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

This study guide covers OCR GCSE (9-1) Biology A, Gateway Science (J247), Topic B1.4 Photosynthesis, statements B1.4a to B1.4f. It follows the OCR specification, Version 4.0 (August 2026), for first assessment in 2018. Statement B1.4f, the interaction of limiting factors, is printed in bold in the specification and is tested only in the Higher Tier papers. Everything else here is for both tiers.

Related pages: the OCR GCSE Biology hub, the printable checklist, the revision notes, the practice questions, the wider Cell level systems guide and the exam preparation page. Not sure where you stand? Try a free 10-minute diagnostic.

What this unit covers

Statement	What you must be able to do	Tier
B1.4a	Describe photosynthetic organisms as the main producers of food, and so of biomass, for life on Earth	Both
B1.4b	Describe photosynthesis: reactants and products, the two-stage process, and where it happens (chloroplasts)	Both
B1.4c	Describe photosynthesis as an endothermic reaction	Both
B1.4d	Describe experiments to investigate photosynthesis	Both
B1.4e	Explain the effect of temperature, light intensity and carbon dioxide concentration on the rate	Both
B1.4f	Explain how the three factors interact in limiting the rate, using graphs	Higher Tier only

The specification’s maths outcomes for this topic are rates as compound measures, moving between graphs and tables, plotting graphs, reading graphs, and the inverse square law for light intensity. All of them appear below.

B1.4a: Producers of food and biomass

Green plants and algae capture light energy from the Sun and use it to build organic compounds from carbon dioxide and water. Almost every food chain starts with one of these organisms, which is why they are called producers. The energy and the carbon in the bodies of consumers (herbivores, carnivores, decomposers) were once in a producer.

Biomass is the mass of living material, usually measured as dry mass. When producers grow, they add biomass. In the oceans the main producers are microscopic algae, so most of the biomass entering marine food chains comes from them, not from large plants.

B1.4b: The process of photosynthesis

Photosynthesis takes in carbon dioxide and water, uses light energy, and makes glucose and oxygen.

• Word equation: carbon dioxide + water → glucose + oxygen (in light, using chlorophyll)
• Symbol equation: 6CO2 + 6H2O → C6H12O6 + 6O2

Count the atoms to confirm it balances: each side has 6 C, 12 H and 18 O. The light energy is absorbed by chlorophyll, the green pigment in chloroplasts. Chloroplasts are the organelles where photosynthesis takes place, so they are most abundant in the palisade cells of the leaf.

OCR describes the reaction as a two-stage process. The summary in the specification says plants and algae trap light from the Sun to fix carbon dioxide with hydrogen from water, making organic compounds. In words:

1. Stage one uses light energy. Water is split, releasing oxygen as a waste gas and supplying hydrogen.
2. Stage two does not need light directly. The hydrogen is used to fix carbon dioxide into glucose.

Glucose is then used in respiration or built into other compounds, or stored as starch. The starch can be detected, which is how many of the practicals below work.

B1.4c: Photosynthesis is endothermic

An endothermic reaction takes in energy from the surroundings. In photosynthesis the energy comes from light. The products, glucose and oxygen, hold more chemical energy than the reactants, carbon dioxide and water. That stored energy is what respiration later releases. Respiration is the opposite pattern: it releases energy to the surroundings, so it is exothermic.

A common slip is to say that photosynthesis “makes” energy. Energy is not made. Light energy is transferred to chemical energy stored in glucose.

B1.4d: Investigating photosynthesis

Light exclusion and the starch test

OCR suggests testing geranium leaves for starch to show the effect of excluding light.

1. Destarch the plant: leave it in a dark cupboard for 24 to 48 hours so the leaves use up their stored starch.
2. Cover part of one leaf with foil or black paper, then put the plant in bright light for several hours.
3. Remove the leaf. Dip it in boiling water for about a minute to kill it and soften the cell walls.
4. Warm the leaf in hot ethanol (in a water bath, never over a flame) to remove the chlorophyll.
5. Rinse the leaf in water and spread it on a white tile. Add iodine solution.

Starch turns iodine solution from orange-brown to blue-black. The uncovered area turns blue-black. The covered area stays orange-brown. This shows that light is needed for photosynthesis to produce starch. Removing the chlorophyll first makes the colour change easy to see.

Historical and mass-gain experiments

Priestley, in the 1770s, showed that a plant could restore air in which a candle had stopped burning. Later work by Ingenhousz showed that light was needed and that the green parts of the plant were responsible for the gas released. OCR’s practical suggestions also pair the Ingenhousz experiment with showing a gain in mass. The idea is that plants given only water, air and light still gain dry mass, so most of the new mass must come from carbon dioxide and water.

Cabomba (pondweed) and gas collection

A cut stem of Cabomba under water gives off bubbles of oxygen in the light.

• Place the stem upside down in a boiling tube of water with a little sodium hydrogencarbonate dissolved in it, to supply carbon dioxide.
• Put a lamp at a measured distance. Place the tube in a water bath or beaker of water, which absorbs heat from the lamp and keeps the temperature steady.
• Let the plant settle for a few minutes. Count bubbles for a fixed time, or collect the gas in a capillary tube or small syringe and measure its volume.
• Repeat at each distance, and repeat the whole run to calculate a mean.

