- Are either made from polypeptide or lipid.
- Secreted by endocrine tissues/glands directly into the blood.
- Transported in the blood until they reach specific target cell.
- Target cells have specific receptor molecules on the plasma membrane/ inside the cell.
- When they link to receptors they initiate reactions that activate enzymes inside the cell.
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Nervous control v hormonal control
- Nervous control has a more localised effect as neurones link directly to their target organs/cells.
- Hormonal control has a widespread effect, because hormones travel in the blood they effect any cell with an appropriate receptor.
- Nervous control is very rapid because neurones transmit electrical impulses very quickly.
- Hormonal control is slower as hormones are transported at the speed that blood flows.
- The response of nervous control is short lived.
- The effects of hormonal control are longer lasting because the hormones have to be removed from the blood by the liver or kidneys.
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- Directional response towards or away from a stimulus (in plants).
- e.g phototropism - response to light.
- Caused by IAA which is a growth regulator.
- IAA causes stem elongation.
- IAA diffuses away from the light side of the plant and accumulates on the shady side.
- This causes cell elongation on the shady side.
- Therefore the stem grows more on the shady side, so the stem bends.
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- Maintaining the internal environment within restricted limits.
- Uses negative feedback.
- There are three parts to the control system - A receptor (detects a stimulus), an integrator (determines and coordinates response) and an effector (carries out the response).
- When conditions return to normal the receptors aren't stimulated so the control system is switched off.
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Control of core body temperature
- Core temperature is monitered by receptors in the hypothalamus.
- If temp falls - Impulses are sent to the heat gain centre in the hypothalamus
- nerve impulses stimulate vasocontriction, raising body hair and shivering.
- If temp rises - Impulses are sent to the heat loss centre in the hypothalamus.
- nerve impulses stimulate vasodilation and sweating.
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Control of blood glucose concentration
When blood glucose conc increases:
- Glucose is absorbed into beta cells (found in islets of langerhans) by facilitated diffusion.
- Insulin is released from the beta cells.
- This stimulates glucose uptake in muscle, liver and adipose tissue.
- In the liver glucose is converted into glycogen (glycogensis)
When blood glucose conc falls:
- Alpha cells (in IoL) secrete glucagon.
- This affects the liver - activates enzymes that break glycogen down to glucose (glycogenolysis).
- Glucose is made from non-carbs such as amino acids and lactate.
- Glucose is added to the blood.
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- Endotherms maintain a constant body temperature.
- There muscles and enzymes are always at a favourable temperature to operate efficently.
- Endotherms need a lot of food (energy) to generate enough heat.
- The body temp of ectotherms varies with external temperature.
- Their muscles and enzymes may not work efficiently if too cold.
- They use less energy for heat so do not need a lot of food.
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Adrenaline and the second messenger
Adrenaline also stimulates the breakdown of glycogen to glucose :
- It combineswith a receptor on the cell surface membrane which activates an enzyme.
- This converts ATP to cAMP.
- cAMP activates the enzymes that breakdown glycogen to glucose.
- This mechanism gives amplification as each molecule of hormone stimulates the production of many molecules of cAMP.
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- Causes blood glucose levels to rise to very high levels.
- Excess glucose is lost in the urine.
Type 1 :
- Caused by the immune system destroying Beta cells.
- This causes a lack of insulin.
- Can be treated by insulin injections.
- insulin is present but cells fail to respond to it.
- Can be treated with diet - avoid sugary foods and eat starchy carbohydrates.
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