Revision notes for Biology ALevel OCRA Biology Module 5 (5.3) Excretion+homeostasis
OCRA Biology ALevel and OCRA Biology AS Level.
H020 and H420.
OCR, OCRA Specification. Breadth and depth of biology.
Featured in Paper 1,2,3.
Chapter 15 – Homeostasis
15.1 – The Principles of Homeostasis
Homeostasis is the maintenance of a constant internal environment/dynamic equilibrium within a
narrow range.
To carry out homeostasis, the body needs:
➢ A receptor to detect changes.
➢ A communication system (nervous
system or hormonal system) so
transfers the information from
receptors to effectors.
➢ Effectors to carry out the response.
Why is homeostasis so important:
Temperature as an example
• If body temperature gets too low, enzymes and substrates have less kinetic energy meaning
they move slower.
• Fewer enzyme-substrate complexes will form. So the rate of metabolic reactions decreases
• If body temperature gets too high, the increase in temperature will break some of the bonds
holding together the tertiary structure of enzymes.
• Active sites will change shape and will no longer be complementary to the substrate shape.
Metabolic reactions would stop.
Negative feedback
• A negative feedback mechanism is one where the receptors detect a change and the
effectors respond to counteract the change, restoring levels to normal
• Negative feedback keeps conditions constant in the body
• E.g. Control of blood sugar levels by insulin and glucagon
• Temperature and water balance control
Positive feedback
• In positive feedback, receptors detect a change and then effectors respond by further
increasing the level away from the normal
• Positive feedback isn’t involved in homeostasis because it doesn’t keep your internal
environment constant. Positive feedback is useful to rapidly activate processes in the body
e.g.
• When a blood vessel is damaged, platelets stick to the region and release factors
that attract more platelets
• Head of a child presses against cervix in childbirth, causing oxytocin to be released,
causing uterus to contract, pushing head further, releasing more.
, Controlling body temperature
All animals are classed as either ectotherms (eg reptiles and fish) or endotherms (eg mammals and
birds) depending on how they control their body temperature
• Thermoregulation – the maintenance of a relatively constant core body temperature to
maintain optimum enzyme activity
• Organisms change temperature due to many reasons:
o Exothermic chemical reactions
o Latent heat of evaporation – objects cool down as water evaporates from surface
o Radiation – transmission of electromagnetic waves to and from the air, water or
ground
o Convection – heating and cooling by currents of air or water
o Conduction – heating because of collision of molecules e.g., from a rock
• Animals can be classed as endotherms or ectotherms depending on how they maintain and
control their body temperature
Ectotherms
• Ectotherms cannot control their body temperature internally.
• Instead they control their temperature by changing their behaviour.
• This means the internal temperature of ectotherms depends on the external temperature
(their surroundings).
• Because ectotherms cannot control their body temperature, they have a variable metabolic
rate – they are more active at higher temperatures and less active at lower temperatures.
• Ectotherms living in water do not need to thermoregulate.
• High heat capacity means temperature of their environment does not change much.
Examples of behavioural responses in ectotherms
Body temperature too low:
• Basking in the Sun.
• Orientate bodies so that the maximum surface area is exposed to the Sun.
• Pressing bodies against the warm ground.
• Increase movement of certain body parts (eg muscles in iguanas or wings in moths) to
increase cellular metabolism.
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