Solid Fat and Liquid Oil
Recall the structure of a typical triglyceride:
Let's simplify this diagram so that it's easier to work with:
This gives us a better image, in terms of the size relationship of the glycerol group on the left end, compared to the hydrocarbon tails. This is, as mentioned before, the structure of the storage form of fat in animals.
Now, let's do the same simplification with the cellularly-useful form of fat, the phospholipid. We'll use lecithin as the example:
Simplified, symbolizing the charged portion with an oval:
We can simplify this even further, and represent the charged "head" with a circle, and the hydrocarbon tails with a single squiggly line:
With this diagram, we can conveniently draw a representation of a cell membrane. The hydrocarbon tails are not soluble in water, so they wiggle around together in a thin layer of oily liquid. The charged heads do dissolve in water, so they line up on the outside of the "lipid bilayer," interacting with water.
We can see in this diagram that the tails of the phospholipids are all lined up neatly. They pack together easily and tightly, and form a kind of liquid crystal. The packing is so good, in fact, that this kind of cell membrane would be solid at room temperature. Similarly, a blob of fat (triglyceride) made with these same fully-saturated fats would be solid at room temperture.
In short: the close packing of saturated fat molecules makes them solidify at room temperature.
The hydrocarbon tails of unsaturated phosophlipids are kinked. That's what the double bond of unsaturation does to the shape of the molecule. Most material that contains unsaturated fats contains a variety of different types of hydrocarbons, with different numbers of double bonds -- so the tails have lots of kinks. The kinked lipids cannot pack together well at all. Therefore, to solidify, they must be cooled significantly.
In short: the kinks of unsaturated fat molecules keep them liquid at room temperature.
Therefore, animal fat (mostly saturated) is solid at room temperature. Plant oils (mosly unsaturated) is liquid at room temperature.
using this diagrammatic representation (the ball with the squiggly line), trans-fat looks just like saturated fat. The kinks do not form with trans-fatty acids; only with cis-fatty acids. This is why partially-hydrogenated plant oils behave like saturated fats in cooking. They solidify at room temperature because the molecules can pack together.
Stored triglycerides vs phospholipids in cell membranes
We spoke above of cell membranes, and presented diagrams of cell membranes. Yet, we also spoke of triglycerides, the storage form of fats. This is, in part, because the same rules apply. Also, in part, it is because it's the functional forms of these fats that matter biologically.
To function correctly, cell membranes must be liquid. Thus, cells of all species have mechanisms to change the relative amounts of saturated fats and unsaturated fats in their cell membranes. The higher the cell's growth temperature, the less unsaturated material; the lower the temperature the more unsaturated material. This turns out to be essential for survival.
Animals cannot produce some of the types of unsaturated fats, however. We must obtain them from species that can produce them, specifically plants and algae. We can also obtain them from grass-fed animals or fish. Grass-fed cows, for example, obtain their unsaturated fatty acids from grass, and we obtain it when we eat the cows. Fish obtain their unsaturated fatty acids from phytoplankton (the tiny photosynthetic algae of lakes and the open ocean).
The functional vs storage forms of fats is significant. The table below illustrates how:
|Storage Form||Form Used By Cells|
|Animal fat||saturated triglycerides||little present|
|Muscle||little present, exept as fat that is inter-leaved with the muscle fibers||mix of saturated and unsaturated phospholipids|
|Brain||little present||mix of phospholipids, with significant amounts of unsaturated lipid|
|Seeds||unsaturated triglycerides, mostly ω-6||mix of phospholipidss, with little ω-3|
|Green leaves||little present||mix of phospholipids, with significant amounts of ω-3|
|non-green plant material||little present||mix of phospholipid|
In general, the best dietary sources of unsaturated fats are green, leafy vegetables -- the greener the better. This is because the most highly-unsaturated fats (the ω-3 fatty acids) are primarily used in chloroplasts.
The storage forms of fats are less highly unsaturated, even in plants.
In animals, the best source of unsaturated fats might be brain. However, certain pathogenic agents (cf. the prions responsible for spongiform neuroencephalopathies like Mad Cow Disease) can be transmitted between species; this argues that the food chain should no longer include brain tissue.