IB DP Biology Topic 6: Human physiology : 6.5 Neurons and synapses Question Bank SL Paper 2

Question

Explain the propagation of electrical impulses along a neuron including the role of myelin.

▶️/Explanation

Markscheme

Neurotransmitter attaches to receptor site, initiating transmission

Nerve impulses are action potentials propagated along the axons of neurons

Resting potential is more negative inside/–70 mV/ more positive outside the membrane
OR
a resting potential has greater concentration of Na ions outside than K ions inside the axon

«volted gated» channels open and Na ions diffuse in

Causes depolarization of the membrane/–70 mV to +40 mV

Local currents affect adjacent channels/cause action potential

Depolarization is followed by repolarization of the neuron 

«voltage gated» channels open and K ions diffuse out/repolarize the membrane

Na-K pumps restore Na/K balance/resting potential

Myelin around the neuron insulates the axon
OR
speeds the transmission

Myelin permits saltatory conduction
OR
permits jumping from node to node

Award [6 max] if no mention of the role of myelin.

 

Question

Draw a labelled diagram of the human heart showing the attached blood vessels.

[6]
a.

Describe the action of the heart in pumping blood.

[5]
b.

Nerves connecting the brain and heart contain neurons that control heart rate.  Explain how a nerve message passes from one neuron to another neuron.

[7]
c.
▶️Answer/Explanation

Markscheme

Remember, up to TWO “quality of construction” marks per essay.

NBDrawings must be correctly proportioned and clearly drawn showing connections between structures. The drawing may show the heart without contraction or in any stage of contraction. Award [1] for any correctly labelled part that has been drawn to the stated standards.
a. atria/right atrium/left atrium – shown above the ventricles and must not be bigger than ventricles;
b. ventricle/left ventricle/right ventricle – shown below the atria, must have thicker walls than atria;
c. vena cava/superior vena cave/inferior vena cava – connected to right atrium;
d. pulmonary artery – shown from right ventricle (to lungs);
e. pulmonary vein(s) – shown (from lungs) to left atrium;
f. aorta – shown as large artery from left ventricle out of heart;
g. AV valves/atrioventricular valves / mitral/bicuspid and tricuspid – named correctly and shown between both atria and ventricles and labelled at least on one side;
h. semilunar valves – shown in aorta/pulmonary artery;
Valves need to open in correct direction.

a.

Remember, up to TWO “quality of construction” marks per essay.

a. (both) atria collect blood (from veins);
b. sinoatrial/SA node sends impulses to muscle/fibres initiating contraction;
c. blood is pushed to ventricles by contraction of atria/atrial systole;
d. AV (atrioventricular) valves are open (as atria contract);
e. semilunar valves are closed so that ventricles fill with blood;
f. ventricles contract / ventricular systole;
g. AV (atrioventricular) valves close (and preventing backflow);
h. blood is pushed out through the semilunar valves/into pulmonary artery and aorta;
i. when ventricles relax/diastole, semilunar valves close preventing backflow of blood;
Do not accept the description of blood flow without a clear action.
Do not accept general statements such as systole = heart contraction and diastole = heart relaxation.

[4 max] if suggests that left and right sides are contracting at different times or simultaneous contraction not indicated.

b.

Remember, up to TWO “quality of construction” marks per essay.

a. nerve impulse reaches the end of the presynaptic neuron;
b. (depolarization causes) calcium channels in membrane (to) open;
c. calcium diffuses into the presynaptic neuron;
d. vesicles of/containing neurotransmitter move to and fuse with presynaptic membrane;
e. (neurotransmitter) released (by exocytosis) into synaptic space/cleft;
f. (neurotransmitter) diffuses across the space/synapse;
g. (neurotransmitter) attaches to receptors on postsynaptic neuron;
h. receptors cause ion channels to open and sodium diffuses into the postsynaptic neuron;
i. the postsynaptic neuron membrane is depolarized;
j. (depolarization) causes a new action potential;
k. (neurotransmitter) on postsynaptic membrane is broken down;
l. (neurotransmitter) is reabsorbed into the presynaptic neuron;

c.

Question

Outline how nerve impulses are transmitted along a nerve fibre.

