3.6.1.1 Survival and response
10 March 2022 08:52
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.
Tropism is when plants respond to a stimuli via growth.
Positive tropism is when the plant grows towards the stimulus and negative is when the plant grows
away from the stimulus.
Stimuli includes light, water, gravity.
The effect of different concentrations of indoleacetic acid (IAA) on cell elongation in the roots and
shoots of flowering plants as an explanation of gravitropism and phototropism in flowering plants.
Tropisms are controlled by growth factors. Indoleacetic acid (IAA) is a type of auxin (growth factor)
that controls cell elongation in shoots and inhibits growth in the roots.
It is made in the tip of the roots/shoots but can diffuse to other cells.
- Positive phototropism
Stimuli is light.
Light is needed by plants in photosynthesis. So plants grow towards the stimuli of light.
IAA is produced in tip of shoot. If there is unilateral light, IAA will diffuse to the shaded side,
resulting in a higher concentration there. The IAA causes cells on the shaded side to elongate
more, causing the plant to bend and grow towards the light.
In roots, high concentration of IAA inhibits growth.
- Gravitropism
Stimuli is gravity.
IAA diffuses from tips of shoots downwards. If plant is vertical, plant cells elongate so plant
grows upwards.
In roots, IAA diffuses to the lower side of the roots. It causes inhibition of growth in the cells
on the lower side, so upper side elongates and roots bend down towards gravity (positive
gravitropism).
Taxes and kineses as simple responses that can maintain a mobile organism in a favourable
environment.
Taxes is a simple response in which an organism can move its entire body towards a favourable
stimulus (positive taxis) or away from an unfavourable stimulus (negative taxis).
Earthworms show negative phototaxis, moves away from light, to prevent
dehydration/predation/able to locate prey.
Bacteria show positive chemotaxis, as they move their body towards chemicals.
Kinesis is when organisms change the speed at which they move/rate at which it changes direction.
If an organism moves from an area of favourable stimuli to unfavourable stimuli, there is an increase
to the rate at which it changes direction, to increase the chance of returning to favourable
environment.
If an organism is surrounded by negative stimuli, rate of changing directions decreases to keep it
moving in a straight line to increases chances of it finding a new location.
Woodlice need to be in damp areas to prevent water loss. If it crosses a border from wet to dry area,
rate of kinesis increases to increase probability it returns to wet area. If it is completely surrounded
Organisms respond to changes in their internal and Page 1
,rate of kinesis increases to increase probability it returns to wet area. If it is completely surrounded
by dry area, rate of kinesis decreases so it can find new area of wet.
The protective effect of a simple reflex, exemplified by a three neurone simple reflex. Details of
spinal cord and dorsal and ventral roots are not required.
Organisms respond to changes in their internal and Page 2
, 3.6.1.2 Receptors
10 March 2022 08:53
The Pacinian corpuscle should be used as an example of a receptor to illustrate that:
• receptors respond only to specific stimuli
• stimulation of a receptor leads to the establishment of a generator potential.
Stimulus is a detectable change in the environment.
A stimulus is detected by receptors, that triggers a response.
Stimulus --> receptor --> coordinator --> effector --> response
The nervous system is made up of peripheral (receptors, motor/sensory neurones) and central
nervous system (brain and spinal cord - co-ordinators).
Receptors:
- Pacinian corpuscle
- Rods
- Cones
The basic structure of a Pacinian corpuscle.
Pacinian corpuscle responds to pressure changes. These receptors are found in skin (feet and
fingers).
Organisms respond to changes in their internal and Page 3
10 March 2022 08:52
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.
Tropism is when plants respond to a stimuli via growth.
Positive tropism is when the plant grows towards the stimulus and negative is when the plant grows
away from the stimulus.
Stimuli includes light, water, gravity.
The effect of different concentrations of indoleacetic acid (IAA) on cell elongation in the roots and
shoots of flowering plants as an explanation of gravitropism and phototropism in flowering plants.
Tropisms are controlled by growth factors. Indoleacetic acid (IAA) is a type of auxin (growth factor)
that controls cell elongation in shoots and inhibits growth in the roots.
It is made in the tip of the roots/shoots but can diffuse to other cells.
- Positive phototropism
Stimuli is light.
Light is needed by plants in photosynthesis. So plants grow towards the stimuli of light.
IAA is produced in tip of shoot. If there is unilateral light, IAA will diffuse to the shaded side,
resulting in a higher concentration there. The IAA causes cells on the shaded side to elongate
more, causing the plant to bend and grow towards the light.
In roots, high concentration of IAA inhibits growth.
- Gravitropism
Stimuli is gravity.
IAA diffuses from tips of shoots downwards. If plant is vertical, plant cells elongate so plant
grows upwards.
In roots, IAA diffuses to the lower side of the roots. It causes inhibition of growth in the cells
on the lower side, so upper side elongates and roots bend down towards gravity (positive
gravitropism).
Taxes and kineses as simple responses that can maintain a mobile organism in a favourable
environment.
Taxes is a simple response in which an organism can move its entire body towards a favourable
stimulus (positive taxis) or away from an unfavourable stimulus (negative taxis).
Earthworms show negative phototaxis, moves away from light, to prevent
dehydration/predation/able to locate prey.
Bacteria show positive chemotaxis, as they move their body towards chemicals.
Kinesis is when organisms change the speed at which they move/rate at which it changes direction.
If an organism moves from an area of favourable stimuli to unfavourable stimuli, there is an increase
to the rate at which it changes direction, to increase the chance of returning to favourable
environment.
If an organism is surrounded by negative stimuli, rate of changing directions decreases to keep it
moving in a straight line to increases chances of it finding a new location.
Woodlice need to be in damp areas to prevent water loss. If it crosses a border from wet to dry area,
rate of kinesis increases to increase probability it returns to wet area. If it is completely surrounded
Organisms respond to changes in their internal and Page 1
,rate of kinesis increases to increase probability it returns to wet area. If it is completely surrounded
by dry area, rate of kinesis decreases so it can find new area of wet.
The protective effect of a simple reflex, exemplified by a three neurone simple reflex. Details of
spinal cord and dorsal and ventral roots are not required.
Organisms respond to changes in their internal and Page 2
, 3.6.1.2 Receptors
10 March 2022 08:53
The Pacinian corpuscle should be used as an example of a receptor to illustrate that:
• receptors respond only to specific stimuli
• stimulation of a receptor leads to the establishment of a generator potential.
Stimulus is a detectable change in the environment.
A stimulus is detected by receptors, that triggers a response.
Stimulus --> receptor --> coordinator --> effector --> response
The nervous system is made up of peripheral (receptors, motor/sensory neurones) and central
nervous system (brain and spinal cord - co-ordinators).
Receptors:
- Pacinian corpuscle
- Rods
- Cones
The basic structure of a Pacinian corpuscle.
Pacinian corpuscle responds to pressure changes. These receptors are found in skin (feet and
fingers).
Organisms respond to changes in their internal and Page 3
