Syllabus coverage
- Describe photosynthesis as the process by which plants synthesise carbohydrates from raw materials using energy from light
- State the word equation for photosynthesis as: carbon dioxide + water -> glucose + oxygen, in the presence of light and chlorophyll
- State the balanced chemical equation for photosynthesis as: 6CO2 + 6H2O -> C6H12O6 + 6O2
- State that chlorophyll is a green pigment that is found in chloroplasts, and that it transfers energy from light into energy in chemicals, for the synthesis of carbohydrates
- Outline the subsequent use and storage of the carbohydrates made in photosynthesis, limited to: starch as an energy store, cellulose to build cell walls, glucose used in respiration to provide energy, sucrose for transport in the phloem, and nectar to attract insects for pollination
- State that most leaves have a large surface area and are thin, and explain how these features are adaptations for photosynthesis
- Identify in diagrams and images the following structures in the leaf of a dicotyledonous plant: chloroplasts, cuticle, guard cells and stomata, upper and lower epidermis, palisade mesophyll, spongy mesophyll, air spaces, vascular bundles, xylem and phloem, and explain how these structures adapt leaves for photosynthesis
- Explain the importance of nitrate ions for making amino acids, and of magnesium ions for making chlorophyll
- Investigate the need for chlorophyll, light and carbon dioxide for photosynthesis, using appropriate controls, and investigate and describe the effects of varying light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis
- Identify and explain the limiting factors of photosynthesis in different environmental conditions
Part of the IGCSE Biology syllabus 0610 / 0970.
Photosynthesis: the process and the equations
Photosynthesis is the process by which plants synthesise carbohydrates from raw materials using energy from light. The raw materials are carbon dioxide and water; the energy comes from light, and it is trapped by the green pigment chlorophyll. The carbohydrate made is glucose, and oxygen is released as a by-product.
The word equation is: carbon dioxide + water -> glucose + oxygen, in the presence of light and chlorophyll. Learn the conditions as well as the two reactants. A question that asks what a plant needs for photosynthesis wants carbon dioxide, water, light and chlorophyll - not just the gases.
The Extended paper also wants the balanced chemical equation: 6CO2 + 6H2O -> C6H12O6 + 6O2. Check that it balances: 6 carbon atoms, 12 hydrogen atoms and 18 oxygen atoms on each side.
| Equation | |
|---|---|
| Word equation | carbon dioxide + water -> glucose + oxygen (in the presence of light and chlorophyll) |
| Balanced chemical equation | 6CO2 + 6H2O -> C6H12O6 + 6O2 |
| Energy | Light energy is transferred into the chemical energy stored in glucose |
Chlorophyll and chloroplasts
Chlorophyll is a green pigment found in chloroplasts, the small green structures inside plant cells. It is the pigment that absorbs light, and it is the reason leaves are green.
Chlorophyll transfers energy from light into energy in chemicals, for the synthesis of carbohydrates. In other words, it converts light energy into the chemical energy stored in glucose - this is the link between the light that falls on a leaf and the glucose that is made inside it.
A plant cell with no chlorophyll cannot photosynthesise, because it cannot absorb light. This explains two things you are asked about: a variegated leaf makes starch only in its green regions, and a plant kept in the dark turns yellow as it loses its chlorophyll and can no longer photosynthesise.
Leaf structure and how it adapts a leaf for photosynthesis
A leaf is a photosynthesis factory, and every part of its structure is an adaptation for that job. Most leaves have a large surface area and are thin. A large surface area absorbs more light, and a thin leaf lets carbon dioxide and oxygen diffuse in and out over a short distance, so gas exchange is fast.
Inside, the leaf is built in layers, and each layer has a job. Examiners ask you to name the layer and then explain how it helps photosynthesis, so learn the two together.
| Leaf structure | How it helps photosynthesis |
|---|---|
| Waxy cuticle | A thin waterproof layer on the upper surface; it is transparent, so light passes through, and it reduces water loss by evaporation |
| Upper epidermis | A single layer of transparent cells with no chloroplasts; it lets light through to the palisade layer below and protects the leaf |
| Palisade mesophyll | Packed cells near the top of the leaf, each containing many chloroplasts; they absorb the most light and are the main site of photosynthesis |
| Spongy mesophyll | Loosely packed cells with many air spaces between them; the air spaces let carbon dioxide diffuse quickly to the cells and oxygen diffuse away |
| Vascular bundle (xylem and phloem) | Xylem brings water and mineral ions in from the roots; phloem carries sucrose away to the rest of the plant |
| Lower epidermis | The underside of the leaf, which contains the stomata; it protects the leaf while allowing gases in and out |
| Stomata and guard cells | Pores that let carbon dioxide in and oxygen and water vapour out; the guard cells open and close them |
Stomata and guard cells: the leaf's gas doors
Stomata (singular: stoma) are tiny pores, mostly in the lower epidermis of the leaf. They are the openings through which carbon dioxide diffuses in and oxygen and water vapour diffuse out.
