AQA A Level Biology

Revision Notes

5.2.1 Mitochondria & the Need for Energy

Structure & Function of the Mitochondria

  • Mitochondria are rod-shaped organelles 0.5 – 1.0 µm in diameter
  • They are the site of aerobic respiration in eukaryotic cells
  • The function of mitochondria is to synthesize ATP
  • Synthesis of ATP in the mitochondria occurs during the last stage of respiration called oxidative phosphorylation
    • This relies on membrane proteins that make up the ‘electron transport chain’ and the ATP synthase enzyme – the details of this are covered later in the notes


  • Mitochondria have two phospholipid membranes
  • The outer membrane is:
    • Smooth
    • Permeable to several small molecules
  • The inner membrane is:
    • Folded (cristae)
    • Less permeable
    • The site of the electron transport chain (used in oxidative phosphorylation)
    • Location of ATP synthase (used in oxidative phosphorylation)
  • The intermembrane space:
    • Has a low pH due to the high concentration of protons
    • The concentration gradient across the inner membrane is formed during oxidative phosphorylation and is essential for ATP synthesis
  • The matrix:
    • Is an aqueous solution within the inner membranes of the mitochondrion
    • Contains ribosomes, enzymes and circular mitochondrial DNA necessary for mitochondria to function

Mitochondria Structure, downloadable AS & A Level Biology revision notes

The structure of the mitochondria

Relationship between structure & function

  • The structure of mitochondria makes them well adapted to their function
  • They have a large surface area due to the presence of cristae (inner folds) which enables the membrane to hold many electron transport chain proteins and ATP synthase enzymes
  • More active cell types can have larger mitochondria with longer and more tightly packed cristae to enable the synthesis of more ATP because they have a larger surface area
  • The number of mitochondria in each cell can vary depending on cell activity
    • Muscle cells are more active and have more mitochondria per cell than fat cells

Exam Tip

Exam questions can sometimes ask you to explain how the structure of a mitochondrion helps it carry out its function effectively. Make sure to follow through with your answer. It is not enough to say that cristae increase the surface area of the inner membrane. You need to explain that an increased surface area of the inner membrane means there are more electron transport chain carriers and ATP synthase enzymes which results in more ATP being produced.

Be prepared to identify the different structures and locations in a mitochondrion from an electron micrograph.

Respiration Produces ATP

  • Living organisms are composed of cells, and within each cell, many activities and processes are constantly being carried out to maintain life
  • Work in a living organism requires energy and usable carbon compounds

Essential work within organisms table

Essential Work Within Organisms Table, downloadable AS & A Level Biology revision notes

The source of energy & materials

  • For nearly all organisms the sun is the primary source of energy
  • The reactions of photosynthesis store energy in organic molecules
    • Light energy from the sun is transformed into chemical potential energy in the synthesis of carbohydrates
    • The carbohydrates formed are then used in the synthesis of ATP (from their breakdown) or are combined and modified to form all the usable organic molecules that are essential for all metabolic processes within the plant
    • Photosynthesis is carried out by the first organism in a food chain, such as plants and some other small organisms
  • Respiration in all living cells releases energy from the breakdown of organic molecules
  • Respiration involves the transfer of chemical potential energy from nutrient molecules (such as carbohydrates, fats and proteins) into a usable energy form (through the synthesis of ATP) that can be used for work within an organism

Glucose equations

glucose + oxygen →  carbon dioxide + water + energy

C6H1206 + 6 O2 →  6 CO2 + 6 H20 + 2870kJ

  • Autotrophs are organisms that are able to synthesise their own usable carbon compounds from carbon dioxide in the atmosphere through photosynthesis
  • Heterotrophs don’t have this ability. They require a supply of pre-made usable carbon compounds which they get from their food

Exam Tip

According to the laws of thermodynamics, energy cannot be created or destroyed; it is transformed from one form into another. Be careful not to say that energy is “created” when talking about photosynthesis and respiration.

You may also be expected to name examples of energy-requiring reactions in organisms:

  • The sodium-potassium pump that is found on many cell membranes is a great example of active transport. Three sodium ions are taken out of the cell while two potassium ions are taken in, both against their respective concentration gradients
  • The movement and contraction of muscles also requires substantial amounts of energy


Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.

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