OCR GCSE Biology A: Photosynthesis: process, investigation and limiting factors (J247) – Revision Notes

These revision notes condense OCR GCSE (9-1) Biology A, Gateway Science (J247), Topic B1.4 Photosynthesis, statements B1.4a to B1.4f, from the specification Version 4.0 (August 2026). For full explanations, start with the study guide. Statement B1.4f is printed in bold in the specification and is tested only in the Higher Tier papers. Use the OCR GCSE Biology hub and the printable checklist to track progress, then try the practice questions. If you are unsure where to begin, the free diagnostics show your weak spots.

Key ideas at a glance

Statement	Remember	Tier
B1.4a	Photosynthetic organisms (plants, algae) are the main producers of food and biomass	Both
B1.4b	CO2 + water → glucose + oxygen; light; chlorophyll; chloroplasts; two stages	Both
B1.4c	Endothermic: takes in energy (light) from the surroundings	Both
B1.4d	Light exclusion, starch test, Priestley/Ingenhousz, Cabomba, alginate beads	Both
B1.4e	Temperature, light intensity, CO2 concentration each affect the rate	Both
B1.4f	Interaction of the three as limiting factors, from graphs	Higher Tier only

B1.4a and B1.4b: Producers and the process

• Producers make organic compounds from carbon dioxide and water using light energy. All consumers depend on them for food, energy and carbon.
• Biomass is the mass of living material, usually dry mass. Producers add biomass when they grow.
• Equations to learn exactly:

Type	Equation
Word	carbon dioxide + water → glucose + oxygen (light)
Symbol	6CO2 + 6H2O → C6H12O6 + 6O2

• Atom check: 6 C, 12 H, 18 O on each side.
• Light is absorbed by chlorophyll in the chloroplasts.
• Stage one needs light: water is split, oxygen is released, hydrogen is made available.
• Stage two: the hydrogen is used to fix carbon dioxide into glucose.
• Glucose is used in respiration or stored as starch.

B1.4c: Endothermic

• Endothermic means energy is taken in from the surroundings. For photosynthesis the energy is light.
• Energy ends up as chemical energy in glucose. Respiration later releases it (exothermic).
• Energy is transferred, never created.

B1.4d: Experiments in steps

Light exclusion with a geranium

1. Destarch: 24 to 48 hours in darkness uses up stored starch.
2. Cover part of a leaf with foil or black paper. Leave the plant in bright light.
3. Boil the leaf for about a minute (kills it and softens it).
4. Hot ethanol in a water bath removes chlorophyll.
5. Rinse, then add iodine solution.
6. Result: exposed area blue-black (starch present); covered area orange-brown (no starch).

Cabomba and gas collection

• Cut stem under water, in a tube with sodium hydrogencarbonate for carbon dioxide.
• Vary one factor: lamp distance, or hydrogencarbonate concentration, or water temperature.
• Control the others. Use a water bath as a heat shield for the lamp.
• Count bubbles for a fixed time or measure gas volume. Volume is more reliable.
• Glowing splint relights in oxygen.

Algae in alginate beads

• Beads of immobilised algae sit in hydrogencarbonate indicator.
• Indicator colours: red (normal carbon dioxide), yellow (more), purple (less).
• Photosynthesis removes carbon dioxide, so the indicator moves towards purple. In the dark, respiration adds carbon dioxide, so it moves towards yellow.

Historical experiments

• Priestley: a plant restored air in which a candle had stopped burning.
• Ingenhousz: light was needed, and green parts released the gas.
• Mass-gain experiments: plants gain dry mass from carbon dioxide and water.

B1.4e: Rate and the three factors

Rate is a compound measure: amount per unit time. Rate = volume (or number) ÷ time.

Factor	Effect on rate	Graph shape	Explanation
Light intensity	Rises, then levels off	Straight rise then plateau	More light energy absorbed by chlorophyll, until something else caps it
Carbon dioxide concentration	Rises, then levels off	Rise then plateau	CO2 is a reactant, so more of it allows more glucose to be made
Temperature	Rises to an optimum, then falls fast	Peak then steep drop	Enzymes work faster, then denature above the optimum

• Limiting factor: the condition that sets a ceiling on the rate at that moment.
• Light intensity follows the inverse square law: intensity ∝ 1/d².
  • Double the distance: one quarter the intensity.
  • Halve the distance: four times the intensity.
  • From 20 cm to 5 cm: (20 ÷ 5)² = 16 times the intensity.

Worked reminder. A lamp is moved from 60 cm to 30 cm. Intensity is multiplied by (60 ÷ 30)² = 4. While light is the limiting factor, the rate also rises about four times.

Percentage change. A rate goes from 30 to 45 bubbles per minute. Change = 15. 15 ÷ 30 × 100 = 50% increase.

