2.90 understand the sources, roles and effects of the following hormones: ADH, adrenaline, insulin, testosterone, progesterone and oestrogen.

ADH
Lack of water is detected by the hypothalamus in the brain, it causes the pituitary gland to produce Anti-diuretic hormone, or ADH. This makes the kidneys to reabsorb more water- so less is lost from the body.

Adrenaline
Produced in the adrenal glands in stressful situations. Heart rate quickens to increase the flow of blood to muscles- this means that they can respire more (as there is more oxygen available) to provide energy if you need to 'fight or flee'.

Insulin
Produced in the pancreas when there is too much glucose in the blood. It stimulates cells to convert the glucose into glycogen which is a from that can be stored. This means that you always have the right amount of glucose in your blood.

Testosterone
Produced in ovaries in girls and testicles in boys. Plays a key role in puberty, developing sex organs and inspiring hair growth.

Progesterone
Produced in the ovaries it maintains the lining ready for pregnancy, and continues to do so if the egg is fertilised.

Oestrogen
Produced in the ovaries, it is controls other hormones to regulate the menstrual cycle. It stops the production of FSH and starts the production of LH.

2.89 describe the role of the skin in temperature regulation, with reference to sweating, vasoconstriction and vasodilation

Sweating- when too hot, glands under the skin secrete sweat, this increases heat loss by evaporation.
Vasoconstriction- blood vessels by the skin shrink, this reduces the blood which runs by the surface meaning less heat can be lost to the air.
Vasodilation- blood vessels by the skin grow, this means that more blood, and so more heat, is travelling near the surface of your body, in this way heat will be lost as it is conducted by the air.

2.88 understand the function of the eye in focusing near and distant objects, and in responding to changes in light intensity

In response to increased light you pupil will shrink, in dim light your pupil will dilate (grow bigger). This happens because you iris will contract to make the pupil smaller or relax to make it bigger. Radial muscles also make the pupil bigger by contracting.
To focus at different distances the lens in your eye adapts its shape:
If an object is near, ciliary muscles will contract which relaxes the suspensory ligaments so that the lens is fat;
If an object is far, ciliary muscles will relax making the suspensory ligaments tight so they pull the lens thin.

Recomend a look at this page:
http://www.bbc.co.uk/schools/gcsebitesize/science/edexcel_pre_2011/electrical/thenervoussystemrev4.shtml

2.87 describe the structure and function of the eye as a receptor

The eye is a receptor of light, it has light receptor cells in its retina. These cells turn stimuli into electrical impulses.

2.86 describe the structure and functioning of a simple reflex arc illustrated by the withdrawal of a finger from a hot object

A reflex is an automatic reaction, for example removing your hand from extreme heat. A reflex arch is the path of the reaction.
It starts at a sense organ e.g a finger;
receptors pick up stimuli e.g heat;
Sensory neurones carry an electrical impulse to the CNS;
A relay neuron carries the impulse through the CNS where a response is decided;
The new impulse is sent through a motor neurone;
This makes an effector carry out a response e.g muscle contracts to bring finger away from heat.

2.85 understand that stimulation of receptors in the sense organs sends electrical impulses along nerves into and out of the central nervous system, resulting in rapid responses

Receptors send a electrical impulse through nerves when stimulate by a stimulus. This message goes to the CNS, here a response is decided and then sent strait back out in electrical impulses through nerves to the effector. The impulses are very fast, as is the reaction time.

2.84 understand that the central nervous system consists of the brain and spinal cord and is linked to sense organs by nerves

The CNS is the centre of the nervous system which decides a response for a stimulus. Receptors in sense organs (eg eyes or skin) send messages through nerves to the CNS- either to you brain or spinal chord- it creates a response which it will send in electrical impulses down nerves to effectors to carry out the response.

2.83 describe how responses can be controlled by nervous or by hormonal communication and understand the differences between the two systems

The nervous system and hormones both coordinate responses with in the body. The nervous does this by electrical impulses so it is very fast. Hormones do this with chemicals which travel, a little slower, at the speed of the blood stream they are travelling in.

2.82 describe positive phototropism of stems

Stems experience positive photo-tropism, this means they always grow towards light.
In a place where light shines there will be fewer auxins (growth hormones) this encourages the stem to bend towards the source of light.

2.81 describe the geotropic responses of roots and stems

Geo-tropism is when a plant grows in response to gravity.
Roots always carry out posotive geo-tropism, towards gravity/ down.
Shoots always carry out negative geo-tropism, away from gravity/ up.

2.80 understand that plants respond to stimuli

Plants respond to stimuli. They will react to changes in the environment, like temperature and light, as they have receptors which can detect a change and effectors to carry out the response. Usually the response is plant hormones- commonly auxin- which stimulate plant growth, stimuli for this are often light (photo-tropism), water (hydro-tropism) or gravity (geo-tropism).

2.79 understand that a coordinated response requires a stimulus, a receptor and an effector

To be able to carry out a response several things are needed:
A stimulus- a change in the internal or external environment- is needed to prevoke a response.
A receptor is needed to detect a stimulus, so that it can send messages to a coordinator to coordinate a response,
An effector is needed to carry out the response to the stimulus.

2.78 understand that homeostasis is the maintenance of a constant internal environment and that body water content and body temperature are both examples of homeostasis

Homeostasis is the regulation of conditions inside the body. For example osmoregulation is the control of water levels in the body. Temperature regulation also needs to take place (as body process work best at 37 degrees) it is called thermoregulation. An example is when you are hot you sweat- so the heat is absorbed from your skin- and when you are cold you body hairs stick out- to trap air as a layer of insulation.

2.77 understand that organisms are able to respond to changes in their environment

Sensitivity is one of the life processes (mrs gren); it is responding to the environment around. Living things must have receptors to be able to detect the change and effectors to be able to carry out a response.