Photosynthesis stores light energy as chemical energy in glucose. Animals depend on that glucose, and on the oxygen released with it.
The idea
In the green parts of a plant, chlorophyll absorbs light. Carbon dioxide enters through the stomata and water comes up from the roots. Using light energy, the leaf builds glucose and releases oxygen. Some glucose is used at once in respiration. Some is stored as starch. Some is converted into other substances, including cellulose for cell walls and proteins when mineral salts such as nitrates are available. A test for starch with iodine is evidence that photosynthesis has been storing food. It is not itself photosynthesis.
The rate depends on what is in short supply. In dim light, light is the limiting factor. In bright light with little carbon dioxide, carbon dioxide is limiting. Temperature affects the enzymes of the process, so a cold leaf works slowly, and a leaf that is too hot may be damaged. Water is a raw material, but on the school syllabus it is not usually the factor that limits the rate in the short term. When you draw a sketch of rate against light, the line rises and then flattens. The flat part means some other factor has taken over.
What earns the marks
Products and raw materials must not be swapped. Oxygen out, carbon dioxide in. A limiting factor is the one holding the rate down, not every substance involved. If the question gives a graph that levels off, say what the plateau means: increasing light further does nothing because another factor is now limiting. Iodine turning blue black shows starch, which is stored glucose, not the gas produced.
Practice questions
These are practice questions in the style of the paper, with a model answer and a mark scheme. They are not a promise of what your paper will ask.
1Question 1
4 marks
(a) Write the word equation for photosynthesis. (2 marks)
(b) State two conditions required, apart from the raw materials. (2 marks)
Model answer
(a) Carbon dioxide + water → glucose + oxygen, in the presence of light and chlorophyll.
(b) Light and chlorophyll.
Mark scheme
Carbon dioxide and water on the reactant side1 mark
Glucose and oxygen on the product side1 mark
Light1 mark
Chlorophyll, or a green leaf. "Chloroplast" is accepted as the place, but the condition named in the equation is chlorophyll1 mark
2Question 2
3 marks
A crop in an open field is photosynthesising quickly at noon. The rate does not rise when the sun becomes even brighter.
(a) What is a limiting factor? (2 marks)
(b) Suggest one factor that may now be limiting this crop. (1 mark)
Model answer
(a) A limiting factor is the condition in shortest supply, the one that sets the rate. Making the others more generous does not speed the process until this one is increased.
(b) Carbon dioxide, or temperature.
Mark scheme
The factor in shortest supply, or the factor that controls the rate1 mark
Increasing other factors does not raise the rate while this one is unchanged1 mark
Carbon dioxide or temperature. "Water" is not the expected short term answer here1 mark
3Question 3
2 marks
Explain one reason animals depend on photosynthesis, even though they do not carry it out. (2 marks)
Model answer
Photosynthesis produces the food that enters food chains, and it releases the oxygen that animals use in respiration.
Mark scheme
It produces organic food, or glucose, that animals eat directly or through a food chain1 mark
It releases oxygen used in respiration. Either point can stand if the other is replaced by a clear equivalent. Both are here, and either one with the idea of dependence earns the marks if both ideas are present as written1 mark
Practise Integrated Science with Anna
More questions on this topic are inside GLearn, marked as you answer, with the reason written out. The first ones need no account.
It is the condition that is in short supply and therefore sets the rate. Light, carbon dioxide and temperature are the usual three. When a graph of rate against light levels off, more light is no longer helping. Another factor has become limiting.
Does iodine test for oxygen?+
No. Iodine solution tests for starch. A blue black colour means starch is present, which is how the leaf has stored sugar made in photosynthesis. Oxygen is tested with a glowing splint, which relights. All WASSCE Integrated Science past questions