6.4 Homeostasis Flashcards
What is homeostasis?
- The maintenance of a constant internal environment within restricted limits in organisms.
- It involves trying to maintain the chemical make-up, volume and other features of blood and tissue fluid.
- It ensures that cells are in an environment that meets their requirements.
Why is it important that core temperature remains stable?
- Maintains stable rate of enzyme-controlled reaction and prevent damage to membranes
- temperature too low = enzyme and substrate molecules have insufficient kinetic energy
- temperature too high = enzymes denature
Why is it important that blood pH remains stable?
- Maintain stable rate of enzyme controlled reaction and optimum conditions for other proteins
- Acidic pH = H+ ions interact with H-bonds and ionic bonds in tertiary structure of enzymes -> shape of active site changes so no ES complexes form
Why is it important that blood glucose concentration remains stable?
- Maintain constant blood water potential: prevent osmotic lysis/crenation of cells
- Maintain constant concentration of respiratory substrate: organism maintains constant level of activity regardless of environmental conditions
Why is homeostasis important for enzymes?
- Enzymes that control reactions, and other proteins, are sensitive to pH and temperature changes.
- Changing these reduces the rate of reaction or may prevent them working at all.
- Maintaining a fairly constant internal environment means reactions take place at a suitable rate.
Why is homeostasis important for organisms?
- Organisms who can maintain a constant internal environment are more independent of changes in the external environment.
- They may have a wider geographical range and therefore greater chance of finding food, shelter etc.
E.g. Mammals are found in most habitats.
What are the control mechanisms?
- The optimum point, at which the system works best, monitored by a:
- Receptor, which detects deviation from the optimum, and informs the:
- Coordinator, which coordinates information from receptors and sends information to an appropriate:
- Effector, a muscle or gland, which brings about the changes needed to return to the optimum, this return creates a:
- Feedback mechanism, where a receptor responds to a stimulus created by a change to the system brought about by the effector.
What is negative feedback?
- When the change produced by the control system leads to a change in the stimulus detected by the receptor and turns the system off
- self regulatory mechanisms return internal environment to optimum when there is a fluctuation
What is positive feedback?
- Occurs when a deviation from an optimum causes changes that result in an even greater deviation from the normal.
E.g. a stimulus leads to a small influx of sodium ions, which increases membrane permeability to sodium ions, more enter, so further increase permeability and more ions.
What is the classification of control systems?
There are many receptors and effectors, so they have separate mechanisms that each produce a positive movement towards an optimum.
This allows a greater degree of control of the factor being regulated.
Having separate mechanisms that controls departures in different directions from the original state is a general feature of homeostasis.
Why is it important information from receptors is analysed by coordinators before action?
Temperature receptors in skin may signal that the skin is cold, so to raise the temperature.
But, information from the hypothalamus in the brain may indicate that the blood temperature is above average, e.g. during exercise.
Also, the control centre must coordinate the action of the effectors so they operate harmoniously.
Why do coordinators analyse inputs from several receptors before sending an impulse to effectors?
- receptors may send conflicting information
- optimum response may require multiple types of effector
When does negative feedback occur?
when the stimulus causes the corrective measures to be turned off, and returns the system to its optimum level.
Why does negative feedback mechanisms control fluctuations in different directions?
Provide more control, especially in case of ‘overcorrection’, which would lead to a deviation in the opposite direction from the original one
What is negative feedback of glucose?
- A fall in glucose concentration is detected by receptors on α-cells.
- These cells secrete glucagon, which causes liver cells to convert glycogen to glucose, which increased blood glucose concentration.
- This blood circulates back to the pancreas, and there is reduced stimulation of α-cells, so they secrete less glucagon.
- So the secretion of glucagon leads to a reduction in its own secretion.
What is negative feedback of increased glucose?
If blood glucose concentration rises, insulin is produced from the β-cells in the pancreas.
Insulin increases the uptake of glucose by cells and its conversion to glycogen and fat.
The fall in blood glucose concentration that results reduces insulin production.
Why is it important there are separate negative feedback mechanisms?
- It gives greater homeostatic control, because there are positive actions in both directions.
