Structures and Functions in Living Organisms · 5 question types
Past paper frequency (2018 to 2024)
This topic accounts for approximately 20% of your exam marks.
Photosynthesis equation, limiting factors, and leaf adaptations are tested on almost every paper.
Photosynthesis is the process by which green plants use light energy absorbed by to combine carbon dioxide and water into glucose, releasing oxygen as a by-product
Photosynthesis is what makes plants autotrophs: they manufacture their own food rather than eating it. This is also why plants sit at the base of every food chain. Almost all the energy flowing through living things on Earth started as sunlight captured by chlorophyll.
The word equation:
carbon dioxide + water → glucose + oxygen
(in the presence of sunlight and chlorophyll)
The balanced symbol equation:
6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂
In plain English: six molecules of carbon dioxide combine with six molecules of water to produce one molecule of glucose and six molecules of oxygen. The reaction is endothermic: it absorbs light energy from the sun and stores it as chemical energy in the glucose.
Writing the balanced symbol equation for photosynthesis
What comes up: complete or give the balanced chemical symbol equation for photosynthesis (2 marks).
Write (two marks): (1) correct formulas on each side — 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ — and (2) the equation must be balanced; if the formulas are right but the balancing numbers are wrong, you still earn 1 mark.
Watch out: the mark scheme awards no credit at all if you write only the word equation instead of the symbol equation. Always check that the balancing numbers (all sixes) are present and correct.
Compare this to aerobic respiration:
glucose + oxygen → carbon dioxide + water (releases energy)
The two equations look like mirror images: the reactants of one are the products of the other. That makes them a useful pair to learn together. They are not literally the reverse reaction, though. Photosynthesis and respiration happen by different pathways, using different enzymes, in different parts of the cell, so one is not simply the other run backwards.
Glucose has many uses inside the plant:
| Where it comes from | Where it goes | |
|---|---|---|
| Carbon dioxide (raw material) | The air, diffuses in through stomata | Used in photosynthesis |
| Water (raw material) | Absorbed from soil by root hair cells, carried up through the xylem | Used in photosynthesis |
| Light energy (not a raw material, but required) | Sunlight, absorbed by chlorophyll | Provides the energy for the reaction |
| Glucose (product) | Made in the chloroplasts | Stored as starch, used for respiration, converted to other molecules |
| Oxygen (product) | Made in the chloroplasts | Released into the air through stomata |
A quick exam note: light is not a raw material. The exam mark scheme only counts CO₂ and water as raw materials, because they are the substances that combine in the reaction. Light is the energy source.