1. In an investigation of the light dependent reaction of photosynthesis, chloroplasts were placed in a tube containing water in which some of the molecules were labelled with 18O (H2180). The tube had a few drops of DCPIP, a hydrogen acceptor, added.
DCPIP is a blue dye which goes colourless when reduced. All carbon dioxide was removed from the tube.
The tube was exposed to light. After one hour the air space above the chloroplast suspension was found to contain oxygen molecules labelled with 180. The DCPIP had turned from blue to colourless.
(i) the presence of the 1802 in the tube;
(ii) why the DCPIP turned colourless.
b) Name the two products produced in the light dependent reaction that are used in the light independent reaction.
(Marks available: 5)
Answer outline and marking scheme for question: 1
Give yourself marks for mentioning any of the points below:
a) (i) oxygen produced during photolysis/using light energy; from breakdown of water
(ii) DCPIP accepted hydrogen/protons/H+
b) NADPH/reduced NADPH: ATP
(Marks available: 5)
2. Write an essay on Photosynthesis and its importance in ecosystems.
(Marks available: 25)
Answer outline and marking scheme for question: 2
You can give yourself up to 2 marks for discussing any of the following issues up to a maximum of 16:
Mechanism of photosynthesis
he structure and role of chloroplasts in relation to photosynthesis;
Light-dependent stage, light energy excites electrons in chlorophyll;
energy in excited electrons used to generate ATP & reduced NADP;
hotolysis of water donates hydrogen & oxygen released;
Light-independent stage - carbon dioxide is accepted by ribulose bisphosphate to form two molecules of glycerate 3-phosphate;
ATP and reduced NADP used to reduce GP to carbohydrate;
ribulose bisphosphate is regenerated, cyclical nature of scheme;
Importance to ecosystems
you have explained how process uses light energy to synthesise organic molecules, necessary for heterotrophs from inorganic materials;
you have explained that photosynthesis is the major route by which energy enters an ecosystem, and is transferred through trophic levels and dissipated;
you have explained how photosynthesis is responsible for recycling the gases oxygen (and carbon dioxide) and so its significance to aerobes/animals;
you have explained how the rate of photosynthesis varies in ecosystems, due to limiting factors of temperature, carbon dioxide concentration, and light intensity;
you have considered other factors in ecosystems affecting rate or productivity e.g. sun & shade plants, leaf structure, different types of
you have considered an aspect of human activity related to photosynthesis in ecosystems e.g. crop production, glasshouse management, greenhouse effect;
Breadth (max 3 marks)
uses more than one ecosystem to illustrate; considers process from the viewpoints of the whole organism and the ecosystem;
considers factors affecting its rate;
Depth (max 3 marks)
deals clearly with both stages;
considers photosynthesis in terms of the energy it contributes to ecosystems
makes fully clear the importance of photosynthesis to ecosystems;
In the real exam you will also be awarded marks for quality of language (in this case, there were 3 marks available).It may be difficult for you to judge your own text, however you may be able to get someone else to give you an opinion.