ORGANISMS RESPOND TO CHANGES IN THEIR Key Terms
INTERNAL & EXTERNAL ENVIRONMENTS
3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Stimulus
Tropism
Responses for survival MISSESTRUCH 2021
Indoleacetic acid (IAA)
A stimulus is a detectable change in the environment. These
Elongation
changes can be detected by cells, which are called receptors.
Phototropism
Organisms increase their chance of survival by responding to
stimuli via different response mechanisms.
Responses in flowering plants
Tropism describes the response of plants to stimuli via growth. Tropisms
can be positive (growing towards a stimulus) or negative (growing away
from a stimulus). Plants respond to light, gravity and water. Tropisms are
controlled by specific growth factors and one key example is indoleacetic
acid (IAA).
IAA is a type of auxin and can control cell elongation in shoots and inhibit
the growth of cells in the roots. It is made in the tip of the roots and shoots
but can diffuse to other cells.
Phototropisms
Shoots need light for the LDR in photosynthesis which is
why plants grow and then bend towards the light. This is
controlled by IAA. This is positive phototropism. Shoot tip
cells produce IAA, which causes cell elongation in shoots,
and this IAA diffuses to other cells. If there is unilateral light,
the IAA will diffuse towards the shaded side of the shoot resulting in a higher concentration of IAA
there. The IAA causes the cells on the shaded side to elongate more and this causes the plant to
bend towards the light source.
Roots do not require light as they do not photosynthesis, and are more able to anchor the plant if they
are deep in the soil away from light. In roots, a high concentration of IAA inhibits cell elongation,
causing roots cells to elongate more on the lighter side and so the root bends away from light.
This is negative phototropism.
1t
MISSESTRUCH 2021
,ORGANISMS RESPOND TO CHANGES IN THEIR Key Terms
INTERNAL & EXTERNAL ENVIRONMENTS
3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Gravitropism
Taxes
Gravitropism MISSESTRUCH 2021
Kinesis
In the shoots, IAA will diffuse from the upper side to the lower Phototaxis
side of a shoot in response to gravity. If a plant is vertical, Chemotaxis
this causes the plant cells to elongate and the plant grows
upwards. If a plant is on its side, it will cause the shoot to bend upwards. This is negative
gravitropism.
In roots, the IAA moves to the lower side of roots in response to gravity so that the upper side
elongates more and the root bends down towards gravity and anchors the plant in. This is positive
gravitropism.
Taxes and kinesis
These are two simple responses that keep organisms within the favourable
conditions of their environment (light, moisture, chemicals).
Taxes
This is a simple response in which an organism will move its entire body
towards a favourable stimulus or away from an unfavourable stimulus.
When an organism moves towards a stimulus this is known as positive taxis, and when an organism
moves away it is described as negative taxis. Earthworms will show negative phototaxis, meaning
they move away from light. They will move towards dark environments, such as in the soil, to help
them avoid dehydration, predators and to locate food. Bacteria can show positive chemotaxis, as
they move towards certain chemicals to aid survival.
Kinesis
This is when an organism changes its speed of movement and its rate it changes direction.
If an organism moves from an area where there are beneficial stimuli to an area with harmful stimuli,
its response will be to increase the rate it changes direction to return to the favourable conditions
quickly. If it is surrounded by negative stimuli, the rate of turning decreases to keep it moving in a
relatively straight line to increase the chances of it finding a new location with favourable conditions.
Woodlice respond to water by inhabiting damp areas to prevent excessive water loss across their
surface. If a woodlouse crossed a border from a damp to a dry area, the response would be to turn
rapidly to increase the probability that it will end up back in the damp area. If the woodlouse was in a
completely dry area, the turning rate would decrease so that it would move in a straight line to
increase the chances of finding a new damp area.
