This comprehensive set of A-Level Biology notes is meticulously crafted to provide students with essential information and streamlined content that reinforces understanding of key concepts. The notes are organized thematically, covering all major topics in the A-Level Biology curriculum, including:
6.3 Skeletal muscles are stimulated to contract by nerves
and act as effectors
6.4 Homeostasis is 6.4.1 Principles of homeostasis and
the maintenance of a negative feedback
stable internal
environment 6.4.2 Control of blood glucose
concentration
Required practical 11
6.4.3 Control of blood water potential
,Topic 6 Organisms respond to changes in their internal & external environments stan.store/biologywitholivia
6.1 Stimuli, both internal and external,
are detected and lead to a response
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 a 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) eg. Auxins (such as IAA) move
(via phloem or diffusion) from growing regions eg. 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
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,Topic 6 Organisms respond to changes in their internal & external environments stan.store/biologywitholivia
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 to shaded side of shoot / root (so conc. ↑)
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 (eg. 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
● Protects from harmful stimuli eg. escape predators / prevents damage to body tissues
Exam insight: common mistakes ❌
Mistake Explanation
“The root tip contains IAA.” To get this mark you need to state that the root tip produces IAA.
“There is more growth / elongation on This is too vague. You need to be clear that it is the cells on
[named side] of the shoot / root”. one side that are elongating more.
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, Topic 6 Organisms respond to changes in their internal & external environments stan.store/biologywitholivia
Required practical 10
Investigation into the effect of an environmental variable on the movement
of an animal using either a choice chamber or a maze.
Describe how the effect of an environmental variable on the movement of
an animal (eg. woodlice) can be investigated using a choice chamber
1. Set up choice chamber (different compartments) to create different environmental conditions
○ Eg. humidity → add a drying agent to one side and damp filter paper to other
○ Eg. light → shine a light but cover one half with black card
2. Control other environmental conditions
○ Eg. if investigating humidity control light intensity with a dim even light above
3. Use a teaspoon to place 12 animals eg. woodlice on centre of mesh platform and cover with lid
4. After a set amount of time eg. 10 minutes record the number of animals in each section
5. Repeat after gently moving woodlice back to centre
Common questions:
The woodlice were left for 15 minutes ● Time to establish humidity / for substance to absorb
before their movement was recorded water / water from paper to evaporate
when investigating the effect of humidity. ● Woodlice no longer affected by handling
Explain why. ● So that behaviour is typical of that humidity
Explain how you would ensure the safe and ● Safely - cover open wounds / wash hands with soap after
ethical handling of animals. ○ To minimise risk of infection
● Ethical - handle carefully / return to habitat ASAP
Explain why a mesh platform is used when ● To keep woodlice a safe distance from drying agent
investigating the effect of humidity.
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