I have read somewhere that the plasma membrane has little cardiolipin but excess cholesterol whereas the inner mitochondrial membrane is rich in cardiolipin and has little cholesterol.I just wanted to know why there is this difference.
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$\begingroup$ I don't know the answer for sure but the thing with cholesterol is that it makes the membranes rigid. So, it might not be very suitable for mitochondria. Cardiolipin could also be just an evolutionary remnant $\endgroup$– WYSIWYGCommented Aug 17, 2014 at 11:04
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According to wikipedia, cardiolipin is found in two places, the inner mitochondrial membrane, and bacterial membranes. Given the endosymbiotic origin of mitochondria, it makes sense that they would retain some remnant of their bacterial ancestry. But more importantly, cardiolipin plays a role in the enzymatic functions of mitchondrial membranes. Cardiolipin forms a bicyclic resonance structure that allows it to trap a proton, which helps oxidative phosphorylation.
Because this structure also responds to pH and divalent cations, it can change it's conformation and aggregation behavior as cellular conditions change.
Cardiolipin also plays a role in holding enzyme complex quaternary structures together. Mitochondrial Complex III requires 1 cardiolipin, Complex IV requires 2 cardiolipin, and Complex V binds 4 cardiolipins.
Cardiolipin also plays a role in apoptosis, where it is oxidized and undergoes conformational changes that cause it to move from inner to outer mitochondrial membranes and generate pores that cytochrome C can escape through and trigger apoptosis.
Cholesterol can be easily oxidized into oxysterols, which are thought to perform a variety of functions, but are not fully understood. If cholesterol levels in the mitochondria were high, I think it would be oxidized and cause trouble. http://en.wikipedia.org/wiki/Oxysterol
