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synapses Flashcards

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1
Q

The sequence of events which leads to the transmission of an impulse at a cholinergic synapse

A

calcium ions move into synaptic knobs

causes synaptic vesicles to move

towards presynaptic membrane

release acetyl choline into gap

diffuses across

binds onto receptor

on postsynaptic neurone

causing depolarisation opening of sodium gates

postsynaptic cell membrane

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2
Q

Different substances are involved in coordinating responses in animals. Hormones are different from local chemical mediators such as histamine in the cells they affect.
Describe how hormones are different in the cells they affect. (1)










A

Hormones have widespread effect

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3
Q

Describe how hormones and local chemical mediators reach the cells they affect. (2)

A

Hormones in blood;

Local chemical mediators spread by diffusion

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4
Q

There are differences in the properties of the cuticle on the upper and lower surfaces of leaves.
Suggest how these differences in the cuticle might explain the differences in rates of uptake of NAA by the two surfaces. (2)

A

Thick cuticle on upper surface;

So shorter diffusion pathway on lower surface

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5
Q

NAA is a similar substance to IAA. It is used to control the growth of cultivated plants. Plant physiologists investigated the effect of temperature on the uptake of NAA by leaves. They sprayed a solution containing NAA on the upper and lower surfaces of a leaf. The graph shows their results. (Lower surface of leaf has a greater rate of uptake of NAA than upper surface)
Explain the effect of temperature on the rate at which NAA is taken up by the lower surface of the leaf. (2)

A

More kinetic energy;

More diffusion

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6
Q

When a young shoot is illuminated from one side, IAA stimulates growth on the shaded side. Explain why growth on the shaded side helps to maintain the leaves in a favourable environment. (2)

A

Causes plant to grow towards light;
Light is required for photosynthesis;


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7
Q

IAA is a specific growth factor. Name the process by which IAA moves from the growing regions of a plant shoot to other tissues. (1)

A

Diffusion

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8
Q

Synapses are unidirectional. Explain how acetylcholine contributes to a synapse being unidirectional. (2)

A

Acetylcholine released from presynaptic side;

Diffusion is from higher concentration to lower concentration

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9
Q

During an action potential, the permeability of the cell-surface membrane of an axon changes. The graph shows changes in permeability of the membrane to sodium ions (Na+) and to potassium ions (K+) during a single action potential.

Explain the shape of the curve for sodium ions between 0.5 ms and 0.7ms (3)

A

Ion channel proteins open;
Influx of sodium ions;
This makes the inside of axon less negative


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10
Q

After exercise, some ATP is used to re-establish the resting potential in axons. Explain how the resting potential is re-established. (2)

A

Active transport;

Of sodium out of axon and potassium in

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11
Q

During an action potential, the membrane potential rises to +40 mV and then falls. Use information from the graph to explain the fall in membrane potential. (3)

A

Potassium channels open;
Potassium diffuses out;
Sodium channels close

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12
Q

Explain what causes the conduction of impulses along a non-myelinated axon to be slower than along a myelinated axon. (3)

A

in a myelinated axon there is ion movement only at the nodes;
and the impulse jumps from node to node;
whereas in non-myelinated axons the impulse has to travel along the whole membrane

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13
Q

Describe the sequence of events which allows information to pass from one neurone to the next neurone across a cholinergic synapse. (8)

A

impulse causes calcium ions to enter axon;
causes vesicles to fuse with presynaptic membrane;
acetylcholine released;
acetylcholine diffuses across synaptic cleft;
and binds with receptors on sodium ion channels on postsynaptic membrane;
sodium ions diffuse into postsynaptic neurone;
acetylcholinesterase hydrolyses acetylcholine to prevent further action potentials;
ATP from mitochondria used to recombine choline and ethanoic acid into acetylcholine

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14
Q

Explain the importance of reflex actions. (3)

A

automatic adjustments to changes in environment;
preventing injury;
helpful for escaping from predators

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15
Q

A different form of epilepsy has been linked to an abnormality in GABA receptors. Suggest and explain how an abnormality in GABA receptors may result in epilepsy. (3)

