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B2.3 · Compare photosynthesis and cellular respiration in ecosystems
Learn to compare photosynthesis and cellular respiration in ecosystems through clear examples and targeted practice.
Ontario Grade 9 Science
Biology: Sustainable Ecosystems and Climate Change
How two processes connect living things, matter, and energy
A tree in a sunny park grows leaves and releases oxygen. A nearby animal breathes in oxygen and releases carbon dioxide. These everyday observations lead to a science question: how are photosynthesis and cellular respiration connected? Both processes involve living things and the movement of matter and energy. A model can show their connections, but it simplifies what happens inside cells.
What you will learn
- Describe photosynthesis and cellular respiration using words and simple models.
- Compare what each process takes in and produces.
- Explain how the processes connect in an ecosystem without treating them as exact opposites.
Start with the two processes
Living things need matter to build and maintain their bodies. They also need energy to carry out life processes. Matter is the material things are made of. Energy is what allows changes and activity to happen.
Photosynthesis is a process in which plants and algae use light energy to make sugar from carbon dioxide and water. Sugar is a food molecule that contains stored energy. Photosynthesis also produces oxygen.
Cellular respiration is a process in which cells break down sugar using oxygen. The process releases energy that cells can use. It produces carbon dioxide and water. Cellular respiration occurs in plants and animals, as well as many other living things.
Plants do both processes. They photosynthesize when they have suitable light, and their cells carry out cellular respiration day and night. Animals do cellular respiration but do not photosynthesize.
- Photosynthesis uses light energy to make sugar.
- Cellular respiration releases usable energy from sugar.
- Plants carry out both processes; animals carry out cellular respiration.
Compare the inputs and outputs
An input is something a process takes in. An output is something it produces. Comparing inputs and outputs helps show how photosynthesis and cellular respiration are related.
For photosynthesis, the main inputs in the simple model are carbon dioxide, water, and light energy. Its outputs are sugar and oxygen. For cellular respiration, the inputs are sugar and oxygen. Its outputs are carbon dioxide, water, and released energy.
The substances made by one process can be used by the other. For example, photosynthesis produces oxygen and sugar that can be used in cellular respiration. Respiration produces carbon dioxide and water that can be used in photosynthesis.
The connection does not mean the processes are identical or happen in the same place or at the same time. It means that matter can move between them. The energy also changes form: light energy becomes stored in sugar during photosynthesis, and some of that stored energy becomes usable by cells during respiration.
- Some outputs of photosynthesis are inputs for respiration.
- Some outputs of respiration are inputs for photosynthesis.
- Matter is rearranged; energy is transferred and changed in form.
Read the model carefully
The flow diagrams in this lesson show the main inputs and outputs. Arrows show the direction of a process, not the movement of a whole plant or animal. The diagrams are simplified: they leave out many details inside cells.
The word equation gives the same comparison in a compact form. In the formulas, carbon dioxide is written as ; water as ; oxygen as ; and sugar as . The correct symbolic forms are shown in the equations below.
For photosynthesis, the simple equation is . For cellular respiration, it is . The number in front of a formula tells how many units take part in the model. These equations summarize the overall changes; they are not instructions for a home experiment.
In an ecosystem, organisms are connected through food and through the exchange of substances with their surroundings. Photosynthesis brings energy from sunlight into sugar. Cellular respiration makes some of that stored energy available to cells. Matter such as carbon, oxygen, and hydrogen continues to be part of substances as they move through these processes.
- A model highlights selected features and leaves out detail.
- Photosynthesis and respiration have related, but different, inputs and outputs.
- Energy enters the model as light and is later released from sugar for cell use.
Evidence, models, and limits
Scientists can investigate these processes by observing living things and comparing evidence about the substances they take in or release. A classroom model or diagram helps organize these ideas, but it is not itself a measurement. No particular result should be assumed from a diagram alone.
The equations and diagrams here are simplified representations. They show the main substances and energy changes used to compare the processes. They do not show every step inside a cell, how quickly a process occurs, or how much of each substance is involved in a real organism.
A scientific claim about how a process works is different from a social or ethical judgment about what people should do. This lesson compares biological processes; it does not decide what choices a community should make.
- Evidence comes from observations or investigations, not from a model by itself.
- A simple equation does not describe every detail or rate.
- Scientific explanations and value judgments are different kinds of statements.
A side-by-side comparison
| Feature | Photosynthesis | Cellular respiration |
|---|---|---|
| Main role | Makes sugar using light energy | Releases usable energy from sugar |
| Main inputs | Carbon dioxide, water, light energy | Sugar, oxygen |
| Main outputs | Sugar, oxygen | Carbon dioxide, water, energy |
| Where it occurs in this comparison | Plants and algae | Plants, animals, and many other living things |
Worked example
Follow the products of photosynthesis
A student is making a model of a leafy plant in sunlight. Which products of photosynthesis could later be used in cellular respiration, and what does each contribute?
