A change in the internal or external environment.

What are the roles of receptor, coordinator and effector?

The receptor detects the stimulus, the coordinator formulates a suitable response and the effector produces the response.

What does it mean that receptors are specific?

Each type of receptor responds to only one type of stimulus.

Indoleacetic acid, a plant growth factor that moves from growing regions to other tissues to regulate growth.

Explain phototropism in a shoot.

IAA moves to the shaded side, stimulating cells there to elongate more, so the shoot bends towards the light.

Explain gravitropism in a root.

IAA accumulates on the lower side and inhibits elongation, so the upper side grows faster and the root bends downwards.

A directional movement of an organism towards or away from a stimulus.

A change in the speed of movement or rate of turning depending on stimulus intensity, with no directed movement.

Why do woodlice tend to gather in damp places?

They move faster and turn more in dry air and slow down in damp air (a kinesis), which reduces water loss.

List the parts of a three-neurone simple reflex arc.

Receptor, sensory neurone, relay neurone, motor neurone, effector.

How does a simple reflex help survival?

It is fast and automatic, with few synapses, so the body is protected from harm before the brain is aware.

In a choice chamber investigation, why is gauze used?

So that the animals cannot touch the chemicals that control humidity, yet the air above them is dry or humid.

Survival and responseAQA A-Level Biology: Flashcards

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What is a stimulus?

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What is a stimulus?: A change in the internal or external environment.
What are the roles of receptor, coordinator and effector?: The receptor detects the stimulus, the coordinator formulates a suitable response and the effector produces the response.
What does it mean that receptors are specific?: Each type of receptor responds to only one type of stimulus.
What is IAA?: Indoleacetic acid, a plant growth factor that moves from growing regions to other tissues to regulate growth.
What is the effect of IAA on cell elongation in shoots?: It stimulates cell elongation.
What is the effect of a high concentration of IAA on roots?: It inhibits cell elongation.
Explain phototropism in a shoot.: IAA moves to the shaded side, stimulating cells there to elongate more, so the shoot bends towards the light.
Explain gravitropism in a root.: IAA accumulates on the lower side and inhibits elongation, so the upper side grows faster and the root bends downwards.
What is a taxis?: A directional movement of an organism towards or away from a stimulus.
What is a kinesis?: A change in the speed of movement or rate of turning depending on stimulus intensity, with no directed movement.
Why do woodlice tend to gather in damp places?: They move faster and turn more in dry air and slow down in damp air (a kinesis), which reduces water loss.
List the parts of a three-neurone simple reflex arc.: Receptor, sensory neurone, relay neurone, motor neurone, effector.
How does a simple reflex help survival?: It is fast and automatic, with few synapses, so the body is protected from harm before the brain is aware.
In a choice chamber investigation, why is gauze used?: So that the animals cannot touch the chemicals that control humidity, yet the air above them is dry or humid.

Exam questions on Survival and response

A person touches a hot saucepan handle and snatches her hand away before she feels any pain. The response is a simple reflex that uses a three-neurone pathway.
Explain why this reflex helps the person to survive.2 marks
A pea seedling is laid on its side in complete darkness. After two days the root has curved downwards and the shoot has curved upwards. Measurements show that indoleacetic acid (IAA) has moved to the lower side of both the root and the shoot.
Explain how the movement of IAA causes the shoot to curve upwards.2 marks
A student uses a choice chamber to investigate the response of woodlice to humidity. The chamber has two halves that can be set to different humidities, with a gauze floor above them. She places 20 woodlice in the centre and counts how many are in each half after 10 minutes. She repeats the investigation five times. The numbers of woodlice in the humid half were 15, 16, 13, 17 and 14.
Describe how the student should set up the choice chamber so that humidity is the only variable that differs between the two halves.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).