4.4.1.1Photosynthetic reaction
Photosynthesis is represented by the equation:
carbon dioxide + water → (light) → glucose + oxygen
You should recognise the chemical symbols CO₂, H₂O, O₂ and C₆H₁₂O₆.
Photosynthesis is an endothermic reaction: energy is transferred from the environment to the chloroplasts by light. Endothermic means energy is taken in, and that word is worth learning here because it is the one the question asks for — a plant is storing the Sun's energy in glucose rather than releasing energy.
Common mistake
Light is not a reactant. The reactants are carbon dioxide and water; light is the energy source, which is why it sits above the arrow rather than on the left of it. Writing "carbon dioxide + water + light" in the equation loses the mark.
- Photosynthesis
- the reaction in which carbon dioxide and water are converted into glucose and oxygen using energy transferred from light
- Endothermic reaction
- a reaction in which energy is taken in from the environment; photosynthesis is endothermic because energy is transferred from the environment to the chloroplasts by light
4.4.1.2Rate of photosynthesis
Four things affect the rate of photosynthesis, and you should be able to explain the effect of each: temperature, light intensity, carbon dioxide concentration, and the amount of chlorophyll.
• Light intensity — more light means more energy for the reaction, so the rate rises until something else runs short. • Carbon dioxide concentration — more of a reactant means a faster rate, on the same terms. • Temperature — the reaction is enzyme controlled, so the rate rises with temperature up to an optimum and then falls as the enzymes denature. That fall is what makes temperature different from the other two. • Amount of chlorophyll — less chlorophyll means less light absorbed. Disease such as tobacco mosaic virus, or magnesium deficiency, reduces it.
Higher tier only: these factors interact, and any one of them may be the factor that limits photosynthesis. Reading a graph with two or three factors on it means finding where each line levels off — while the line is rising, the factor on the x-axis is limiting; once it is flat, something else is.
Higher tier only: light intensity follows an inverse square law. Double the distance from the lamp and the light intensity falls to a quarter, not a half, because intensity is proportional to 1 ÷ distance². That is why a rate-against-distance graph curves the way it does.
Handling the numbers is examined as much as the biology. Measure and calculate rates as amount ÷ time. When you plot, put the factor you changed on the x-axis and the rate on the y-axis, select a scale for the axes that fills at least half the grid, and draw a line of best fit.
Reading in the other direction, extract and interpret the graph rather than describing its shape: quote figures with units, say where the line stops rising, and name what is limiting the rate on each part of it. Translating information between graphical and numeric form — reading a value off a line, or turning a table of counts into a graph — is a skill the paper tests directly, and rearranging rate = amount ÷ time to find a missing quantity is the simple algebra it expects with it.
Worked exampleHigher tier only
A lamp is moved from 10 cm to 20 cm from the pondweed. By what factor does the light intensity change?
- Light intensity is proportional to 1 ÷ distance².
- At 10 cm: 1 ÷ 10² = 1 ÷ 100.
- At 20 cm: 1 ÷ 20² = 1 ÷ 400.
- Compare: (1/400) ÷ (1/100) = 100 ÷ 400.
Answer: The intensity falls to a quarter — doubling the distance divides the intensity by four.
Worked example
Pondweed releases 45 bubbles of oxygen in 3 minutes at one distance from the lamp, and 15 bubbles in 3 minutes at another. Calculate the rate of photosynthesis at each, in bubbles per minute.
- Rate = number of bubbles ÷ time.
- 45 ÷ 3 = 15 bubbles per minute.
- 15 ÷ 3 = 5 bubbles per minute.
- Compare: 15 ÷ 5 = 3.
Answer: 15 and 5 bubbles per minute — the rate is three times higher at the nearer distance.
Required practical 6
Investigate the effect of light intensity on the rate of photosynthesis using an aquatic organism such as pondweed
- Put a piece of pondweed in a test tube of water containing sodium hydrogencarbonate solution, which supplies carbon dioxide.
- Place a lamp a measured distance from the tube and leave it a few minutes to settle.
- Count the bubbles of oxygen given off in one minute, or collect the gas in a capillary tube and measure its volume.
- Repeat at a range of distances, keeping the temperature constant with a heat shield or water bath.
- Repeat each distance at least three times and take a mean, then plot rate against light intensity.
Watch out: The lamp heats the water as well as lighting it, so without a heat shield you are changing two variables at once and cannot say which caused the change. Counting bubbles is quick but rough — bubbles are not all the same size, so measuring the volume of gas is the better method if you have it.
Higher tier only: limiting factors matter economically. In a greenhouse a grower can add heat, light or carbon dioxide to raise the rate of photosynthesis, but each costs money. The decision is whether the extra crop is worth more than the extra fuel or electricity — so data about which factor is limiting tells a grower which input is worth buying, and which would be spent on a factor that is not holding the plant back at all.
- Limiting factor
- the factor in shortest supply, which is holding back the rate of photosynthesis; on a graph, the factor on the x-axis is limiting while the line is still rising
- Inverse square law
- the relationship in which light intensity is proportional to 1 divided by distance squared, so doubling the distance from a lamp reduces the intensity to a quarter
4.4.1.3Uses of glucose from photosynthesis
The glucose produced in photosynthesis may be:
• used for respiration • converted into insoluble starch for storage • used to produce fat or oil for storage • used to produce cellulose, which strengthens the cell wall • used to produce amino acids for protein synthesis.
To produce proteins, plants also use nitrate ions absorbed from the soil — which is why nitrate deficiency stunts growth.
Common mistake
Starch is stored rather than glucose because starch is insoluble. Storing large amounts of soluble glucose would make the cell contents far more concentrated, so water would move in by osmosis. That reason is the mark, not the word "storage". You can identify these products with the qualitative reagents from required practical 4: iodine turns blue-black with starch, Benedict's solution turns orange or red with sugars, and Biuret reagent turns purple with protein.
- Starch
- the insoluble carbohydrate that plants convert glucose into for storage
- Cellulose
- the substance produced from glucose that strengthens the plant cell wall
Before you practise
- carbon dioxide + water → glucose + oxygen, with light above the arrow. Endothermic: energy is taken in.
- Rate factors: light intensity, carbon dioxide concentration, temperature, amount of chlorophyll.
- Temperature is the odd one out — the rate falls again past the optimum as enzymes denature.
- Higher tier: any factor can be the limiting one; a levelling graph means something else has become the limit.
- Higher tier: intensity ∝ 1 ÷ distance², so doubling the distance quarters the light.
- RP6: pondweed, sodium hydrogencarbonate for carbon dioxide, control the temperature, repeat and take a mean.
- Glucose goes to respiration, starch, fats and oils, cellulose, or amino acids — the last needing nitrate ions too.
- Starch is stored because it is insoluble and so does not affect osmosis.