A

receptors have different tertiary structure;
GABA cannot bind;
inhibition of neuronal activity does not occur

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16
Q

Epilepsy may result when there is increased neuronal activity in the brain.
One form of epilepsy is due to insufficient GABA. GABA is broken down on the postsynaptic membrane by the enzyme GABA transaminase. Vigabatrin is a new drug being used to treat this form of epilepsy. The drug has a similar molecular structure to GABA. Suggest how Vigabatrin may be effective in treating this form of epilepsy. (2)

A

inhibits enzyme which breaks down GABA;

more GABA available to inhibit neurone

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17
Q

The binding of GABA to receptors on postsynaptic membranes causes negatively charged chloride ions to enter postsynaptic neurones. Explain how this will inhibit
transmission of nerve impulses by postsynaptic neurones
(3)

A

inside becomes more negatively charged;
stimulation does not reach threshold level;
depolarisation does not occur

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18
Q

Acetylcholine is a neurotransmitter which binds to postsynaptic membranes and stimulates the production of nerve impulses. GABA is another neurotransmitter. It is produced by certain neurones in the brain and spinal cord. GABA binds to postsynaptic membranes and inhibits the production of nerve impulses. The diagram shows a synapse involving three neurones.
Describe the sequence of events leading to the release of acetylcholine and its binding to the postsynaptic membrane. (4)

A

action potential arrives;
calcium ions enter synaptic knob;
vesicles fuse with membrane;
acetylcholine diffuses across synaptic cleft

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19
Q

(Refer to exam q) The resting potential of a neurone is maintained at -70mV. A metabolic poison was applied to a neurone and the change in the resting potential was measured over several hours. The results are shown in the graph. Explain the change in resting potential that takes place after the application of the metabolic poison. (4)

A 
Active transport stops;
Sodium ions no longer pumped out;
Sodium ions continue to diffuse in;
Becomes less negative inside

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20
Q

Describe how the resting potential of a neurone is maintained. (2)

A

Active transport of sodium ions back out and potassium in;
Less permeable to sodium ions


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21
Q

Enkephalins are neurotransmitters released by the brain and spinal cord in response to harmful stimuli. Enkephalin molecules are similar in shape to acetylcholine.
Enkephalin molecules act as pain killers by inhibiting synaptic transmission. Explain how this inhibition occurs. (4)

A

Bind to receptors;
On postsynaptic membrane;
Competes with acetylcholine;
Reduces depolarisation

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22
Q

Nervous transmission is delayed at synapses. Explain why. (2)



A

chemical rather than electrical;
Process of transmission takes time because of transmitter diffusion


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23
Q

The axon of neurone A is myelinated. The axon of neurone B is non-myelinated. Explain why impulses travel faster along the axon of neurone A. (2)

A

Myelinated - impulse jumps from node to node;

Non-myelinated - impulse travels whole length of axon membrane

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24
Q

Give two differences between a cholinergic synapse and a neuromuscular junction. (2)

A

neurone to neurone and neurone to muscle;

action potential in neurone and no action potential in muscle

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25
Q

Explain how a resting potential is maintained in a neurone. (4)

A 
membrane less permeable to sodium ions;
sodium ions actively transported out;
sodium ion carrier proteins;
inside of axon is negative compared to outside

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26
Q

Explain why the transmission of a series of nerve impulses along neurone B uses less energy than transmission along neurone A. (3)

A

Neurone B is myelinated;
Less active transport of ions at nodes of Ranvier;
Less ATP needed for active transport


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27
Q

An action potential is produced in neurone A. Describe how this action potential passes along the neurone. (3)

A

Depolarisation of axon membrane causes local currents to be set up;
Na+ gates open in adjoining region and sodium ions enter;
Adjoining region depolarises


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28
Q

What is the all-or-nothing principle?

A

There is a particular level of stimulus that triggers an action potential. At any level above this threshold, a stimulus will trigger an action potential that is the same size regardless of the size of the stimulus. Below the threshold, no action potential is triggered.

