6.1.1 Survival and response

Question 1

Stimulus:

Answer 1

A stimulus is a change in the internal or external environment.
Organisms increase their chance of survival by responding to changes in their internal and external environment.

A receptor detects a stimulus. Receptors are specific to one type of stimulus. A coordinator formulates a suitable response to a stimulus. An effector produces a response.

Question 2

Survival and response in flowering plants:

Answer 2

Specific growth factors move from growing regions to other tissues, where they regulate growth in response to directional stimuli.

Question 3

IAA

Answer 3

IAA is a specific growth factor produced by the tips of shoots and roots.
IAA moves from the growing regions of a plant shoot to other tissues by diffusion (and sometimes active transport).
IAA stimulates growth in shoots and inhibits growth in roots.
IAA doesn’t stimulate growth by mitosis, rather it causes cell elongation by increasing cell wall plasticity, allowing cells to grow by absorbing water through osmosis.
Uneven cell elongation causes growing shoots or roots to bend.

Question 4

Phototropism:

Answer 4

Growth response to the direction of light (the stimulus).
Can be positive (towards) or negative (away from):
Shoots are positively phototropic, roots are negatively phototropic.
IAA moves to the shaded side in both shoots and roots (IAA stimulates growth in shoots and inhibits growth in roots).

Question 5

Gravitropism:

Answer 5

Growth response to the direction of gravity (the stimulus).
Shoots are negatively gravitropic, roots are positively gravitropic.
IAA moves to underside in both shoots and roots (IAA stimulates growth in shoots and inhibits growth in roots).

Question 6

Taxis:

Answer 6

A directional response to a stimulus.
Taxis is movement towards or away from a stimulus.

Question 7

Kinesis:

Answer 7

A non-directional response to a stimulus.
Kinesis involves changes in the speed of movement and rate of change of direction.

Question 8

Simple reflex:

Answer 8

Reflexes are rapid, involuntary and stereotypical.
The simple reflex has a protective effect by avoiding or reducing damage to tissues, by allowing the organism to quickly obtain food or escape from predators, and to do homeostasis.
The three-neurone simple reflex involves a sensory neurone, relay neurone, and motor neurone.