BiologyMad A-Level Biology



1. The diagram shows dendrites from each of two neurones, A and B, forming synapses with a third neurone, C. Concentrations of some ions are also shown.

[pic]

| |Ion concentration/mmol dm–3 |

| |Na+ |K+ |Cl– |

|Outside neurone C |145 |12 |120 |

|Inside neurone C |12 |155 |5 |

(a) Acetylcholine increases the permeability of the postsynaptic membrane to Na+ ions. Explain how acetylcholine released from neurone A generates an action potential in neurone C.

ACh combines with receptor (on neurone C / on postsynaptic membrane);

causes influx of Na+ ions;

down gradient / diffuse;

less negative/more positive inside/ ‘depolarisation’ occurs;

reference to above threshold (to generate action potential);

(3)

(b) Neurone B releases a substance called GABA as its neurotransmitter. GABA increases the permeability of the postsynaptic membrane to Cl– ions. Suggest how impulses in neurone B would reduce the chances of an action potential in neurone C.

(Cl– entry to neurone C) makes potential more negative inside/hyperpolarises;

requires more stimulation (from A) / ACh / Na+/ more impulses;

(2)

(c) Suggest a benefit of neurone C synapsing with the two types of neurone, A and B.

Reference to modulation of activity of neurone C / impulse transmission not

inevitable / eq;

(1)

(Total 6 marks)

2. The diagram shows some of the pump and channel proteins in the cell surface membrane of a neurone.

[pic]

(a) (i) What evidence is there in the diagram that structure A is involved in active transport?

ATP converted/used/energy from ATP;

(1)

(ii) Explain the role of the Na+ and K+ channels in producing the membrane resting potential.

More potassium pores than sodium pores/more permeable to K+/less

permeable to Na+;

more potassium / + ve ions (leaks) out; R.ref to active transport R.refs to

channel opening/closing

therefore more positive on outside/ORA;

(2)

(b) Explain how structure A and the Na+ gate allow depolarisation of this membrane.

Sodium ions pumped out (to create diffusion gradient);

sodium gates open/via Na+ gate to allow sodium ions (to flood) in; 2

I refs to K+

(2)

(Total 5 marks)

3. (a) The graph shows the changes in the permeability of an axon membrane to sodium ions and to potassium ions during an action potential.

[pic]

Use the information in the graph to explain how:

(i) at the start of an action potential, the potential difference across the membrane rapidly changes from negative to positive;

Increase in permeability to sodium ions;

which enter the axon;

(2)

(ii) the resting potential is restored.

Increase in permeability to potassium ions which move out;

(1)

(b) Suggest why, during a period of intense nervous activity, the metabolic rate of a nerve cell increases.

More nerve impulses more sodium has to be removed from axon;

active transport of sodium ions;

requires ATP;

(2)

(Total 5 marks)

4. Answers should be written in continuous prose. Credit will be given for biological accuracy, the organisation and presentation of the information and the way in which the answer is expressed.

The diagram represents a reflex arc.

[pic]

(a) Describe and explain the events which occur in the effector neurone at point A during the passage of a nerve impulse.

“Depolarisation” / reduced P.D. / –70 to –40 mV;

to threshold;

by local currents;

increased permeability of membrane to sodium ions / sodium gates open;

sodium ions enter;

by diffusion;

positive pd inside / eq via figures;

then potassium gates open / permeability to potassium ions increases;

potassium ions leave;

by diffusion (ONCE only)

resting potential re-established; )

) N.B. could

by ion pump / by active transport of ions / by Na-pump; ) start here

(7)

(b) Describe the events which allow transmission to take place across the synapse

labelled B.

Increased permeability of (presynaptic) membrane to calcium ions;

Ca 2+enter;

vesicles fuse with membrane;

exocytosis of / release of acetylcholine / neurotransmitter / other named e.g.;

diffuses across synaptic cleft;

binds to receptors on postsynaptic membrane / side;

increased Na + permeability / opens sodium channels / depolarises membrane /

reference e.p.s.p.;

acetylcholine broken down by acetylcholinesterase;

(6)

(c) Reflexes are described as being rapid, automatic responses. Use the information in the diagram to explain those features of a reflex arc which result in the response being rapid and automatic.

Rapid:

Only involves three neurones / receptor, relay and effector neurone;

myelination / saltatory conduction;

and two / a few synapses;

chemical / synaptic transmission is slow OR electrical / nervous transmission is

fast;

Automatic;

does not necessarily involve passage to brain / only spinal cord;

same pathway used each time;

higher brain centres not involved / no thinking;

(4)

(Total 17 marks)

5. The diagrams show the connections between light sensitive cells and neurones leading to the optic nerve of the eye. The connections will result in either greater acuity or greater sensitivity of vision.

[pic]

[pic]

(a) Pattern A results in greater visual acuity. What is meant by visual acuity?

Resolving power/ability to distinguish small objects/close objects/tell close 1

objects apart;

(1)

(b) Pattern B means that there are far fewer neurones in the optic nerve than photoreceptors in the retina. This is known as retinal convergence. Explain how this enables a person to see in dimly lit conditions.

