Biology A Level Revision (AQA)
Paper 1 - Year 12 / AS Level
Biological Molecules
Cells
Organisms Exchanging Substances
Genetic Information, Variation
Paper 2 - Year 13 / A Level
Energy Transfers In and Between Organisms
Organisms Responses to their Environment
Genetics, Evolution and Ecosystems
The Control of Gene Expression
Some diagrams used in this document are sourced from Cognito (https://cognitoedu.org).
All rights to those diagrams belong to Cognito.
, Biology Revision A Level AQA
Organisms Responses to their Environment
Survival
- Detecting and responding to a stimulus can increase an organism's survival chance.
● Key words
- Stimulus - a change in the internal (blood glucose concentration) or external
(temperature) environment.
- Receptors - detect the change in the environment.
- Effectors - produce a response.
Animal Responses
- Animals respond to a stimulus to aid their survival by moving them to a more
favourable environment.
● Tactic Response (taxis):
- If an organism responds to a stimulus directionally, it is a tactic response.
- The stimuli can be positively or negatively directional.
- If the organism moves away from a factor, it is negatively directional. For example,
woodlice move away from light so are negatively phototactic.
- However, single-celled algae move towards light to help them survive making them
positively phototactic.
● Kinetic Response (kineses):
- If an organism shows no direction to a stimulus, it is a non-directional kinetic
response.
- For example, woodlice move slower and turn less in high humidity but move faster
and turn more in a lower humidity. Since less water is lost in the damper
environments, woodlice are in a more favourable environment and so move less to
stay in that environment. However, if they are in a dry environment, they move
faster to increase their survival as they are more likely to find a favourable
environment.
● Choice Chambers:
- Used to study the response animals produce to environmental stimuli.
1. Divide a petri dish into two halves with a divider.
2. Place damp filter paper on one half and none on the other half and lay a piece of
mesh on top.
3. Then, cover half with black card and leave the other half clear.
➢ ¼ should be dark and damp.
➢ ¼ should be dark and dry.
➢ ¼ should be light and damp.
➢ ¼ should be light and dry.
4. Place 10 woodlice on the centre of the mesh and close the lid.
5. After 10 minutes, record the number of woodlice in each
compartment.
6. Carefully move the woodlice back to the centre and repeat.
Page 1
, Biology Revision A Level AQA
Plant Responses
- Plants are not mobile so can not move away from danger.
- Instead, plants can get directionally in response to a stimulus.
● Phototropism
- Light causes a plant to grow in certain directions.
- The plant’s shoots are positively phototropic (grow towards light).
- The plant's roots are negatively phototropic (grow away from light).
● Gravitropism
- Gravity also causes plants to grow in certain directions.
- The plant's shoots are negatively gravitropic (grow upwards / against gravity).
- The plant’s roots are positively gravitropic (grow downwards / with gravity).
● Indoleacetic Acid (IAA)
- Indoleacetic acid is a type of auxin which controls a plant's tropic response.
- IAA is transported via diffusion / active transport (over short distances) and through
the phloem (over longer distances).
- IAA is transported to different parts of the plant when it detects a directional
stimulus which can create an uneven distribution.
- This uneven distribution can cause a directional growth response.
- IAA inhibits growth in the roots and
promotes growth in the shoots.
● IAA’s Role in Phototropism
- IAA is transported to the more shades
area if a shoot is exposed to an uneven
light source which causes the shaded
area to elongate. This causes the plants
to bend towards the light.
- Likewise, IAA is also transported to the
shaded part of the plant’s roots.
However, instead, IAA inhibits cell
elongation making the roots bend away
from light.
● IAA’s Role in Gravitropism
- When shoots are exposed to an uneven
gravitational pull, IAA is transported to the
underside where the pull of gravity is
closer. The IAA causes the cell on the
underside to elongate and grow away from
the gravitational pull (upwards).
- When roots are exposed to an uneven
gravitational pull, IAA is transported to the
underside and inhibits cell elongation at the
lower end. This causes the roots to bend
towards the gravitation pull (downwards).