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29
Q

Explain how the refractory period ensures that nerve impulses are kept separate from one another.

A

During the refractory period the sodium channels are closed so no sodium ions can move inwards and no action potential is possible. This means there must be an interval between one impulse and the next.


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30
Q

Why is it necessary for acetylcholine to be hydrolysed by acetylcholinesterase?

A

To recycle the choline and ethanoic acid, to prevent acetylcholine from continuously generating a new action potential in the postsynaptic neurone.

31
Q

Explain why hyperpolarisation reduces the likelihood of a new action potential being created.

A

As the inside of the membrane is more negative than at resting potential, more sodium ions must enter in order to reach the potential difference of an action potential, i.e. it is more difficult for depolarisation to occur. Stimulation is less likely to reach the threshold level needed for a new action potential.


32
Q

Describe the basic events in the transmission of a nerve impulse from one neurone to another.

A

Neurotransmitter is released from vesicles in the presynaptic neurone into the synaptic cleft when an action potential reaches the synaptic knob. The neurotransmitter diffuses across the synapse to receptor molecules on the postsynaptic neurone to which it binds, thereby setting up a new action potential.


33
Q

How is the postsynaptic neurone adapted to receive the neurotransmitter?

A

It has receptor molecules on its membrane.

34
Q

How is a presynaptic neurone adapted for the manufacture of neurotransmitter?

A

It possesses many mitochondria and large amounts of endoplasmic reticulum.

35
Q

In a myelinated axon, sodium and potassium ions can only be exchanged at certain points along it.
Explain why ions can only be exchanged at these points.

A

The remainder of the axon is covered by a myelin sheath that prevents ions being exchanged.


36
Q

State two differences between animal hormones and plant growth factors

A

Animal hormones are made in particular organs and affect other organs some distance away.
Plant growth factors are made by cells located throughout the plant and have localised effects.


37
Q

Suggest two advantages to a plant of having roots that respond to gravity by growing in the direction of its pull.

A

Response ensures that roots grow downwards into the soil, thus anchoring the plant firmly and bringing it closer to water, needed for photosynthesis.


38
Q

Name two chemical mediators and state the effects they each have on blood vessels.

A

Histamine and prostaglandins - both cause dilation of small arteries and arterioles and increased permeability of capillaries.

39
Q

Acetylcholine is the neurotransmitter at neuromuscular junctions.
Describe how the release of acetylcholine into a neuromuscular junction causes the cell membrane of a muscle fibre to depolarise. (3)


A

movement by diffusion;
binding to receptors on post-synaptic membrane;
causing sodium channels to open

40
Q

State three ways in which a response to a hormone differs from a response to a nerve impulse.

A

Hormone response is slow, widespread and long lasting.

Nervous response is rapid, localised and short lived.

41
Q

What is meant by the refractory period? (1)

A

no new nerve impulse be produced in this time

42
Q

Explain how movements of these sodium and potassium ions are involved in producing the action potential. (2)

A

sodium ions diffuse in;

causing depolarisation

43
Q

The insecticide DFP combines with the active site of the enzyme acetylcholinesterase. Explain why the muscles stay contracted until the insecticide is lost from the neuromuscular junction. (2)

A

acetylcholinesterase is unable to breakdown acetylcholine;

acetylcholine still available to depolarise the membrane

44
Q

The molecular structure of cobra toxin is similar to the molecular structure of acetylcholine. Explain why the toxin permanently prevents muscle contraction. (2)

A

toxin competes for receptors;

the toxin does not cause depolarisation

45
Q

Describe three changes in the root tip cells between stages A and D. (3)

A

Formation of vacuole;
Formation of starch grains;
Movement of grains towards bottom of cell

46
Q

In this investigation, the higher the concentration of sucrose in a rat’s mouth, the higher the frequency of nerve impulses from each taste receptor to the brain.
If rats are given very high concentrations of sucrose solution to drink, the refractory period makes it impossible for information about the differences in concentration to reach the brain.
Explain why. (2)

A

Refractory period limits the frequency of nerve impulses;
When maximum frequency reached, all higher concentrations of sucrose seem the same


