Photosynthesis: Equation, Minerals and Investigations

Q: Do I need to learn the balanced symbol equation for Core exams?

No, Core candidates only need to recall the word equation for photosynthesis. The balanced symbol equation is only required for Extended candidates.

Biology· 6.1· 12 min read

1. Photosynthesis: Core Concept and Word Equation★★☆☆☆⏱ 3 min

Photosynthesis is the process by which plants make glucose from carbon dioxide and water, using light energy trapped by chlorophyll in chloroplasts. Oxygen is released as a waste product. The glucose produced is used for respiration, stored as starch, or used to make other molecules like cellulose for cell walls.

📘 Definition

Photosynthesis

The process by which plants manufacture carbohydrates from raw materials using energy from light

Example:

Leaves are the main site of photosynthesis in most plants, as they contain high concentrations of chlorophyll.

📐 Worked Example

Write the word equation for photosynthesis required for Core CIE IGCSE Biology 0610 exams.

1
1. List the reactants on the left side of the arrow: carbon dioxide + water
2
1. Add the required conditions above the arrow: light energy, chlorophyll
3
1. List the products on the right side of the arrow: glucose + oxygen
4
Final equation: carbon dioxide + water --(light energy, chlorophyll)--> glucose + oxygen

Exam tip:

Core candidates will lose marks if you write the symbol equation instead of the explicitly requested word equation. Always check the question requirements carefully.

2. Required Mineral Ions for Plant Growth★★☆☆☆⏱ 3 min

Plants absorb mineral ions from the soil through their root hair cells by active transport. Two key minerals are required for healthy growth, with distinct deficiency symptoms if they are missing from the soil.

📘 Definition

Mineral ion

Dissolved inorganic nutrient absorbed from soil for plant growth and function

Mineral Ion	Use in Plant	Deficiency Symptom
Nitrate	Making amino acids for protein synthesis and growth	Stunted growth, yellow older leaves
Magnesium	Making chlorophyll for photosynthesis	Yellow leaves (chlorosis), poor growth

📐 Worked Example

A gardener notices their tomato plant has yellow leaves and very slow growth. Identify the two minerals the plant is likely lacking, and explain your answer.

1
1. Yellow leaves indicate chlorophyll is not being produced: this means the plant lacks magnesium, which is required to make chlorophyll.
2
1. Slow, stunted growth means the plant cannot make enough proteins for new cell growth: this indicates a lack of nitrate ions, needed for amino acid and protein synthesis.
3
1. Final answer: The plant is deficient in magnesium and nitrate ions.

Exam tip:

You must link the use of the mineral directly to the deficiency symptom in exam answers, not just list the symptom alone to get full marks.

3. Photosynthesis Practical Investigations★★★☆☆⏱ 4 min

There are four standard practical investigations you need to recall for your exams, including the Alternative to Practical paper (Paper 6). Most investigate factors affecting the rate of photosynthesis or confirm that photosynthesis has occurred; one uses hydrogencarbonate indicator solution to show the gas exchange of an aquatic plant in light and dark conditions.

📐 Worked Example

Describe how you would investigate the effect of light intensity on the rate of photosynthesis using pond weed (Elodea).

1
1. Set up a beaker of water with a piece of Elodea pond weed submerged, inverted under a water-filled test tube to collect oxygen gas. Use a lamp as the light source.
2
1. Measure the distance between the lamp and the beaker to set different light intensities: the closer the lamp, the higher the light intensity.
3
1. Count the number of oxygen bubbles released by the pond weed in one minute for each distance, repeat three times for each distance to calculate a mean.
4
1. Control variables: keep the temperature of the water the same, use the same piece of Elodea, keep CO2 concentration constant by adding a fixed amount of sodium hydrogencarbonate to the water.
5
1. Expected result: The closer the lamp (higher light intensity), the more oxygen bubbles are released per minute, up to a point where another factor becomes limiting.

✓ Quick check

What test would you use to confirm a leaf has carried out photosynthesis?
Reveal answer
Iodine test for starch —
Starch is a storage product of glucose made during photosynthesis, so a positive iodine test (blue-black colour) confirms photosynthesis has occurred.

