Cellular Respiration Basics
- Rachel Taylor
- Mar 31
- 3 min read
Updated: May 2
HSC Biology | Free Study Notes
This topic matters because cells need a usable source of energy for processes such as active transport, growth, repair and movement, and the Module 1 syllabus specifically includes investigating cell respiration in eukaryotic cells as part of cell function.
In this lesson
the word equation for cellular respiration
the role of mitochondria
the reactants and products of respiration
how ATP is released
how respiration links to photosynthesis
What is cellular respiration?
Cellular respiration is the process cells use to release usable energy from glucose.
This energy is released in a form the cell can use, called ATP.
Cellular respiration is important because cells need energy for:
active transport
growth
repair
chemical reactions
movement in specialised cells
The word equation for cellular respiration
The word equation for aerobic cellular respiration is:
glucose + oxygen → carbon dioxide + water + (energy)
The energy released is captured in ATP.
A fuller version is:
glucose + oxygen → carbon dioxide + water + (ATP)
Reactants and products
Reactants
Reactants are the substances needed at the start of the reaction.
In cellular respiration, the reactants are:
glucose
oxygen
Products
Products are the substances made by the reaction.
In cellular respiration, the products are:
carbon dioxide
water
ATP is released, which stores usable energy for the cell
Simple summary
Part of the process | Substance |
Reactant | Glucose |
Reactant | Oxygen |
Product | Carbon dioxide |
Product | Water |
Released | ATP |
Mitochondria
Mitochondria are the main site of aerobic respiration in eukaryotic cells.
Why mitochondria matter
Mitochondria are important because they:
help release energy from glucose
produce ATP for cell activities
Cells with high energy demands often contain many mitochondria.
Examples include:
muscle cells
cells involved in active transport
ATP release
ATP stands for adenosine triphosphate.
Why ATP is important
ATP is the cell’s immediate energy source.
The energy released during respiration is transferred into ATP so the cell can use it when needed.
What ATP is used for
Cells use ATP for:
moving substances against concentration gradients
building larger molecules
cell division
movement of cilia or flagella
muscle contraction in animals
Key idea
Respiration does not just “make energy”. It releases energy from glucose and transfers that energy into ATP.
That wording is more accurate and more exam-friendly.
Where respiration happens
Respiration happens continuously in living cells.
In eukaryotic cells
Most aerobic respiration happens in the mitochondria.
In plants and animals
Both plant and animal cells carry out cellular respiration.
This is a very common point of confusion. Plants do not only photosynthesise, they also respire.
Role of respiration in cells
Respiration allows cells to use the energy stored in food molecules.
Without respiration:
cells could not release enough usable energy
ATP supply would fall
many cell processes would stop
This is why respiration is essential for both plants and animals.
Link to photosynthesis
Cellular respiration and photosynthesis are closely linked.
Photosynthesis
uses carbon dioxide and water
requires light energy
produces glucose and oxygen
Cellular respiration
Cellular respiration:
uses glucose and oxygen
produces carbon dioxide and water
releases energy into ATP
The link
The products of photosynthesis are the reactants of respiration.
The products of respiration are the reactants of photosynthesis.
This is why the two processes are often described as complementary.
Important difference
They are linked, but they are not opposites in a simple sense:
photosynthesis stores energy in glucose
respiration releases energy from glucose into ATP
Worked example
Exam-style question
State the word equation for cellular respiration and explain why mitochondria are important.
Worked answer
The word equation for cellular respiration is:
glucose + oxygen → carbon dioxide + water
Mitochondria are important because they are the main site of aerobic respiration in eukaryotic cells, where energy from glucose is released and transferred into ATP.
Why this works
This answer:
states the correct equation
names the organelle
links mitochondria to ATP production and energy release
Common mistakes
Saying respiration only happens in animals. Plant cells also respire.
Mixing up the word equations for photosynthesis and respiration.
Saying mitochondria create energy from nothing. They release energy from glucose.
Forgetting that oxygen is a reactant in aerobic respiration.
Saying ATP is a reactant. It is produced during respiration.
Quick quiz
What is the word equation for cellular respiration?
In which organelle does most aerobic respiration happen?
Name the two reactants of cellular respiration.
What is ATP used for in cells?
How does respiration link to photosynthesis?
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