These are my notes for the sixth unit of alevel biology - organisms respond to changes in their internal and external environment!
I got an A in my alevel AQA biology exam using these notes.
Happy revising :)
,Stimulus
• STIMULUS = a change in the internal/external environment of an
organism that leads to a response in the organism
,Taxes
• TAXES = the direction of the organism is determined by the directio
of the stimulus
• An organism will move its whole body towards a favourable stimulu
(positive taxis) or away from an unfavourable stimulus (negative
taxis)
,Examples of taxis:
• Algae move towards light (positive phototaxis) which increases their
chance of survival as they need light for photosynthesis to produce fo
• Earthworms move away from light (negative phototaxis) which increa
their chance of survival as this takes them into soil where they are abl
conserve water, find food and avoid predators
• Bacteria will move towards a region of high glucose concentration
(positive chemotaxis) which increases their chance of survival becaus
glucose is a source of food
• Bacteria will move away from waste products they produce (negative
chemotaxis)
,Kinesis
• KINESIS = the organism changes the speed it moves and the rate it
changes direction
• If an organism crosses a dividing line between a favourable and an
unfavourable environment, its rate of turning increases. This increa
its chance of returning quickly to a favourable environment
• If an organism moves a considerable distance into an unfavourable
environment, its rate of turning decreases so it moves in long straig
lines to reach a favourable environment
,Example of kinesis:
• Woodlice lose water in their bodies in dry condition so they move
quickly and change direction more in dry conditions. This increases
their chance of moving back into a damp area. Once they are in a
damp area, they slow down and change direction less so they are
more likely to stay within the damp area
,Plant growth
factors
,Tropism
• TROPISM - A tropism is the directional growth of a plant in respons
to a directional stimulus
, Plants respond to…
• LIGHT – shoots grow towards light because its needed for photosynthe
(positive phototropism)
• They can also grow away from the light (negative phototropism)
• GRAVITY – plants need to be anchored into the soil for stability and to
absorb water and nutrients so the roots grow in the direction of gravity
(positive gravitropism)
• They can also grow against the direction of gravity (negative gravitropis
• WATER – plant roots grow towards water because its needed for
photosynthesis and other metabolic processes (positive hydrotropic)
• They can also grow away from water (negative hydrotropic)
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