6.1.1 Survival and response

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 is a tropism?

• growth of a plant in response to directional stimulus

• Positive tropism = towards a stimulus; negative tropism = away from stimulus

Summarise the role of growth factors in flowering plants

• 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)

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

• in shoots, high concentrations of IAA stimulates cell elongation

• In roots, high concentrations of IAA inhibits cell elongation

Explain gravitropism in flowering plants

1. Cells in tip of shoot / root produce IAA

2. IAA diffuses down shoot / root (evenly initially)

3. IAA moves to lower 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 away from gravity whereas roots bend towards gravity

Explain phototropism in flowering plants

1. Cells in tip of shoot / root produce IAA

2. IAA diffuses down shoot / root (evenly initially)

3. IAA moves down 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

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

1. Taxes (tactic response)

• 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

• Depending on intensity of stimulus

(Examples: taxis - woodlice moving away from light to avoid predators; kinesis - woodlice moving faster in drier environments to increase their chance of moving to an area with higher humidity to prevent drying out)

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

• rapid as only 3 neurones and few synapses (synaptic transmission is slow)

• Autonomic (doesn’t involve conscious regions of brain) so doesn’t have to be learnt

• Protect from harmful stimuli e.g. escape predators / prevents damage damage to body tissues

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