topic 6 - organisms respond to changes in their environments

What is a stimulus?

A change in an organism’s internal or external environment

Why is it important that organisms can respond to stimuli?

Organisms increase their chance of survival by responding to stimuli

What are kineses?

Non-directional responses to stimuli.

Speed of movement or rate of directional change can change with the intensity of the stimuli.

What is an example of kineses?

Woodlice moving faster in drier environments to increase their chance of moving in an area with higher humidity to prevent drying out.

What are taxis?

Directional responses to a directional stimulus - involves moving towards (positive) or away from (negative) the stimulus.

What is an example of taxis?

Woodlice moving away from light to avoid predators.

How do plants respond to directional stimuli?

Specific growth factors which are hormone-like growth substance that can speed up or slow down plant growth.

Growth factors move from growing regions e.g. shoot / root tips where they’re produced to other tissues where they regulate growth in response to directional stimuli

What are auxins?

A class of growth factors found in root & shoot tips that diffuse backwards to stimulate growth in the cells just behind where they’re produced.

What is an example of an auxin?

IAA (indoleacitic acid)

Where is IAA synthesised?

In the growing regions of the plant - tips of the roots and shoots

How does IAA move around the plant?

By diffusion and active transport from cell to cell, and in the phloem for long distances

How does indoleactic acid (IAA) affect cells in roots and shoots?

• In shoots, high concentrations of IAA stimulates cell elongation

• In roots, high concentrations of IAA inhibits cell elongation

What happens in cell elongation?

The cell walls become loose and stretchy so the cells become longer.

What is a tropism?

Growth of a plant in response to a directional stimulus - there are two types.

• Positive tropism = towards a stimulus

• Negative tropism = away from stimulus

What is phototropism?

The growth of a plant in response to light.

• Shoots are positively phototropic so the grow towards light.

• Roots are negatively phototropic so grow away from light.

What is gravitropism?

The growth of a plant in response to gravity.

• Shoots are negatively gravitropic so grow away from gravity

• Roots are positively gravitropic so grow towards gravity

What happens in gravitropism in the shoots vs the roots?

In both the IAA is produced by cells in the tip of the shoot vs root and diffuses down. The IAA then moves to the lower side of the shoot vs root increasing the conc of IAA there.

Shoots: 

IAA stimulates cell elongation

Cells on the lower side elongate.

The shoot tip curves up /away from gravity.

Negative tropism

Roots: 

IAA inhibits cells elongation.

Cells on the lower side stay short.

The root tip curves down/towards gravity.

Positive tropism

What happens in phototropism in the shoots vs the roots?

In both the IAA is produced by cells in the tip of the shoot vs root and diffuses down. The IAA then moves to the shaded side of the shoot vs root increasing the conc of IAA there.

Shoots: 

IAA stimulates cell elongation

Cells on the shaded side elongate.

The shoot tip curves towards the light.

Positive tropism

Roots: 

IAA inhibits cells elongation.

Cells on the shaded side stay short.

The root tip curves away from the light.

Negative tropism