Negative feedback restores systems to their original level. The possession of separate mechanisms involving negative feedback controls departures in different directions from the original state, giving a greater degree of control. Positive feedback results in greater departures from the original levels. Positive feedback is often associated with a breakdown of control systems, e.g. in temperature control. Candidates should be able to interpret diagrammatic representations of negative and positive feedback.

A feedback is when a receptor senses a change in stimuli due to a response that it coordinated. This means that it can make an informed decision to change the response it is coordinating.

Negative feedback is when feedback makes the response stop.

There is a norm for conditions, and if this is deviated from in either direction a different response will be coordinated. A response in either direction (if there is too much or too little of something) will have its own negative feedback loop. For example, the alpha cells stop producing glucagon when the blood glucose concentration is back up to normal, and the beta cells stop producing insulin when the blood glucose concentration is back down to normal.

Positive feedback is when feedback makes the response carry on, making the conditions get further and further from the norm. One example of this is in neurones when sodium is detected sodium ion channels are opened so more can flood in.

Mostly positive feedback is a bad thing caused by a disease or due to a break down, for example hypothermia.

Homeostasis in mammals involves physiological control systems that maintain the internal environment within restricted limits. The importance of maintaining a constant core temperature and constant blood pH in relation to enzyme activity. The importance of maintaining a constant blood glucose concentration in terms of energy transfer and water potential of blood.

Homeostasis is the control of internal conditions.

It makes sure that the cells in the body are functioning efficiently.

It does this by bringing conditions, for example temperature, PH and concentration of ions, back to a suitable level.

If the conditions were not controlled some processes would be disturbed for example proteins could denature and stop functioning or osmosis could cause cells to burst.

Having these things internally controlled means that an organism can live different environments and still function properly.

If blood glucose levels are too low there is not enough to respire sufficiently and supply cells with energy, if it is too high it decreases the water potential in the blood meaning water moves out of cells.

Nerve cells pass electrical impulses along their length. They stimulate their target cells by secreting chemical neurotransmitters directly on to them. This results in rapid, short-lived and localised responses. Mammalian hormones are substances that stimulate their target cells via the blood system. This results in slow, long-lasting and widespread responses. Histamine and prostaglandins are local chemical mediators released by some mammalian cells and affect only cells in their immediate vicinity. In flowering plants, specific growth factors diffuse from growing regions to other tissues. They regulate growth in response to directional stimuli. The role of indoleacetic acid (IAA) in controlling tropisms in flowering plants.

There are some chemical mediators which are released at the site which needs a response. In animals these are histamine (involved in immune and allergic response) and prostaglandins (involved in response to injury and illness).

In plants chemicals used to control growth, in terms of direction, are released from the growing root or shoot and diffuse down the chemical gradient to other parts of the plant. IAA is an example of this: it is released from the growing shoot and causes cells to elongate, however it is no present in direct sunlight, which means that the side of a plant with the most sun will not grow in comparison to the rest of the plant, curving it towards the light.