▶️/Explanation

Markscheme

a. (sodium channels/voltage-gated channels open and) Na+ diffuse into neuron (down concentration gradient);
b. inside of neuron becomes positive compared to outside / potential is reversed / depolarization;
c. wave of depolarization moves (down the membrane);
d. (potassium channels open and) K+ diffuse out (down concentration gradient);
e. inside becomes negative compared with outside / potential across membrane restored / repolarization;
f. active transport of K+ (into neuron) and Na+ (out of neuron) restores resting potential;

Question

Draw a labelled diagram of a motor neuron.

[5]
a.

Explain how an impulse passes along the membrane of a neuron.

[8]
b.

Describe the process of endocytosis.

[5]
c.
▶️Answer/Explanation

Markscheme

Award [1] for each of the following clearly drawn and correctly labelled.

a. cell body – shown with a nucleus;

b. nucleus correctly labelled;

c. axon – shown as double line longer than the longest dendrite;

d. myelin sheath/Schwann cells – surrounding the axon;

e. nodes of Ranvier – shown in axon;

f. dendrites – shown extending from the cell body;

g. motor end plates – not covered by myelin sheath and ending with buttons/dots;

a.

a. resting potential is –70mV / relatively negative inside in comparison to the outside;

b. Na+/Kpumps maintain/re-establish (the resting potential);

c. more sodium ions outside than inside (when at the resting potential);

d. more potassium ions inside than outside (when at the resting potential);

e. nerve impulse is an action potential that stimulates a (wave of) depolarization along the membrane/axon;

f. if neuron is stimulated/threshold potential/–50mV is reached sodium ion channels open;

g. sodium ions diffuse/move in;

h. (Namove in) causing depolarization;

i. potassium ion channels open / potassium ions diffuse/move out;

j. (Kmove out) causing repolarization;

k. local currents / description of Naion diffusion between depolarized region and next region of axon to depolarize;

Accept any of the above points clearly explained in an annotated diagram.

b.

a. (plasma) membrane encloses/engulfs solid particles/droplets of fluid/molecules;

b. fluidity of the membrane allows endocytosis;

c. (plasma) membrane forms pit/forms indentation/pulled inwards/invaginates;

d. membrane pinches off /seals back on itself/edges fuse;

e. vesicle/vacuole formed;

f. inside of plasma membrane becomes outside of vesicle membrane / converse;

g. vesicle breaks away from plasma membrane/moves into cytoplasm;

h. active process / endocytosis/vesicle formation requires energy;

Accept any of the above points clearly described in an annotated diagram.

c.

Question

Draw a labelled diagram of a motor neuron.

[5]
a.

Explain how an impulse passes along the membrane of a neuron.

[8]
b.

Describe the process of endocytosis.

[5]
c.
▶️Answer/Explanation

Markscheme

Award [1] for each of the following clearly drawn and correctly labelled.

a. cell body – shown with a nucleus;

b. nucleus correctly labelled;

c. axon – shown as double line longer than the longest dendrite;

d. myelin sheath/Schwann cells – surrounding the axon;

e. nodes of Ranvier – shown in axon;

f. dendrites – shown extending from the cell body;

g. motor end plates – not covered by myelin sheath and ending with buttons/dots;

a.

a. resting potential is –70mV / relatively negative inside in comparison to the outside;

b. Na+/Kpumps maintain/re-establish (the resting potential);

c. more sodium ions outside than inside (when at the resting potential);

d. more potassium ions inside than outside (when at the resting potential);

e. nerve impulse is an action potential that stimulates a (wave of) depolarization along the membrane/axon;

f. if neuron is stimulated/threshold potential/–50mV is reached sodium ion channels open;

g. sodium ions diffuse/move in;

h. (Namove in) causing depolarization;

i. potassium ion channels open / potassium ions diffuse/move out;

j. (Kmove out) causing repolarization;

k. local currents / description of Naion diffusion between depolarized region and next region of axon to depolarize;

Accept any of the above points clearly explained in an annotated diagram.

b.

a. (plasma) membrane encloses/engulfs solid particles/droplets of fluid/molecules;

b. fluidity of the membrane allows endocytosis;

c. (plasma) membrane forms pit/forms indentation/pulled inwards/invaginates;

d. membrane pinches off /seals back on itself/edges fuse;

e. vesicle/vacuole formed;

f. inside of plasma membrane becomes outside of vesicle membrane / converse;

g. vesicle breaks away from plasma membrane/moves into cytoplasm;

h. active process / endocytosis/vesicle formation requires energy;

Accept any of the above points clearly described in an annotated diagram.

c.