Each stoma is surrounded by two guard cells. When the guard cells take in water they swell and become turgid, and because of the way their cell walls are thickened, the stoma opens. When the guard cells lose water they become flaccid and the stoma closes.
This opening and closing controls gas exchange and, just as importantly, controls water loss. Having most stomata on the lower surface, out of direct sunlight, reduces evaporation.
- Carbon dioxide diffuses in through the stomata; oxygen and water vapour diffuse out.
- Guard cells turgid (full of water) -> the stoma is open.
- Guard cells flaccid (short of water) -> the stoma is closed.
- Closing the stomata in drought or at night reduces water loss, but it also stops carbon dioxide entering, so photosynthesis slows.
Limiting factors and the shape of the graphs
The rate of photosynthesis is controlled by three factors: light intensity, carbon dioxide concentration and temperature. At any moment the rate is limited by whichever of these is in shortest supply - that is the limiting factor.
For light intensity and carbon dioxide concentration the graph has the same basic shape: the rate rises as the factor increases, then levels off (plateaus). The rise is the region where that factor is limiting; the plateau is where it is no longer limiting, and one of the other two factors has become the limiting factor.
Temperature is the exception at the top end. Instead of levelling off, the rate rises to an optimum and then falls sharply, because the enzymes that control photosynthesis are denatured by high temperature.
| Factor | What the graph shows | Why |
|---|---|---|
| Light intensity | Rises steeply at first, then levels off | More light is absorbed by chlorophyll, so the rate rises; once another factor is in shortest supply the rate stops rising |
| Carbon dioxide concentration | Rises, then levels off | More carbon dioxide is available to make glucose; the rate plateaus when light intensity or temperature becomes limiting |
| Temperature | Rises to an optimum, then falls sharply | Warm conditions give the enzymes more kinetic energy and more collisions, until high temperature denatures the enzymes and the rate falls |
Mineral requirements: nitrate and magnesium ions
Plants need mineral ions from the soil as well as carbon dioxide and water. Cambridge requires two: nitrate ions and magnesium ions. Both are absorbed from the soil by the roots.
Nitrate ions are needed for making amino acids, and therefore proteins. Proteins are needed for growth, so a plant short of nitrates cannot build new cells and its growth is stunted.
Magnesium ions are needed for making chlorophyll. A plant short of magnesium cannot make enough chlorophyll, so its leaves turn yellow and it can photosynthesise less, which also slows its growth.
| Mineral ion | Used to make | Deficiency symptom |
|---|---|---|
| Nitrate ions | Amino acids, and therefore proteins | Stunted growth - the plant cannot make enough protein to build new cells |
| Magnesium ions | Chlorophyll | Yellow leaves, because the plant cannot make enough chlorophyll, so photosynthesis is reduced |
Investigating photosynthesis
To show that a leaf has photosynthesised, test it for starch with iodine solution. Iodine turns blue-black where starch is present, and starch is made from the glucose of photosynthesis.
First destarch the plant by leaving it in the dark for 24-48 hours. This uses up any starch already stored in the leaves, so any starch found afterwards must have been made during the experiment. Destarching is the control that makes the test valid.
To test a leaf: boil it in water to kill it and break down the cell membranes, then boil it in ethanol in a hot water bath to remove the green chlorophyll so the colour change can be seen, then rinse it and add iodine solution. A blue-black colour means starch is present.
Variegated leaves (green and white patches) show that chlorophyll is needed: after a starch test only the green parts turn blue-black, because only they contain chlorophyll. A leaf with part of it covered with black paper shows that light is needed, because only the uncovered part contains starch. An aquatic plant such as pondweed can show the effect of light intensity by counting the bubbles of oxygen released per minute.
| Experiment | What it shows |
|---|---|
| Destarch the plant (24-48 hours in the dark), then test a leaf with iodine | Any starch found was made during the experiment, not stored beforehand |
| Test a variegated leaf (green and white patches) with iodine | Only the green parts turn blue-black, so chlorophyll is needed for photosynthesis |
| Cover part of a leaf with black paper, leave the plant in light, then test with iodine | Only the uncovered part contains starch, so light is needed for photosynthesis |
| Pondweed in water with a lamp at different distances, counting oxygen bubbles per minute | The effect of light intensity on the rate of photosynthesis |
| An aquatic plant with hydrogencarbonate indicator solution | Shows the effect of light and dark on gas exchange: the indicator turns purple in the light (carbon dioxide used) and yellow in the dark (carbon dioxide released) |
Key terms you must be able to define
- Photosynthesis
- The process by which plants synthesise carbohydrates from raw materials using energy from light; carbon dioxide and water are converted into glucose and oxygen.
