Co-ordination and Response — Edexcel Linear - OL Revision Notes & Topical Questions

Organisms must detect changes (stimuli) in their environment and respond to them to survive. In animals a stimulus is detected by a receptor, a coordinator (brain or spinal cord) decides the response, and an effector (muscle or gland) carries it out. Plants have no nervous system; they respond to stimuli by growth, controlled by hormones.

Homeostasis is the maintenance of a constant internal environment.

A coordinated response, such as that required when there is not enough water in the blood, needs a stimulus (low blood water potential detected by the brain), a coordinator (the brain and pituitary gland), a hormone (ADH) and an effector (the kidney), producing a response (more water reabsorbed).

Plants grow in response to stimuli. Plant shoots display positive phototropism (growing towards a light source) and negative geotropism; roots display positive geotropism (growing down) and negative phototropism. The growth responses are controlled by the hormone auxin, produced in the shoot tip.

The benefit

• shoots growing towards light can photosynthesise more;
  • roots growing downwards anchor the plant and reach water and minerals.
• Experiments with a shoot tip covered by foil, removed or with a mica barrier show that the tip detects light and makes auxin.

The human nervous system consists of the central nervous system (CNS: brain and spinal cord) and the peripheral nervous system (all nerves branching out from the CNS).

Neurones are not directly connected. At a synapse, an impulse arriving triggers release of neurotransmitter from vesicles; it diffuses across the gap and binds to receptors on the next neurone, starting a new impulse. Synapses ensure impulses pass in one direction only.

A reflex is an involuntary, automatic and rapid response that does not involve the conscious part of the brain. The pathway is: stimulus → receptor → sensory neurone → relay neurone in the CNS → motor neurone → effector → response. Reflexes such as pain withdrawal, blinking and coughing protect us from damage and choking.

A sensory neurone has a long dendron carrying impulses from the receptor to the cell body, which lies in a swelling outside the spinal cord, and a short axon entering the spinal cord.

For a data question about reaction time, remember that the time for a reflex is short because there are only one or two synapses, whereas a voluntary response needs more neurones and the involvement of the conscious brain. Use these reasons to explain why a reflex is faster than a decision to move.

The lens is elastic, and fine focusing is called accommodation. It is brought about by the contraction or relaxation of the ciliary muscles, which changes the tension in the suspensory ligaments and so the shape of the lens.

The cornea does most of the refraction; the lens provides the fine adjustment so light is focused on the retina.

The iris contains radial and circular muscles that work antagonistically to change the pupil size and so the amount of light reaching the retina; this protects the retina from damage by bright light and allows vision in dim light.

The skin contains receptors for touch, pressure, pain, heat and cold. Structures in the skin play an important role in regulating body temperature, an example of homeostasis; the temperature must be maintained at about 37 °C. Temperature receptors send impulses to the brain, which coordinates a cooling or heating response.

If asked how the skin responds in a cold environment, focus on vasoconstriction and reduced sweating. Temperature regulation is an example of homeostasis because it must be maintained within a constant range.

A hormone is a chemical substance produced by a gland and carried by the blood, which alters the activity of one or more specific target organs. Hormones transmit information from one part of the organism to another and bring about a change.

Adrenaline (adrenal glands) prepares the body for fight or flight: it increases heart rate and breathing rate, diverts blood to muscles, dilates the pupils and raises blood glucose, supplying muscles with more oxygen and glucose for respiration.

Blood glucose concentration must be kept within a narrow range, another example of homeostasis like body temperature.

Oestrogen and progesterone both prepare and maintain the uterus lining.

Diabetes is a condition in which blood glucose is not controlled properly.

In the exam, state the hormone, its source, its target and its effect in this order, because mark schemes follow this pattern. For example: insulin is released by the pancreas, acts on the liver and muscle cells, and lowers blood glucose by converting glucose to glycogen.