47
Q

Serotonin is a neurotransmitter released in some synapses in the brain. It is transported back out of the synaptic gap by a transport protein in the pre-synaptic
membrane.
Serotonin diffuses across the synaptic gap and binds to a receptor on the post-synaptic membrane.
Describe how this causes depolarisation of the post-synaptic membrane. (2)

A

Causes sodium ion channels to open;
Sodium ions enter cell and cause depolarisation


48
Q

t is important that a neurotransmitter such as serotonin is transported back out of synapses. Explain why. (3)

A

If not removed keeps binding to receptors;
Keeps causing action potentials in post-synaptic membrane
Prevents information being carried across synapse

49
Q

Outline how IAA controls tropisms. (5)



A

Cells in the tip of a shoot produce IAA, which is transported down the shoot;
Light causes IAA to move to shaded side of shoot;
Greater concentration of IAA on shaded side than light side;
IAA causes elongation of cells, so cells on shaded side elongate more;
Shaded side grows faster, causing shoot to bend towards light

50
Q

Early experiments into the effects of light on shoot growth used gelatin and mica to test how the bending response was created.
Why was
(i) gelatin used?
(ii) mica used? (2)

A

(i) Gelatin conducts chemicals but not electricity

(ii) Mica conducts electricity but not chemicals

51
Q

Describe the structure of neurones




A

Cell body containing a nucleus and lots of RER;
Dendrites which carry nerve impulses towards cell body;
Axon, long fibre which carries nerve impulses away from cell body;
Schwann cells which surround axon, protect it and provide electrical insulation;
Myelin sheath, made of membranes of Schwann cells, rich in the lipid ‘myelin’ which transmits nerve impulses faster;
Nodes of ranvier which are gaps between Schwann cells

52
Q

The black mamba’s toxin kills prey by preventing their breathing. It does this by inhibiting the enzyme acetylcholinesterase at neuromuscular junctions. Explain how this prevents breathing. (3)

A

Acetylcholine not broken down;
Na+ ions continue to enter;
Intercostal muscles stay contracted

53
Q

Why is it necessary for acetylcholine to be hydrolysed by acetylcholinesterase? (2)

A

To recycle the choline and ethanoic acid;
To prevent acetylcholine from continuously generating a new action potential in the postsynaptic neurone


54
Q

How may a drug inhibit the nervous system? (2)

A

Inhibit the release of the neurotransmitter;

Block the receptors on the ion channels on postsynaptic neurone

55
Q

How may a drug stimulate the nervous system? (3)

A

Mimick a neurotransmitter;
Stimulate the release of more neurotransmitter;
Inhibit the enzyme that breaks down the neurotransmitter


56
Q

How may drugs effect synapses? (2)

A

They stimulate the nervous system by creating more action potentials in postsynaptic neurones;
They inhibit the nervous system by creating fewer action potentials in postsynaptic neurones


57
Q

Suggest an advantage of responding to high-level stimuli but not to low-level ones. (3)

A

Reacting to low-level stimuli can overload central nervous system;
Organisms fail to respond to more important high-level stimuli;
Which are more likely to represent danger

58
Q

When walking along a street we barely notice the background noise of traffic. However, we respond to louder noises, such as the sound of a horn. From your knowledge of summation, explain this difference. (5)

A

Produces low-level frequency action potentials;
Neurotransmitter insufficient to exceed threshold value to create a new action potential;
Loud noises create higher frequency action potentials, neurotransmitter exceeds threshold value;
Sufficient to trigger action potential in postsynaptic neurone;
Example of temporal summation


59
Q

Describe how a resting potential is established in an axon. (7)




A

Inside of axon negatively charged relative to outside;
Because active transport of sodium ions out of the axon by sodium-potassium pumps is faster than active transport of potassium ions into the axon;
Potassium ions diffuse out of axon but few sodium ions diffuse in as sodium gates are closed;
Potassium ions stop diffusing out because they are attracted to negative state of axon and repelled by outside;
Further increasing potential difference;
More positive ions outside than inside, creating a chemical gradient;
Equilibrium is reached, no net movement of ions

