Photosynthesis and respiration are two of the most important biological processes on Earth, and two of the most commonly confused topics in GCSE Biology. Students often mix them up because both involve gases, both happen in living things, and the equations look similar at first glance. But they are actually opposite processes. This guide explains the difference between photosynthesis and respiration clearly with a comparison table, real examples, a memory trick, and a quiz.
Photosynthesis is the process plants use to make food (glucose) from sunlight, water, and carbon dioxide. It takes in carbon dioxide and releases oxygen. Respiration is the process all living things use to release energy from glucose. It takes in oxygen and releases carbon dioxide and water. Photosynthesis builds energy. Respiration releases it.
Difference Between Photosynthesis and Respiration: Comparison Table
| Feature | Photosynthesis | Respiration |
|---|---|---|
| What it does | Makes glucose (stores energy) | Breaks down glucose (releases energy) |
| Who does it | Plants, algae, some bacteria | All living things including plants |
| Raw materials needed | Carbon dioxide and water | Glucose and oxygen (aerobic) |
| Products made | Glucose and oxygen | Carbon dioxide, water, and energy (ATP) |
| Gas taken in | Carbon dioxide (CO2) | Oxygen (O2) |
| Gas released | Oxygen (O2) | Carbon dioxide (CO2) |
| Energy | Requires light energy from the sun | Releases chemical energy (ATP) |
| Where it happens | Chloroplasts (in plant cells) | Mitochondria (in all cells) |
| When it happens | Only in light | All the time, day and night |
What is Photosynthesis?
Photosynthesis is the process by which plants, algae, and some bacteria convert light energy from the sun into chemical energy stored as glucose. It is essentially how plants make their own food. Without photosynthesis, there would be no oxygen in the atmosphere and no food for almost any living thing on Earth.
Photosynthesis happens in the chloroplasts of plant cells, specifically using a green pigment called chlorophyll. Chlorophyll is what makes plants green and it is the molecule that captures light energy to power the reaction.
The equation for photosynthesis:
Carbon dioxide + Water + Light energy → Glucose + Oxygen
6CO2 + 6H2O + light energy → C6H12O6 + 6O2
Key things to remember about photosynthesis:
- It only happens when light is available
- It takes place in the chloroplasts
- It removes carbon dioxide from the atmosphere
- It produces the oxygen we breathe
- The glucose produced is used for growth, energy, and storage
What is Respiration?
Respiration is the process by which all living cells release energy from glucose. This energy is used to power everything a living thing does, from moving muscles to building proteins to maintaining body temperature. Every single cell in your body is carrying out respiration right now.
It is important not to confuse respiration with breathing. Breathing is the physical process of moving air in and out of your lungs. Respiration is the chemical process happening inside cells. They are related but they are not the same thing.
Respiration takes place in the mitochondria of cells. There are two types worth knowing:
- Aerobic respiration – uses oxygen and is far more efficient. This is what happens during normal activity
- Anaerobic respiration – happens without oxygen, produces less energy, and causes lactic acid buildup in muscles during intense exercise
The equation for aerobic respiration:
Glucose + Oxygen → Carbon dioxide + Water + Energy (ATP)
C6H12O6 + 6O2 → 6CO2 + 6H2O + ATP
The equation for anaerobic respiration (in animals):
Glucose → Lactic acid + Energy
Example 1 – A plant on a sunny windowsill:
During the day, the plant is photosynthesising. It is absorbing carbon dioxide through its leaves, taking in water through its roots, and using sunlight to produce glucose and oxygen. The oxygen it releases is what makes plants valuable for indoor air quality. At night, with no light, photosynthesis stops but respiration continues around the clock.
Example 2 – You running a race:
When you run, your muscle cells need energy quickly. They use aerobic respiration to break down glucose using oxygen. Your breathing speeds up to bring in more oxygen and remove the extra carbon dioxide being produced. If you sprint too hard and your body cannot supply oxygen fast enough, your muscles switch to anaerobic respiration and lactic acid builds up, causing that burning feeling.
Example 3 – A forest:
A forest is doing both processes simultaneously. Every leaf is photosynthesising in daylight, pulling carbon dioxide out of the air and releasing oxygen. Every cell in every tree, root, and organism in the soil is also respiring, using oxygen and releasing carbon dioxide. During the day, photosynthesis outpaces respiration in plants, so the net effect is oxygen production and carbon dioxide absorption.
Example 4 – Bread rising:
Yeast carries out anaerobic respiration when making bread. As the yeast breaks down glucose without oxygen, it produces carbon dioxide gas (which makes the bread rise) and ethanol. This is the same process used to make beer and wine. It is respiration but without oxygen.
