6.1 Stimuli, both internal and external, are detected and lead to a response

What is a stimulus?

• A change in an organism’s internal or external environment.

Why is it important for organisms to respond to stimuli?

• To increase the organism’s chance of survival.

What is a tropism?

• Growth of a plant in response to a directional stimulus.

Distinguish between positive and negative tropism.

• Positive tropism: Growth towards the stimulus.

• Negative tropism: Growth away from the stimulus.

What are plant growth factors and what is their role?

• Hormone-like substances, e.g., auxins such as IAA.

• Produced in growing regions (shoot/root tips).

• Transported from growing regions via phloem or diffusion to other tissues.

• Regulate growth in response to directional stimuli (tropisms).

How does IAA affect cell elongation in shoots versus roots?

• In shoots, high IAA stimulates cell elongation.

• In roots, high IAA inhibits cell elongation.

Describe the process of gravitropism in flowering plants.

• Cells in the tip of a shoot or root produce IAA.

• IAA diffuses evenly at first.

• Gravity causes IAA to accumulate on the lower side.

• In shoots, high IAA stimulates elongation → shoot bends away from gravity (negative gravitropism).

• In roots, high IAA inhibits elongation → root bends towards gravity (positive gravitropism).

Why do roots show positive gravitropism?

• IAA accumulates on the lower side.

• High IAA inhibits cell elongation on that side.

• Lower side grows slower → root bends towards gravity.

Why do shoots show negative gravitropism?

• IAA accumulates on the lower side.

• High IAA stimulates cell elongation on that side.

• Lower side grows faster → shoot bends away from gravity.

What is the directional growth response of plants called?

• Tropism.

What is phototropism in flowering plants?

• Growth of a plant in response to a directional light stimulus.

Describe the process of phototropism in a shoot.

• Cells in the tip produce IAA.

• IAA diffuses evenly at first.

• Light causes IAA to move to the shaded side, so concentration increases there.

• In shoots, high IAA stimulates cell elongation on the shaded side.

• Shaded side grows faster → shoot bends towards the light (positive phototropism).

How does IAA distribution differ between the light and shaded sides?

• IAA concentration is higher on the shaded side.

What is a taxis (tactic response)?

• A directional movement towards or away from a directional stimulus.

What is a kinesis (kinetic response)?

• A non-directional response where the speed of movement or rate of turning changes in response to a non-directional stimulus.

• Response depends on the intensity of the stimulus.

Give an example of a taxis.

• Woodlice moving away from light to avoid predators (negative phototaxis).

Give an example of a kinesis

• Woodlice moving more rapidly and turning less in a dry environment to increase the chance of finding a damp area (prevents drying out).

What is the purpose of a simple reflex arc?

• To provide a rapid, automatic, protective response to harmful stimuli.

Why are simple reflexes rapid?

• They involve only 3 neurones (sensory, relay, motor) and few synapses as synaptic transmission is slow.

Why are reflexes described as autonomic?

• They do not involve conscious regions of the brain and do not need to be learned.

How do reflexes protect an organism?

• By enabling a quick escape from predators or withdrawal from harmful stimuli to prevent tissue damage.