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Topic 6 · 6.1

Photosynthesis

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • Describe photosynthesis, state its word equation and the role of chlorophyll; state the balanced equation EXTENDED.
  • Outline how the carbohydrates made are used and stored, and why nitrate and magnesium ions are needed.
  • Investigate the need for chlorophyll, light and carbon dioxide, with controls.
  • Investigate the effects of light intensity, carbon dioxide and temperature on the rate, and gas exchange with hydrogencarbonate indicator.
  • Identify and explain limiting factors EXTENDED.

📚The biology

What photosynthesis is

Photosynthesis is the process by which plants synthesise carbohydrates from raw materials using energy from light.

carbon dioxide + water → glucose + oxygen  (in the presence of light and chlorophyll)

\[ \mathrm{6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2} \]

EXTENDED The balanced equation is Supplement. Chlorophyll is a green pigment found in chloroplasts; it transfers energy from light into energy in chemicals, for the synthesis of carbohydrates.

A leaf with arrows: light energy from the Sun and carbon dioxide from the air (entering through stomata) go in, and water arrives from the roots in the xylem. Oxygen leaves through the stomata, and glucose is made, to be used or stored. Inside, chlorophyll in chloroplasts absorbs energy from light.
Raw materials: carbon dioxide and water. Energy: light, absorbed by chlorophyll. Products: glucose and oxygen.

What happens to the glucose

Glucose at the top with arrows to five uses: starch, an energy store; cellulose, for cell walls; respiration, which releases energy; sucrose, transported in the phloem; nectar, which attracts insects.
Glucose is converted to starch for storage because starch is insoluble and does not affect osmosis; it is transported as sucrose.

Plants also need mineral ions from the soil: nitrate ions to make amino acids (and so proteins), and magnesium ions to make chlorophyll. A plant short of magnesium has yellow leaves because it cannot make enough chlorophyll.

Showing what photosynthesis needs

Photosynthesis is detected by testing a leaf for starch. First destarch the plant by keeping it in the dark for 48 hours, so any starch found later must have been made during the experiment. To test a leaf: boil it in water (kills the cells, softens the leaf), boil it in ethanol in a water bath — never over a flame — to remove the chlorophyll, rinse it in warm water, spread it on a white tile and add iodine solution.

Three experiments. A variegated leaf, green in the middle and white at the edge: after the iodine test only the green part is blue-black. A leaf partly covered with a strip of foil: after the test the covered strip is orange-brown and the rest blue-black. Two plants in sealed bags, one with soda lime and one without: the leaf from the soda lime bag stays orange-brown, the other is blue-black.
Blue-black = starch made. Chlorophyll, light and carbon dioxide are all needed. In each, the uncovered part or the second plant is the control.

The rate of photosynthesis

An aquatic plant such as pondweed releases bubbles of oxygen; counting them per minute gives a measure of the rate. Moving the lamp changes the light intensity; sodium hydrogencarbonate in the water supplies carbon dioxide; a water bath changes the temperature.

A lamp shining through a tank of water, the heat shield, on to a test-tube of pondweed standing in a beaker of water containing sodium hydrogencarbonate. Bubbles of oxygen rise from the cut stem. The distance d between lamp and pondweed is marked.
The water tank absorbs heat from the lamp, so light intensity changes without changing the temperature.

Increasing light intensity, carbon dioxide concentration or temperature (up to the enzymes’ optimum) increases the rate.

Gas exchange in the light and dark. Hydrogencarbonate indicator changes colour with the carbon dioxide concentration.

Three sealed tubes of hydrogencarbonate indicator. With pondweed in the light: purple, because photosynthesis is faster than respiration and carbon dioxide falls. With pondweed in the dark: yellow, because only respiration happens and carbon dioxide rises. With no pondweed: stays orange-red.
Plants respire all the time; in bright light photosynthesis uses carbon dioxide faster than respiration releases it.

