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

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.

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

Size, shape and distribution Average 0.5-1.0µm 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

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

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

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

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

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)

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

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