Unit 6 - Organisms respond to changes in their internal and external environments
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Summary homeostasis
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Unit 6 - Organisms respond to changes in their internal and external environments
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AQA
Summary notes for AQA A-level Biology homeostasis topic. Includes clear information on homeostasis, negative/positive feedback, control of blood glucose, diabetes, kidney structure, ultrafiltration, osmoregulation, ADH. Summarised from class notes and the official course textbook. From an A* studen...
Unit 6 - Organisms respond to changes in their internal and external environments
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Homeostasis
Principles of homeostasis
• Homeostasis is the physiological control systems that maintain the Factors that are controlled by
internal environment within restricted limits. homeostasis:
• The internal environment is made up of tissue fluids that bathe cells, • Glucose
supplying nutrients and removing waste. • Inorganic ions
• It ensures the cells can function despite changes in the external • Osmoregulation
environment • Thermoregulation
• There are still continuous fluctuations around an optimum point e.g. • Blood pressure
in temperature and pH • pH
• Waste removal
Importance
Temperature and pH
• Enzymes/proteins that control biochemical reactions are sensitive to changes in pH and
temperature. Fluctuations in these factors may reduce the rate of reactions or prevent them
from working
• Transport and channel proteins are also effected
• Key terms: tertiary bonds, active site changes shape, no longer complementary to substrate, no
ES complexes
Equilibrium
• Many reactions are reversible and a finely balanced equilibrium is maintained. Any factors
altering this balance will alter the reactions
Osmosis
• Changes to the water potential of the blood and tissue fluid may cause cells to shrink/expand
through osmosis.
• Isotonic solutions are ideal in the body
Blood glucose
• A constant blood glucose concentration is important to maintain a constant water potential and
it also ensures reliable source of glucose for respiration
• Vital for the glycolysis stage of aerobic respiration, where glucose is split to release 38 ATP
which is then used for many processes e.g. contraction of spindle fibres in anaphase of mitosis
Independence from the external environment
• Organisms that can maintain a constant internal environment have a wider geographical range
and are more independent of external environment which is beneficial
• Independence from constantly fluctuating external environments is useful
Control mechanisms
• The optimum point: the point at which the system operates best.
• Receptor: detects any deviation from the optimum point (ie., a stimulus)
• Coordinator: coordinates information from receptors and sends instructions to an effector
• Effector: often a muscle or gland, which brings about the changes needed to return the system
to the optimum point.
, 1. Stimulus
2. Detected by a receptor
3. Information sent via nervous or endocrine system
4. Control centre - co-ordinator sends instructions
5. Effector brings about changes
Feedback mechanisms
Negative feedback
Negative feedback is when a deviation from normal is detected by a receptor, which causes a
correcting response from an effector. Once the normal is re-established, the system is turned off.
This is important because if information is not fed back to the receptor then stimulation of the
effector will continue, causing over correction
Example: blood glucose
• Fall in concentration detected by pancreatic cells (alpha cells)
• These cells secrete glucagon which causes liver cells to convert glycogen to glucose which is
released in the blood
• Reduced stimulation of alpha cells means they secrete less glucagon and so liver cells stop
converting glycogen into glucose
• If blood glucose concentration rises, insulin is produced from beta cells in the pancreas and this
increases the uptake of glucose by cells
Positive feedback
• Positive feedback involves a deviation from the set/normal level
with the response produced causing further deviations in the same
direction.
• Often harmful as it causes unstable conditions
• Can be useful in some situations
• Deviation intensifies until it finally removes the stimulus
In an exam, say … causes more release of … which causes more
release of …
Examples:
Childbirth • Baby head pushes against cervix
• Nerve impulses transmitted to the brain
• Oxytocin released which causes contractions
• Baby’s head pushes baby towards cervix which releases more oxytocin
Hyperthermia/ • Thermoregulatory mechanisms break down
hypothermia • Above 41 or below 35
Nerves • A few sodium ions move into the neuron
• This causes a large influx of sodium ions which forms an action potential
Menstruation • Oestrogen is released and causes the production of more oestrogen
Blood clotting • Damage in blood vessel causes platelets to aggregate
• Activated platelets release platelet activating factors which attract more platelets
Positive feedback Negative feedback
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