Organisms responses to changes in the environment

Every living organism is surrounded by a constantly changing world — and conditions inside its body change too. To stay alive, an organism must be able to detect these changes and do something about them.

A stimulus (plural: stimuli) is a change in the environment that an organism can detect. Stimuli can be:

• External — changes outside the body, e.g. a drop in temperature, a bright light, a loud sound, a smell, the touch of a predator.
• Internal — changes inside the body, e.g. a fall in blood glucose, a rise in body temperature, feeling thirsty.

The action the organism takes is called the response.

Why does this matter? Detecting stimuli and responding correctly improves the chance of survival. For example:

• A rabbit that detects a fox and runs away is more likely to survive than one that does not.
• A plant that grows towards light captures more light for photosynthesis and grows better.
• A person whose body detects a rise in temperature and sweats cools down and avoids overheating.

Exam tip. Learn the definition exactly: a stimulus is a change in the (internal or external) environment that is detected by the organism. Do not just say "a stimulus is the environment" — it is a change in it.

Organisms do not respond to a stimulus instantly and magically — the information passes through a clear sequence of stages. This is the single most important idea in this subtopic, so learn the chain in order:

1. Stimulus — a change is detected (e.g. you touch something hot).
2. Receptor — a structure that detects the stimulus (e.g. temperature/pain receptors in your skin).
3. Coordination — the information is processed and a decision is made about how to respond (in animals this is done by the nervous system and hormonal system).
4. Effector — the part that carries out the response. Effectors are muscles (which contract) or glands (which release substances such as hormones).
5. Response — the action that results (e.g. you pull your hand away).

A worked example of the chain — touching a hot object:

• Stimulus: heat from the object.
• Receptor: temperature/pain receptors in the skin.
• Coordination: the nervous system (spinal cord/brain) processes the signal.
• Effector: a muscle in the arm.
• Response: the muscle contracts and you pull your hand away.

Exam tip. If a question asks you to "describe how an organism responds to a stimulus", write the chain in the right order and name a real receptor, effector and response for that example. Each correct stage usually scores a mark.

Two parts of the chain are easy to mix up, so make sure you can tell them apart.

Receptors — the detectors

• A receptor detects a stimulus and starts the response.
• Examples: light receptors in the eye (detect light), sound receptors in the ear, temperature and pressure receptors in the skin, taste receptors on the tongue, smell receptors in the nose.
• Receptors may be single cells or grouped together into a whole sense organ (e.g. the eye).

Effectors — the doers

• An effector is the part of the body that carries out the response.
• There are just two kinds of effector to remember:
  • Muscles — respond by contracting (e.g. to move a limb, blink, or move food along the gut).
  • Glands — respond by secreting (releasing) substances, such as hormones or enzymes.

Putting it together: a receptor at the start of the chain detects the change; an effector at the end of the chain does something about it. The job of the coordination stage in the middle is simply to connect the right receptor to the right effector.

Exam tip. A classic 1-mark trap: "Name an effector." Acceptable answers are a muscle or a gland — not the brain, not a nerve, and not a receptor.

Animals carry out the coordination stage of the response chain using two communication systems working together.

1. The nervous system

• Uses electrical impulses carried along nerves (nerve cells / neurones).
• Responses are very fast and short-lasting, and aimed at a precise part of the body.
• Good for quick reactions — pulling your hand from a hot pan, blinking, catching a ball.

2. The hormonal (endocrine) system

• Uses chemicals called hormones that are released by glands and carried in the blood.
• Responses are slower to start but often longer-lasting and can affect many parts of the body at once.
• Good for longer-term changes — growth, controlling blood glucose, the changes during puberty.

Together these systems let an animal detect a stimulus and trigger an effector (muscle or gland) to produce a suitable response.

Exam tip. Keep this at the overview level for spec 2.80. You only need to know that animals use the nervous and hormonal systems to coordinate responses — the detailed structure of neurones, reflex arcs and specific hormones belongs to other spec points.

Plants cannot run, swim or fly away — but they do respond to changes in their environment. A plant responds by growing towards or away from a stimulus. A growth response of a plant towards or away from a stimulus is called a tropism.

• Growing towards a stimulus is a positive tropism.
• Growing away from a stimulus is a negative tropism.

Two common stimuli that plants respond to:

• Light — a shoot grows towards light (a response called phototropism). This helps the leaves capture more light for photosynthesis.
• Gravity — a root grows downwards, in the direction of gravity (a response called gravitropism / geotropism). This anchors the plant and reaches water and minerals in the soil.

These responses improve the plant's survival: better access to light, water and support.

Exam tip. The key word for this statement is tropism — a plant's response is a growth response, which is slower than an animal's response. At this overview level you only need to know that plants respond by growing (tropisms); the detailed mechanism (auxins) is covered elsewhere.