A2 1 Questions Flashcards
Suggest why paramecium numbers may not be accurate using haemocytometre
Paramecium are mobile and can move around haemocytometre
Why is there a high degree of variability of concentration of P730 during month of May (graph)
Differences in lead shading
Suggest why Gibberellins produces the greatest growth when cytokinin levels are not limiting
Cytokinins increases the rate of cell division - more cells produced in division; more cells for gibberellins to act on.
Propagation of action potentials quotes to remember :
Disturbs adjacent part of the membrane
Positive ions from depolarized zone pass along the inside of the membrane towards the polarised zone immediately in front.
Similar effect occurs on outside of the membrane where positive ions love back from the polarised zone into the depolarised zone.
Wave of action potentials
Continuous disturbance
What factors affect the speed of an impulse
Myelin sheath: acts an an electrical insulator in myelinated neurones. Depolarisation only occurs at the part of the sheath that is disrupted between adjacent Schwann cells known as the nodes of ranvier. Local circuits form at the nodes of ranvier so action potentials jump from one node to another, known as saltatory conduction.
Diameter of axon - the thicker the axon, the faster the impulse. Less ‘leakage’ Of ions in neurones with a larger and thicker diameter. Too much leakage in smaller diameters of axons makes it difficult to maintain potential gradients required to form a resting and action potential.
Temperature - temperature affects the rate of diffusion of the ions involved in neurone action. It affects the speed that neurones can conduct impulses.
Describe the transmission at synapses
When an impulse/action potential arrives at the synaptic bulb it causes calcium channels to open within the synaptic bulb and calcium ions rush in down a concentration gradient - high in cleft, low in bulb.
The influx of calcium ions causes vesicles containing the neurotransmitter acetylcholine to move towards the pre synaptic cleft.
At the pre synaptic bulb the neurotransmitter is released by exocytosis into the cleft. It diffuses across the cleft and attaches to specific receptor sites on the post synaptic membrane. The receptor sites are proteins that are complementary in shape to the neurotransmitter.
The binding of the neurotransmitter to the receptor sites causes gated sodium channels to open within the post synaptic membrane, and Na+ ions rush into the post synaptic neurone. This generates an excitatory post synaptic potential (EPSP) and an action potential is generated as the post synaptic membrane is depolarised if the threshold stimulus is met.
On the specific receptor sites there is an enzyme present known as acetylcholinesterase which breaks acetylcholine down to its constituent components, acetate and choline. The choline is taken up by the post synaptic cell and combined with acetyl co-enzyme A to reform acetylcholine.
Mitochondria present in the synaptic bulb to provide ATP for this action
Function of synapses
Ensure unidirectionality - nerve impulses can only past from pre synaptic neurone to the post synaptic neurone as neurotransmitter is only made in ore synaptic neurone and neurotransmitter receptors only made in post synaptic neurone.
Prevent over stimulation of effectors (eg muscles) - too many impulses passing along the same neurone in a short period of time will exhaust the supply of neurotransmitter quicker than it can be built up, the synapses fatigue.
Provide intergration and synapses provide flexibility.
