1. Structure and function of mitochondria
Learning Outcome:
Explain with the aid of diagrams and electron micrographs, how the structure of a mitochondrion enables it to carry out its functions

2. Mitochondrial ultrastructure
All mitochondria contain an inner and outer membrane.
Outer membrane is smooth
Inner membrane is folded into cristae that give the inner membrane a larger surface area.

3. Mitochondrial ultrastructure
Inner and outer membranes separate the two compartments within the mitochondrion, creating an intermembrane space
The matrix is enclosed by the inner membrane.
Semi-rigid gel-like, consisting of lipids and proteins
Contains looped mitochondrial DNA, mitochondrial ribosomes and enzymes

4. Size, shape and distribution
Average µm in diameter x 2-5µm in length
Athletes' mitochondria in muscle tissue can be up to 10µm in length
Longer and more densely packed cristae for greater ATP production (more on this later)
Mammalian liver cells contain up to 2500 mitochondria, which equates to 20% of the cell’s volume
Cytoskeleton (microtubules) can move mitochondria around to locations where they are required, although some are permanently in places where there is a high ATP consumption

5. the matrix
The matrix is where link reaction and Krebs cycle takes place (stages 2 and 3 of aerobic respiration)
The matrix contains the following molecules to ensure these reactions occur:
Enzymes
Coenzyme NAD
Oxcaloacetate
Mitochondrial DNA (to code for enzymes etc)
Mitochondrial ribosomes – for protein synthesis
More on these later

6. the outer membrane
Similar structure to membranes around other organelles
Contains proteins that contain channels or carriers to allow pyruvate made in the cytoplasm to enter the mitochondrion
Other proteins can be enzymes

7. The inner membrane
Has a different lipid composition to the outer membrane and is more impermeable to small ions, including hydrogen ions.
Is folded into many cristae to increase surface area
Has many electron carriers embedded in the membrane, as well as the enzyme ATP synthase

8. The inner membrane
Electron carriers in the inner membrane are arranged in electron transport chains.
Each electron carrier is an enzyme with an associated cofactor.
The cofactors are haem groups and contain an iron atom
Iron atoms can accept and lose electrons i.e. can be reduced (Fe2+ or oxidised (Fe3+ ). The passage of electrons through the membrane has energy which is used later

9. the inner membrane
Some proteins in the inner membrane pump electrons from the matrix to the intermembrane space
They are able to do this by using the energy released from the passage of electrons through the membrane
The inner membrane is impermeable to ions so the build up of ions in here causes a proton gradient (source of potential energy)

10. Structure relating to function the inner membrane
The inner membrane is where ATP is synthesised. The enzyme that does this is called ATP synthase.
ATP synthase is a large protein that protrudes into the matrix
They allow protons to pass through them down the proton gradient (known as chemiosmosis)
Protons flowing through drives the rotation of part of this enzyme that converts ADP + Pi into ATP

11. the inner membrane
The coenzyme FAD becomes reduced during one stage of the Krebs cycle.
Here, it becomes bound to a dehydrogenase enzyme in the inner membrane of the mitochondrion
The liberated hydrogen atoms get passed back to the mitochondrial matrix