Pretty, concise notes covering AQA A-Level Biology Topic/Unit 6 - organisms respond to their internal and external environments
Used to get an A* at A-Level.
Topic 6: Organisms respond to change in their internal and external environments
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Survival and Response
Key words:
Stimulus - can be detected by an organism. Can either be internal in multicellular organisms or external in any organism.
Receptor - organ or specialised cell that can detect the change that is causing the stimulus.
Response - as a result of stimulus that is detected by receptor a response is caused. This may be movement of the organism or a change in behaviour.
Taxes and Kinesis:
Taxis is response that involves movement in a specific direction.
Positive taxis is towards the stimulus and negative taxis is away from the stimulus.
E.g., positve chemotaxis is mobile bacteria moving to an area where there is higher concentration of glucose.
Kinesis is response that involves movement but in random directions.
Both speed and frequency of direction change increase.
Response is carried out in order to increase the chance that the organism will enter different conditions more rapidly.
E.g., if you place a woodlouse in a dry area it will speed ip and change direction more frequently in order to increase the chance it enters a damp region
which are its favoured conditions.
Plants response to stimuli:
Plants respond to external stimuli to increase their chance of survival. They exhibit tropisms that is growth responses controlled by a direction stimulus.
E.g., phototropism where direction of growth is determined by the direction of light. The shoots of the plant are positively phototropic and grow towards the
light whereas roots are negatively phototropic and grow away from the light.
Plant growth is controlled by indoleacetic acid (IAA) which is an important auxin produced in the tips and shoots of flowering plants.
The distribution of IAA around the plant controls tropisms.
If IAA is unevenly distributed, it causes uneven growth of the plant.
When the shoot is illuminated from all sides, the auxins are distributed evenly and move down the shoot tip thus causing elongation of cells across the zone
of elongation.
If the shoot is only illuminated from one side, the auxins move towards the shaded part of the shoot thus causing elongation of the shaded side only which
results in the bending of the shoot towards the light.
Gravitropism in roots is the opposite. IAA will build up on the lower side of the roots.
In roots IAA inhibits growth therefore causing cells on the upper side to grow faster causing the root to bend downwards.
Simple reflex arc:
Reflexes are rapid, automatic responses that can protect an organism from harmful stimuli and help them to survive and avoid danger.
These are fast responses which bypass the brain meaning that no decision has to be made.
Stimulus -> Receptor -> Sensory neuron -> Intermediate neuron -> Motor neuron -> Effector -> Response
Sensory neuron carries the nerve impulse from the receptor to the spinal cord.
Motor neuron carries the nerve impulse from the spinal cord to the effector which can be muscle or gland.
Intermediate neuron is located entirely in the spinal cord and relays the nerve impulse from the sensory neuron to motor neuron.
, Topic 6: Organisms respond to changes in their internal and external environments
Receptors
Detect the changes in the internal and external environment.
Many types of receptors each specific to a particular kind of stimuli e.g., photoreceptors detect changes in light whereas mechanoreceptors like
Pacinian Corpuscle detect mechanical stimuli in the form of pressure and vibrations.
Pacinian Corpuscle
Located deep in the skin and are mostly found on fingers, soles of the feet and external genitalia.
They are also found in joints, tendons and ligaments.
Have a single sensory neuron located in the centre of connective tissue called lamellae which forms layers separated by gel.
Contains stretch mediated sodium channels in the cell surface membrane.
When not under pressure these channels are closed, however, under pressure these become deformed.
As a result they open and allow the rapid influx of sodium ions to occur.
The positive charge on the sodium changes the membrane potential causing the membrane to become depolarised.
This results in a generator potential being created which goes on to create an action potential in the axon.
Photoreceptors in the eye
Light receptors.
Light enters the eye through the pupil and the amount of light entering is controlled by muscles in the iris.
The lens focuses the light on the retina where photoreceptors are located specifically the fovea.
The nerve impulses received by the photoreceptor cells are carried via optic nerve to the brain.
The point where the optic nerve leaves the eye is the blind spot as there are no photoreceptor cells there.
2 types of photoreceptors in the retina:
-> Cones involved in colour vision.
-> Rods involved in monochromatic vision.
Cone cells are present at greatest density in the fovea and contain pigment iodopsin. They are not sensitive to light therefore require bright light to work.
3 different types of cone each sensitive to the primary colours (red, green and blue).
Cone cells provide good visual acuity because each cone cell has its own synapse via a bipolar neuron which connects to the optic sensory neuron.
Rod cells are mainly concentrateed in the highest density outside of the fovea and contain pigment rhodopsin. They are very sensitive to light and are
stimulated in low light conditions.
Rod cells provide low visual acuity as more than one rod cell shares the same synapse with a bipolar cell.
Multiple rods need to be stimulated to cause the creation of a generator potential.
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