Biology editorial review is supported by the
named review panel.
Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
Mitochondria are the organelles where most of the reactions of aerobic respiration take place. Cells with high energy demands: such as muscle cells, sperm cells and liver cells: contain many mitochondria. The number of mitochondria in a cell indicates its metabolic activity.
Mitochondria appear across multiple topics: cell structure, respiration, specialised cells and even active transport. Understanding that they are the site of aerobic respiration helps you answer questions in all these areas consistently.
Structure of mitochondria
Mitochondria are small organelles with a double membrane. The inner membrane is highly folded into cristae, which increase the surface area for the chemical reactions of aerobic respiration. The matrix inside contains enzymes that catalyse these reactions.
At IGCSE level, you do not need to know the details of the electron transport chain or Krebs cycle. The key points are: mitochondria are the site of aerobic respiration, and their structure (folded inner membrane) maximises the rate of the reaction.
Why some cells have more mitochondria
Cell type
Why many mitochondria
Muscle cells
High energy demand for contraction
Sperm cells
Energy needed for swimming to reach the egg
Root hair cells
Active transport of mineral ions requires energy
Liver cells
Many metabolic reactions including deamination and detoxification
Nerve cells
Energy for active transport of ions to maintain resting potential
When a question asks "why does this cell have many mitochondria?", the answer is always: "because it has a high energy demand for [specific process], and mitochondria are the site of aerobic respiration which releases this energy."
Linking mitochondria to other topics
Whenever the syllabus mentions energy from respiration, mitochondria are involved. Active transport requires energy from respiration (mitochondria). Muscle contraction requires energy (mitochondria). Even temperature regulation in mammals depends on heat released during respiration in mitochondria.
When comparing plant and animal cells, both contain mitochondria because both carry out aerobic respiration. Plant cells additionally have chloroplasts for photosynthesis, but the mitochondria are still present and essential for releasing energy from the glucose that photosynthesis produces.
Try a focused question (3 marks)
Explain why sperm cells contain many mitochondria.
Show the answer and marking guidance
Mitochondria are the site of aerobic respiration.
Aerobic respiration releases energy from glucose.
Sperm cells need a large amount of energy to power the tail (flagellum) for swimming to reach and fertilise the egg.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Saying mitochondria produce energy: they release energy already stored in glucose.
Confusing mitochondria with chloroplasts: chloroplasts are for photosynthesis in plant cells.
Claiming that having more mitochondria means the cell is bigger: it means the cell has higher energy demands.
Forgetting to connect mitochondria to a specific function when explaining why a cell has many.