Homeostasis

Homeostasis is the maintenance of a constant internal environment.

It means that there are continuous fluctuations brought about by variations in the internal and external conditions, e.g changes in temperature, pH and water potential. These changes occur around a set point. Homeostasis is the ability to return to that set point and so maintain organisms in a balenced equilibrium.

Importance of homeostasis:

• prevents changes to pH and temperature that would reduce the efficiency of enzymes that control biochemical reactions. It means that reactions take place at a constant and predictable rate.
• prevents changes to water potential that would cause cells to shrink and expand (burst) due to water leaving by osmosis. By maintaining a constant blood glucose concentration, water potential is also maintained.
• homeostasis allows organisms to be more independent so they have a greater chance of finding food and shelter as they can explore a range of external environments.

• Set Point: the desired level (norm) that the system operates on. It is monitered by a...
• Receptor: detects any deviation from the set point. It informs the...
• Controller: coordinates information from various receptors and sends instructions to a...
• Effector: brings about the changes needed to return the system to the set point. This creates a ...
• Feedback Loop: informs the receptor of the changes to the system brought about by the effector.

Heat Gain:

• production of heat by the metabolism of food during respiration
• gain of heat from the environment by conduction, convection, radiation

Heat Loss:

• evaporation of water (e.g sweating)
• loss of heat from the environment by conduction, convection and radiation

Conduction: occurs in solids and is the transfer of energy through matter from particle to particle

Convection: occurs in fluids and is the transfer of heat as a result of the movement of the warmed matter itself

Radiation: energy is not transferred by the movement of particles but by electromagnetic waves when the waves hit an object

ENDOTHERM: an animal that maintains its body temperature by physiological mechanisms

ECTOTHERM: an animal that uses the environment to regulate its body temperature

Ectotherms gain most of their heat from the environment, so their body temperature fluctuates with the environment's. They control their body temperature by adapting their behaviour to changes in the external temperature. They cannot warm up by exercise because, if their body temperature is low, they cannot respire fast enough to provide energy for rapid movements.

Lizards control their body temperature by:

• exposing themselves to the sun when cold
• taking shelter when hot
• gaining warmth from the ground
• generating metabolic heat by respiration, although this is not the main source of heat
• colour variations as darker colours absorb more heat, while lighter colours reflect heat

Endotherms gain most of their heat from internal metabolic activities. Their body temperature remains constant, despite fluctuations of the external environment. 

Conserving and gaining heat:

• vasoconstriction so less blood reaches the skin surface through capillaries
• shivering to produce metabolic heat
• raising of hair so a thick layer of insulator air is trapped next to the skin
• increased metabolic rate increases heat from respiration
• decrease in sweating 
• behavioural mechanisms e.g sheltering from the wind or basking in the sun

Losing heat:

• vasodilation so more blood passes close to the skin surface through capillaries, which is then radiated away from the body
• increased sweating as evapouration uses heat energy
• lowering of hair reduces the thickness of the insulating layer, so more heat is lost
• behavioural mechanisms e.g sheltering from the sun

Regulation of core body temperature in endotherms is an example of homeostasis. 

The hypothalamus moniters the temperature of blood passing through it. Thermoreceptors in the skin also measure skin temperature, and send impulses along the autonomic nervous system to the hypathalamus. The animal can therefore take measures to conserve or lose heat as appropriate, before core temperature is affected.

The hypothalamus consists of two parts:

• heat gain centre which is activated by a fall in blood temperature and controls the mechanisms that increase body temperature
• heat loss centre which is activated by a rise in blood temperature and controls the mechanisms that decrease body temperature

Hormones are:

• produced by glands, which secrete the hormone directly into the blood
• carried in the blood plasma to the target cells, which have complementary receptors on the cell surface membrane
• effective in very small quantities, with widespread and long-lasting effects

The second messenger model is used by adrenaline and glucagon to regulate blood glucose:

• the hormone is the first messenger; it binds to specific receptors on the cell-surface membrane of target cells to form a hormone-receptor complex
• the hormone-receptor complex activated an enzyme inside the cell that results in the production of a chemical (the second messenger)
• this second messenger causes a series of chemical changes that produce the required response. For adrenaline, this response is the conversion of gylcogen to glucose

The pancreas is a large gland that is situated in the upper abdomen, behind the stomach. It produces enzymes (protease, amylase and lipase) for digestion and hormones (insulin and glucagon) for regulating blood glucose.

The pancreas contains groups of hormone-producing cells, the islets of Langerhans, which are of two types:

• a cells: larger and produce glucagon
• b cells: smaller and produce insulin

Blood glucose comes from three sources:

• directly from the diet in the form of glucose from the breakdown of other carbohydrates
• glycogenolysis (breakdown of glycogen) stored in liver and muscle cells
• gluconeogenesis (production of new glucose) other than carbohydrates, e.g glycerol and amino acids

B cells in the pancreas detect a rise in blood glucose level and respond by secreting insulin directly into the blood plasma. When it combines with receptors it causes:

• a change in the tertiary structure of the glucose transport protein channels, causing them to change shape and open, allowing more glucose to enter the cells
• an increase in the number of carrier molecules in the cell-surface membrane
• activation of the enzymes that convert glucose to glycogen and fat

So blood glucose is lowered by:

• increasing the rate of absorption of glucose into cells
• increasing the respiratory rate of cells, which use up more glucose
• increasing the rate of conversion of glucose into glycogen (glycogenesis)
• increasing the rate of conversion of glucose to fat

A cells detect a fall in blood glucose and respond by secreting glucagon directly into the blood plasma. Only the cells in the liver have receptors that bind to glucagon, so only liver cells respond by:

• activating an enzyme that converts glycogen to glucose
• increasing the conversion of amino acids and glycerol into glucose (gluconeogenesis)

The overall effect is increasing the amount of glucose in blood so it returns to its normal level, which causes a cells to reduce the secretion of glucagon (negative feedback). 

Role of Adrenaline:

At times of excitement and stress, adrenaline is produced by the adrenal glands. It raises the blood glucose level by:

• activating the enzyme that causes the breakdown of glycogen to glucose in the liver
• inactivating the enzyme that synthesis glycogen from glucose

Diabetes is a metabolic disorder caused by an inability to control blood glucose levels due to a lack of the hormone insulin or a loss of responsiveness to insulin.

• Type I: body is unable to produce insulin
• Type II: due to glycoprotein receptors on body cells losing their responsiveness to insulin
  

Control

• Type I: controlled by injections 2 or 4 times a day (as, if taken orally, the protein would be digested). The dose of insulin must be matched to the glucose intake.
• Type II: controlled by regulating the intake of carbohydrate in the diet and matching this amount to the amount of exercise taken. It can be supplemented by injections or drugs.