PhotosynthesisIB MYP Sciences: Revision notes
Section 1
What is photosynthesis?
Photosynthesis is the process by which plants make glucose from carbon dioxide and water, using light energy. Oxygen is released as a by-product.
Word equation: carbon dioxide + water → glucose + oxygen (in light, with chlorophyll)
Balanced symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Photosynthesis stores light energy as chemical energy in glucose. Plants are producers: nearly all food chains begin with them.
Light is not a reactant: it is the energy source. Write 'in light' above or below the arrow, not as a substance in the equation.
Section 2
Balancing the equation
In a balanced equation, each element has the same number of atoms on both sides.
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
- Carbon: 6 on the left (6 × 1) and 6 on the right
- Hydrogen: 12 on the left (6 × 2) and 12 on the right
- Oxygen: 12 + 6 = 18 on the left; 6 + 12 = 18 on the right
You must keep the small subscript numbers the same. Only change the large numbers in front of each formula.
Section 3
Chloroplasts and chlorophyll
Photosynthesis takes place in chloroplasts. These are small structures found mainly in the cells of the leaf, especially the palisade cells.
Chloroplasts contain chlorophyll, the green pigment that absorbs light energy (mainly red and blue light). The energy is used to join carbon dioxide and water to make glucose.
Chlorophyll is why leaves are green: green light is reflected, not absorbed.
Section 4
Uses of glucose
Plants use the glucose they make in several ways:
- Respiration: glucose is broken down to release energy for all life processes
- Starch: glucose is converted to starch, an insoluble store, in leaves, stems and roots
- Cellulose: glucose is used to build cell walls
- Proteins: glucose is combined with nitrates from the soil to make amino acids and proteins for growth
- Fats and oils: stored in seeds and fruits
Starch is used because it is insoluble, so it does not affect the movement of water into and out of cells.
Section 5
Testing a leaf for starch
If a leaf has photosynthesised, it contains starch. To test for starch:
- Boil the leaf in water for about a minute. This kills the leaf and makes it permeable.
- Heat the leaf in hot ethanol in a water bath (never over a naked flame, because ethanol is flammable). This removes the chlorophyll.
- Rinse the leaf in water to soften it.
- Add iodine solution.
Result: blue-black = starch present; orange-brown = no starch.
In an investigation (criteria B and C), a plant is first destarched by leaving it in the dark for 24 hours. Part of a leaf is then covered with foil to show that light is needed. Control other variables and repeat for reliability.
Learn the reason for each step: water kills the leaf, ethanol removes chlorophyll, iodine detects starch.
Must know
- Word equation: carbon dioxide + water → glucose + oxygen (light, chlorophyll)
- Balanced equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
- Chlorophyll in the chloroplasts absorbs light energy
- Glucose is used for respiration, starch, cellulose and proteins
- Starch test: boil, ethanol, rinse, iodine; blue-black shows starch
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Photosynthesis
- A greenhouse grower in the Netherlands grows tomato plants. The leaves make glucose from carbon dioxide and water using energy from light, in a process that also releases a gas into the greenhouse air.State the role of chlorophyll in photosynthesis.2 marks
- Photosynthesis can be summarised by an incomplete symbol equation in which two of the numbers have been left out: ? CO₂ + 6 H₂O → C₆H₁₂O₆ + ? O₂. The glucose produced is used by the plant in several ways.State two ways in which a plant uses the glucose made in photosynthesis.2 marks
- A student wants to find out whether light is needed for photosynthesis. She keeps a geranium plant in a dark cupboard for 24 hours. She then covers one leaf completely with aluminium foil and places the plant in bright light for two days. She removes the covered leaf and an uncovered leaf and tests both for starch. She boils each leaf in water, heats it in hot ethanol, rinses it and adds iodine solution. The uncovered leaf turns blue-black. The covered leaf stays orange-brown.Describe the purpose of each of the following steps in the starch test: boiling the leaf in water, heating the leaf in hot ethanol, and adding iodine solution.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).