- Chlorophyll
- The green pigment found in chloroplasts that absorbs light and transfers energy from light into energy in chemicals, for the synthesis of carbohydrates.
- Chloroplast
- The green structure inside a plant cell that contains chlorophyll; the site of photosynthesis.
- Stoma (plural stomata)
- A tiny pore, mostly in the lower epidermis of a leaf, through which carbon dioxide diffuses in and oxygen and water vapour diffuse out.
- Guard cell
- One of the pair of cells surrounding a stoma; it opens the stoma when turgid (full of water) and closes it when flaccid.
- Limiting factor
- The factor in shortest supply that is holding back the rate of photosynthesis at that moment - light intensity, carbon dioxide concentration or temperature.
- Palisade mesophyll
- The layer of tightly packed cells near the upper surface of the leaf, containing many chloroplasts; the main site of photosynthesis.
- Destarch
- To keep a plant in the dark before an experiment so that the starch already stored in its leaves is used up, making the starch test valid.
Applied to the real world
Cambridge sets biology in unfamiliar contexts - a hospital, a farm, a water-treatment plant. Use these as models for how to apply the science, then write your own.
A commercial grower burns a paraffin heater or pipes in extra carbon dioxide and adds artificial lamps to raise the rate of photosynthesis. More carbon dioxide and more light mean more glucose is made, so the crop grows faster and the yield is higher. The grower also keeps the greenhouse warm, but not too warm, because above the optimum the enzymes of photosynthesis are denatured.
A gardener tests a variegated leaf (green with white patches) for starch and finds that only the green parts turn blue-black with iodine. The white patches contain no chlorophyll, so they cannot absorb light and cannot make starch. For the same reason, a houseplant left in a dark room loses its chlorophyll and its leaves turn yellow, because it is no longer photosynthesising.
A farmer spreads nitrate fertiliser on a cereal crop so the plants can make the amino acids and proteins they need for growth. A crop short of nitrate is stunted and yields little. If the soil is short of magnesium instead, the plants cannot make chlorophyll, so their leaves turn yellow and photosynthesis slows - a different deficiency with a different symptom.
Pondweed is used to show the effect of light intensity on photosynthesis: the closer the lamp, the more oxygen bubbles are released per minute, until the rate levels off because carbon dioxide or temperature becomes the limiting factor. In an aquarium, aquatic plants in a brightly lit tank release oxygen that helps keep the fish healthy, while in the dark they respire and release carbon dioxide.
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Exam-style questions
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Mark scheme — tick each point you actually made
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Mark scheme — tick each point you actually made
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Exam technique for this topic
- Learn the word equation and the balanced equation together, and include the conditions. The word equation is carbon dioxide + water -> glucose + oxygen in the presence of light and chlorophyll; the balanced equation is 6CO2 + 6H2O -> C6H12O6 + 6O2.
- When a question asks about leaf structure, name the part and then give its job. 'The palisade mesophyll is packed with chloroplasts, so it absorbs the most light' scores; naming the layer on its own does not.
- For limiting-factor graphs, describe the shape and then explain it: the rate rises while that factor is limiting, then plateaus because another factor - usually carbon dioxide concentration or temperature - has become limiting.
- Never write that plants respire only at night. Plants respire all the time; photosynthesis happens only in the light. In daylight a plant does both at once.
- Keep the mineral ions apart. Nitrate ions make amino acids and proteins (deficiency: stunted growth); magnesium ions make chlorophyll (deficiency: yellow leaves).
- In any practical question, mention destarching and iodine. Destarching shows the starch was made during the experiment, and iodine turning blue-black shows starch is present. For the effect of light intensity, count oxygen bubbles per minute from pondweed.
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Where this fits in IGCSE Biology
This page is one topic of Cambridge IGCSE Biology 0610 / 0970. IGCSE Biology is the course home: every area and every topic in Cambridge's own order, the papers you sit and what each one tests, and the exam-practice tools. The full topic list, with each topic linked to its revision page, is on the IGCSE Biology syllabus page.