Topic 6 Organisms respond to changes in their
internal & external environments v2 Model
Notes with Correct Answers.
Latest Updated 2026/2027
Downloaded by charles nguyo ()
,Topic 6 Organisms respond to changes in their
internal & external environments v2 Model
Notes with Correct Answers.
Latest Updated 2026/2027
AQA A Level Biology
Topic 6
Organisms respond to
changes in their internal &
external environments
Model answer notes by @biologywitholivia
Topic Sub-topic Understand Memorise Practise
6.1 Stimuli, both 6.1.1 Survival and response
internal and external,
are detected and Required practical 10
lead to a response
6.1.2 Receptors
6.1.3 Control of heart rate
6.2 Nervous 6.2.1 Nerve impulses
coordination
6.2.2 Synaptic transmission
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
Downloaded by charles nguyo ()
Required practical 11
,Topic 6 Organisms respond to changes in their
internal & external environments v2 Model
Notes with Correct Answers.
Latest Updated 2026/2027
Downloaded by charles nguyo ()
, Topic 6 Organisms respond to changes in their internal &
Topic 6 Organisms respond to changes in their internal & external environments stan.store/biologywitholivia
external environments v2 Model Notes with Correct
Latest Updated 2026/202
Answers.
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
Downloaded by charles nguyo ()
internal & external environments v2 Model
Notes with Correct Answers.
Latest Updated 2026/2027
Downloaded by charles nguyo ()
,Topic 6 Organisms respond to changes in their
internal & external environments v2 Model
Notes with Correct Answers.
Latest Updated 2026/2027
AQA A Level Biology
Topic 6
Organisms respond to
changes in their internal &
external environments
Model answer notes by @biologywitholivia
Topic Sub-topic Understand Memorise Practise
6.1 Stimuli, both 6.1.1 Survival and response
internal and external,
are detected and Required practical 10
lead to a response
6.1.2 Receptors
6.1.3 Control of heart rate
6.2 Nervous 6.2.1 Nerve impulses
coordination
6.2.2 Synaptic transmission
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
Downloaded by charles nguyo ()
Required practical 11
,Topic 6 Organisms respond to changes in their
internal & external environments v2 Model
Notes with Correct Answers.
Latest Updated 2026/2027
Downloaded by charles nguyo ()
, Topic 6 Organisms respond to changes in their internal &
Topic 6 Organisms respond to changes in their internal & external environments stan.store/biologywitholivia
external environments v2 Model Notes with Correct
Latest Updated 2026/202
Answers.
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
Downloaded by charles nguyo ()
