O-Level Biology · Secondary 3 and 4
Nutrition in Plants
Green plants make their own food by photosynthesis, using light energy to turn carbon dioxide and water into glucose. In this topic you learn how a leaf is built for photosynthesis and gas exchange, the equation for photosynthesis, the factors that limit its rate, and how to show experimentally that light, chlorophyll and carbon dioxide are needed. These ideas link to cell structure, diffusion and osmosis, and to food chains, because almost all the energy in living things first enters through photosynthesis.
What the syllabus expects
- Identify the parts of a leaf in cross-section and relate them to photosynthesis and gas exchange
- State the word and balanced chemical equations for photosynthesis
- Explain the effects of light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis
- Describe the starch test on a leaf and experiments showing that light, chlorophyll and carbon dioxide are needed
- Describe how the glucose made is used by the plant
- Explain the role of stomata and guard cells in gas exchange
Leaf structure
A leaf is broad and thin, giving a large surface area to absorb light and a short distance for carbon dioxide to diffuse to the cells. A waxy cuticle on the upper surface reduces water loss. The upper epidermis is transparent, so light passes through to the palisade mesophyll. Palisade cells are long, tightly packed and contain many chloroplasts, so most photosynthesis happens there. The spongy mesophyll below has irregular cells with large air spaces between them, allowing carbon dioxide and oxygen to diffuse through the leaf. The lower epidermis contains most of the stomata, each surrounded by two guard cells that open and close the pore. Vascular bundles in the veins contain xylem, which brings water and mineral ions to the leaf, and phloem, which carries away sugars made in the leaf. The mesophyll cells are covered by a thin film of water in which gases dissolve before diffusing into them.
The photosynthesis equation
Photosynthesis is the process in which light energy, absorbed by chlorophyll, is used to make glucose from carbon dioxide and water, with oxygen released as a by-product. In words: carbon dioxide + water gives glucose + oxygen, in the presence of light and chlorophyll. The balanced equation is 6CO₂ + 6H₂O gives C₆H₁₂O₆ + 6O₂, with light energy and chlorophyll written over the arrow in your answer. Light energy is converted into chemical energy stored in glucose. Water is absorbed by root hair cells and travels up the xylem. Carbon dioxide enters through the stomata by diffusion. During the day, photosynthesis usually happens faster than respiration, so the leaf takes in carbon dioxide and gives out oxygen overall. At night there is no photosynthesis, so the leaf only respires.
Limiting factors
The rate of photosynthesis depends on light intensity, carbon dioxide concentration and temperature. A limiting factor is the factor in shortest supply, which holds the rate back. On a graph of rate against light intensity, the rate rises at first because light is limiting. Then the line levels off: increasing the light no longer raises the rate, because another factor, such as carbon dioxide concentration or temperature, is now limiting. Raising that factor gives a higher plateau. Temperature affects the enzymes that control photosynthesis, so the rate rises up to an optimum and then falls as the enzymes are denatured. Growers use these ideas in greenhouses, adding lighting, extra carbon dioxide and heating so that no single factor limits growth.
Testing a leaf for starch and using the glucose
Glucose made in photosynthesis is quickly turned into starch, so testing for starch shows whether photosynthesis has taken place. First destarch the plant by keeping it in the dark for 48 hours. To test a leaf, boil it in water for about a minute to kill the cells and make them permeable to iodine. Then place it in a test tube of ethanol in a hot water bath to remove the chlorophyll, which would hide the colour. Dip it in hot water to soften it, spread it on a white tile and add iodine solution. Parts that contain starch turn blue-black and parts without starch stay brown. The plant uses glucose for respiration, converts it to starch for storage, to sucrose for transport in the phloem, to cellulose for cell walls, and, with nitrate ions, into amino acids for proteins.
Worked example
A destarched plant with variegated leaves (green and white parts) was placed in sunlight for six hours. Part of one leaf was covered on both sides with black paper. The leaf was then tested for starch. Predict the colour of (i) the green uncovered parts, (ii) the white uncovered parts and (iii) the green covered parts, and explain each result.
- Recall that the plant was destarched, so any starch found must have been made during the six hours in light.
- Green uncovered parts had both chlorophyll and light, so they photosynthesised, made glucose and converted it to starch. Iodine turns blue-black.
- White uncovered parts had light but no chlorophyll, so no light energy was absorbed and no photosynthesis took place. Iodine stays brown.
- Green covered parts had chlorophyll but no light reached them, so no photosynthesis took place. Iodine stays brown.
- Conclude by comparing pairs: green uncovered versus white uncovered shows chlorophyll is needed, and green uncovered versus green covered shows light is needed.
Answer: (i) Blue-black, starch present. (ii) Brown, no starch because there is no chlorophyll. (iii) Brown, no starch because there is no light. Photosynthesis needs both light and chlorophyll.
Common mistakes
- Saying plants make energy or that photosynthesis produces energy. Light energy is converted into chemical energy in glucose.
- Saying the iodine test shows glucose. Iodine tests for starch. The glucose made is stored as starch, which is why the starch test works.
- Leaving out why the leaf is boiled in ethanol. It removes chlorophyll so that the colour change can be seen. Ethanol is flammable, so heat it in a water bath, never over a flame.
- Explaining a plateau on a light intensity graph by saying 'the plant has enough light'. Say that another factor, such as carbon dioxide concentration or temperature, has become limiting.
- Writing that plants do not respire during the day. Plants respire all the time. In the day, photosynthesis is usually faster, so it masks respiration.
Exam tips
- In variegated leaf or covered leaf questions, state what each part had or lacked (light, chlorophyll), then link to photosynthesis, glucose and starch, then the iodine colour.
- For limiting factor graphs, name the limiting factor on each part of the curve: the rising part is limited by the factor on the x-axis, the plateau by something else.
- Learn the steps of the starch test with the reason for each step. Questions often give one step and ask why it is done.
Practice questions
1. Easy: Write the word equation for photosynthesis and name the substance that absorbs light energy.
Show answer
Carbon dioxide + water gives glucose + oxygen, in the presence of light and chlorophyll. Chlorophyll, in the chloroplasts, absorbs the light energy.
2. Medium: A pondweed releases 30 bubbles of gas in 2 minutes when a lamp is 20 cm away. When the lamp is moved to 10 cm, it releases 52 bubbles in 2 minutes. Calculate the rate at each distance and explain the change.
Show answer
At 20 cm: 30 ÷ 2 = 15 bubbles per minute. At 10 cm: 52 ÷ 2 = 26 bubbles per minute. Moving the lamp closer increases light intensity, and light was limiting, so the rate of photosynthesis increases and more oxygen is released.
3. Harder: Explain how two features of the palisade mesophyll and one feature of the spongy mesophyll help photosynthesis.
Show answer
Palisade cells contain many chloroplasts, so more light energy is absorbed. They are long and closely packed near the upper surface, so many cells receive light. The spongy mesophyll has large air spaces, so carbon dioxide can diffuse quickly from the stomata to the photosynthesising cells.
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