2jCo-ordination and response

Why organisms respond

The environment around an organism keeps changing — light, temperature, water availability, chemicals, touch. An organism that detects these changes and responds to them survives better: it can find food, avoid harm, and keep its internal conditions suitable for enzymes to work.

The three parts of a co-ordinated response

Every co-ordinated response needs all three:

The response is what the effector does — a muscle contracts, or a gland secretes a hormone.

Receptors are grouped into sense organs: eye (light), ear (sound), skin (touch, temperature, pain), tongue and nose (chemicals).

Homeostasis

Homeostasis is the maintenance of a constant internal environment.

The two examples you must know:

• body water content
• body temperature

It matters because enzymes in your cells only work properly within narrow ranges. Too hot and enzymes are denatured (the active site changes shape so the substrate no longer fits). Too much or too little water and cells swell or shrink by osmosis.

The skin and temperature regulation

Core body temperature is held at about 37 °C. Temperature receptors detect the change; the skin is the main effector organ.

When you are too hot

• Sweating — sweat glands release sweat onto the skin surface. The water in it evaporates, and the energy needed for evaporation is transferred from the skin, cooling the body.
• Vasodilation — the small arterioles supplying the capillaries near the skin surface get wider. More blood flows close to the surface, so more energy is transferred to the surroundings by radiation. The skin looks red/flushed.

When you are too cold

• Sweating stops (or is greatly reduced), so no cooling by evaporation.
• Vasoconstriction — the arterioles supplying the surface capillaries get narrower. Less blood flows near the surface, so less energy is lost. The skin looks pale.

Common mistake: writing that "the blood vessels move up and down in the skin", or that "the capillaries dilate". The capillaries cannot change diameter — it is the arterioles supplying them that widen (vasodilation) or narrow (vasoconstriction). Say more/less blood flows near the skin surface, not the vessels move.

Common mistake: saying "sweat cools you down". Sweat only cools you when it evaporates — that word must appear.

Examiner tip. Read every word of the question stem before you write. If it asks about the response to being too cold, do not write a paragraph on sweating; if it says "explain", you must give a mechanism (evaporation transfers energy from the skin), not just "you sweat". Examiners repeatedly report marks lost because candidates spot a key word, rush into an answer, and describe the wrong half of the topic.

Plants respond too

Plants have no nerves and no muscles, so they respond by growing in a particular direction. These growth responses are called tropisms. They are slow, but they are still co-ordinated responses to a stimulus.

• Positive tropism = grows towards the stimulus.
• Negative tropism = grows away from the stimulus.

The four responses you must know

Why these responses are useful

• Shoots growing towards light get more light for photosynthesis.
• Shoots growing upwards lift leaves and flowers clear of the soil.
• Roots growing downwards anchor the plant and reach water and mineral ions in the soil.

Auxin and the phototropic response of a stem

Auxin is a plant hormone. You only need it for the phototropic response of stems:

1. Auxin is made in the tip of the shoot.
2. It moves down the shoot, just below the tip.
3. When light shines from one side, auxin accumulates on the shaded side of the shoot.
4. Auxin causes the cells to elongate (get longer).
5. So the cells on the shaded side grow longer than those on the lit side.
6. The shoot therefore bends towards the light.

In all-round light, or in darkness, auxin is spread evenly and the shoot grows straight up.

Common mistake: saying "auxin makes cells divide more on the dark side". For this spec, say cell elongation — the cells get longer, so that side of the stem is longer and the stem curves the other way (towards the light).

Common mistake: "the plant moves towards the light". Plants grow; they do not move.

Examiner tip. When you are given data or a diagram from a tropism experiment (e.g. shoots with tips removed, or covered with foil caps), examiners report that weaker answers only describe what they can see — "this one bent and this one didn't". You must explain: say where auxin was produced, where it accumulated, and that unequal cell elongation caused the bend. Description alone scores almost nothing on an "explain" question.

Two communication systems

Common mistake: if a question says "give differences other than the speed of transmission", do not mention speed. Examiners report candidates throwing away a mark by writing the one difference the question has already excluded.

The central nervous system (CNS)

The CNS consists of the brain and the spinal cord only.

