AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
Organisms increase their chance of survival by responding to changes in…
Full specification wording
Survival and response (A-level only): Organisms increase their chance of survival by responding to changes in their environment.
A stimulus is a detectable change in the internal or external environment; a response is the change in behaviour or physiology that follows.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 1 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A stimulus is a detectable change in the internal or external environment; a response is the change in behaviour or physiology that follows. Responding raises the chance of surviving long enough to reproduce. The general pathway involves a stimulus, receptor, coordinator, effector and response. Receptors are specific to one form of energy, such as a Pacinian corpuscle detecting pressure. Coordinators include the nervous system, hormonal systems, or plant growth factors. Effectors vary: animals use muscles or glands, whereas plants use elongating cells. Responses range from reflex withdrawals to taxes and kineses that keep invertebrates in favourable conditions.
What earns the marks
- Defining a stimulus as a detectable change in the internal or external environment.
- Identifying the general pathway: stimulus, receptor, coordinator, effector, response.
- Linking a specific response to an increased chance of survival.
- Identifying the correct effector for a given response, such as a muscle in animals or elongating cells in plants.
Common mistakes and how to put them right
Writing that organisms respond 'in order to survive', which implies intent rather than natural selection.
Put it right: State that responses increase survival chances.
Omitting the coordinator, making it appear as though the stimulus acts directly on the effector.
Treating the receptor and the effector as the same structure, rather than distinct parts of a pathway.
Exam tips
- Use the general pathway (stimulus, receptor, coordinator, effector, response) to structure answers about unfamiliar animal responses.
- Always explicitly state how a specific response provides a survival advantage, such as avoiding predators or finding food.
Try the lesson
The start of the Survival and response (A-level only): Organisms increase their chance of survival by responding to changes in their environment. lesson, made from checked teaching notes for this specification point.
Have a go first
Which statement describes the benefit to an organism of responding to a change in its environment?
Listen
A stimulus is a detectable change in the internal or external environment; a response is the change in behaviour or physiology that follows. Responding raises the chance of surviving long enough to reproduce. The general pathway involves a stimulus, receptor, coordinator, effector and response. Receptors are specific to one form of energy, such as a Pacinian corpuscle detecting pressure. Coordinators include the nervous system, hormonal systems, or plant growth factors. Effectors vary: animals use muscles or glands, whereas plants use elongating cells. Responses range from reflex withdrawals to taxes and kineses that keep invertebrates in favourable conditions. A strong answer earns marks for these points. Defining a stimulus as a detectable change in the internal or external environment. Identifying the general pathway: stimulus, receptor, coordinator, effector, response. Linking a specific response to an increased chance of survival. Identifying the correct effector for a given response, such as a muscle in animals or elongating cells in plants. A common slip: Writing that organisms respond 'in order to survive', which implies intent rather than natural selection. Instead, state that responses increase survival chances. Exam tip: Use the general pathway (stimulus, receptor, coordinator, effector, response) to structure answers about unfamiliar animal responses.
Key words
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stimulus
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response
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receptor specificity
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effectors
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See every definition
- stimulus
- A detectable change in the internal or external environment.
- response
- The change in behaviour or physiology that follows a stimulus.
- receptor specificity
- Receptors are specific to one form of energy, such as a Pacinian corpuscle detecting pressure.
- effectors
- Animals use muscles or glands, whereas plants use elongating cells.
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