| Date | May 2013 | Marks available | 2 | Reference code | 13M.3.SL.TZ2.13 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 2 |
| Command term | Compare | Question number | 13 | Adapted from | N/A |
Question
The hypothesis that aging involves loss of brain cells was investigated in mice. The olfactory bulb (OB) of the brain was studied because its layered arrangement of neurons resembles large regions of the human brain. Sensory input about smell is sent to the OB by axons of receptor cells that line the upper nasal cavity. These axons synapse with relay neurons in the OB where interpretation of smell perception begins. The bar charts show the total volume of neurons in the OB and the density of synapses (number of synapses per unit area) in two regions of a mouse’s OB.
State when the total volume of OB neurons is the greatest.
Compare the total synapse density of neurons in the outer and inner OB layers.
Evaluate, using the data in the bar charts, the hypothesis that aging involves loss of brain cells.
Suggest the implications of the data for humans.
Markscheme
6 months
a. no clear trend in the inner layer whereas there is a decrease in the outer layer;
b. outer layer is higher (than inner layer) at 2 months and lower at 18 months;
(do not accept statements that are not comparisons)
Accept any other correct comparisons.
a. volume of neurons remains the same;
b. synapse density in the outer but not the inner layer decreases with age;
c. number of neurons may not be reduced even with fewer synapses;
a. smell perception may fall in aging humans;
b. changes in smell perception may change food eating habits/reduce quality of life;
c. ageing human brains may lose synapses but not neurons (as previously thought);
d. losing synapses in one part of the brain may be repeated in other parts of the brain;
Examiners report
Most were able to state that the total volume was greatest at 6 months.
Most were able to gain a mark for spotting that there is no clear trend in the inner layer but a decrease in the outer. Few gained the second mark. There were many correct statements, but few correct comparisons.
In c most were able to state that the volume of neurons remains the same and that the synapse density in outer but not inner decreases with age.
In d most were able to gain the mark for loss of synapses, but not neurons, but only the more astute candidates could link it back to the introduction and talk about smell perception.
