What is the sequence of events in a response to a stimulus?

  1. Stimulus - a detectable change in the internal or external environment of an organism
  2. Receptor - detects specific types of stimulus
  3. Coordinator - coordinates a suitable response to a stimulus
  4. Effector - produces a response to a stimulus
  5. Response - the action

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What is a taxis?

A taxis is a simple response whose direction is determined by the direction of the stimulus.

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What is the difference between positive phototaxis and negative phototaxis?

Positive phototaxis is when an organism moves towards light.


For example, algae need to move towards light for photosynthesis.


Negative phototaxis is when an organism moves away from light.


For example, earthworms move away from light as the heat of the sun can dry out their skin.

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What is positive chemotaxis?

Positive chemotaxis is when an organism moves towards a region where a specific chemical is more concentrated.


For example bacteria may move towards a region rich in glucose to obtain food.

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What is a kinesis, and how does it help increase the chance of survival in some organisms?

A kinesis is a response in which the organism changes the speed at which it moves and the rate at which it changes direction.


A kinesis increases an organism's chances of survival by enhancing its chances of a quick return to a favourable environment.


For example, kinesis in woodlice helps them maintain favourable damp conditions, preventing them from drying out and increasing their chances of survival.

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What is a tropism?

A tropism is the growth of a part of an organism like a plant in response to a directional stimulus.

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What are positive phototropism and negative phototropism, and how are they helpful to plants?

Positive phototropism is when a part of an organism grows towards light.

This increases the probability that plant shoots and the leaves will be able to capture light for photosynthesis.


Negative phototropism is when a part of an organism grows grow away from light.

This increases the probability that plant roots will grow into the soil, where they can absorb more water and mineral ions.

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What are positive gravitropism and negative gravitropism, and how are they helpful to plants?

Positive gravitropism is when a part of an organism grows towards gravity.

This increases the probability that plant roots will grow into the soil, where they can absorb more water and mineral ions.


Negative gravitropism is when a part of an organism grows away from gravity. 

This increases the probability that plant shoots and the leaves will grow upwards towards the sun, so they can capture more light for photosynthesis.

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