| Date | May 2012 | Marks available | 3 | Reference code | 12M.3.SL.TZ1.8 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Explain | Question number | 8 | Adapted from | N/A |
Question
Distinguish between oxidation and reduction.
Outline the process of glycolysis.
Using light as an example, explain the concept of limiting factors in photosynthesis.
Markscheme
oxidation is loss of electrons, reduction is gain of electrons;
oxidation frequently involves gaining oxygen, reduction is losing oxygen;
oxidation is losing hydrogen, reduction is gaining hydrogen;
oxidation increases oxidation state / number of elements, reduction lowers it;
takes place in cytoplasm;
glucose is phosphorylated/two molecules of ATP are used;
one hexose sugar/glucose is converted into two three-carbon/3C molecules/hydrolysis;
pyruvate is formed/oxidation of glucose to pyruvate;
small yield/net gain of two ATP;
net gain of two NADH + H+;
does not require/use oxygen/anaerobic process;
limiting factors can determine the rate of photosynthesis / if the level of a factor is changed the rate of photosynthesis changes;
only changes to one factor will affect rate of photosynthesis at a particular time;
light intensity affects the light-dependent reactions/production of ATP/NADPH;
at low light levels the rate of photosynthesis is directly proportional to light intensity/light is limiting;
at high light levels there is no further increase in the rate of photosynthesis/some other factor is limiting (e.g. CO2/temperature);
Accept the above points illustrated by a suitable correctly sketched graph with both axes labelled and correct shape (see example below).
Examiners report
This was answered well by most students, the majority getting two marks for the idea of loss/gain of electrons and loss/gain of oxygen.
Many candidates had no difficulty in outlining the process of glycolysis and most gained three marks. Some of the weaker answers were too vague with few precise statements about the different stages and substances produced.
The understanding of many candidates was a little limited in this question, and their answers were less well presented. Many had the idea of limiting factors, but few could express clearly the way in which light could affect the rate of photosynthesis.