2 MISSESTRUCH 2021
,ORGANISMS RESPOND TO CHANGES IN THEIR Key Terms
INTERNAL & EXTERNAL ENVIRONMENTS
3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Central nervous system
Neurones
MISSESTRUCH 2021
Simple reflex
Pacinian corpuscle
More complex organisms have a nervous system. A Pressure
receptor detecting a stimulus triggers the following response: Channel proteins
Stimulus --> Receptor -->Coordinator-->Effector-->Response
The nervous system is made up of the peripheral
and central nervous system. The PNS includes
the receptors, sensory and motor neurones,
whilst the CNS is the coordination centres such
as the brain and spine.
Receptors
Each receptor responds only to specific stimuli and this
stimulation of a receptor leads to the establishment of a
generator potential which can cause a response.
Three receptors you must know are:
1. Pacinian corpuscle
2. Rods
3. Cones
The Pacinian corpuscle
This receptor responds to pressure changes. These receptors
occur deep in the skin, mainly in fingers and feet. It consists of a
single sensory neurone wrapped with layers of tissue separated
by gel. The sensory neurone in the Pacinian corpuscle has
special channel proteins in its plasma membrane.
3 MISSESTRUCH 2021
,ORGANISMS RESPOND TO CHANGES IN THEIR Key Terms
INTERNAL & EXTERNAL ENVIRONMENTS
3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Stretch-mediated sodium channel
Generator potential
The Pacinian corpuscle MISSESTRUCH 2021
Photoreceptors
The plasma membranes contain channel proteins that allow ion
Rods
transportation and the membranes surrounding the sensory
Cones
neurones have stretch-mediated sodium channels. These
channels will open and allow sodium ions to diffuse into the
sensory neurone only when they are stretched and deformed.
In the resting state, sodium ion channels are too narrow for
sodium ions to diffuse into the sensory neurone. Therefore, the
resting potential is maintained. When pressure is applied, the
stretch-mediated sodium ion channels are deformed and
widen. This allows sodium ions to diffuse in which leads to the
establishment of a generator potential.
The human retina
The retina contains two types of photoreceptors: rods and cones.
Rods, so-called because of their shape, cannot distinguish
between different wavelengths of light and process images in black
and white. Rods can, however, detect light of very low intensity
though because many rod cells connect to one sensory neurone
(retinal convergence).
To create the generator potential, the pigment of
rod cells (rhodopsin) must be broken down by
light energy. There is enough energy from low-
intensity light to cause the breakdown. Enough
pigment has to be broken down for the threshold
to be met in the bipolar cell. This threshold can be
reached even in low light because so many rod
cells are connected to a single bipolar cell, this is
an example of spatial summation.
4 MISSESTRUCH 2021
, ORGANISMS RESPOND TO CHANGES IN THEIR Key Terms
INTERNAL & EXTERNAL ENVIRONMENTS
3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Visual acuity
Summation
However, this retinal convergence means MISSESTRUCH 2021
that the brain cannot distinguish between the separate sources Iodopsin
of light that stimulated it. Two light sources close together Retinal convergence
cannot be seen as separate so rod cells give low (poor) visual Fovea
acuity.
Cone cells
There are three types of cone cells that contain different types of iodopsin pigment
(red, green and blue) which all absorb different wavelengths of light. Depending
on the proportion of each cone cell that is stimulated, we perceive coloured images.
Iodopsin is only broken down if there is high light intensity, so action potentials can only be
generated with enough light.
Only one cone cell connects to a bipolar cell. Therefore, no spatial summation occurs
and cones can only respond to high light intensity, which is why we can’t see colour when it is
dark. As each cone cell is connected to one bipolar cell, the brain can distinguish between the
separate sources of light that were detected. So, cone cells give high visual acuity.
The distribution of rods and cones
The distribution in the retina is uneven. Light is focused by the
lens on the part of the retina opposite the pupil (the fovea)
which will receive the highest intensity of light.
Therefore, most cone cells are
located near the fovea as they
only respond to high light
intensities and rod cells are
located further away as these
can respond at lower light
intensities.
