biology paper 2 revision

homeostasis

Homeostasis is the regulation of the internal conditions of a cell or organism to maintain optimum conditions for function in response to internal and external changes.

Homeostasis maintains optimal conditions for enzyme action and all cell functions.

In the human body, these include control of:

• blood glucose concentration

• body temperature

• water levels

These automatic control systems may involve nervous responses or chemical responses.

All control systems include:

• cells called receptors, which detect stimuli (changes in the environment)

• coordination centres (such as the brain, spinal cord and pancreas) that receive and process information from receptors

• effectors, muscles or glands,

the nervous system

The structure of the nervous system is adapted to its functions.

The nervous system enables humans to react to their surroundings and to coordinate their behaviour.

Information from receptors passes along cells (neurones) as electrical impulses to the central nervous system (CNS). The CNS is the brain and spinal cord. The CNS coordinates the response of effectors which may be muscles contracting or glands secreting hormones.

            Stimulus →    receptor →     coordinator  →     effector  →     response

The various structures in a reflex arc – including the sensory neuron, synapse, relay neuron and motor neurons – relate to their function.

Reflex actions are important. Reflex actions are automatic and rapid; they do not involve the conscious part of the brain.

The data from graphs, charts and tables can be extracted to make interpretations about the functioning of the nervous system.

the endocrine system

The endocrine system is composed of glands which secrete chemicals called hormones directly into the bloodstream. The blood carries the hormone to a target organ where it produces an effect. Compared to the nervous system the effects are slower but act for longer.

The pituitary gland in the brain is a ‘master gland’ which secretes several hormones into the blood in response to body conditions. These hormones in turn act on other glands to stimulate other hormones to be released to bring about effects.

The endocrine glands and their function

The pituitary gland: lies under the base of the skull.

It secretes eight hormones, some of which are responsible for controlling the other endocrine glands of the body.

The thyroid gland: produces thyroxine that controls the speed at which oxygen and food products are burned up to produce energy.

The pancreas secretes digestive juices. It also secretes insulin that regulates the amount of sugar in the blood.

The ovaries: in females, secretes oestrogen the hormone that controls the development of secondary sexual characteristics and plays an important part during pregnancy.

The testes: in males secretes testosterone the hormone that controls the development of secondary sexual characteristics.

The adrenal glands: lie just in front of each kidney. They secrete the hormones adrenalin and noradrenalin times of stress.

inheritance and variation

The number of chromosomes is halved during meiosis and then combined with new genes from the sexual partner to produce unique offspring. Gene mutations occur continuously and on rare occasions can affect the functioning of the animal or plant. These mutations may be damaging and lead to a number of genetic disorders or death. Very rarely a new mutation can be beneficial and consequently, lead to increased fitness in…