Unit 6 - Organisms respond to changes in their internal and external environments
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AQA A Level Biology A* Essay Example - Nervous Coordination
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Unit 6 - Organisms respond to changes in their internal and external environments
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AQA
A* level aqa biology essay entitled: Importance of responses to changes in internal and external environments of an organism. Detailed explanations about multiple topics - including those studied in year 1 of the course. Essay includes sections on: homeostasis, tropisms, taxes and kinesis, controll...
Unit 6 - Organisms respond to changes in their internal and external environments
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Importance of responses to changes in the internal and external environment of an organism
a. Nervous coordination
b. Controlling heart rate
c. Tropisms
d. Homeostasis
e. Taxes and kinesis
Homeostasis is the maintenance of a constant internal environment via physiological control systems.
Homeostasis controls factors such as temperature, blood glucose and blood water potential. The
body’s internal temperature is kept constant at 37ºc meaning that it is ideal for most metabolic
processes to occur. Maintaining an optimum temperature is important as if internal temperature gets
too low, there is less kinetic energy so there are fewer collisions between particles and as a result,
less enzyme-substrate complexes are formed. If temperature is too high, enzymes denature and the
active site changes shape, making it no longer complementary to the substrate. Homeostasis ensures
that changes in the internal environment are detected and restorative systems are put in place in
different processes such as negative feedback. Homeostasis is important as it maintains the correct
internal conditions and that everything works at the rate that it is supposed to. Homeostasis also
regulates blood glucose levels - this is important as extra glucose creates the tissue fluid surrounding
cells more concentrated. Blood water potential is negative so water moved out of cells by osmosis.
Restorative systems as a result of homeostasis detects the changes and ensures that cells do not
shrivel or burst.
Phototropism is a plant response controlled by auxins and growth factors such as indoleacetic acid
(IAA). IAA is a type of auxin that causes cell elongation in shoot tips and inhibits cell elongation in the
roots. Plants require light for the light dependent stage of photosynthesis, so diffusion of auxins
across the shoot tips or roots enable the plant to absorb as much light as possible, meaning that
enough energy and oxygen to respire is produced. Positive phototropism occurs in shoots, and
negative phototropism occurs in roots as they do not require light as they don’t photosynthesise.
Gravitropism is a plant response due to gravity. If a plant is vertical, IAA diffused from the upper side
to the lower side of shoot, causing the cells to enlongate and the plant grows upwards - this is
negative gravitropism. Positive gravitropism occurs when IAA diffuses towards the lower side of
roots, causing the cells to elongate downwards to anchor the plant into the ground. Tropisms are
important because if the plants are unable to respond to their environment, they may not receive
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