Hydrogencarbonate (bicarbonate) indicator solution changes colour with the amount of carbon dioxide dissolved in the water: it is red-orange at normal atmospheric CO₂, turns yellow if CO₂ increases (more acidic), and turns purple if CO₂ decreases (less acidic). Sealing an aquatic plant such as pondweed into tubes of this indicator lets you follow its net gas exchange in the light and in the dark. Always include a control tube containing indicator but no plant.

📐 Worked Example

A student seals equal pieces of pondweed into two tubes of red-orange hydrogencarbonate indicator solution. Tube A is left in bright light and tube B in darkness for two hours. Predict and explain the colour change of the indicator in each tube.

1
1. In tube A (light), the pondweed photosynthesises faster than it respires, so it removes more carbon dioxide from the water than it releases. The falling CO₂ makes the solution less acidic, so the indicator changes from red-orange to purple.
2
1. In tube B (dark), the pondweed cannot photosynthesise but still respires, releasing carbon dioxide into the water. The rising CO₂ makes the solution more acidic, so the indicator changes from red-orange to yellow.
3
1. A control tube containing indicator but no pondweed should stay red-orange, showing that the colour changes are caused by the gas exchange of the plant.

Exam tip:

For practical questions, always state you will repeat measurements 3 times to calculate a mean and reduce random error, to gain extra marks.

4. Extended Only: Balanced Symbol Equation for Photosynthesis★★★★☆Extended only⏱ 2 min

Extended candidates must recall and balance the symbol equation for photosynthesis, including state symbols. You may be asked to balance the equation or explain how it matches the word equation.

📘 Definition

Balanced chemical equation

An equation where the number of atoms of each element is the same on both the reactant and product sides.

6CO_2(g) + 6H_2O(l) \xrightarrow{\text{light energy, chlorophyll}} C_6H_{12}O_6(aq) + 6O_2(g)

📐 Worked Example

Explain why the balanced symbol equation for photosynthesis has 6 molecules of carbon dioxide and 6 molecules of water as reactants.

1
1. Count the number of carbon atoms in the glucose product: there are 6 carbon atoms in C₆H₁₂O₆.
2
1. Each CO₂ molecule has 1 carbon atom, so 6 CO₂ molecules are needed to provide all 6 carbon atoms for glucose.
3
1. Count the number of hydrogen atoms in glucose: 12 hydrogen atoms. Each H₂O molecule has 2 hydrogen atoms, so 6 H₂O molecules provide 12 hydrogen atoms.
4
1. The remaining oxygen atoms from the reactants form 6 molecules of O₂ as the waste product, so the equation is balanced.

Exam tip:

You must include the conditions (light energy and chlorophyll) above the arrow in the symbol equation to get full marks in Extended exams.

5. Extended Only: Limiting Factors of Photosynthesis★★★★☆Extended only⏱ 4 min

📘 Definition

Limiting factor

The factor that is in shortest supply and so directly limits the rate of photosynthesis; increasing it raises the rate until another factor becomes limiting.

Three environmental factors can limit the rate of photosynthesis: light intensity, carbon dioxide concentration, and temperature. At any moment, the rate is set by whichever of these is in shortest supply. You can identify the limiting factor from a rate-of-photosynthesis graph: where the line is still rising as a factor is increased, that factor is limiting; where the line levels off into a plateau, that factor is no longer limiting and something else has taken over.

Light intensity: as light increases the rate rises, until another factor becomes limiting.
Carbon dioxide concentration: as CO₂ increases the rate rises, until another factor becomes limiting.
Temperature: the rate rises as temperature increases because reactions are enzyme-controlled, but above the optimum the enzymes are denatured and the rate falls.
Growers apply this in greenhouses by adding extra light, raising the CO₂ concentration, and warming the air to keep photosynthesis fast.

📐 Worked Example

A graph of the rate of photosynthesis against light intensity rises steeply at low light and then levels off into a flat plateau at high light. State the limiting factor in each region and explain how you know.