5 MISSESTRUCH 2021
INTERNAL & EXTERNAL ENVIRONMENTS
3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Stimulus
Tropism
Responses for survival MISSESTRUCH 2021
Indoleacetic acid (IAA)
A stimulus is a detectable change in the environment. These
Elongation
changes can be detected by cells, which are called receptors.
Phototropism
Organisms increase their chance of survival by responding to
stimuli via different response mechanisms.
Responses in flowering plants
Tropism describes the response of plants to stimuli via growth. Tropisms
can be positive (growing towards a stimulus) or negative (growing away
from a stimulus). Plants respond to light, gravity and water. Tropisms are
controlled by specific growth factors and one key example is indoleacetic
acid (IAA).
IAA is a type of auxin and can control cell elongation in shoots and inhibit
the growth of cells in the roots. It is made in the tip of the roots and shoots
but can diffuse to other cells.
Phototropisms
Shoots need light for the LDR in photosynthesis which is
why plants grow and then bend towards the light. This is
controlled by IAA. This is positive phototropism. Shoot tip
cells produce IAA, which causes cell elongation in shoots,
and this IAA diffuses to other cells. If there is unilateral light,
the IAA will diffuse towards the shaded side of the shoot resulting in a higher concentration of IAA
there. The IAA causes the cells on the shaded side to elongate more and this causes the plant to
bend towards the light source.
Roots do not require light as they do not photosynthesis, and are more able to anchor the plant if they
are deep in the soil away from light. In roots, a high concentration of IAA inhibits cell elongation,
causing roots cells to elongate more on the lighter side and so the root bends away from light.
This is negative phototropism.
1t
MISSESTRUCH 2021
,ORGANISMS RESPOND TO CHANGES IN THEIR Key Terms
INTERNAL & EXTERNAL ENVIRONMENTS
3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Gravitropism
Taxes
Gravitropism MISSESTRUCH 2021
Kinesis
In the shoots, IAA will diffuse from the upper side to the lower Phototaxis
side of a shoot in response to gravity. If a plant is vertical, Chemotaxis
this causes the plant cells to elongate and the plant grows
upwards. If a plant is on its side, it will cause the shoot to bend upwards. This is negative
gravitropism.
In roots, the IAA moves to the lower side of roots in response to gravity so that the upper side
elongates more and the root bends down towards gravity and anchors the plant in. This is positive
gravitropism.
Taxes and kinesis
These are two simple responses that keep organisms within the favourable
conditions of their environment (light, moisture, chemicals).
Taxes
This is a simple response in which an organism will move its entire body
towards a favourable stimulus or away from an unfavourable stimulus.
When an organism moves towards a stimulus this is known as positive taxis, and when an organism
moves away it is described as negative taxis. Earthworms will show negative phototaxis, meaning
they move away from light. They will move towards dark environments, such as in the soil, to help
them avoid dehydration, predators and to locate food. Bacteria can show positive chemotaxis, as
they move towards certain chemicals to aid survival.
Kinesis
This is when an organism changes its speed of movement and its rate it changes direction.
If an organism moves from an area where there are beneficial stimuli to an area with harmful stimuli,
its response will be to increase the rate it changes direction to return to the favourable conditions
quickly. If it is surrounded by negative stimuli, the rate of turning decreases to keep it moving in a
relatively straight line to increase the chances of it finding a new location with favourable conditions.
Woodlice respond to water by inhabiting damp areas to prevent excessive water loss across their
surface. If a woodlouse crossed a border from a damp to a dry area, the response would be to turn
rapidly to increase the probability that it will end up back in the damp area. If the woodlouse was in a
completely dry area, the turning rate would decrease so that it would move in a straight line to
increase the chances of finding a new damp area.