Simplified flow model. It shows the main inputs and outputs, not every event inside a cell.
- Identify the outputsThe photosynthesis model shows sugar and oxygen as its products.
- Compare with respiration inputsCellular respiration uses sugar and oxygen. Therefore, both photosynthesis products can serve as inputs to respiration. Sugar contains stored energy, and oxygen is used in the process.
Answer: Sugar and oxygen. Sugar provides stored energy, and oxygen is an input for cellular respiration.
Check: This follows by matching the products of photosynthesis to the inputs of cellular respiration.
Worked example
Trace respiration products
An animal cell carries out cellular respiration. Which products could become inputs to photosynthesis in a plant, and what else is produced by respiration?
Simplified flow model. The arrows show main products and do not show all cell details.
- Name respiration outputsThe simple respiration model produces carbon dioxide, water, and released energy.
- Match plant inputsPhotosynthesis uses carbon dioxide and water. These two respiration products can therefore be inputs to photosynthesis. Released energy is not a matter input for photosynthesis; photosynthesis uses light energy.
Answer: Carbon dioxide and water can be used in photosynthesis. Respiration also releases energy that cells can use.
Check: The comparison distinguishes matter products from released energy.
Worked example
Compare a plant and an animal
A class model includes a plant and an animal. Which process or processes should be shown for each organism, and why?
Comparison model, not a complete map of cell activity. It does not show amounts or rates.
- Apply what each process doesPhotosynthesis uses light to make sugar, so it belongs in the plant part of this model. Cellular respiration releases usable energy from sugar, and both plant and animal cells need usable energy.
- Show both organisms accuratelyShow photosynthesis and cellular respiration for the plant. Show cellular respiration for the animal. Do not show animal photosynthesis in this comparison.
Answer: The plant carries out photosynthesis and cellular respiration. The animal carries out cellular respiration.
Check: The answer includes respiration in plants as well as animals.
Common mistakes and how to avoid them
Plants only photosynthesize and do not respire.
Correction: Plants carry out cellular respiration as well as photosynthesis.
Photosynthesis and respiration are exactly the same process running backward.
Correction: Their main inputs and outputs are related, but they have different roles. Photosynthesis stores light energy in sugar; respiration releases usable energy from sugar.
The diagram proves how fast a process happens.
Correction: A flow diagram shows a simplified relationship. It does not provide measurements or rates.
Lesson summary
- Photosynthesis uses light energy, carbon dioxide, and water to make sugar and oxygen.
- Cellular respiration uses sugar and oxygen and produces carbon dioxide, water, and released energy.
- Some matter products of each process can be inputs to the other.
- Plants carry out both processes; animals carry out cellular respiration.
Check your understanding
Question 1
Which pair is an output of photosynthesis and also an input to cellular respiration?
- Sugar and oxygen
- Carbon dioxide and water
- Light and water
- Oxygen and released energy
Show answer and explanation
Sugar and oxygen
Photosynthesis produces sugar and oxygen, and cellular respiration uses both.
Question 2
Which statement about a plant is correct?
- It photosynthesizes but does not carry out cellular respiration.
- It carries out cellular respiration but never photosynthesizes.
- It can carry out both photosynthesis and cellular respiration.
- It carries out neither process because it makes its own food.
Show answer and explanation
It can carry out both photosynthesis and cellular respiration.
Plants photosynthesize and their cells also carry out cellular respiration.
Question 3
What does a simple flow diagram of these processes not tell you by itself?
- Which main substances are shown as inputs and outputs
- How photosynthesis and respiration are connected in the model
- The exact rate of either process in a real organism
- That energy is involved in both processes
Show answer and explanation
The exact rate of either process in a real organism
A simplified diagram shows relationships, but it does not give measurements or exact rates.
Key terms
- Matter
- The material that makes up substances and living things.
- Energy
- What allows changes and activity to happen.
- Photosynthesis
- A process in which plants and algae use light energy to make sugar from carbon dioxide and water.
- Cellular respiration
- A process in which cells use oxygen to break down sugar and release usable energy.
- Input
- A substance or form of energy taken in by a process.
- Output
- A substance or form of energy produced by a process.
Continue through SNC1W
View the complete SNC1W Ontario Grade 9 Science curriculum and lessons
- B2.2 · Explain matter cycling and energy flow in ecosystem balance
- B2.4 · Investigate biodiversity, water, soil, and succession in sustainability
- B1.1 · Assess climate impacts on ecosystems and ways to respond
- B1.2 · Assess climate impacts on Canadian communities, including Indigenous communities
- B1.3 · Investigate community practices that support ecosystem balance
- B2.1 · Explain interactions among Earth’s four spheres and ecosystems
About this lesson and its review
Published by DoAssignment. This reviewed lesson follows Ontario Grade 9 Science (SNC1W), expectation B2.3. It is a study resource, not an official curriculum publication.
Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.