Photosynthesis Explained for CIE IGCSE Biology (With Equations, Experiments & Exam Questions)
Only 40% of IGCSE students score full marks on photosynthesis questions. Are you one of them? This guide covers everything — equations, limiting factors, the pondweed experiment, and real exam questions. Read this once and you will know exactly what to write in your paper.
What Is Photosynthesis?
Plants do not eat food like we do. They make their own food using sunlight. This process is called photosynthesis. It happens inside the leaves of green plants.
The leaf takes in carbon dioxide from the air. It absorbs water through the roots. Using light energy, the plant converts these into glucose and oxygen. Glucose gives the plant energy to grow. Oxygen is released into the air.
The Photosynthesis Equation for IGCSE Biology
This is the one equation every IGCSE student must know cold.
Word equation: Carbon dioxide + Water → Glucose + Oxygen
Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (light energy required)
CIE examiners ask for both versions. Write light energy above the arrow in the symbol equation. Missing that costs you marks every single time.
The reaction happens inside chloroplasts — tiny structures found in leaf cells. Chloroplasts contain a green pigment called chlorophyll. Chlorophyll absorbs red and blue light to power the reaction. Green light is mostly reflected, which is why leaves look green.
Limiting Factors of Photosynthesis — IGCSE
A limiting factor is what slows photosynthesis down. Even if everything else is available, one shortage will hold the rate back. This topic appears in almost every CIE IGCSE Biology paper.
There are three main limiting factors you must know:
1. Light Intensity More light means more energy for the reaction. Photosynthesis speeds up as light increases. But once light is no longer the shortage, something else takes over as the limit.
2. Carbon Dioxide Concentration CO₂ is a raw material for photosynthesis. More CO₂ in the air means the reaction can go faster. In greenhouses, farmers add extra CO₂ to increase crop yield.
3. Temperature Enzymes control the reactions inside the chloroplast. Warmer temperatures speed up enzyme activity. But above around 40°C, enzymes start to denature and the rate drops sharply.
How to answer limiting factor graph questions: Look at where the graph levels off. At that flat part, increasing the current factor no longer helps. A different factor is now limiting the rate. Always name that factor and explain why.
The Pondweed Experiment — CIE IGCSE Biology Practical
The pondweed experiment (using Elodea) is the most tested practical in CIE IGCSE Biology. You measure how fast a water plant produces oxygen bubbles when you change one variable at a time.
Equipment: Elodea cutting, beaker of water, lamp, ruler, stopwatch, sodium hydrogen carbonate solution (to maintain CO₂ supply)
Method:
- Place the Elodea cutting in a beaker of water with sodium hydrogen carbonate dissolved in it.
- Set the lamp at a fixed distance from the plant.
- Wait two minutes for the plant to adjust.
- Count the number of bubbles produced in one minute.
- Move the lamp to a new distance and repeat.
- Record all results in a table.
What do the bubbles mean? Each bubble is oxygen gas — a product of photosynthesis. More bubbles per minute means a faster rate of photosynthesis.
Common exam mistakes to avoid:
- Forgetting to state that only one variable changes at a time
- Not mentioning that other factors must be kept constant
- Saying “light” instead of “light intensity”
- Forgetting that sodium hydrogen carbonate keeps CO₂ levels steady
A graph tip: When you plot bubble count against light intensity, the graph rises steeply at first, then levels off. That flat section means light is no longer the limiting factor — CO₂ or temperature is now holding the rate back.
Real IGCSE Exam Questions on Photosynthesis
Try each question yourself before reading the answer.
Question 1 Write the balanced chemical equation for photosynthesis.
Model Answer: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, with light energy shown above the arrow. Both the reactants and products must be correct. Coefficients must balance.
Question 2 A student placed Elodea in bright light with very little CO₂. The bubble rate was slow. What is the limiting factor? Explain your answer.
Model Answer: The limiting factor is carbon dioxide concentration. Even though light is not a problem, there is not enough CO₂ available as a raw material. This slows the rate of photosynthesis.
Question 3 Explain why increasing temperature beyond 40°C causes the rate of photosynthesis to fall.
Model Answer: Above 40°C, the enzymes that control photosynthesis begin to denature. Their active sites change shape. Substrates can no longer bind properly. The rate of reaction falls as a result.
Question 4 In the pondweed experiment, a student counted more bubbles when the lamp was moved closer. Explain why.
Model Answer: Moving the lamp closer increases light intensity. More light energy is available for photosynthesis. The rate of the reaction increases, so more oxygen is produced and more bubbles are released per minute.
Who We Are — BioKatalyst
Hi, I am Karishma. I run BioKatalyst with my partner Khushbu. Together, we have been teaching Biology for 13 years — first inside Cambridge-affiliated schools, and now fully online. We have won several teaching awards along the way.
Here is what makes us different. We teach every student ourselves. No other tutors. No assistants. No middlemen. When your child books a class with us, they get Karishma or Khushbu — directly, every single time.
We do not teach in batches. Each session is built around what your child needs that week. If they are stuck on limiting factors, we stay on limiting factors until it makes sense. If they keep forgetting the equation, we use memory techniques until it sticks. We check, we repeat, we move forward only when the student is ready.
Many of our students came to us struggling with topics just like this one. After a few weeks of focused one-to-one sessions, they were not just passing — they were confident going into the exam.
Frequently Asked Questions
Which photosynthesis equation do I need for CIE IGCSE? You need both. The word equation and the symbol equation can both appear in your paper. For the symbol equation, always show light energy above the arrow: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
How many limiting factors do I need to know? Three — light intensity, carbon dioxide concentration, and temperature. Know how each one affects the rate of photosynthesis and what happens when the factor is too high or too low.
Is the pondweed experiment always in the paper? It appears very regularly. You should know the full method, what the bubbles represent, how to identify the variable being changed, and how to read or describe the graph.
My child keeps forgetting the photosynthesis equation. What should we do? This is one of the most common problems we see. In our one-to-one classes, we use recall techniques and repeat testing until the equation becomes automatic. Most students have it down within two sessions.
Do you teach only photosynthesis or the full syllabus? We cover the full CIE IGCSE Biology syllabus. But we always start from where the student is struggling. If photosynthesis is the weak point, that is where we begin — and we rebuild from there.
How is BioKatalyst different from other online biology tutors? Karishma and Khushbu teach every class personally. Your child will not be passed around to different tutors. They get the same teacher every week — someone who knows exactly where they are and what they need next.
Book a Free Demo Class
Photosynthesis does not have to be confusing. One session with us and your child will understand the equation, the experiment, and how to answer graph questions with confidence.
Book a free demo class with us today. No pressure. Just one class where we see where your child is and show them exactly what we can do together.
BioKatalyst — Online Biology Tutors | Taught directly by Karishma & Khushbu | 13 Years Experience | CIE IGCSE Specialists