2 MISSESTRUCH 2021
,ORGANISMS RESPOND TO CHANGES IN THEIR Key Terms
INTERNAL & EXTERNAL ENVIRONMENTS
3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Central nervous system
Neurones
MISSESTRUCH 2021
Simple reflex
Pacinian corpuscle
More complex organisms have a nervous system. A Pressure
receptor detecting a stimulus triggers the following response: Channel proteins
Stimulus --> Receptor -->Coordinator-->Effector-->Response
The nervous system is made up of the peripheral
and central nervous system. The PNS includes
the receptors, sensory and motor neurones,
whilst the CNS is the coordination centres such
as the brain and spine.
Receptors
Each receptor responds only to specific stimuli and this
stimulation of a receptor leads to the establishment of a
generator potential which can cause a response.
Three receptors you must know are:
1. Pacinian corpuscle
2. Rods
3. Cones
The Pacinian corpuscle
This receptor responds to pressure changes. These receptors
occur deep in the skin, mainly in fingers and feet. It consists of a
single sensory neurone wrapped with layers of tissue separated
by gel. The sensory neurone in the Pacinian corpuscle has
special channel proteins in its plasma membrane.
3 MISSESTRUCH 2021
,ORGANISMS RESPOND TO CHANGES IN THEIR Key Terms
INTERNAL & EXTERNAL ENVIRONMENTS
3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Stretch-mediated sodium channel
Generator potential
The Pacinian corpuscle MISSESTRUCH 2021
Photoreceptors
The plasma membranes contain channel proteins that allow ion
Rods
transportation and the membranes surrounding the sensory
Cones
neurones have stretch-mediated sodium channels. These
channels will open and allow sodium ions to diffuse into the
sensory neurone only when they are stretched and deformed.
In the resting state, sodium ion channels are too narrow for
sodium ions to diffuse into the sensory neurone. Therefore, the
resting potential is maintained. When pressure is applied, the
stretch-mediated sodium ion channels are deformed and
widen. This allows sodium ions to diffuse in which leads to the
establishment of a generator potential.
The human retina
The retina contains two types of photoreceptors: rods and cones.
Rods, so-called because of their shape, cannot distinguish
between different wavelengths of light and process images in black
and white. Rods can, however, detect light of very low intensity
though because many rod cells connect to one sensory neurone
(retinal convergence).
To create the generator potential, the pigment of
rod cells (rhodopsin) must be broken down by
light energy. There is enough energy from low-
intensity light to cause the breakdown. Enough
pigment has to be broken down for the threshold
to be met in the bipolar cell. This threshold can be
reached even in low light because so many rod
cells are connected to a single bipolar cell, this is
an example of spatial summation.
4 MISSESTRUCH 2021
, ORGANISMS RESPOND TO CHANGES IN THEIR Key Terms
INTERNAL & EXTERNAL ENVIRONMENTS
3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Visual acuity
Summation
However, this retinal convergence means MISSESTRUCH 2021
that the brain cannot distinguish between the separate sources Iodopsin
of light that stimulated it. Two light sources close together Retinal convergence
cannot be seen as separate so rod cells give low (poor) visual Fovea
acuity.
Cone cells
There are three types of cone cells that contain different types of iodopsin pigment
(red, green and blue) which all absorb different wavelengths of light. Depending
on the proportion of each cone cell that is stimulated, we perceive coloured images.
Iodopsin is only broken down if there is high light intensity, so action potentials can only be
generated with enough light.
Only one cone cell connects to a bipolar cell. Therefore, no spatial summation occurs
and cones can only respond to high light intensity, which is why we can’t see colour when it is
dark. As each cone cell is connected to one bipolar cell, the brain can distinguish between the
separate sources of light that were detected. So, cone cells give high visual acuity.
The distribution of rods and cones
The distribution in the retina is uneven. Light is focused by the
lens on the part of the retina opposite the pupil (the fovea)
which will receive the highest intensity of light.
Therefore, most cone cells are
located near the fovea as they
only respond to high light
intensities and rod cells are
located further away as these
can respond at lower light
intensities.
5 MISSESTRUCH 2021
