How do plants capture light energy and use it to make sugar?
The word equation for photosynthesis, the two stages of the light reactions and carbon fixation, the uses of the sugar made, and the limiting factors that control the rate of photosynthesis.
An SQA National 5 Biology answer on photosynthesis, covering the word equation, the two stages of the light reactions and carbon fixation, the uses of the sugar made, and the limiting factors that control the rate of photosynthesis.
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What this dot point is asking
The SQA wants you to give the word equation for photosynthesis, describe its two stages (the light reactions and carbon fixation), state the uses of the sugar made, and explain how limiting factors (light, carbon dioxide and temperature) control the rate.
The word equation
Note that this is the reverse of respiration: photosynthesis stores energy in glucose, while respiration releases it.
The two stages
Photosynthesis happens in two linked stages inside the chloroplast.
The hydrogen and ATP made in the first stage are passed to the second stage, which is why the two stages are connected: the light reactions power the carbon fixation.
Uses of the sugar
The glucose made in photosynthesis is used in several ways:
- It is broken down in respiration to release energy.
- It is converted to starch for storage.
- It is converted to cellulose to build cell walls.
Limiting factors
The rate of photosynthesis depends on the conditions, and at any time one factor may be holding back the rate.
On a rate graph, as you increase a factor the rate rises until another factor becomes limiting, at which point the line levels off.
Examples in context
Example 1. Greenhouses. Growers raise crop yields by removing limiting factors in a greenhouse: extra lighting, added carbon dioxide and warmth all let photosynthesis run faster, so the plants make more sugar and grow more. This is a direct, practical use of the limiting-factor idea.
Example 2. Pondweed and bubbles. A classic experiment counts the oxygen bubbles released by pondweed at different light intensities. As the lamp is moved closer, the rate of bubbling rises, then levels off, showing light intensity acting as a limiting factor until another factor takes over.
Try this
Q1. State the word equation for photosynthesis. [1 mark]
- Cue. Carbon dioxide + water produces glucose + oxygen.
Q2. Name the three limiting factors of photosynthesis. [1 mark]
- Cue. Light intensity, carbon dioxide concentration and temperature.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 style4 marksDescribe the two stages of photosynthesis, naming what is produced in each.Show worked answer →
A 4-mark answer should cover both stages and their products.
Stage 1 is the light reactions. Light energy is trapped by chlorophyll in the chloroplast and used to split water into hydrogen and oxygen. ATP is produced, and oxygen is released as a by-product.
Stage 2 is carbon fixation. A series of enzyme-controlled reactions uses the hydrogen and the ATP from stage 1 to fix carbon dioxide, producing sugar (glucose).
Markers reward (1) light trapped by chlorophyll, (2) water split giving oxygen and ATP, (3) carbon fixation using hydrogen and ATP, and (4) sugar produced.
SQA N5 style3 marksA graph shows the rate of photosynthesis rising with light intensity then levelling off. Explain the shape, naming the limiting factor at each part.Show worked answer →
A 3-mark answer should explain the rise and the plateau using limiting factors.
At low light intensity, the rate rises as light increases, because light intensity is the limiting factor controlling the rate.
Where the line levels off, increasing light no longer increases the rate. Light is no longer limiting; another factor, such as carbon dioxide concentration or temperature, has become the limiting factor.
Markers reward (1) light limiting the rate on the rising part, (2) the rate levelling off, and (3) naming another factor (carbon dioxide or temperature) as limiting at the plateau.
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Sources & how we know this
- SQA National 5 Biology Course Specification — SQA (2019)