Unit 6 - Organisms respond to changes in their internal and external environments
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AQA A Level Biology - Organisms respond to changes in their environments
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Unit 6 - Organisms respond to changes in their internal and external environments
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AQA
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AQA Biology A Level Student Book
Complete class notes on Chapters 14-15 of AQA Biology Course.
14 Response to stimuli
15 Nervous coordination and muscles
Written by a recent A* student
Includes pictures, tables, flow diagrams, model answers and extra information for 25 mark synoptic essays.
NB Does not include chapte...
Unit 6 - Organisms respond to changes in their internal and external environments
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Module 6: Organisms Respond to Changes in Environment
Key information 1
Response to stimuli 2
Survival and response 2
Plant growth factors 2
Animal hormones 4
Reflex arc 4
Receptors 6
Pacinian corpuscle 6
The Eye 7
Control of Heart Rate 8
Nervous Coordination and Muscles 9
Neurones 9
The Nerve impulse 11
Propagation of an action potential 12
The Synapse 14
The neuromuscular junction 15
Skeletal muscle structure 15
The contraction of skeletal muscle 17
, Response to stimuli
Survival and response
Stimulus = detectable change resulting in a response
Can be internal/external (ie response to hunger vs response to touch)
Those who are able to respond more quickly/efficiently will result in a greater chance of survival and
passing on favourable alleles to the offspring. Selection pressures favour the organisms with the most
appropriate response.
Taxis - A response whose direction is determined by the direction of the stimulus
Results in a motile animal responding directly to a stimulus by moving its whole body towards a
favourable stimulus or away from an unfavourable stimulus.
- Eg positive phototaxis = movement towards light.
- Eg positive chemotaxis = movement towards certain chemicals
Kinesis - A response in which the organism doesn’t move towards/away from the stimulus
Results in a motile animal rapidly changing direction in a random manner. The more unpleasant the
stimulus the more rapidly it changes direction. This response is designed to bring the animal back into
favourable conditions. This important when the stimulus is less directional (eg temperature/humidity)
- Eg In dry areas, woodlice move more rapidly and change direction.
Tropism - The growth/movement of part of a plant in response to a directional stimulus
Results in a plant moving towards/away from a stimulus (eg light, gravity, water)
- Eg positive phototropism = movement towards light (eg leaves)
- Eg negative phototropism = movement away from light (eg roots)
- Eg negative gravitropism = movement towards the ground (eg roots)
- Eg positive hydrotropism = movement towards water (eg roots)
Plant growth factors
Stimulus Name Response of shoots Response of roots Use
Light Phototropism Positive None Light for LDR
Gravity Gravitropism Negative Positive To obtain nutrients/stability
Water Hydrotropism None Positive To obtain water for photosynthesis etc.
Touch Thigmotropism Varies Varies Eg Venus flytraps, growing around bamboo
, Plant growth factors: Chemicals produced in plants at low concentrations and transported elsewhere
to produce a response
Coleoptile: A protective sheath that covers the first leaf in grasses/cereals
Auxins: A group of growth factors that control cell elongation. Eg IAA
IAA stimulates elongation of shoot cells and inhibits elongation of root cells.
● Stimulates lateral root growth
● Stimulates fruit development
● Involved in leaf fall
● Can be used synthetically:
○ As herbicides
○ To induce fruiting
○ To induce root formation of cuttings
Auxin is produced in the tip and diffuses down
it. It is destroyed by light, leading to an
accumulation of IAA on the shaded side of the
plant. This results in elongation of the shaded side
of the cell, so the tip bends towards the light.
Meristem = areas of high rates of mitosis (in
growth zones)
Cell division → Elongation →
Maturation
This process starts:
- At the top of the tip, working down the shoot
- At the bottom of the root tip, working up the root.
SYNOPTIC ESSAY TOPIC:
Importance of receptors in
biology.
IAA: Auxin responsible for cell
growth in tip.
NB: H+ ions change pH which
affects cell walls which leads
to absorption of water.
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