A leaf makes glucose using light, chlorophyll, carbon dioxide and water, and it gives off oxygen.
The idea
Photosynthesis is how a green plant makes its food. Chlorophyll in the leaf traps light energy. That energy drives a reaction between carbon dioxide from the air and water from the soil. The plant makes glucose and releases oxygen. In words: carbon dioxide plus water, in the presence of light and chlorophyll, gives glucose and oxygen.
The leaf is built for this work. A broad thin blade catches light and lets gases move quickly. Stomata, mostly on the lower surface, let carbon dioxide in and oxygen out. Veins bring water in and carry food away. Glucose may be used at once in respiration, stored as starch, or changed into other substances the plant needs.
What earns the marks
Keep raw materials, products and conditions in separate lists. Oxygen is given off, it is not taken in. Chlorophyll traps the light. It is not a raw material and it is not used up. If the question asks for a starch test, iodine solution turns blue black only when starch is present. A leaf kept in the dark is tested so you can show that light was needed.
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) Name two raw materials used in photosynthesis. (2 marks)
(b) Name the pigment that traps light energy. (1 mark)
(c) State one reason a leaf is broad. (1 mark)
Model answer
(a) Carbon dioxide and water.
(b) Chlorophyll.
(c) A broad leaf catches more light.
Mark scheme
Carbon dioxide named as a raw material1 mark
Water named as a raw material1 mark
Chlorophyll named as the pigment. A chloroplast is the organelle, not the pigment, so it does not earn this mark1 mark
A broad leaf has a larger surface for absorbing light1 mark
2Question 2
4 marks
A potted plant was kept in a dark cupboard for two days. A leaf was then tested with iodine solution and it stayed yellow brown.
(a) Why was the plant kept in the dark first? (1 mark)
(b) Give two reasons the iodine did not turn blue black. (2 marks)
(c) Name the gas the leaf gives off when it is later placed in bright light. (1 mark)
Model answer
(a) So that starch already stored in the leaf would be used up.
(b) There was no light, so photosynthesis stopped, and no new starch was made for the iodine to detect.
(c) Oxygen.
Mark scheme
To use up stored starch, or to destarch the leaf1 mark
No light, so photosynthesis could not take place1 mark
No starch was formed, so iodine stayed yellow brown1 mark
Oxygen1 mark
3Question 3
6 marks
State three features of a leaf that help photosynthesis, and say how each feature helps. (6 marks)
Model answer
The leaf is broad, so it absorbs a lot of light. It has many stomata, so carbon dioxide can enter. Veins carry water to the cells and carry food away.
Mark scheme
Broad shape named, with the idea that it catches more light2 marks
Stomata named, with the idea that carbon dioxide enters through them2 marks
Veins named, with the idea that they supply water or remove food2 marks
Practise Science with Kevin
More questions on this topic are inside GLearn, marked as you answer, with the reason written out. The first ones need no account.
Is chlorophyll a raw material for photosynthesis?+
No. The raw materials are carbon dioxide and water. Chlorophyll is the green pigment that traps light. Light and chlorophyll are conditions, not substances that are used up as reactants.
What does a blue black colour with iodine tell you about a leaf?+
It tells you starch is present. Plants often store the glucose from photosynthesis as starch, so a blue black result is used as evidence that photosynthesis has taken place. All BECE Science past questions