| Date | May 2015 | Marks available | 2 | Reference code | 15M.2.SL.TZ1.4 |
| Level | Standard level | Paper | Paper 2 | Time zone | Time zone 1 |
| Command term | Explain | Question number | 4 | Adapted from | N/A |
Question
Distinguish between absorption of red, green and blue light by chlorophyll.
Draw a graph to show the effect of increasing light intensity on the rate of photosynthesis.
Explain one way of directly measuring the rate of photosynthesis.
Markscheme
a. absorbs at blue and red (in high amount);
b. greatest absorption is of blue light / more blue light than red light absorbed;
c. low absorption of green light / green light is reflected;
Allow above points in an annotated diagram.
either:
a. production of oxygen (which is a by-product of photosynthesis);
b. outline of method to collect gas/monitor gas production per unit of time/over time (eg count bubbles/collect in syringe/oxygen sensors over a time period);
or:
c.uptake of carbon dioxide (as carbon dioxide used as raw material for photosynthesis);
d. outline of method to detect uptake of carbon dioxide over time (eg change/rise in pH of water surrounding a water plant using pH meter or paper/CO2 sensor over time);
Examiners report
Most candidates earned two marks for this two mark question. Most stated that blue and red are absorbed (one mark), but that green is reflected (another mark). Thus, they distinguished green from the other two colors. Very few candidates distinguished red from blue, which was listed as a third marking point.
A whole variety of poor drawings was seen from straight lines starting from 0,0 and going up at a 45° angle to sigmoid curves to bell-shaped curves. Many drawings lacked straight sections for the increase or plateau portions.
Few candidates earned both marks here. The most common answers suggested measuring oxygen production or carbon dioxide uptake. Some added a method for doing so; others gave the rationale for doing so. Only a handful suggested taking the measurements for a set amount of time, or taking a reading before and after a time interval. Hence, rate could not be calculated and the second marking point was not earned. As a teaching point, it could be observed that many of the experiments in our practical programmes involving rates, do not insist on rate calculations because a divisor of one (time unit) has been set in the procedure. That kind of shortcut hurt candidates in this examination.
