Stimuli- Internal/ External- Leading to a Response

What is a stimulus?

A detectable change in the internal or external environment.

Why must organisms detect changes in their internal and external environments?

To maintain conditions favourable for survival and reproduction.

What is a receptor?

A cell or organ that detects a stimulus.

What is an effector?

A cell, tissue or organ that carries out a response.

What is meant by a response?

A change in behaviour or physiology due to a stimulus.

Why do organisms respond to stimuli?

Responses help maintain optimal conditions and avoid danger.

Why does responding to stimuli increase an organism’s chance of survival?

Improves ability to find food, avoid harm, and reproduce.

What does selection pressure favour in terms of response systems?

Systems that efficiently detect and respond to meaningful stimuli.

What is the general pathway linking stimulus to response?

Stimulus → receptor → coordinator → effector → response.

Why is coordination needed between receptors and effectors?

To ensure appropriate responses are produced.

What is meant by “communication” in biological response systems?

The transfer of information between receptors, coordinators and effectors.

Which two main systems coordinate responses in animals?

Nervous system and hormonal system.

Which system coordinates responses in plants?

Chemical and growth factor systems, mainly plant hormones.

What is a simple response?

A basic behavioural response involving simple mechanisms.

Why are simple responses particularly important in organisms with a simple nervous system?

They allow survival without complex processing.

What is taxis?

A directional movement in response to a stimulus.

What does a positive taxis mean?

Movement towards the stimulus.

What does a negative taxis mean?

Movement away from the stimulus.

Give an example of positive phototaxis in an organism.

Algae moving towards light.

Give an example of negative phototaxis.

Earthworms moving away from light.

Give an example of positive chemotaxis.

Bacteria moving towards higher glucose concentration.

Why do microorganisms often use chemotaxis?

To locate nutrients or avoid harmful chemicals.

What is kinesis?

A non-directional change in movement in response to stimulus intensity.

How does an organism’s movement change in kinesis?

The speed or rate of turning changes.

What happens to the rate of turning in an unfavourable environment?

It increases.

How does kinesis increase the chance of returning to favourable conditions?

It keeps the organism in or near favourable conditions longer.

Give an example of an organism showing kinesis.

Woodlice increasing turning rate in dry conditions.

What is a tropism?

A growth response towards or away from a directional stimulus.

In which organisms do tropisms occur?

Plants.

What is phototropism?

Growth in response to light.

What is gravitropism?

Growth in response to gravity.

Why are tropisms important for plant survival?

They optimise access to light, water and minerals.

What is indoleacetic acid (IAA)?

A plant growth regulator (auxin).

What group of substances does IAA belong to?

Auxins.

Where is IAA produced in shoots?

Shoot tips.

How is IAA transported through plant tissues?

By diffusion and active transport via parenchyma cells.

Why does IAA accumulate on the shaded side of a shoot?

Light destroys or inhibits IAA on lit side, so it diffuses to shaded side.

How does IAA affect cell elongation in shoots?

It increases cell elongation in shoots.

Why does accumulation of IAA on one side of a shoot cause bending?

Shaded cells elongate more, causing bending towards light.

How does IAA lead to positive phototropism in shoots?

IAA accumulates on shaded side, stimulating elongation and bending the shoot towards light.

How does IAA affect roots differently from shoots?

IAA inhibits cell elongation in roots at higher concentrations.

What is negative phototropism in roots and why does it occur?

Roots bend away from light; helps them grow into soil.

What is positive gravitropism in roots and why is it beneficial?

Roots grow towards gravity, improving anchorage and nutrient uptake.

What is negative gravitropism in shoots and why does it occur?

Shoots grow against gravity, helping them access light.

Why is the uneven distribution of IAA essential for tropic responses?

It creates differential growth, the basis of curvature.

Why do young plant tissues respond more strongly to IAA?

They have more responsive, less lignified cell walls.

Why does IAA often inhibit rather than stimulate growth in roots?

Root tissues require lower IAA concentrations for stimulation; higher levels inhibit growth.

Why is tropism considered a directional response but kinesis is not?

Tropisms depend on stimulus direction; kinesis depends on stimulus intensity only.

Why are tropisms slower than taxis responses?

They rely on hormone redistribution and growth rather than rapid movement.

Why does the ability to detect and respond to light provide a major survival advantage for plants?

It maximises photosynthesis and therefore energy acquisition.

Why is gravitropism important for stability and resource acquisition?

Ensures roots grow down for water/minerals and shoots grow up for light.

Why do genetically controlled growth responses provide evidence for natural selection?

Organisms with advantageous tropic responses survive better and pass on alleles controlling them.