Rate from a volume. 0.5 cm³ of oxygen in 4 minutes: 0.5 ÷ 4 = 0.125 cm³ per minute.

B1.4f: Interacting factors (Higher Tier only)

• A graph with two curves, drawn for two values of a second factor, shows which factor limits at each point.
• Where the curves overlap, the x-axis factor is limiting.
• Where the lower curve has flattened but the higher one keeps rising, the second factor is limiting the lower curve.
• Where the higher curve also flattens, a third factor limits it. Often this is temperature.
• Evidence statement format: “At X, the rate did not rise when Y was increased, so Y is not limiting. It did rise when Z was increased, so Z is limiting.”

Fair test table for the Cabomba practical

You change (independent)	You measure (dependent)	You keep the same (control variables)
Lamp distance	Bubbles per minute or gas volume per minute	Temperature, hydrogencarbonate concentration, same stem, same lamp, same counting time
Hydrogencarbonate concentration	As above	Lamp distance, temperature, same stem, same counting time
Water temperature	As above	Lamp distance, hydrogencarbonate concentration, same stem, same counting time

Always repeat and take a mean. Let the plant settle for a few minutes after each change before you start counting.

Graph skills checklist

1. Independent variable on the x axis, rate on the y axis.
2. Choose scales that use more than half of each axis, with equal steps.
3. Label axes with quantity and unit, for example rate (bubbles per minute).
4. Plot points accurately and draw a smooth line of best fit, not dot-to-dot, unless told otherwise.
5. To describe a graph, give the trend, quote a pair of values with units, then name the limiting factor in any flat section.
6. To turn a graph into a table, read the y value at each chosen x value. To turn a table into a graph, plot the pairs.

Worked reminder. Temperature data: 15 °C gives 2.0 and 25 °C gives 4.4 cm³ per hour. Increase = 2.4. Percentage change = 2.4 ÷ 2.0 × 100 = 120%. At 35 °C the rate is 3.3, a fall of 1.1, so the change from 25 °C is −1.1 ÷ 4.4 × 100 = −25%.

Must-know distinctions

Pair	Difference
Photosynthesis and respiration	Photosynthesis takes in energy (endothermic), only in light; respiration releases energy (exothermic), all the time
Slows down and denatures	Low temperature slows enzymes; high temperature changes their shape permanently
Gas volume and bubble count	Volume is more accurate; bubble size varies
Producer and consumer	Producers make their own food; consumers eat other organisms

Quick self-test

1. State the balanced symbol equation for photosynthesis.
2. Where in a cell does photosynthesis happen?
3. Why is photosynthesis called endothermic?
4. In the geranium experiment, why is the leaf warmed in hot ethanol?
5. What colour does iodine solution turn with starch?
6. A pondweed releases 3.6 cm³ of oxygen in 12 minutes. Find the rate.
7. A lamp moves from 10 cm to 30 cm. By what factor does intensity change?
8. Give the reason rate falls at high temperature.
9. What does a plateau on a light intensity graph show?
10. (Higher Tier only) Two curves overlap at low light but separate at high light. What does that tell you?

Answers

1. 6CO2 + 6H2O → C6H12O6 + 6O2.
2. In the chloroplasts.
3. It takes in energy (light) from the surroundings and stores it in glucose.
4. To remove chlorophyll so the colour change is visible (the leaf is first dipped in boiling water, then warmed in ethanol).
5. Blue-black.
6. 3.6 ÷ 12 = 0.30 cm³ per minute.
7. Distance is three times larger, so intensity is multiplied by 1/9.
8. Enzymes denature, so they no longer work properly.
9. Another factor, not light intensity, is now limiting.
10. Light limits the rate at low intensity. At high intensity, the factor that differs between the curves is limiting the lower curve.

Where marks are usually lost

• Writing an unbalanced symbol equation, or leaving out light as a condition.
• Saying photosynthesis “makes energy”. It transfers light energy into chemical energy.
• Saying “light” when the question wants “light intensity”, or writing “heat” instead of temperature.
• Describing the starch test without destarching, boiling or removing chlorophyll.
• Giving results with no units, or a rate without “per minute”.
• Saying the enzymes are “killed” rather than denatured.
• In plateau questions, naming the x-axis factor instead of a different one.
• Using the 1/d² rule backwards, so a bigger distance gives a bigger intensity.
• Forgetting to keep other factors constant when asked how to make a test fair.

Official syllabus

OCR GCSE (9-1) Gateway Science Biology A (J247), Version 4.0 (August 2026), for first assessment in 2018, published by OCR (Oxford Cambridge and RSA Examinations). Topic B1.4 Photosynthesis, statements B1.4a to B1.4f.