Question

Exposure to organophosphorus pesticides (OP) is a cause of serious nerve damage. It disrupts synaptic transmission by inhibiting the enzyme acetylcholinesterase, causing death due to cardiovascular and respiratory failure.

Recombinant human acetylcholinesterase (rAChE) was obtained by genetic engineering and produced in Nicotiana benthamiana plants. It was tested as a new therapeutic treatment in mice that were exposed to OP. The following graph shows the severity of the symptoms shown by each mouse at different ratios of rAChE to OP.

To test the effect of OP damage on synapses, mice were treated with rAChE, OP or both. Their diaphragms were dissected 10 days after treatment. The area of the synapse between axons and the diaphragm was measured. When the synapses are damaged by OP there is a  greater area. The box plot shows the effect of different treatments on the area of the synapse.

State the minimum ratio at which some mice showed no symptoms.

[1]
a.

Analyse the effect of increasing the ratio of rAChE to OP on the symptoms in mice.

[3]
b.

Predict what would happen if a mouse received 300 mg of rAChE and 600 mg of OP.

[2]
c.

Calculate the difference in median area of synapse between the control mice and mice treated with rAChE and OP, giving the units.

[1]
d.

Describe the evidence for damage to synapses by OP provided by data in the box plot.

[2]
e.

Using the data from both graphs, evaluate the hypothesis that plant-produced rAChE could be used to protect humans or other mammals from damage caused by exposure to OP.

[2]
f.
▶️Answer/Explanation

Markscheme

0.38 (allow any value in the range 0.37–0.39)

a.

a. as the ratio increases, the symptoms decrease;
b. between 0 and 0.16 (accept 0.14 – 0.18) symptoms decrease/are moderate;
c. between 0.16 (accept 0.14 – 0.18) and 0.4 (accept 0.38 – 0.42) symptoms are mild;
d. between 0.38 and 0.41 mice may have mild or no symptoms;
e. after 0.4 (accept 0.38 – 0.42) there are no symptoms;

b.

a. (ratio would have been) 0.5/1 to 2;
b. no symptoms;

c.

90 μm2 (accept 75 – 100) (units required)

d.

a. higher/highest median area of synapses;
b. higher/highest maximum/minimum area of synapses;
c. higher/highest 25th/75th percentile;

e.

a. the higher the RAChE, the milder the symptoms/damage (first graph);
b. with or without OP, RAChE decreases area of synapses / RAChE reduces the damage to synapses (second graph);
c. (but still) some increase in area/damage to synapses (with OP) even with RAChE;
d. the study was done on mice with no evidence that its results extend to humans;

f.

Question

The diagram shows the release of a neurotransmitter across a synapse.

(a) Identify the structure labelled X. [1]

(b) Outline how the neuron is stimulated to release the neurotransmitter. [2]

(c) Explain the action of neonicotinoid pesticides in insects. [3]

▶️Answer/Explanation

a (Neurotransmitter) vesicle; Do not accept Vacuole 
b a. the arrival of a nerve impulse/action potential/depolarisation (stimulates the release of a neurotransmitter);
b. depolarisation stimulates calcium ion channels to open;
OR
calcium ions enter the presynaptic knob/button
c. (calcium ions) cause the vesicle to fuse with / move to the membrane
OR
vesicles release neurotransmitter by exocytosis;

c a. prevents synaptic/nerve transmission

b. is /acts like /has similar structure to a neurotransmitter/acetylcholine
c. (neonicotinoid pesticides) bind to acetylcholine receptors in the post synaptic membrane;
d. neonicotinoid pesticides are not broken down (as acetylcholine would be) by acetylcholinesterase/enzyme;
e. the receptors are overstimulated
f. paralyses/kills the insects;

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