Control in Biological Systems

Transcription

1 Control in Biological Systems 1. Physiological Homeostasis Negative Feedback Control In animals such as ourselves, the internal environment of our bodies must have certain conditions within tolerable limits to continue the healthy functioning of us. This is done by a process called negative feedback control, where various receptors and effectors bring about a reaction to ensure that such conditions remain favourable. In this tutorial, we investigate the control of blood sugar concentrations, water concentrations and temperature. The principle of negative feedback control is illustrated by the diagram below This occurrence is known as physiological homeostasis, translating in layman's terms to the physical equilibrium. It is essentially a corrective mechanism, consider the following scenario in a person The level of glucose in the bloodstream drops The person requires glucose in cells to meet the demand for ATP The body detects this with a particular receptor designed for this function These receptors release hormones, chemical messages that initiate the start of the feedback mechanism The hormones travel to their target tissue and initiate a corrective response In this case, the corrective response is the secretion of more glucose into the bloodstream Requirement of Negative Feedback Control Because mammals are warm blooded, the enzymes that are part of their make-up as a warm blooded animal require a certain temperature to operate optimally. Also, the water

2 concentration of a cell and its chemical concentration must remain at a certain level to allow normal cellular processes to occur. In light of this, the feedback mechanism in such warm blooded animals is essential in regards to allowing the body to work in optimal conditions - so any change in from the norm in temperature is corrected by the feedback mechanism. Advantages of Homeostasis Homeostasis has survival value because it means an animal can adapt to a changing environment. It can deal with the temperature difference you face when you step our your front door. The body will attempt to maintain a norm, the desired level of a factor to achieve homeostasis. However, it can only work within tolerable limits, where extreme conditions can disable the negative feedback mechanism In these instances, death can result, unless medical treatment is executed to bring about the natural occurrence of these feedback mechanisms 2. Homeostasis of Water Osmoregulation Osmoregulation is the regulation of water concentrations in the bloodstream, effectively controlling the amount of water available for cells to absorb. The homeostatic control of water is as follows A change in water concentration leads to active via negative feedback control Osmoreceptors that are capable of detecting water concentration are situated on the hypothalamus next to the circulatory system The hypothalamus sends chemical messages to the pituitary gland next to it. The pituitary gland secretes anti-diuretic hormone (ADH), which targets the kidney responsible for maintaining water levels. When the hormone reaches its target tissue, it alters the tubules of the kidney to become more / less permeable to water If more water is required in the blood stream, high concentrations of ADH make the tubules more permeable. If less water is required in the blood stream, low concentrations of ADH make the tubules less permeable. This is illustrated by the flow chart below

3 Evolutionary Adaptations in Water Regulation Some of the tutorial pages in the adaptation tutorial investigate some of the evolutionary adaptations that organisms have achieved through natural selection. This looks at Ways in which both animals and plants can be better adapted to cope with extreme environments (desert or wetlands). These changes can be behavioural, physical or anatomical, and in some way promote water regulation. Both plant and animal adaptations are investigated 3. Blood / Sugar Homeostasis Blood / Sugar Regulation As described in the cell biology tutorial, the body requires volumes of glucose in order to create ATP. The amount of ATP demanded will fluctuate, and therefore the body regulates the availability of glucose to maximise its energy making potential. Two hormones are responsible for controlling the concentration of glucose in the blood. These are insulin and glucagon. The diagram illustrates the principle of negative feedback control in action involving blood/sugar levels.

4 Pancreas Receptors The receptors of the pancreas are responsible for monitoring glucose levels in the blood, since it is important in every cell for respiration. Two types of cell release two different hormones from the pancreas, insulin and glucagon. These hormones target the liver, one or the other depending on the glucose concentration In cases where glucose levels increase, less glucagon and more insulin is released by the pancreas and targets the liver In cases where glucose levels decrease, less insulin and more glucagon is released by the pancreas and targets the liver The Liver The liver acts as a storehouse for glycogen, the storage form of glucose. When either of the above hormones target the liver, the following occurs Insulin - Insulin is released as a result of an increase in glucose levels, and therefore promotes the conversion of glucose into glycogen, where the excess glucose can be stored for a later date in the liver Glucagon - Glucagon is released as a result of an decrease in glucose levels, and therefore promotes the conversion of glycogen into glucose, where the lack glucose can be compensated for by the new supply of glucose brought about from glycogen Diabetes Diabetes insipidus is a condition where excess urine is excreted caused by the sufferers inability to produce ADH and promote the retention of water. Diabetes Mellitus is another form of diabetes where the sufferer does not have the ability to produce sufficient insulin, meaning that glucose cannot be converted into glycogen. Anyone who has this condition usually has to take injections of insulin after meals and snacks to maintain their storage of glucose needed in emergencies. Fight or Flight In emergencies, adrenaline is released by the body to override the homeostatic control of glucose. This is done to promote the breakdown of glycogen into glucose to be used in the emergency. These emergencies are often known as 'fight or flight reactions'. Adrenaline is secreted by the adrenal glands. The secretion of it leads to increased metabolism, breathing and heart rate. Once the emergency is over, and adrenaline levels drop, the homeostatic controls are once again back in place

5 4. Temperature Regulation in Homeotherms Control of Temperature in Homeotherms Animals capable of temperature regulation within a given range are deemed homeotherms (alternatively homiotherms or homotherms). They have the ability to regulate temperature via negative feedback control. Temperature is controlled in a variety of ways in these animals. The hypothalamus once again acts as a receptor in regulation, by detecting fluctuations in temperature. These receptors are better known as thermo-receptors. Skin also possesses thermo-receptors which can detect the temperature of the external environment. This information is relayed to the hypothalamus which can in turn transmit nerve pulses for corrective mechanisms to occur Corrective Mechanisms in Temperature Control Increased sweating is a corrective response aimed to reduce the temperature of the organism. Vasodilation is a corrective response where the blood vessels close to the skin surface become more dilated, meaning their is a larger surface area for heat to be lost of the external environment from the blood vessel carrying over-heated blood. Vasoconstriction is the opposite of this and occurs when temperatures in an organism drop. The blood vessels become constricted so that minimal heat loss occurs. The hairs on your body also play an important role in temperature regulation. A corrective response can occur where the hairs 'stand on end', and trap a layer of air between the hair and the skin. This insulation of warmer air next to the skin reduces heat lost, while a thin layer of insulation would increase heat loss. Other corrective mechanisms are involved, such as a drop in metabolic rate and shivering when temperatures drop. Homeotherms: An organism, such as a mammal or bird, having a body temperature that is constant and largely independent of the temperature of its surroundings; an endotherm. Note: The above materials are from Regulation of Biological Systems at web site: For information on Control of Growth and Development of Biological System, visit: