Biology A2 edexcel topic 8: Grey matter

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Topic 8: Grey matter
8.1 Sensitivity in plants
Photoreceptors: A structure in living organisms, especially a sensory cell or sense organ that
responds to light.
The interaction of cells enables them to coordinate their activities appropriately. Organisms
have specialised cells or molecules that are sensitive to changes in the environment
(stimulus), called receptors. These then trigger events to bring about coordinated responses
to environmental changes.
Plant photoreceptors:
Plants use light as their sole energy source in photosynthesis. Light gives plants information
about the seasons and respond by moving into different stages of their life cycles.
Plants respond to:
Light intensity
Light direction
Light quality (wavelength)
Light duration (day length)
Proteins that are sensitive to red light found in leaves. Phytochromes exist in two forms
which are changed from one form to another when they absorb light. Phytochrome red P(R)
absorbs red light and is changed to far-red P(FR) which absorbs far-red light. This changes it
back to P(R) in a reversible reaction.
White light, including sunlight, contains both red and far-red light. This causes more P(FR) to
be produced from P(R). In the dark, P(FR) slowly changes to P(R):
After daylight there is more P(FR)
After darkness there is more P(R)
In some plant species, phytochromes affect seed germination. Dormant seeds only
germinate when they contain plenty of P(FR) which happens when normal sunlight falls onto
them. This is valuable because it prevents germination in unsuitable conditions when there is
not enough light for photosynthesis. If a light-sensitive seed is provided with the correct
conditions (water, temperature) and then is given a short burst of far-red light, it won't
germinate. If the burst of far-red light is followed by a burst of red light it will germinate
because the red light converts P(R) to P(FR).
Phytochromes also determine if the plant produces flowers. Some plants adapt to flower in
spring, when the days are longer. Others are adapted to flower in late summer or autumn
when the days are shorter. The photoperiod determines flowering.
During darkness, P(FR) converts back to P(R). Short-day plants require P(R) in their tissues to
flower, which occurs when they have long uninterrupted nights. If white or red light is shone
on them even briefly during the night, they will not flower because this converts P(R) back to

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Long-day plants need an abundance of P(FR) which only happens when they have short nights
because not all of the P(FR) has been converted and the sun begins the conversion of P(R)
back to P(FR) again.
Phytochromes activate other molecules in plant cells which affect various metabolic
pathways. The phytochromes also act as transcription factors in the nucleus, switching genes
on and off.
A growth response to directional light. Shoots are positively phototrophic, growing towards
the source of light.…read more

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Peripheral nervous system is sub-divided into:
Somatic nervous system ­ voluntary, stimulates skeletal muscle
Autonomic nervous system ­ involuntary, stimulates smooth muscle, cardiac muscle
and glands
Autonomic nervous system divided into:
Sympathetic ­ Prepares body for fight or flight response ­ increases activity e.g.
speeding up heart rate
Parasympathetic ­ Prepares body for rest and digest ­ decreases activity e.g.
lowering breathing rate
The nervous system carries messages around the body using neurones.…read more

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Relay/connector neurones:
Connect sensory and motor neurones
Mostly in CNS
Large number of connections with other nerve cells
Cell body is in the middle of the axon
Stimulus Receptor cells Sensory neurone CNS Motor neurone Effectors
There are two types of main extensions from the cell body of a neurone:
Dendrites ­ Conduct impulses towards cell body
Axons ­ Transmits impulses away from cell body
Myelin sheath:
Some neurones have a fatty insulating layer around the axon
Made of many layers of Schwann cells…read more

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There is a change in permeability, causing Na+ channels to open so Na+ to
enter the cell down the concentration gradient ­ inside of cell becomes more
positive than outside
This reverses the resting potential, the membrane depolarises. The potential
difference is +40mV (action potential)
If this reaches the threshold level, an action potential is generated and an
impulse is fired. If it does not, nothing happens
2) Repolarisation...…read more

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The action potential travels rapidly along the axon or dendron. This is because the
repolarisation of one part of the membrane sets up local currents with the areas either side
of it ­ these regions depolarise too as some Na+ flow sideways.
Propagation of an impulse along an axon:
1. At resting potential there is a positive charge on the outside of the membrane and
negative charge on the inside, with higher sodium ion concentration outside and
higher potassium ion concentration inside
2.…read more

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Two neurones are not in direct contact, there is a small gap called the synaptic cleft. The
synapse is the junction between the two neurones. The presynaptic neurone has a swelling,
called a synaptic knob which contains vesicles filled with neurotransmitter. The presynaptic
knob has mitochondria to provide energy to make neurotransmitters and vesicles. The
electrical impulse cannot cross the synaptic cleft, so a neurotransmitter is released at the end
of the first neurone from the presynaptic membrane.…read more

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Although synapses slow the transmission of impulses, they are useful:
Ensure impulses travel only in one direction because receptors are only on the
postsynaptic membrane
Allows neurones to connect with many other neurones ­ increases range of possible
responses to a particular stimulus
Control nerve pathways an give flexibility of response
Integrate information from different neurones to give a coordinated response
Extent of depolarisation:
Depends on how much neurotransmitter reaches the postsynaptic membrane
- Depends partly on the frequency of impulses reaching the presynaptic membrane…read more

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If enough Na+ ions enter, the potential difference changes significantly and will initiate an
impulse or action potential.
Synapses can amplify or disperse information ­ one neurone connects to many other
neurones ­ information dispersed around the body ­ synaptic divergence or many neurones
connect to one neurone ­ information amplified (made stronger) ­ synaptic convergence.
Sensory receptors
Specialised cells that detect changes in the environment. They are specific to one
type of stimulus
Energy transducers ­ convert one form of energy to another.…read more

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Conjunctiva: Protects the cornea
Cornea: Bends light
Lens: Focuses light on retina
Iris: Controls amount of light entering eye by controlling pupil size
Sclera: Protective layer, allows attachment of external muscles
Blind spot: No light sensitive cells where optic nerve leaves eye
Fovea: Most sensitive part of retina
Retina: Contains light-sensitive cells
Vitreous humour: Transparent jelly
Choroid: Black layer prevents internal reflection of light
Cilary muscle: Alters thickness of lens for focusing
Optic nerve: Transmits impulses to brain
Pupil: Circular opening for directing light to…read more



thanks mate

Former Member

fantastic notes, saved me alot of time and effort thanks!


Thank you! 

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