This mini-lesson walks you through the whole of Eduqas Topic 5 — Photosynthesis: what photosynthesis is (an endothermic reaction using chlorophyll and light), the word and symbol equations, where it happens and what the glucose is used for, and how temperature, light intensity and carbon dioxide concentration act as limiting factors on the rate.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Watch for the Practical work and Key idea flags. Press Start when you're ready.
Photosynthesis · the basics
What is photosynthesis?
Green plants and other photosynthetic organisms use chlorophyll and light energy to convert carbon dioxide and water into glucose, releasing oxygen as a by-product.
carbon dioxide + water → glucose + oxygenlight energy is absorbed by chlorophyll to drive the reaction
Endothermic: photosynthesis takes in energy from the surroundings (light energy). A reaction that absorbs energy is endothermic — the opposite of respiration, which releases energy.
Quick check
Energy in or out?
?Photosynthesis absorbs light energy from its surroundings to build glucose. What kind of reaction is this?
Photosynthesis · where & the symbol equation
Chloroplasts & the symbol equation
Photosynthesis happens inside chloroplasts, the green organelles found mainly in leaf cells. Chloroplasts contain the green pigment chlorophyll, which absorbs the light energy needed for the reaction.
6CO2 + 6H2O → C6H12O6 + 6O2the balanced symbol equation — glucose is C₆H₁₂O₆
Reactants: carbon dioxide + water. Products: glucose + oxygen. Oxygen is the by-product.
Watch out: chlorophyll and chloroplast are not the same word. The chloroplast is the organelle; chlorophyll is the green pigment inside it that traps light.
Quick check
The by-product
?Which gas is released into the air as a by-product of photosynthesis?
Photosynthesis · uses of glucose
What the plant does with glucose
The glucose made in photosynthesis is a versatile building block. The plant uses it in several ways:
Used in respiration to release energy.
Stored as insoluble starch (so it doesn't affect osmosis or dissolve away).
Converted to cellulose to build cell walls.
Used to make lipids (oils) for storage, e.g. in seeds.
Combined with nitrate ions (absorbed from the soil) to make amino acids, which build proteins.
Key idea: starch is the plant's main storage molecule because it is insoluble. Making proteins needs both glucose and nitrate ions from the soil.
Quick check
Which needs nitrate?
?Besides glucose, which substance absorbed from the soil does a plant need in order to make amino acids and proteins?
Sort it
Reactant, product or condition?
Tap a card, then tap the group it belongs to in the photosynthesis reaction.
⬅ Reactants (used up)
➡ Products (made)
⚙ Conditions needed
Photosynthesis · rate & limiting factors
What controls the rate?
Three environmental factors affect the rate of photosynthesis:
Light intensity — more light gives more energy for the reaction.
Carbon dioxide concentration — CO₂ is a raw material (reactant).
Temperature — the reaction is controlled by enzymes.
At any moment the factor in shortest supply caps the rate — it is the limiting factor. On a graph of rate against a factor, the line rises, then levels off (plateaus) once a different factor becomes limiting.
Rate climbs as the factor increases, then flattens when something else becomes limiting.
Temperature is special: raising temperature speeds the rate up to an optimum, but at very high temperatures the enzymes denature and the rate falls.
Quick check
Read the plateau
?On a graph, the rate of photosynthesis rises as light intensity increases, then levels off and stays flat even when the light gets brighter. What does the flat part show?
Calculate
Your turn — rate of oxygen release
1A piece of pondweed produced 60 bubbles of oxygen in 5 minutes. Calculate the rate of photosynthesis in bubbles per minute.
bubbles/min
Hint: rate = number of bubbles ÷ time = 60 ÷ 5.
Key idea · maths
Light intensity & distance
In the practical we change light intensity by moving a lamp closer or further away. Light intensity follows an inverse-square law: it is inversely proportional to the square of the distance.
light intensity ∝ 1 ÷ distance²double the distance → intensity becomes one quarter (¼)
Move the lamp from 10 cm to 20 cm (double the distance) and the intensity drops to a quarter.
Careful: intensity falls with distance squared, not just distance. Triple the distance and intensity becomes 1 ÷ 3² = 1/9.
Calculate
Your turn — inverse-square law
2A lamp is moved from 10 cm to 20 cm away from the pondweed. If the light intensity at 10 cm was 1, what is the relative light intensity at 20 cm?
(relative)
Hint: the distance doubles (×2), so intensity = 1 ÷ 2² = 1 ÷ 4.
Match it
Match each statement to the idea
Tap a statement on the left, then its matching term on the right.
Statement
Idea
Quick check
Turn the heat up too far
?The temperature around a plant is raised to a very high level. The rate of photosynthesis suddenly falls. What is the best explanation?
Practical work · SP5
Investigating rate — pondweed
The specified practical investigates factors that affect the rate of photosynthesis. A common method uses pondweed (such as Cabomba or Elodea):
Place a piece of pondweed in water and shine a lamp on it.
Count the bubbles of oxygen given off per minute — this measures the rate.
Change the distance of the lamp to vary the light intensity.
Keep other factors (temperature, CO₂) the same to make it a fair test.
Bubbles of oxygen per minute measure the rate; the lamp distance sets the light intensity.
Fair test: only the light intensity should change. A water bath keeps temperature constant, and adding sodium hydrogencarbonate keeps CO₂ supply steady.
Practical work · Calculate
Your turn — bubbles per minute
3At a lamp distance of 15 cm, the pondweed gave off 45 bubbles in 3 minutes. Calculate the rate in bubbles per minute.
bubbles/min
Hint: rate = 45 ÷ 3.
Quick check
Keep it fair
?A student is investigating how light intensity affects the rate of photosynthesis in pondweed. Which variable should they change?
Recap
The big ideas to know
What it is: endothermic reaction; chlorophyll + light convert CO₂ + water → glucose + oxygen
Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Where: chloroplasts (contain chlorophyll), mainly in leaves