- E.g. if glucagon raised glucose concentration above the optimum, it would take time for it to decrease again, if it was only by metabolic activity.
- But, by the second hormone insulin, blood sugar concentration is lowered, so is more rapid.
What are the characteristics of hormones?
- They are produced in glands, which secrete the hormone directly in the blood (endocrine).
- carried in blood plasma to the target cells, which have specific receptors on their cell-surface membrane complementary to the hormone.
- effective in very low concentrations, but have widespread, long-lasting effects.
Why is there a time lag between hormone production and response by an effector?
It takes time to:
- produce hormone
- transport hormone in the blood
- cause required change to the target protein
What is the second messenger model?
This mechanism of hormone action is used by adrenaline and glucagon, to regulate blood glucose concentration.
What is the process of the mechanism involving adrenaline?
- Adrenaline binds to a transmembrane protein receptor in the cell-surface membrane of a liver cell.
- binding causes the protein to change shape on the inside of the membrane.
- activating the enzymes adenylate cyclase, which converts ATP to cyclic AMP (cAMP) (removes 2 phosphates).
- cAMP acts as a second messenger that binds to protein kinase enzymes, changing its shape, activating it.
- The enzyme catalyses the conversion of glycogen to glucose, which moves out the liver by facilitated diffusion and into the blood through channel proteins.
What is the pancreas made up of?
- Mainly the cells that produce its digestive enzymes.
- Scattered throughout are hormone-producing cells - islets of Langerhans, these include:
α cells produce glucagon.
β cells, which are smaller and produce insulin.
What is the liver?
- Located above the diaphragm
-weighs up to 1.5kg, and is made up of hepatocytes cells. - involved in regulating blood glucose concentration, by:
Glycogenesis, Glycogenolysis, Gluconeogenesis.
Name the factors that effect blood glucose concentration
- amount of carbohydrates digested from diet
- rate of glycogenolysis
- rate of gluconeogenesis
What is glycogenesis?
- The liver converts glucose into the storage polymer glycogen.
- When blood glucose is higher than normal the liver removes glucose and converts it to glycogen.
- It can store 75-100g of glycogen, sufficient to maintain blood glucose concentration for 12 hours at rest.
What is glycogenolysis?
- the liver hydrolyses glycogen into glucose which can diffuse into blood
- When blood glucose is lower than normal, the liver converts stored glycogen back to glucose, which diffuses into the blood to restore the normal blood glucose concentration.
What is gluconeogenesis?
- The production of glucose from sources other than carbohydrate.
- When its supply of glycogen is exhausted, the liver can produce glucose from non-carbohydrate sources such as glycerol and amino acids.
- liver converts glycerol and amino acids into glucose
Why does blood glucose concentration need regulating?
- If concentration falls too low, cells will be deprived of energy and die - brain cells are especially sensitive because they can only respire glucose.
- If concentration rises too high, it lowers the water potential and creates osmotic problems that can cause dehydration.
Where does blood glucose come from?
Normal concentration is 5mmol dm^-3.
- Directly from the diet in the form of glucose absorbed following hydrolysis of carbs - starch, maltose, lactose and sucrose.
- From hydrolysis in the small intestine of glycogen / glycogenolysis store in the liver and muscle cells.
- From gluconeogenesis, production of glucose from other sources.
What is insulin?
- The β cells of islets of Langerhans have receptors that detect the stimulus of increased concentration, and respond by secreting insulin into the blood.
- Insulin is a globular protein made up of 51 amino acids.
- All body cells par RBCs have glycoprotein receptors that bind to insulin molecules.
What happens when insulin binds to glycoprotein receptors?
- Change in the tertiary structure of the glucose transport carrier proteins, causing them to change shape and open, allowing more glucose in by facilitated diffusion.
- An increase in the number of carrier proteins for glucose.
- Activation of enzymes that convert glucose to glycogen and fat.
What happens with insulin and the number of carrier proteins?
- At low insulin concentrations, the protein from which the carrier proteins are made is part of the membrane of vesicles.
- A rise in insulin concentration results in these vesicles fusing with the cell-surface membrane so increasing the number of glucose transport channels.
