| Date | May 2016 | Marks available | 1 | Reference code | 16M.3.HL.TZ0.4 |
| Level | Higher level | Paper | Paper 3 | Time zone | TZ0 |
| Command term | Draw | Question number | 4 | Adapted from | N/A |
Question
Synaptic density is the number of synapses per unit volume. The graph shows the synaptic density for a 28-week-old fetus, and from birth to age 70.
(i) Determine the age when synaptic density is highest.
(ii) Explain how synaptic density decreases after the age determined in (a)(i).
Label the diagram of the reflex arc with the names of the neurons indicated.
Draw an arrow on the diagram of the reflex arc to show the direction of impulses.
Markscheme
(i) 8 months
(ii) Neural pruning
OR
loss of neurons
Through apoptosis/programmed cell death
Loss of dendrites/axon branches/synapse elimination
Due to lack of use
«In older age» damage to brain/strokes/chemical abuse
I: interneuron/relay neuron
II: sensory neuron
Arrow drawn on diagram to show correct direction of impulse
Arrow can also be a single loop through interneuron showing complete pathway or two arrows to and from interneuron.
Examiners report
(i) Almost all were able to read the correct value off the graph and score one mark.
(ii) This question was poorly answered and one that seemed poorly understood. Many candidates did not ‘explain’ how synaptic density decreases (after 8 months) but instead described the graph. Some were able to get a mark for knowing that the neural pruning occurred and others that the change in density was due to lack of use but little other knowledge was apparent.
Most were able to correctly label the diagram and get two marks.
Some carelessly drew only half the pathway for the reflex arc and therefore did not score the mark for this question.