The CNS is linked to the sense organs by nerves (bundles of neurones).

• Sensory neurones carry impulses into the CNS from receptors.
• Motor neurones carry impulses out of the CNS to effectors.
• Relay neurones are found inside the CNS and connect the two.

Stimulation of a receptor in a sense organ sends electrical impulses along nerves into the CNS. The CNS co-ordinates a response and sends impulses out to effectors (muscles or glands), giving a rapid response.

Synapses and neurotransmitters

Neurones do not touch. The tiny gap between two neurones is a synapse.

1. An electrical impulse arrives at the end of the first neurone.
2. This causes the release of a neurotransmitter (a chemical) into the gap.
3. The neurotransmitter diffuses across the synapse.
4. It binds to receptor molecules on the membrane of the next neurone.
5. This triggers a new electrical impulse in the next neurone.

Because neurotransmitter is only released on one side, impulses travel across a synapse in one direction only.

The simple reflex arc — finger on a hot object

A reflex action is rapid and automatic — it does not involve conscious thought by the brain, which is why it is fast and protective.

Stimulus: heat/pain from the hot object

→ Receptor — temperature/pain receptor in the skin of the finger

→ Sensory neurone — carries the impulse to the spinal cord

→ synapse → Relay neurone (in the spinal cord) → synapse

→ Motor neurone — carries the impulse out to the arm

→ Effector — the biceps muscle

→ Response — the muscle contracts and the finger is withdrawn

The brain is informed afterwards (that is when you feel the pain), but it is not needed for the response itself.

Examiner tip. In reflex arc questions, name the neurones in the correct order and put in the word synapse between them — many marks are for sequence, not for detail. Also be precise about the effector: the effector is the muscle, not "the arm" or "the hand", and the response is that the muscle contracts.

Structure of the eye

The eye is a sense organ containing receptors that detect light.

Rods work in dim light (black and white); cones detect colour and need bright light.

Focusing on near and distant objects (accommodation)

The lens changes shape. The pupil has nothing to do with focusing.

Either way, light is focused sharply onto the retina.

A useful memory hook: the ciliary muscle is a ring. When it contracts the ring gets smaller, so it pulls the ligaments inwards — they slacken and the elastic lens springs into a fatter shape.

Responding to changes in light intensity (the pupil reflex)

This is a reflex action protecting the retina from damage by bright light.

Common mistake: confusing the two responses. Focusing = lens + ciliary muscle + suspensory ligaments. Light intensity = iris + pupil.

Examiner tip. Examiners report that weaker answers to questions on focusing on near and distant objects contain descriptions of pupil changes instead. If the question is about accommodation, do not mention the iris or pupil at all — you will gain nothing and waste time. Check which stimulus the question names: distance of the object → lens; brightness of the light → pupil.

A hormone is a chemical made by a gland, carried in the blood plasma, that affects target organs with the correct receptors.

The hormones everyone must know

How insulin actually works

This is the detail examiners say is missing from weaker answers. Do not just write "insulin controls blood sugar".

After a meal, blood glucose concentration rises. The pancreas detects this and secretes insulin into the blood. Insulin:

1. makes liver and muscle cells more permeable to glucose, so they take glucose out of the blood;
2. makes the liver convert glucose into glycogen for storage.

The result is that the blood glucose concentration falls back to normal — an example of homeostasis.

Separate Biology only — not in Science (Double Award): ADH, FSH and LH

Order to remember: FSH → matures the egg and triggers oestrogen → oestrogen repairs the uterus lining → LH → ovulation → progesterone maintains the lining.

Common mistake: writing that FSH causes ovulation. It does not — LH does. FSH only matures the follicle.

Common mistake: mixing up which gland makes what. FSH and LH come from the pituitary gland; oestrogen and progesterone come from the ovaries.

Examiner tip. Two things examiners flag every year. First, on insulin: candidates say it "controls" or "maintains" blood sugar but never say how — you must state that it makes liver and muscle cells take up glucose and that the liver converts glucose to glycogen. Second, candidates confuse the roles of FSH and LH with those of oestrogen and progesterone, and wrongly say FSH stimulates ovulation. Learn the four in sequence, with the gland that makes each one.