TOPIC 9
9.1 Homeostasis
i Know that homeostasis is the maintenance of a state of dynamic equilibrium.
ii Understand the importance of maintaining pH, temperature and water potential in the body.
Temperature and pH are controlled to maintain optimum enzyme activity and cell membrane
integrity. Water potential is controlled to avoid negative osmotic effects which could damage a cell
iii Understand what is meant by negative feedback and positive feedback control.
Negative feedback – counteracts any change in internal conditions by reversing changes to restore
optimum conditions
Positive feedback – acts in the same direction as the original disturbance and reinforces the original
stimulus (e.g. dilation of cervix during childbirth/blood clotting/urination)
9.2 Chemical control in mammals
i Understand the principles of mammalian hormone production by endocrine glands and their mode
of action involving receptors on target cells.
Hormones – signalling proteins secreted by endocrine glands directly into the bloodstream (only
target organs and cells with complementary receptors on plasma membrane, therefore specific)
ii Know that there are two main modes of action in hormones:
hormones attach to receptor sites and trigger the release of a second messenger that
activates specific enzymes in the cell, including adrenaline
hormones enter cells and bind directly to transcription factors, including oestrogen.
There are two modes of hormone action
1. Hormones (e.g. adrenaline) bind to a receptor on a target cell membrane. This triggers a
serious of intracellular membrane-bound reactions which stimulates the release of a second
messenger e.g. cAMP. The second messenger activates enzymes to alter the metabolism of
the cell e.g. cAMP increases cellular respiration, contraction of muscle cells and relaxation of
smooth muscle etc.
2. Hormones (e.g. oestrogen) pass through the cell membrane and bind to a receptor inside
the cell. They form a hormone-receptor complex which passes into the nucleus and acts as a
transcription factor to regulate gene expression
9.3 Chemical control in plants
i Understand that chemical control in plants is brought about by plant growth substances such as
auxins, cytokinins and gibberellins.
Auxin
Growth stimulants
, Maintain apical dominance and suppress the growth of lateral buds
Promote root growth
Promote trophic responses to unilateral light (directional growth responses e.g.
phototrophism, geotropism)
Functions include rooting powder and weed killers
Increase the plasticity of cell walls by increased turgor pressure
Cytokinins
Stimulate cell division by attaching to receptor sites on cell
surface membranes and triggering the formation of
transcription factors inside the cell
Work synergistically with ethene to promote abscission of
leaves
Adenine-based compound
Produced in meristematic tissues of roots and travels through
xylem
Promotes cell division
How it works
- Binds to a receptor
- Induces signal transduction inside a cell via second
messenger
- A protein enters the nucleus and activates a
transcription factor
- Gene expression changes
2. Binding of cytokinin to its receptor causes a change in shape
that exposes the active site for the protein kinase domain of
the receptor
3. An intermediate protein is phosphorylated and enters the nucleus
4. The intermediate protein phosphorylates a transcription factor
5. The phosphorylated transcription factor binds to promotors of more than 20 cytokinin
response genes, initiating their transcription
Gibberellins
Control of internode length and seed development (the distance along a plant stem between
one side branch and the next)
stimulate growth of fruit
stimulate germination
stimulate ‘bolting’ – rapid growth and/or flowering
To stimulate germination:
1. The seed absorbs water, which activates the embryo.
2. The activated embryo secretes gibberellins, which diffuse to the aleurone layer (outmost
layer of the endosperm).
3. The aleurone layer produces amylase, which diffuses to the endosperm layer and breaks
down starch into glucose.
9.1 Homeostasis
i Know that homeostasis is the maintenance of a state of dynamic equilibrium.
ii Understand the importance of maintaining pH, temperature and water potential in the body.
Temperature and pH are controlled to maintain optimum enzyme activity and cell membrane
integrity. Water potential is controlled to avoid negative osmotic effects which could damage a cell
iii Understand what is meant by negative feedback and positive feedback control.
Negative feedback – counteracts any change in internal conditions by reversing changes to restore
optimum conditions
Positive feedback – acts in the same direction as the original disturbance and reinforces the original
stimulus (e.g. dilation of cervix during childbirth/blood clotting/urination)
9.2 Chemical control in mammals
i Understand the principles of mammalian hormone production by endocrine glands and their mode
of action involving receptors on target cells.
Hormones – signalling proteins secreted by endocrine glands directly into the bloodstream (only
target organs and cells with complementary receptors on plasma membrane, therefore specific)
ii Know that there are two main modes of action in hormones:
hormones attach to receptor sites and trigger the release of a second messenger that
activates specific enzymes in the cell, including adrenaline
hormones enter cells and bind directly to transcription factors, including oestrogen.
There are two modes of hormone action
1. Hormones (e.g. adrenaline) bind to a receptor on a target cell membrane. This triggers a
serious of intracellular membrane-bound reactions which stimulates the release of a second
messenger e.g. cAMP. The second messenger activates enzymes to alter the metabolism of
the cell e.g. cAMP increases cellular respiration, contraction of muscle cells and relaxation of
smooth muscle etc.
2. Hormones (e.g. oestrogen) pass through the cell membrane and bind to a receptor inside
the cell. They form a hormone-receptor complex which passes into the nucleus and acts as a
transcription factor to regulate gene expression
9.3 Chemical control in plants
i Understand that chemical control in plants is brought about by plant growth substances such as
auxins, cytokinins and gibberellins.
Auxin
Growth stimulants
, Maintain apical dominance and suppress the growth of lateral buds
Promote root growth
Promote trophic responses to unilateral light (directional growth responses e.g.
phototrophism, geotropism)
Functions include rooting powder and weed killers
Increase the plasticity of cell walls by increased turgor pressure
Cytokinins
Stimulate cell division by attaching to receptor sites on cell
surface membranes and triggering the formation of
transcription factors inside the cell
Work synergistically with ethene to promote abscission of
leaves
Adenine-based compound
Produced in meristematic tissues of roots and travels through
xylem
Promotes cell division
How it works
- Binds to a receptor
- Induces signal transduction inside a cell via second
messenger
- A protein enters the nucleus and activates a
transcription factor
- Gene expression changes
2. Binding of cytokinin to its receptor causes a change in shape
that exposes the active site for the protein kinase domain of
the receptor
3. An intermediate protein is phosphorylated and enters the nucleus
4. The intermediate protein phosphorylates a transcription factor
5. The phosphorylated transcription factor binds to promotors of more than 20 cytokinin
response genes, initiating their transcription
Gibberellins
Control of internode length and seed development (the distance along a plant stem between
one side branch and the next)
stimulate growth of fruit
stimulate germination
stimulate ‘bolting’ – rapid growth and/or flowering
To stimulate germination:
1. The seed absorbs water, which activates the embryo.
2. The activated embryo secretes gibberellins, which diffuse to the aleurone layer (outmost
layer of the endosperm).
3. The aleurone layer produces amylase, which diffuses to the endosperm layer and breaks
down starch into glucose.