Counting bubbles is quick but poor, because bubble size varies. Measuring volume of gas is more reliable. A glowing splint relights in the collected gas, which identifies it as oxygen.

Algae in alginate beads

Single-celled algae can be trapped in jelly-like alginate beads. The beads are put in hydrogencarbonate indicator. This indicator is red at normal carbon dioxide levels, turns yellow when carbon dioxide rises and purple when it falls. Photosynthesising algae use up carbon dioxide, so the indicator shifts towards purple. Changing the number of beads, the lamp distance or the temperature lets you test each factor.

B1.4e: Factors affecting the rate

The rate of photosynthesis is a compound measure: an amount per unit time, for example cm³ of oxygen per minute or bubbles per minute.

Worked example 1: rate. A pondweed releases 2.7 cm³ of oxygen in 9 minutes. Rate = volume ÷ time = 2.7 ÷ 9 = 0.30 cm³ per minute.

Worked example 2: percentage change. Bubble rate rises from 8.0 to 14.0 per minute when carbon dioxide is added. Change = 14.0 − 8.0 = 6.0. Percentage change = 6.0 ÷ 8.0 × 100 = 75%.

Light intensity

More light means a higher rate, because more energy is absorbed by chlorophyll. At first the rate is directly proportional to intensity. Then the graph levels off.

Light from a small lamp spreads out, so intensity follows the inverse square law: it varies as one over the distance squared, measured from the lamp (intensity ∝ 1/d²). Doubling d cuts intensity to a quarter. Halving d multiplies it by four.

Worked example 3: lamp distance. A lamp is moved from 25 cm to 50 cm. Distance has doubled, so intensity = (1/2)² = 1/4 of the original. Moved from 40 cm to 10 cm, intensity is multiplied by (40/10)² = 16.

Worked example 4: processing results. A Cabomba stem gives these bubble counts in 2 minutes.

Distance (cm)	Bubbles in 2 min	Rate (per min)	Relative intensity (1/d²), 10 cm = 1.00
10	46	23.0	1.00
20	12	6.0	0.25
30	5	2.5	0.11
40	3	1.5	0.06

Rate fell from 23.0 to 6.0 when intensity fell to 0.25. That is about one quarter, so the rate is roughly proportional to intensity here. Plotting rate (y axis) against relative intensity (x axis) gives close to a straight line through the origin.

Carbon dioxide concentration

Carbon dioxide is a reactant, so supplying more of it raises the rate until some other condition caps it. Air contains about 0.04% carbon dioxide, so it often limits the rate on a bright, warm day.

Temperature

The reactions are catalysed by enzymes. Warming the plant towards the optimum gives particles more kinetic energy, so enzyme and substrate meet more often and the rate climbs. Past the optimum the enzymes lose their shape (they denature), and the rate falls steeply. The graph is a rising curve with a peak, then a rapid fall.

Limiting factors

A limiting factor is whichever condition is lowest relative to what the plant could use, so it caps the rate. On a graph with a flat section, the variable plotted on the x axis has stopped being the cap, and something else has taken over.

B1.4f: Interaction of limiting factors (Higher Tier only)

On a graph of rate against light intensity, drawing two curves at two carbon dioxide concentrations shows how factors interact. At low light, the curves overlap, because light is limiting and extra carbon dioxide cannot help. At higher light the lower-carbon-dioxide curve flattens first. Its plateau shows carbon dioxide (or temperature) is now limiting. The higher-carbon-dioxide curve climbs further before flattening.

Worked example 5: reading the interaction. Data at 25 °C, rate in arbitrary units.

Light intensity (au)	0	10	20	30	40	50	60	70
0.04% carbon dioxide	0	8	16	22	24	24	24	24
0.10% carbon dioxide	0	8	16	24	32	38	40	40

1. At 10 and 20 au the rates match, so light is the limiting factor, not carbon dioxide.
2. At 50 au the lower curve is flat at 24 but the upper reaches 38. The difference shows carbon dioxide limits the lower curve. The increase is (38 − 24) ÷ 24 × 100 = 58% (to 2 significant figures).
3. At 60 and 70 au the upper curve is flat at 40. More light does not help, so light is no longer limiting. Temperature is the likely limiting factor now, because it was held at 25 °C and the enzymes may not be at their optimum. Carbon dioxide at 0.10% could also be limiting, so name both and say which test would separate them.

For any plateau, say two things: the factor on the x axis is not limiting there, and a factor that was held constant, or changed between curves, is.

Common errors

• Writing the symbol equation unbalanced, or with oxygen on the wrong side.
• Saying plants do not respire. They respire all the time and photosynthesise only in the light.
• Saying a plateau means the plant has “run out of light”. It means a different factor is now limiting.
• Heating ethanol over a flame, or testing a leaf for starch without removing the chlorophyll.
• Saying temperature “kills” the enzyme above the optimum. Say it denatures.
• Saying a lamp at 20 cm gives half the intensity of one at 10 cm. It gives a quarter.

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.