60
Q

How can a synapse prevent a new action potential being created? (3)

A

Chloride ion channels can be kept open;
Influx of chloride ions;
Making postsynaptic membrane more negative than at resting potential

61
Q

What is summation? (3)

A

When action potentials produce insufficient amounts of neurotransmitter to exceed the threshold value and trigger a new action potential, summation can occur;
Spatial summation: a number of presynaptic neurones together release enough neurotransmitter to trigger an action potential;
Temporal summation: a single presynaptic neurone releases neurotransmitter many times over a short period

62
Q

How do synapses allow a single stimulus to create a number of simultaneous responses? (1)

A

A single impulse along one neurone can be transmitted to a number of different neurones at a synapse

63
Q

Why do synaptic knobs possess many mitochondria and large amounts of endoplasmic reticulum? (2)

A

Mitochondria provides ATP for synthesis of proteins and neurotransmitters;
Endoplasmic reticulum synthesises proteins and neurotransmitters

64
Q

What is meant by the all-or-nothing principle? (4)

A

There is a certain level of stimulus that triggers an action potential;
This threshold value must be exceeded to produce an action potential;
At any level above this threshold, a stimulus will trigger an action potential of the same size;
Regardless of the size of the stimulus


65
Q

Why is the refractory period important? (3)

A

Ensures that an action potential occurs in one direction only;
Ensures that action potentials are separated from one another;
Limits the number of action potentials


66
Q

What is the refractory period? (2)

A

The period after an action potential when inward movement of sodium ions is prevented because the sodium channels are closed;
preventing a further action potential from being generated


67
Q

How does temperature affect the speed of a nerve impulse? (5)

A

The higher the temperature the faster the nerve impulse;
Sodium-potassium pump uses active transport;
Which requires energy from ATP from respiration;
Respiration is enzyme-controlled;
More kinetic energy so more enzyme-substrate complexes formed and faster diffusion

68
Q

How does the diameter of an axon affect the speed of a nerve impulse? (2)

A

The greater the diameter of an axon, the faster the speed of conductance;
due to less leakage of ions from a large axon

69
Q

Why does an action potential pass along a myelinated neurone faster than an unmyelinated one? (4)

A 
Saltatory conduction;
Myelin sheath acts as an insulator;
Prevents action potentials from forming;
Action potentials jump between nodes of Ranvier

70
Q

Describe how an action potential moves across an axon. (6) 

A

At resting potential, high concentration of sodium ions outside membrane compared to inside, so inside is more negative;
Stimulus causes some sodium channels to open so sodium diffuses into axon and membrane is depolarised;
Localised electrical circuits cause sodium channels further along to open;
Influx of sodium ions in this region causes depolarisation;
Behind this new region of depolarisation, sodium channels close and potassium channels open and membrane is repolarised;
Action potential continues in this way along the neurone

71
Q

Describe how an action potential is produced in an axon. (6)

A

A stimulus causes a temporary reversal of charge and inside of axon becomes positive (depolarisation);
Energy of stimulus causes some sodium channels to open;
So sodium diffuses into axon;
Once an action potential of +40mV has been established, channels close and there is no more influx of sodium;
Potassium channels open and potassium ions diffuse out, causing repolarisation of axon;
Axon becomes more negative than usual, so potassium gates close and sodium-potassium pump again actively transports sodium ions out and potassium ions in


72
Q

How is the movement of ions controlled? (3)

A

Phospholipid bilayer of axon membrane prevents ions diffusing across it;
Intrinsic proteins contain channels which open and close accordingly;
Some intrinsic proteins actively transport ions in and out (sodium-potassium pump)

73
Q

How would the binding of another neurotransmitter on receptors on postsynaptic membranes inhibit the transmission of impulses by postsynaptic neurones?

A
  • the postsynaptic neurone is depolarised
  • stimulation doesn’t reach threshold level so action potential not produced
  • not depolarised
  • reduces effect of Na+ ions entering

biology unit 5 (8 decks)

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