Relevant reference to summation;

action potentials or generator potentials from several receptors/rods sent to

single synapse/neurone;

sufficient neurotransmitter released;

action potential/electrical impulse more likely to be generated in this

neurone/threshold more likely to be exceeded;

(2)

(Total 3 marks)

6. (a) The perception of objects seen by the eye often depends upon the difference in brightness or contrast between them. When light strikes rod cells, nerve impulses are generated in the bipolar cells. In turn the bipolar cells stimulate the ganglion cells of the optic nerve leading to the brain. The bipolar cells have cross connections to neighbouring synapses which are inhibitory. The effect of this lateral inhibition is shown in the diagram.

[pic]

[pic]

(i) Explain how one axon of a neurone can be stimulatory, while another axon of the same neurone can be inhibitory.

Different (neuro)transmitters produced / named transmitters;

(1)

(ii) Lateral inhibition causes an increase in contrast between the bright light and the dim light when perceived by the brain. Use information from the diagram to explain why.

Both bright light and dim light rates reduced/dim light rate reduced;

but proportional difference between bright and dim light rates

increased/owtte;

(2)

(b) Describe how rod cells act as transducers of energy.

Transducer converts one form of energy into another;

OR light to chemical energy; convert light/chemical energy to electrical energy;

rhodopsin /visual purple absorbs light energy/light energy causes change in

rhodopsin/rhodopsin to opsin and retinal/eq;

causes action potentials/electrical changes in neurones/depolarisation; (2)

(Total 5 marks)

7. A person was instructed to close the left eye and stare with the right eye at a cross drawn on a plain white board. Different objects were then moved into the field of view from one side. The points where the objects were first seen and where the colours were first identified were recorded. The results are shown below.

[pic]

(a) On what part of the right eye would the cross be focused?

Fovea;

(1)

(b) Explain why an object might be seen at point Z but its colour would not be identifiable.

Image falls on rod cells;

(1)

(c) (i) Explain how possession of different sorts of cone cell allows the colour purple to be identified.

Different cones sensitive to red and blue light;

purple stimulates both;

(2)

(ii) Draw an arrow below the line on the diagram showing where you would expect the colour purple to be first identified.

Arrow opposite point where blue first identified;

(1)

(Total 5 marks)

8. The diagram shows the distribution of rods and cones in the retina of a human eye.

[pic]

(a) Using information in the diagram, explain how:

(i) rod cells enable us to see in conditions of low light intensity;

Several rod cells to each neuron / bipolar cell/each synapse/convergence

principle of additive effect of light striking several rod cells/(spatial)

summation;

(2)

(ii) cone cells enable us to distinguish between objects close together.

Each cone connects to a single neurone/no convergence;

brain receiving information from each cone cell individually;

(2)

(b) The graphs show the changes in membrane potential in a presynaptic neurone and a postsynaptic neurone when an impulse passes across a synapse.

[pic] [pic]

(i) What is the resting potential of the presynaptic neurone?

-60 millivolts;

(1)

(ii) Explain what causes the change in the membrane potential in the presynaptic neurone between 1 and 1.8 milliseconds.

Increase in membrane permeability/gates open/channels open;

to sodium ions;

sodium ions enter;

by diffusion/down gradient;

(sodium) pump inhibited/eq;

(3)

(iii) How long is the delay between the maximum depolarisation in the presynaptic and the maximum depolarisation in the postsynaptic membrane?

0.6 milliseconds;

(1)

(iv) Describe the events that occur at the synapse during this delay.

Calcium ion/Ca2+ entry;

vesicles fuse with preSM ( and rupture);

exocytosis of/release (neuro)transmitter substance / named e.g.;

diffuse across gap;

attach to receptors on post SM; (not “fuse with....”)

increase permeability to sodium (ions) open Na channels/ref. e.p.s.p.; MAX 3

(3)

(c) The point at which an individual neurone makes contact with a striated muscle fibre is called a neuromuscular junction. Acetylcholine solution was added to a neuromuscular junction.

The graph shows the effect of the acetylcholine on the length of the muscle fibre.

[pic]

Acetylcholine is normally hydrolysed by an enzyme at the neuromuscular junction. Some insecticides inhibit this enzyme. Suggest how these insecticides are effective in killing insects.

Insecticide binds to enzyme;

ACh / neurotransmitter not hydrolysed/not broken down;

remains attached to receptor;

Na channels remain open;

continued stimulus to muscles;

tetanus/fatigue/continuous contraction/spasms/no relaxation;

insect unable to move/fly/breathe;

(I refs. energy/ATP)

(3)

(d) The diagram shows the positions of the muscle proteins, actin and myosin, in a non-contracted sarcomere.

Diagram showing sarcomere shorter/Z lines closer together;

[pic]

Using the same scale as in the diagram, draw on the grid below, a sarcomere after contraction.

[pic]

(1)

(e) Explain the role of the following during muscle contraction.

(i) Calcium ions

Ca2+: moves/detaches/changes shape of switch protein/blocking

molecule/tropomyosin/troponin;

expose binding sites/allows cross-bridge formation/eq; MAX 2

activates myosin ATP-ase/enables myosin head to split ATP;

(2)

(ii) Mitochondria

Mitochondria: production of ATP;

to attach/release/cock myosin head/

move myosin head/removal of Ca2+ ions;

(2)

(Total 20 marks)

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