First 534 words of the document:
Organisms respond to changes in their internal and external
environments (AQA A2 Biology) PART 1 of 9 TOPICS
Survival and response:
Organisms increase their chance of survival by responding to changes in their environment.
In flowering plants, specific growth factors move from growing regions to other tissues, where they
regulate growth in response to directional stimuli e.g. light and gravity.
Phototropism is where shoots and roots grow in response to light. This is because of a plant hormone
(auxin) called indoleacetic acid which is produced in the meristem of shoots (NB: Indoleacetic acid
can be abbreviated into IAA and the meristem is the tip of shoots. This is the only plant auxin that
you need to know for AQA). IAA has different effects in shoots and roots:
Shoots: IAA diffuses to the side of the shoot behind the meristem (in cell elongation) that is
shaded. This promotes growth and the cells elongate causing the shoot to bend towards light.
This is called positive phototropism.
Roots: IAA diffuses to the side of the root that is shaded. This inhibits growth making the root
bend away from light. This is called negative phototropism.
Geotropism/Gravitropism is where shoots and roots grow in response to gravity. This is also because
of an auxin called IAA. IAA also has different effects in shoots and roots:
Shoots: IAA diffuses to the side of the shoot that is towards gravity behind the meristem (in
cell elongation). This promotes growth making the shoot grow away from gravity. This is called
Roots: IAA diffuses to the side of the root that is towards gravity. This inhibits growth making
the root bend towards gravity. This is called positive geotropism/gravitropism.
Taxis is a directional response in movement because of a stimuli. Positive taxis is a directional
movement towards a stimuli with negative taxis being a directional movement away from stimuli. NB:
The different types of taxis do no need to be known but must be familiarised with. This includes
chemotaxis - chemically generated taxis, phototaxis - light-sensitive taxis, geotaxis - taxis in
response to gravity and rheotaxis - taxis in response to movement.
Kinesis is a non-directional response in movement because of stimuli. There is no positive or negative
kinesis but is random in order to increase the survival chances of an organism. If an organism is in an
environment where it is increasing its survival chances then it would turn slowly with many turns to
keep in the same spot. If it is in danger then it would turn faster with less turns to exit.
Reflex arc is a process to create a protective effect. It starts with a stimulus which is picked by the
receptors in the skin. A nerve impulse is carried along the sensory neurone to the spinal cord. It then
travels through the relay/intermediate neurone which connects the sensory neurone to the motor
neurone. The nerve impulse then travels along the motor neurone which connects the spinal cord to