Page 2
Paper 1 - Year 12 / AS Level
Biological Molecules
Cells
Organisms Exchanging Substances
Genetic Information, Variation
Paper 2 - Year 13 / A Level
Energy Transfers In and Between Organisms
Organisms Responses to their Environment
Genetics, Evolution and Ecosystems
The Control of Gene Expression
Some diagrams used in this document are sourced from Cognito (https://cognitoedu.org).
All rights to those diagrams belong to Cognito.
, Biology Revision A Level AQA
Organisms Responses to their Environment
Survival
- Detecting and responding to a stimulus can increase an organism's survival chance.
● Key words
- Stimulus - a change in the internal (blood glucose concentration) or external
(temperature) environment.
- Receptors - detect the change in the environment.
- Effectors - produce a response.
Animal Responses
- Animals respond to a stimulus to aid their survival by moving them to a more
favourable environment.
● Tactic Response (taxis):
- If an organism responds to a stimulus directionally, it is a tactic response.
- The stimuli can be positively or negatively directional.
- If the organism moves away from a factor, it is negatively directional. For example,
woodlice move away from light so are negatively phototactic.
- However, single-celled algae move towards light to help them survive making them
positively phototactic.
● Kinetic Response (kineses):
- If an organism shows no direction to a stimulus, it is a non-directional kinetic
response.
- For example, woodlice move slower and turn less in high humidity but move faster
and turn more in a lower humidity. Since less water is lost in the damper
environments, woodlice are in a more favourable environment and so move less to
stay in that environment. However, if they are in a dry environment, they move
faster to increase their survival as they are more likely to find a favourable
environment.
● Choice Chambers:
- Used to study the response animals produce to environmental stimuli.
1. Divide a petri dish into two halves with a divider.
2. Place damp filter paper on one half and none on the other half and lay a piece of
mesh on top.
3. Then, cover half with black card and leave the other half clear.
➢ ¼ should be dark and damp.
➢ ¼ should be dark and dry.
➢ ¼ should be light and damp.
➢ ¼ should be light and dry.
4. Place 10 woodlice on the centre of the mesh and close the lid.
5. After 10 minutes, record the number of woodlice in each
compartment.
6. Carefully move the woodlice back to the centre and repeat.
Page 1
, Biology Revision A Level AQA
Plant Responses
- Plants are not mobile so can not move away from danger.
- Instead, plants can get directionally in response to a stimulus.
● Phototropism
- Light causes a plant to grow in certain directions.
- The plant’s shoots are positively phototropic (grow towards light).
- The plant's roots are negatively phototropic (grow away from light).
● Gravitropism
- Gravity also causes plants to grow in certain directions.
- The plant's shoots are negatively gravitropic (grow upwards / against gravity).
- The plant’s roots are positively gravitropic (grow downwards / with gravity).
● Indoleacetic Acid (IAA)
- Indoleacetic acid is a type of auxin which controls a plant's tropic response.
- IAA is transported via diffusion / active transport (over short distances) and through
the phloem (over longer distances).
- IAA is transported to different parts of the plant when it detects a directional
stimulus which can create an uneven distribution.
- This uneven distribution can cause a directional growth response.
- IAA inhibits growth in the roots and
promotes growth in the shoots.
● IAA’s Role in Phototropism
- IAA is transported to the more shades
area if a shoot is exposed to an uneven
light source which causes the shaded
area to elongate. This causes the plants
to bend towards the light.
- Likewise, IAA is also transported to the
shaded part of the plant’s roots.
However, instead, IAA inhibits cell
elongation making the roots bend away
from light.
● IAA’s Role in Gravitropism
- When shoots are exposed to an uneven
gravitational pull, IAA is transported to the
underside where the pull of gravity is
closer. The IAA causes the cell on the
underside to elongate and grow away from
the gravitational pull (upwards).
- When roots are exposed to an uneven
gravitational pull, IAA is transported to the
underside and inhibits cell elongation at the
lower end. This causes the roots to bend
towards the gravitation pull (downwards).
Page 2
