Biology- Chemical coordination and homeostsis

• 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.

• 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.

• 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.

• 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.

• 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.

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.  

• 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.

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.

• 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.

Type 2:

• insulin is present but cells fail to respond to it.
• Can be treated with diet - avoid sugary foods and eat starchy carbohydrates.