In-depth summary for content covered in the Homeostasis topic of A-level AQA Biology. This will still be applicable to other exam boards, but take caution when looking at key-words and the order of specific processes.
Homeostasis is the maintenance of an internal environment within restricted limits
-> to ensure they don’t stray too far from optimal
Ensuring health of cells, tissues and organs in an organism
And helping with their function
The most common factors controlled are temperature, pH, water potential and blood glucose
-> these all are important when it comes to metabolism
Enzymes are a significant reason for homeostasis, since they only work in a very specific
range of conditions – and therefore rely on these being maintained to function
Water potential changes can also cause severe damage to cells and tissues, causing them to
shrink, or burst via lysis depending on what way the change goes
Water potential also changes blood pressure – which can have some nasty consequences
Control mechanisms:
Control of conditions relies on a series of stages:
An optimum point – this is where a system operates best
This is monitored by a receptor
This leads to changes (stimuli) being detected, leading to impulses which are linked to coordinators
and effectors
Coordinator – brain / spinal cord
Effector – muscle or gland which causes the response
Feedback mechanisms are also essential to homeostasis, and involve this series of events leading to
a specific response – this can be either negative or positive
Figure 1- reproduced from [1]
Negative feedback – the change produced by a control system leads to a change in the
stimulus, turning the control response off and returning conditions to optimum
Positive feedback – the system continues to increase the deviation from ‘optimum’ – which
is usually harmful
However, in processes such as sodium influx in neurones – this is essential for function
, Oliver Dyson
Ectotherms and Endotherms:
There are different types of organisms when it comes to maintenance of temperature
-> endotherms and ectotherms:
Ectotherms:
These animals gain heat from the environment
-> and so their body temperature is dependent on this
Reptiles such as lizards are example of this
They commonly expose themselves to sunlight in order to gain heat
Or take shelter to prevent overheating
Endotherms:
These organism gain their heat form internal metabolic activities
-> meaning internal temperature is held constant for most of the time
Apart from when extreme environmental conditions act
They have a wide range of mechanisms to control body Figure 2 - reproduced from [1]
temperature
Vasoconstriction – increases the diameter of the blood vessels near the skin, increasing heat loss
-> shivering causes the muscles to contract and produce metabolic heat
Raising of hair traps a layer of insulating air – preventing heat loss
Increased metabolic rate leads to more respiration and heat
Sweating can be used to control heat – as evaporating water causes energy losses from the
body
Behavioural mechanisms are also used by these animals – but it isn’t the main way temperature is
controlled
Hormones:
Hormones all vary in structure, but share a range of features
-> mostly in the way they function
They are produced in endocrine glands, which secrete the hormone directly into the blood
They are carried in the blood plasma to cells they act on
This are called ‘target cells / tissues / organs’
They take longer to cause a response than nerve impulses
But have longer lasting effects
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