| Date | May 2011 | Marks available | 1 | Reference code | 11M.3.SL.TZ1.7 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Suggest | Question number | 7 | Adapted from | N/A |
Question
The rate of photosynthesis in the marine seagrass, Zostera marina, was investigated under a range of pH conditions. After a period of darkness, the plants were illuminated at a constant light intensity at 15°C and the rate of photosynthesis was measured. Zostera marina can use both dissolved carbon dioxide ( CO2 ) and hydrogen carbonate ions for photosynthesis. The rate of photosynthesis is plotted on the y-axis on the left. In addition, the concentration of carbon dioxide was measured for each pH investigated and is plotted on the y-axis on the right.
State the carbon dioxide concentration at pH 7.2.
Calculate the percentage decrease in the rate of photosynthesis from pH 7 to pH 7.5.
Outline the relationship between pH and the rate of photosynthesis.
Suggest how Zostera marina can perform photosynthesis even at very low carbon dioxide concentrations.
Based on the information and data provided, discuss the role of one limiting factor, other than carbon dioxide, and suggest how this would affect the rate of photosynthesis.
Markscheme
200 μM (units required)
(77–51)/77×100 = 35 % (Units required. Allow answers in the range of 32–37 %.)
highest rate of photosynthesis at pH 7;
decrease (in rate of photosynthesis) between pH 7 and pH 7.5;
little change (in rate of photosynthesis) at higher pH values;
rate of photosynthesis falls again (slightly) at pH 9;
uses hydrogen carbonate ions;
uses stored carbon dioxide / carbon dioxide from respiration;
pH
optimum pH may be less than 7;
reducing the pH below 7 may lead to a higher rate (of photosynthesis);
(but) enzyme activity can be affected by low pH;
or
Temperature
optimum temperature may not be 15ºC;
enzyme activity is affected by temperature;
temperatures above (or below) 15ºC may lead to a higher rate (of photosynthesis);
or
Light intensity
light intensity may not be optimal/may be limiting;
too low light intensity produces less ATP/NADPH + H+ ;
higher light intensities may result in a higher rate (of photosynthesis);
as light intensity/temperature increases rate of photosynthesis may not increase as another factor becomes limiting;
[1] for named limiting factor and [1] for effect on photosynthesis.
Examiners report
This was mainly answered correctly, and again, units were required.
The majority gave correct answers within the required range of 32-37%.
This question required candidates to refer to the data given in the graph, but many answered in general terms without any referral to the pHs shown in the data. Consequently, answers tended to be too vague to gain marks.
Most gained a mark for “use of hydrogen carbonate ions”, but few gave the idea of carbon dioxide from respiration being used.
Many students could identify a limiting factor correctly, but did not then explain how it could affect photosynthesis.
