[3.6.1.1] Survival & Response

Question 1

What is a stimulus?

Answer 1

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

Question 2

Why is it important that organisms can respond to stimuli?

Answer 2

Organisms increase their chance of survival by responding to stimuli.

Question 3

What is a tropism?

Answer 3

• Growth of a plant in response to a directional stimulus.
• Positive tropism = towards a stimulus; negative tropism = away from stimulus.

Question 4

Summarise the role of growth factors in flowering plants.

Answer 4

• Specific growth factors (hormone-like growth substances) e.g. Auxins (such as IAA) move (via phloem or diffusion) from growing regions e.g. shoot / root tips where they’re produced.
• To other tissues where they regulate growth in response to directional stimuli (tropisms).

Question 5

Describe how indoleacetic acid (IAA) affects cells in roots and shoots.

Answer 5

• In shoots, high concentrations of IAA stimulates cell elongation.
• In roots, high concentrtions of IAA inhbitis cell elongation.

Question 6

Explain gravitropism in flowering plants.

Answer 6

1. Cells in tip of shoot / root produce IAA.
2. IAA diffuses down shoot / root (evenly initally).
3. IAA moves to lower side of shoot / root (so concentration increases).
4. In shoots this stimulates cell elongation whereas in roots this inhbits cell elongation.
5. So shoots bend away from gravity wheras roots bend towards gravity.

Question 7

Explain phototropism in flowering plants.

Answer 7

1. Cells in tip of shoot / root produce IAA.
2. IAA diffuses down shoot / root (evenly initally).
3. IAA moves to shaded side of shoot / root (so concentration increases).
4. In shoots this stimulates cell elongation whereas in roots this inhibits cell elongation.
5. So shoots bend towards light whereas roots bend away from light.

Question 8

Describe the simple responses that can maintain a mobile organism in a favourable environment.

Answer 8

1. Taxes (tactic responses).
   
   • Directional response.
   • Movement towards or away from a directional stimulus.
2. Kinesis (kinetic responses).
   
   • Non-directional response.
   • Speed of movement or rate of direction change changes in response to a non-directional stimulus.
   • Dependning on intensity of stimulus.

Question 9

Explain the protective effect of a simple (e.g. 3 neurone) reflex.

Answer 9

• Rapid as only 3 neurones and few synapses (synaptic transmission is slow).
• Autonomic (doesn’t involve conscipus regions of brain) so doesn’t have to be learnt.
• Protects from harmful stimuli e.g. escape predators / prevents damage to body tissues.