Response to Changes in the Environment

Section 1 of 7Detecting and responding

Stimulus

• Stimulus A stimulus is any change in the surroundings that an organism can detect.
• Examples include light, temperature, sound, chemicals, touch and pressure.

Receptors

• Receptors Receptors are the cells or organs that detect stimuli.
• In humans they are found in sense organs such as the eyes, ears, skin, nose and tongue.

Plant detection

• Plant detection Plants have no sense organs, but specialised cells can detect light, gravity and water.

Response

• Response After detecting a stimulus the organism produces a response, which is the action that follows.

Section 2 of 7Kinds of response

Responses

Movement

• Movement The organism may move towards a stimulus such as food, or away from one such as a predator or a hot object.

Secreting chemicals

• Secreting chemicals A gland may release a substance, for example a hormone or sweat.

Adjusting body processes

• Adjusting body processes Internal changes, such as altering breathing rate or heart rate, may follow a stimulus.

Section 3 of 7A response in sequence

1Stimulus

• Stimulus A change occurs, for example a loud bang nearby.

2Receptor

• Receptor Sensory cells in the ear detect the sound.

3Coordination

• Coordination Information is passed on and processed so that a decision is made.

4Response

• Response Muscles contract and the person ducks or moves away.

Section 4 of 7Why responding matters

Stimulus	Possible response	Benefit to survival
Predator approaching	Run or hide	Avoids being eaten
Smell of food	Move towards the source	Finds a source of energy
Body gets too cold	Shivering and reduced heat loss	Keeps internal conditions suitable for enzymes
Light direction (plant)	Shoot grows towards the light	More light for photosynthesis

Section 5 of 7Worked examples of stimulus and response

Hand on a hot surface

• Hand on a hot surface The stimulus is heat, which is detected by temperature receptors in the skin of the hand.
• The response is a rapid withdrawal of the hand, produced by muscles in the arm, and it prevents tissue damage.

Light in the eye

• Light in the eye Bright light is detected by receptor cells in the retina of the eye.
• The response is a narrowing of the pupil, which protects the retina from being damaged by too much light.

Plant roots and water

• Plant roots and water Specialised cells in the root detect water and gravity.
• The root grows downwards towards moisture, which gives the plant a steady supply of water and minerals.

Linking to survival

• Linking to survival Every response should be linked to why it helps: escaping predators, finding food, or keeping internal conditions stable.
• Organisms that respond well are more likely to survive and reproduce.

Section 6 of 7Key terms

Term	Meaning
Stimulus	A change in the environment that can be detected by an organism.
Receptor	A cell or organ that detects a stimulus.
Response	The action an organism takes after detecting a stimulus.
Sense organ	A group of receptor cells forming an organ, such as the eye or ear.

Section 7 of 7Exam technique

1

Confusing the stimulus with the receptorlight is the stimulus, while the retina contains the receptors.

2

Assuming only animals respond to stimuli; plants also respond, for instance by growing towards light.

3

Saying that organisms respond 'to be able to move' — the key idea is that responding increases the chance of survival.

4

Listing responses without linking them to a benefit, such as escaping danger, finding food or keeping conditions stable.