- Response by plants to directional stimuli.
- Growth towards is positive. Growth away is negative.
- Response is stimulated by plant growth regulators called auxins.
- Auxins stimulate cells in the stem/roots to grow faster.
- When one site is stimulated the stem/roots bends.
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- A simple response that some mobile organisms use to keep themselves in a favourable environment.
- Is a directional response to the stimulus.
- The organism doesn't depend on chance to get into a favourable environment.
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- Simple response that some mobile organisms use to keep themselves in a favourable position in their environment.
- Non-directional response to a change in the environment.
- The rate of random movement increases.
- If the organism finds itself in a more favourable environment, it stops moving.
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Simple reflex arc
Stimulus > receptor > sensory neurone > CNS > motor neurone > effector > response.
- The response doesn't need to be thought out.
- The response is always the same.
- This type of control is very important for protecting the organism from damage.
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Autonomic control of heart rate
- Heartbeat is controlled by the autonomic nervous system.
- The cardioregulatory centre is found in the medulla (brain stem).
- There are two parts - acceleratory centre and inhibitory centre.
- Acceleratory sends impulses along the sympatheic nerve to the SAN.
- Noradrenaline is released and heart rate increases.
- Inhibitory sends inpulses along the vagus (parasympatheic) nerve.
- Acetyl choline is release and heart rate decreases.
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Control of blood pressure
If blood pressure is too low:
- Cartoid sinus (baroreceptor) is less stretched.
- Impulses sent at a lower frequency.
- Acceleratory centre is stimulated.
- Impulses sent along sympathetic nerve.
- SAN increases heart rate and blood pressure.
If blood pressure is too high:
- Cartoid sinus (baroreceptor) is more stretched.
- Impulses sent at a higher frequency.
- Inhibitory centre is stimulated.
- Impulses sent along vagus nerve.
- SAN slows heart rate and decreases blood pressure.
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Control of blood pH/ Carbon dioxide
Rise in CO2/ fall in pH:
- Chemoreceptors send impulses to ventilation centre (VC) and cardioregulatory centre (CR).
- VC increases rate and depth of breathing.
- CR sends impulses along sympathetic nerve, speeding up heart beat and increasing blood pressure.
- CO2/ pH returns to normal levels.
Fall in CO2/ rise in pH:
- Chemoreceptors send impulses to VC and CR.
- VC reduces rate and depth of breathing.
- CR sends impulses along vagus nerve, decreasing heart rate and blood pressure.
- CO2 / pH returns to normal levels.
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- detects changes in pressure on the skin and around joints and tendons.
- Contains two protein channels - a sodium ion pump and a channel protein that allows sodium ions into the cell by facilitated diffusion.
- The channel protein is normally closed.
- When pressure is applied the channel proteins are stretched and change shape, causing them to open.
- Sodium ions flood in along their gradient causing an influx of positive charges.
- This creates an electrical potential difference.
- The difference initiates an electrical impulse.
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Rods & cones
- Are light sensitive cells found in the retina.
- Transduce light energy into electrical impulses.
- Rods are sensitive to low light levels and give us night vision (no colour).
- Cones need high levels of light to work and give day vision (colour).
- Both synapse with bipolar cells.
- Several rods connect to one bipolar cell.
- Cones connect to an individual bipolar cell.
- Rods are found all over the retina except to fovea and blind spot.
- Cones are concentrated in the fovea.
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- The ability to distinguish to (close together) spots of light seperately.
- During the day cones give high visual acuity.
- As cones are connected to their own bipolar cell, when they are stimulated they send their individual nerve impulses to the brain.
- In order to see two points of light, they must fall onto two seperate cones with a gap of at least one cone between them.
- Light rays falling on rods that are close together result in nerve impulses going to the same ganglion cell, so they can't be percieved seperately.
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Sensitivity to low light levels
- Rod cells give better sensitivity to dim light.
- Because many rods are connected to one bipolar cell, if several rods are stimulated their generator potentials combine.
- This means there is a big enough generator potential to produce a nerve impulse.
- This is called summation because the effects are added together.
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