Example 5 – Why we need plants:
All animals, including humans, can only respire. We cannot photosynthesise. We depend entirely on plants and other photosynthetic organisms to produce the oxygen we breathe and the food chain that feeds us. The glucose plants make through photosynthesis is the starting point for almost all food energy on Earth.
The opposite process trick:
Photosynthesis = uses light (photo means light) to BUILD glucose. Takes in CO2, releases O2.
Respiration = rests and releases energy from glucose. Takes in O2, releases CO2.
Think of them as exact opposites. The inputs of one are the outputs of the other:
Photosynthesis: CO2 in, O2 out, glucose made.
Respiration: O2 in, CO2 out, glucose used.
If you can remember one equation, you can write the other by simply reversing the arrow and swapping inputs and outputs.
Quick Quiz: Photosynthesis or Respiration?
1. Which process takes in carbon dioxide and releases oxygen?
2. Which process happens in the mitochondria?
3. Which process only happens when light is available?
4. Which process do ALL living things carry out, including plants?
5. Where in the plant cell does photosynthesis take place?
Difference Between Photosynthesis and Respiration in Exams
The difference between photosynthesis and respiration is one of the most heavily tested topics in GCSE Biology. Common exam questions include writing out both equations, explaining where each process takes place, comparing the raw materials and products, explaining why plants do both, and calculating net gas exchange in different conditions. Always learn both equations, know the organelles involved, and be ready to explain why respiration happens continuously while photosynthesis only happens in light.
Common Mistakes to Avoid
Confusing respiration with breathing:
Breathing moves air in and out of your lungs. Respiration is the chemical reaction inside cells that releases energy from glucose. They are related but completely different things. In an exam, never describe respiration as “breathing in and out.”
Thinking plants only photosynthesise:
Plants do both. They photosynthesise in light and respire all the time. A common exam question asks what happens to a plant at night — the answer is that photosynthesis stops but respiration continues.
Getting the gas exchange backwards:
Photosynthesis takes in CO2 and releases O2. Respiration takes in O2 and releases CO2. Students frequently get these the wrong way round. Remember: they are opposites. One is the reverse of the other.
Forgetting that respiration is not just aerobic:
Anaerobic respiration is also respiration. It happens without oxygen and produces lactic acid in animals or ethanol in yeast. Examiners will specifically test whether you know both types and when each one is used.
Frequently Asked Questions
Do plants photosynthesise and respire at the same time?
Yes. During daylight hours, plants do both simultaneously. Photosynthesis happens in the chloroplasts while respiration happens in the mitochondria, and these are separate organelles. During the day, the rate of photosynthesis in most plants is higher than the rate of respiration, so the plant is a net absorber of CO2 and net producer of O2. At night, photosynthesis stops but respiration continues.
Why do plants need to respire if they can photosynthesise?
Photosynthesis produces glucose and stores energy in it. Respiration releases that energy so the plant can actually use it for growth, repair, reproduction, and all other life processes. You cannot use the energy locked in glucose directly. It has to be released through respiration first. Every living cell needs respiration to function.
What is the compensation point?
The compensation point is the light intensity at which the rate of photosynthesis exactly equals the rate of respiration in a plant. At this point, the plant is neither gaining nor losing carbon dioxide overall. Below the compensation point, respiration exceeds photosynthesis and the plant loses carbon. Above it, photosynthesis exceeds respiration and the plant gains carbon. This concept appears frequently in A-Level Biology.
Can animals photosynthesise?
Almost no animals can photosynthesise. There are a few rare exceptions, such as certain sea slugs that incorporate chloroplasts from algae they eat and use them temporarily. But as a general rule, animals cannot photosynthesise and depend entirely on plants and other photosynthetic organisms for their energy supply.
Why is photosynthesis important for the whole planet?
Photosynthesis is the foundation of almost all life on Earth. It produces the oxygen in our atmosphere, removes carbon dioxide, and creates the organic molecules that form the base of every food chain. Without photosynthesis, atmospheric oxygen would disappear within thousands of years, and the food chains that support all animal life would collapse. It is arguably the most important chemical process on the planet.
For more Biology help visit Khan Academy: Photosynthesis.
Also read: Difference Between Mitosis and Meiosis | Difference Between Speed and Velocity | Difference Between Mass and Weight
The difference between photosynthesis and respiration is the difference between building energy and releasing it. One captures sunlight and locks it into glucose. The other unlocks that energy so living things can use it. They are perfectly complementary opposites. Understanding the difference between photosynthesis and respiration is not just important for exams. It explains how almost all life on Earth actually works. The difference between photosynthesis and respiration is at the heart of biology.
The best Biology students do not just memorise the equations for the difference between photosynthesis and respiration. They understand why the two processes exist and how they depend on each other. Every time you eat food or breathe air, you are experiencing the difference between photosynthesis and respiration in action. Keep that connection in mind and the topic will always make sense.