1
1. In the rising region (low light), increasing the light intensity increases the rate, so light intensity is the limiting factor here — it is in shortest supply.
2
1. At the plateau (high light), increasing the light intensity no longer increases the rate, so light is no longer limiting. Another factor — such as carbon dioxide concentration or temperature — has become the limiting factor.
3
1. To increase the rate at the plateau, the grower must increase whichever factor is now limiting (for example, raise the CO₂ concentration or the temperature), not the light.

Exam tip:

On a rate graph, the rising part shows the factor being varied is limiting; the flat plateau shows a different factor has become limiting. Always name which factor is limiting in each region.

6. Common Pitfalls

Wrong move:

Writing the symbol equation instead of the word equation for Core questions.

Why:

Core exams only assess the word equation, you will lose marks for giving an unrequired symbol equation, especially if it is unbalanced.

Correct move:

Always check if the question is for Core or Extended, and only give the equation type explicitly asked for.

Wrong move:

Stating that magnesium is used for growth directly.

Why:

Magnesium is only used to make chlorophyll; poor growth from magnesium deficiency is an indirect effect of reduced photosynthesis.

Correct move:

Link the mineral's function directly to the symptom: magnesium deficiency reduces chlorophyll production, which reduces photosynthesis, leading to poor growth.

Wrong move:

Forgetting to list control variables in practical investigation descriptions.

Why:

Examiners require you to show you understand that only one variable should be changed at a time to get valid results.

Correct move:

Always list at least 2 controlled variables for any practical question, e.g. temperature, CO₂ concentration for photosynthesis investigations.

Wrong move:

Writing the photosynthesis equation with glucose as C₆H₁₂O₆ in Core papers.

Why:

Core candidates are not expected to know the chemical formula of glucose, and writing it will not get extra marks, and may lead to errors if you write it wrong.

Correct move:

Only use the word 'glucose' in Core exam answers for the photosynthesis equation.

Wrong move:

Stating that oxygen bubbles measure the rate of respiration in pond weed experiments.

Why:

The oxygen released by pond weed in the light is a product of photosynthesis, not respiration.

Correct move:

State that the number of oxygen bubbles per minute is a measure of the rate of photosynthesis, as more bubbles mean more oxygen is being produced.

7. Quick Reference Cheatsheet

Content Type	Core Requirement	Extended Requirement
Photosynthesis Equation	carbon dioxide + water --(light, chlorophyll)--> glucose + oxygen	Word equation + balanced symbol equation with state symbols and conditions
Mineral Ions	Recall uses and deficiency symptoms of nitrate and magnesium	Same as Core, plus ability to explain how mineral deficiency reduces photosynthesis
Practical Investigations	Describe method for light intensity, CO₂, and chlorophyll effect investigations, and starch test	Same as Core, plus ability to evaluate investigation validity and suggest improvements
Limiting factors	Not required at Core	Identify and explain the limiting factor (light intensity, CO₂ concentration, temperature) from rate-of-photosynthesis graphs

8. Frequently Asked

Do I need to learn the balanced symbol equation for Core exams?

No, Core candidates only need to recall the word equation for photosynthesis. The balanced symbol equation is only required for Extended candidates.

What is the test for starch produced in photosynthesis?

Boil the leaf in ethanol to remove chlorophyll, add iodine solution. A blue-black colour change confirms starch is present.

Going deeper

syllabusCIE IGCSE Biology 0610 2026-2028 SyllabusRefer to section 6.1 for official content
practical guideIGCSE Biology Required Practicals Checklist

What's Next

Now that you have mastered photosynthesis equations, mineral requirements, the practical investigations (including the hydrogencarbonate indicator experiment) and the limiting factors of photosynthesis, you are ready to move on to leaf structure and adaptations, which explains how the leaf is adapted to maximise the rate of photosynthesis and is often paired with photosynthesis questions in exam papers. For Extended candidates, keep practising interpreting rate-of-photosynthesis graphs to identify limiting factors, and balancing the symbol equation. Test your knowledge with past-paper practical-design questions, which carry a high number of marks in Paper 3 and Paper 4.