (osmoregulation) is a similar method, using membranes as a semi-permeable barrier. It has been found by Abbe Nollet in 1748, that if water was put on one side of an animal membrane, and more concentrated solution on the other side, water crossed from the pure to the solution side. That is, water went from the side where there was more of it (no space being taken by the solute molecules) to the side with less. There was a net flow of water down its concentration gradient. The water moves in random thermal motion. In this way, it is always ensured that water concentration is equal in and outside of a cell. It should be noted however, that this is not always the optimal solution, because the concentration of water outside a cell may be above or below the desired level, and the cell cannot very easily resist the flow of water. As a result, if a person drinks too much water, cells in the body, may uptake an excess of water and rupture, which can lead to fatal illness.
Plants regulate their internal environment, and because they have no automated pump, as in the heart in animals, they rely almost entirely on the movement of solutes and solvents by diffusion. For example, plants move respiration products from the source (the area it was made), often the leaves, to the sink (the area that needs the products), often the roots. The products travel in the phloem by mass transport and are then distributed to cell that need the products most. This happens because cells that need it most are the cells lacking in solutes, and thus the concentration gradient is greater in these cells, so more of the products can diffuse across the cell membrane. The same principle applies to the uptake of water, except in this case, water travels from the roots to the leaves via the xylem.
Plants use guard cells to open and close their stomata (pore on the underside of the leaf) to preserve or flush out water from within the plant system. This opening and closing of the guard cells primarily depends on the concentration of water in the stomatal cells with respect to the humidity of the air surrounding them.
Since the surrounding environment is always changing, most organisms are said to be in constant dynamic equilibrium, meaning that their internal ionic concentration, temperature, pH etc are always being adjusted to cope with these changes.
Since the process of homeostasis is found so widely, if not unanimously, in organisms, it speaks for itself in explaining its importance in survival. It is especially the organisms which inhabit locations of extreme environmental change and diversity, from which we can learn the most, and perhaps make use of their fast response time in research into antidotes for poisoning, for example, which happens due to an imbalance in certain toxic molecules, or in starvation or malnutrition in the third world.
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