Simple Responses to Stimuli
All successful organisms must detect changes in external and external environment and respond
appropriately to increase chances of survival.
The basic model of response:
Stimulus Receptor Coordination Centre Effector Response
1. Taxis – This is a directional response of a whole organism towards/away from a stimulus.
The appropriate response will increase chances of survival. For example:
o Phototaxis – Movement in response to light
o Geotaxis/Gravitaxis – Movement in response to gravity
o Hydrotaxis - Movement in response to water
o Thermotaxis - Movement in response to temperature
o Chemotaxis - Movement in response to a chemical stimulus
2. Kinesis – This is a non-directional response of a whole organism to a stimulus. The Humid Dry
organism will slow down/turn more frequently in the preferred environment in
order to stay in this environment. For example:
o Woodlice prefer a humid environment so will slow down and turn more
frequently in this environment.
3. Tropisms – This is growth towards/away from a stimulus. Tropisms are found in
plants and fungi. For example:
o Phototropism – Growth towards a light source
o Thigmotropism – Growth in response to pressure
o Geotropism – Growth in response to pressure
Phototropism – The role of plant growth factors
Phototropism increases chances of survival as the plant is growing towards a light source. This
means that more light energy hits the leaf which means more protons hitting the photosystem II.
This leads to more products of photosynthesis so more energy for growth.
IAA (indolacetic acid) is responsible for phototropism in
plants.
In the shoot tip, there are cells that make and secrete
IAA. IAA diffuses down the plant stem. IAA stimulates cell
elongation in the shoot. This causes the shoot to grow
upwards towards the light.
However, in directional light IAA diffuses towards the
shaded side. This causes differential cell elongation. This
means that the cells on the shaded side grow longer
causing the plant to bend towards the light.
Evidence to support the role of IAA:
• When the tip is cut off the plant there is no response to light
• When the tip is covered up there is no response to light
• When movement is blocked down the illuminated side the same response is given
• When movement is blocked down the dark side there is no response to light
, Simple Reflex
Motor neuron – The motor neuron transmits electrical impulses from the CNS to the effectors.
Sensory neuron – The sensory neuron transmits electrical impulses from the receptors to the CNS.
Interneuron – The interneuron transmits electrical impulses between the sensory and motor
neurones.
Simple reflexes have evolved to increase chances of survival by responding to changes in the
external environment. The responses are very rapid because the body responds to a stimulus
without conscious decision. Because there is no time spent making this decision, the information
travels very fast from the receptors to the effectors. However if there is an interneuron present in
the reflex arc, it is possible to override the reflex.
Some examples of simple reflexes in humans are:
• The knee jerks when tapped in the right place
• Pupils get smaller in bright light
• Babies suck on anything placed on the cheek
, Nervous System Organisation
Nervous System
Central Nervous Peripheral Nervous
System System
Sensory Nervous Motor Nervous
System System
Voluntary Involuntary
Responses Responses
Sympathetic Parasympathetic
Nervous System Nervous System
• Central nervous system – This consists of the brain and spinal cord.
• Peripheral nervous system – This consists of all the nerves in the body other than in the
brain and spinal cord.
• Sensory nervous system – This is where information comes IN.
• Motor nervous system – This is where information comes OUT.
• Voluntary responses – Responses that we consciously control. For example skeletal muscles.
• Involuntary responses – Responses that we do not consciously control. For example smooth
muscles.
• Sympathetic nervous system – This is where things are sped up, for example heart rate
• Parasympathetic nervous system – This is where things are slowed down, for example hear
rate.
Functions of the nervous system:
• Sensory input – Detecting changes in the internal and external environment. This is when
information is carried into the CNS.
• Interpretation – Receiving information and processing the output. The CNS decides on an
appropriate response
• Motor output – Giving an appropriate response such as a muscle contraction or gland.
, Neuron Structure
All successful organisms must detect changes in external and external environment and respond
appropriately to increase chances of survival.
The basic model of response:
Stimulus Receptor Coordination Centre Effector Response
1. Taxis – This is a directional response of a whole organism towards/away from a stimulus.
The appropriate response will increase chances of survival. For example:
o Phototaxis – Movement in response to light
o Geotaxis/Gravitaxis – Movement in response to gravity
o Hydrotaxis - Movement in response to water
o Thermotaxis - Movement in response to temperature
o Chemotaxis - Movement in response to a chemical stimulus
2. Kinesis – This is a non-directional response of a whole organism to a stimulus. The Humid Dry
organism will slow down/turn more frequently in the preferred environment in
order to stay in this environment. For example:
o Woodlice prefer a humid environment so will slow down and turn more
frequently in this environment.
3. Tropisms – This is growth towards/away from a stimulus. Tropisms are found in
plants and fungi. For example:
o Phototropism – Growth towards a light source
o Thigmotropism – Growth in response to pressure
o Geotropism – Growth in response to pressure
Phototropism – The role of plant growth factors
Phototropism increases chances of survival as the plant is growing towards a light source. This
means that more light energy hits the leaf which means more protons hitting the photosystem II.
This leads to more products of photosynthesis so more energy for growth.
IAA (indolacetic acid) is responsible for phototropism in
plants.
In the shoot tip, there are cells that make and secrete
IAA. IAA diffuses down the plant stem. IAA stimulates cell
elongation in the shoot. This causes the shoot to grow
upwards towards the light.
However, in directional light IAA diffuses towards the
shaded side. This causes differential cell elongation. This
means that the cells on the shaded side grow longer
causing the plant to bend towards the light.
Evidence to support the role of IAA:
• When the tip is cut off the plant there is no response to light
• When the tip is covered up there is no response to light
• When movement is blocked down the illuminated side the same response is given
• When movement is blocked down the dark side there is no response to light
, Simple Reflex
Motor neuron – The motor neuron transmits electrical impulses from the CNS to the effectors.
Sensory neuron – The sensory neuron transmits electrical impulses from the receptors to the CNS.
Interneuron – The interneuron transmits electrical impulses between the sensory and motor
neurones.
Simple reflexes have evolved to increase chances of survival by responding to changes in the
external environment. The responses are very rapid because the body responds to a stimulus
without conscious decision. Because there is no time spent making this decision, the information
travels very fast from the receptors to the effectors. However if there is an interneuron present in
the reflex arc, it is possible to override the reflex.
Some examples of simple reflexes in humans are:
• The knee jerks when tapped in the right place
• Pupils get smaller in bright light
• Babies suck on anything placed on the cheek
, Nervous System Organisation
Nervous System
Central Nervous Peripheral Nervous
System System
Sensory Nervous Motor Nervous
System System
Voluntary Involuntary
Responses Responses
Sympathetic Parasympathetic
Nervous System Nervous System
• Central nervous system – This consists of the brain and spinal cord.
• Peripheral nervous system – This consists of all the nerves in the body other than in the
brain and spinal cord.
• Sensory nervous system – This is where information comes IN.
• Motor nervous system – This is where information comes OUT.
• Voluntary responses – Responses that we consciously control. For example skeletal muscles.
• Involuntary responses – Responses that we do not consciously control. For example smooth
muscles.
• Sympathetic nervous system – This is where things are sped up, for example heart rate
• Parasympathetic nervous system – This is where things are slowed down, for example hear
rate.
Functions of the nervous system:
• Sensory input – Detecting changes in the internal and external environment. This is when
information is carried into the CNS.
• Interpretation – Receiving information and processing the output. The CNS decides on an
appropriate response
• Motor output – Giving an appropriate response such as a muscle contraction or gland.
, Neuron Structure
