How do plants make their own food, and how is the leaf adapted for the job?
State the word equation for photosynthesis, describe the conditions it needs, and explain how the leaf is adapted to carry it out
A focused N(A)-Level answer on photosynthesis. The word equation, the raw materials and conditions needed, the test for starch, and how the leaf is adapted to photosynthesise.
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What this dot point is asking
SEAB wants you to state the word equation for photosynthesis, to describe the conditions a plant needs to carry it out, and to explain how the leaf is adapted for the job. The central idea is that plants make their own food from simple raw materials using light energy, which is the start of almost every food chain.
The answer
What photosynthesis is
Photosynthesis is the process by which green plants make glucose (a food) from carbon dioxide and water, using light energy trapped by the green pigment chlorophyll. The word equation is:
The glucose is used for energy or stored as starch, and the oxygen is released into the air.
Conditions and raw materials
For photosynthesis a plant needs:
- carbon dioxide, taken in from the air through tiny holes in the leaf,
- water, taken up from the soil through the roots,
- light, usually from the Sun,
- chlorophyll, the green pigment in the chloroplasts that absorbs the light.
If any of these is missing, photosynthesis stops or slows.
Testing a leaf for starch
A leaf can be tested for starch to show it has photosynthesised. After removing the green colour, iodine solution is added: it turns blue-black if starch is present, and stays orange-brown if it is not.
How the leaf is adapted
The leaf is well designed for photosynthesis:
- it is broad and flat, giving a large surface area to absorb light,
- it is thin, so gases can diffuse in and out quickly,
- it contains many chloroplasts full of chlorophyll, especially near the top,
- it has tiny holes (stomata) on the underside that let carbon dioxide in and oxygen out,
- it has veins that bring water and carry away the glucose.
Examples in context
Example 1. Why a houseplant grows towards the window. Light is needed for photosynthesis, so a plant grows towards the brightest light to capture more of it. More light (up to a point) means faster photosynthesis and more food made.
Example 2. Why farmers add carbon dioxide in greenhouses. Carbon dioxide is a raw material for photosynthesis. Raising its level in a greenhouse (along with enough light and warmth) speeds up photosynthesis, so the crops grow faster and yields increase.
Try this
- Cue. State the two products of photosynthesis. Glucose and oxygen.
- Cue. Name the green pigment that absorbs light for photosynthesis. Chlorophyll.
- Cue. Explain why a leaf is broad and flat. To provide a large surface area for absorbing light.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marks(a) Write the word equation for photosynthesis. (b) State the two raw materials a plant takes in and where each comes from.Show worked answer →
(a) Carbon dioxide plus water, using light energy and chlorophyll, gives glucose plus oxygen.
(b) Carbon dioxide is taken in from the air (through the leaves), and water is taken up from the soil (through the roots).
What markers reward: a correct word equation including light and chlorophyll, and the two raw materials with their correct sources.
Original3 marksA leaf is tested for starch and turns blue-black with iodine solution. (a) State what this shows. (b) Explain why a plant kept in the dark would not give this result.Show worked answer →
(a) The blue-black colour shows that starch is present, so the leaf has been photosynthesising.
(b) Without light, photosynthesis cannot happen, so no glucose (and therefore no starch) is made. The leaf would stay brown with iodine.
What markers reward: blue-black showing starch from photosynthesis, and no light meaning no photosynthesis so no starch is made.
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