Membrane Fluidity – Biological Membranes

00:00 Now membrane fluidity is another way that organisms deal or have to deal with the environments they find themselves in. Now to illustrate this, I need to introduce a couple of concepts.

00:13 So the first concept is that of transition temperature or what we call Tm. The Tm defines a conversion, and in simpler terms that conversion is defining something that's changing the lipid bilayer from acting like a solid to acting like a fluid. Now that turns out to be really important for a bilayer because fluidity is important. Cells need to have that flexibility and fluidity in order to move. As we see the Tm here, the Tm is the halfway point in the transition. Now one of the things that happen with fatty acids is, they have different amounts of saturation and unsaturation. Fatty acids that are unsaturated have much lower melting points than those that are saturated. And similarly the lipid bilayers that contain fatty acids that are unsaturated will tend to have a much lower Tm than those that are saturated. Now this turns out to be really important, and a good illustration of this for example is with fish. Fish live in the ocean, and the ocean environment is typically cooler than the environment that we find ourselves in. Further fish don't thermoregulate, so they're pretty much dependant upon the temperature of the environment they find themselves in. We all have heard about the value of the unsaturated fish oil fatty acids, and one of the reasons that fish have high unsaturated fatty acids is so they can put these into their membranes and keep the membranes fluid at the lower temperatures that the fish finds itself in within the ocean. Now another important consideration for lipid bilayers is cholesterol. I mentioned it earlier in this talk and cholesterol is important, not for changing the Tm, but for widening the range over which the Tm actually exists. That widening of the range allows the membrane to have a broader range of environments in which the cell can actually exist.