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Photosynthesis is one of the most important biological processes on Earth. It is the process by which plants, algae and some bacteria convert light energy into chemical energy stored in glucose. This lesson covers the equations, conditions and details of photosynthesis as required by the Edexcel GCSE Biology (1BI0) specification, Topic 6: Plant Structures and their Functions.
Photosynthesis is the process by which green plants and algae absorb light energy and use it to convert carbon dioxide and water into glucose and oxygen. It is an endothermic reaction because it takes in energy from the surroundings in the form of light.
Photosynthesis takes place in the chloroplasts of plant cells. Chloroplasts contain a green pigment called chlorophyll, which absorbs light energy (particularly red and blue wavelengths) and reflects green light — this is why plants appear green.
Exam Tip: The examiners often ask whether photosynthesis is exothermic or endothermic. Remember: endothermic = energy is taken IN from the surroundings. Light energy is absorbed and converted to chemical energy in glucose.
carbon dioxide + water → glucose + oxygen
This reaction requires light energy and the presence of chlorophyll.
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
You must learn this symbol equation. Notice that it is balanced:
| Element | Reactant side | Product side |
|---|---|---|
| Carbon (C) | 6 | 6 |
| Hydrogen (H) | 12 | 12 |
| Oxygen (O) | 6 + 6 = 18* | 6 + 12 = 18* |
*Counting all oxygen atoms across both compounds on each side.
Exam Tip: You may be asked to write either the word equation or the symbol equation. Make sure you can recall both. The symbol equation must be balanced. Do not forget to state that light energy is needed and that chlorophyll acts as the catalyst/absorber of light.
Photosynthesis occurs primarily in the leaves of a plant, specifically inside the chloroplasts of mesophyll cells.
Chloroplasts are organelles found in plant cells and algal cells. They have a double membrane and contain:
At GCSE level, you need to know that chloroplasts contain chlorophyll and that photosynthesis occurs in two broad stages, but the detailed biochemistry of each stage is not required.
Exam Tip: If asked "where in the cell does photosynthesis take place?", the answer is the chloroplast. If asked "where in the plant?", the answer is mainly the leaves (specifically the palisade and spongy mesophyll cells).
Although GCSE does not require detailed knowledge of the two stages, understanding the overview helps you answer higher-tier questions:
Photosynthesis is essential because it:
Plants do not simply make glucose and leave it as it is. Glucose produced by photosynthesis is used in several important ways:
Glucose is broken down during cellular respiration to release energy for life processes. This happens in every living cell of the plant.
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ energy)
Glucose molecules are joined together to form cellulose, a structural polysaccharide. Cellulose makes up plant cell walls, providing strength and rigidity.
Glucose is combined with nitrate ions (NO₃⁻) absorbed from the soil through the roots to produce amino acids. Amino acids are then assembled into proteins needed for growth and repair.
Glucose is converted into starch for storage. Starch is insoluble in water, which is important because:
Glucose can be converted into lipids (fats and oils) for long-term energy storage. Seeds, for example, often contain high levels of lipids.
Glucose is also used to produce sucrose (for transport in phloem), nucleic acids (DNA and RNA) and other organic molecules.
| Use of glucose | Product | Purpose |
|---|---|---|
| Respiration | CO₂ + H₂O + energy | Energy for life processes |
| Cellulose synthesis | Cellulose | Structural — cell walls |
| Amino acid synthesis | Amino acids → proteins | Growth, repair, enzymes |
| Starch synthesis | Starch | Energy storage (insoluble) |
| Lipid synthesis | Fats and oils | Long-term energy storage |
Exam Tip: A common question is "Why do plants store glucose as starch rather than as glucose?" The answer is that starch is insoluble, so it does not dissolve in cell sap and therefore does not affect osmosis (water movement) in the cell. Glucose is soluble and would affect the water potential of the cell.
To show that photosynthesis has occurred, you can test a leaf for starch:
Exam Tip: If a question asks you to explain the purpose of each step, make sure you know: boiling water = kill cells; ethanol = remove chlorophyll; iodine = test for starch (blue-black = positive).
For photosynthesis to take place, the plant needs:
If any one of these is missing, photosynthesis cannot occur.
Photosynthesis and respiration are closely linked processes that are often compared in exams:
| Feature | Photosynthesis | Aerobic Respiration |
|---|---|---|
| Equation | 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ | C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O |
| Energy | Endothermic (takes in light energy) | Exothermic (releases energy) |
| Where | Chloroplasts (in cells with chlorophyll) | Mitochondria (in all living cells) |
| When | Only in the light | All the time (day and night) |
| Gas taken in | CO₂ | O₂ |
| Gas released | O₂ | CO₂ |
Notice that the equations are essentially the reverse of each other. The glucose and oxygen produced by photosynthesis are the reactants for respiration, and vice versa.
During the daytime, a plant carries out both photosynthesis AND respiration. If photosynthesis is faster than respiration, there is a net uptake of CO₂ and a net release of O₂. At night, only respiration occurs, so the plant takes in O₂ and releases CO₂.
The compensation point is the light intensity at which the rate of photosynthesis exactly equals the rate of respiration. At this point, there is no net gas exchange.
Exam Tip: Be clear that plants respire ALL the time (24 hours a day), not just at night. Photosynthesis only occurs in the light, but respiration is continuous. A very common misconception is that "plants photosynthesise during the day and respire at night" — in reality, they respire constantly.
Plants need more than just glucose to grow. They require mineral ions absorbed from the soil:
These minerals are absorbed by the roots through active transport and carried upward in the xylem.
Exam Tip: Be prepared to link photosynthesis to other topics such as respiration, food chains and the carbon cycle. Examiners love cross-topic questions.