Limiting factors EXTENDED

A limiting factor is the factor in shortest supply, which stops the rate increasing further. On a graph, where the rate rises as light intensity rises, light is limiting; where the line levels off, something else (carbon dioxide concentration or temperature) is limiting.

A graph of rate of photosynthesis against light intensity with two curves, for high and low carbon dioxide. Both rise steeply at first, where light intensity limits, then level off; the low carbon dioxide curve levels off at a lower rate, where carbon dioxide limits.
Raising carbon dioxide raises the plateau, proving CO2 was the limiting factor there. EXTENDED

EXTENDED Growers use this in glasshouses: extra lighting on dull days, burning paraffin to add carbon dioxide and warmth — but only up to the point where another factor limits, or the cost exceeds the extra yield.

✏️Worked example

Using the pondweed apparatus, a student counted bubbles per minute at four distances: 10 cm, 58; 20 cm, 34; 30 cm, 19; 40 cm, 11. (a) Describe the relationship between distance and rate. [2] (b) Explain why the pondweed was left for 5 minutes at each new distance before counting. [1] (c) Suggest one reason why counting bubbles is not a very accurate measure of the rate. [1] (d) EXTENDED The rate increases less between 20 cm and 10 cm than the extra light would predict. Suggest why. [2]

(a) As the distance increases, the rate decreases: moving the lamp further away lowers the light intensity, so less energy is available for photosynthesis.

(b) To let the plant adjust to the new light intensity, so the rate is steady when counted.

(c) Any: bubbles are different sizes; some oxygen dissolves in the water; bubbles may be missed or run together.

(d) Light is no longer the only limiting factor: another factor, such as carbon dioxide concentration or temperature, is limiting the rate.

Check it. Every step closer gives more bubbles, but the jumps shrink as light stops being the limiting factor — the start of a plateau, as on the limiting-factor graph above.
“The lamp heats the plant.” That is exactly what the heat shield is for. If asked about controls, name temperature as the variable kept constant and say how.

📝Practise

In the style of the multiple-choice, theory and practical papers. EXTENDED marks Supplement content.

1. (Multiple choice.) Which are the raw materials of photosynthesis? A: glucose and oxygen. B: carbon dioxide and oxygen. C: carbon dioxide and water. D: water and glucose.
C.
2. (Theory.) State the role of chlorophyll in photosynthesis. [1]
It transfers energy from light into energy in chemicals (absorbs light energy) for making carbohydrates.
3. (Theory.) A plant grown in soil short of nitrate ions is small and weak. Explain why. [2]
Nitrate ions are needed to make amino acids, and so proteins, which are needed for growth.
4. (Practical.) Explain why a plant is destarched before an experiment on photosynthesis, and describe how. [2]
So that any starch found must have been made during the experiment. Keep the plant in the dark for about 48 hours.
5. (Practical.) When testing a leaf for starch, why is it boiled in ethanol, and why must a water bath be used? [2]
To remove the chlorophyll, so the colour of the iodine can be seen. Ethanol is flammable, so it must not be heated over a flame.
6. (Practical.) Tubes of hydrogencarbonate indicator are set up with pondweed in bright light, in dim light and in the dark. Predict the final colour of each and explain the dim light result. [4]
Bright light: purple. Dark: yellow. Dim light: stays orange-red — photosynthesis uses carbon dioxide at the same rate as respiration releases it, so the concentration does not change.
7. (Theory.) EXTENDED Write the balanced equation for photosynthesis. [2]
\( \mathrm{6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2} \).
8. (Theory.) EXTENDED On a warm, sunny summer afternoon the rate of photosynthesis in a crop stops increasing as the light gets brighter. Suggest the limiting factor and one way a glasshouse grower could raise the rate. [2]
Carbon dioxide concentration. Increase the carbon dioxide in the glasshouse (e.g. burn paraffin or release CO2).

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • Science and Plants for Schools (SAPS) — the algal balls and hydrogencarbonate indicator practical
  • Royal Society of Biology — investigating the rate of photosynthesis with pondweed