Lipids

Lipids are an extremely important class of molecules for all life that we know of. Lipids are fats, but they do much more than you might know. Lipids form the major component of every cell membrane. They form smaller compartments in cells such as vesicles. They form the crucial insulation around neurons that allow them to transmit nerve signals at high speed.

Fat, a tissue composed of cells (fat cells) containing lipids is important as a lightweight way to store energy, and it cushions many organs of higher organisms. Fat is crucial for the absorption of certain vitamins, like vitamin-D, and it helps animals control their temperature. And fat tastes good, too. Mmmmm ... butter.


The simplest lipids: Fatty acids

The simplest kind lipids are the fatty acids (shown). They're called that because of the carboxylic acid (COOH) "head" and the fatty or aliphatic (CH-rich) "tail".

A molecule that is aliphatic contains long chains of hydrocarbons, usually saturated with all the hydrogens the chain can incorporate.

The acidic head group is soluble in water because it contains elements that form hydrogen bonds (H-bonds) with water. The head group is also called hydrophilic (water loving).

The aliphatic tail, on the other hand is hydrophobic (water fearing). The tail does not dissolve in water and tends to bury itself in substances, like other aliphatic tails, that are similar to it: Like dissolves like. This will lead to some profound effects that determine much of the structure of cells, organelles and tissues, as we shall see.


Double bonds in the fatty-acid tail

Many fatty acids contain one or more (but usually one) double bond in the aliphatic tail.

Single bonds create pivot points about which parts of molecules can rotate freely (unless obstructed by another part of the molecule). But double bonds do not allow free pivoting. That loss of freedom of motion imparts different properties to such fatty acids.

Further, atoms joined by double bonds can be arranged in cis or trans form, as shown at the top of the figure. The trans form of oleic acid has a kink in it, which determines some of the properties that structures composed of trans-fatty acids will have.

[In Latin, cis means "on this side of" and trans means "across from."]

Fatty acids without any double bonds are called saturated fats because they have as many hydrogens as they can possibly have. Insertion of a double bond eliminates two hydrogens because two electrons that would be necessary to bind hydrogens are occupied in the bond. Any fatty acid without a double bond is called unsaturated.


Lipids in water – micelles

When placed in aqueous (water) solution, fatty acids and other lipids tend to form structures that minimize the exposure of the aliphatic tails to water. One such structure is the micelle shown here.

We'll use this little abbreviation for a fatty acid. The blue circle is the hydrophilic head and the curvy line is the hydrophobic tail:

single fatty acid cartoon

Fatty acid hydrophilic head groups orient themselves out toward the solvent, protecting the aliphatic tails on the inside, where much of the water of solution can be excluded.

Micelles form spontaneously whenever lipids are dissolved in water.


Liposomes

In a similar manner, lipids can also spontaneously arrange into this configuration (cross section shown), a liposome. The core is filled with aqueous solution, so the head groups are still exposed to water, and the tails are clearly buried in a region protected from water. This arrangement is called a lipid bilayer ("two layers").

The membranes around cells are a more complicated kind of liposome, generally made of a different kind of lipid called a phospholipid, but the idea is the same.

Source: Wikipedia Commons

Here is an electron micrograph of a lipid bilayer vesicle, a large liposome that contains things in a cell, such as substances to be transported from one place to another.

You can clearly see the two dark edges of the bilayer membrane, the hydrophilic head groups of the lipids.


Phospholipids

Phospholipids are the most abundant kind of lipid in cells. They form the bulk of cell membranes and do a few other jobs.

Phospholipids have two main characteristics that distinguish them from the simple lipids we looked at above. First, the head group contains a phosphate (OPO3) group as the hydrophilic portion of the molecule. Second, phospholipids have two hydrophobic tails.

Many phospholipids are variations of the phosphatidyl choline molecule shown schematically below. It contains a choline ion, (CH3)3N+C2H4OH, a phosphate group (OPO3-) and two aliphatic tails; one usually has a kink formed by a double bond.

There are a few more common polar head groups, including derivatives of glycerol, the amino acid serine, and ethanolamine.


Phospholipids with two hydrophobic tails are the most common lipids in living organisms

Phospholipid bilayers

The diagram on the right shows a common shorthand drawing for phospholipids, a circle with a couple of wavy lines sticking out from it. To the right is a schematic of a phospholipid bilayer, which forms the major part of most cell membranes.

It is thought that the presence of a kink in one of the tails causes the membrane to have a little more space—be a little looser—so that water and other molecules can move more freely through it.

Note: This sketch is highly schematic. Phospholipid membranes don't look quite this organized, and they contain many other structures like trans-membrane ("across the membrane") proteins and other molecules.


Cholesterol

Cholesterol is another very important class of lipid molecule. The head group of cholesterol is a flat and relatively rigid system of four cyclic hydrocarbons which, when inserted into a phospholipid membrane, serve to stiffen it.

While cholesterol is a key component of many (but not all) kinds of cell membranes and other structures that involve lipids, it can also be a problem, particularly for older humans with high-fat diets. Because cholesterol is hydrophobic, it tends to clump to itself in aqueous solution, like blood. Cholesterol in the blood system (serum cholesterol) can stick to itself in clumps and "plaques" large enough to block important arteries, causing cardiovascular disease.

The root of cholesterol is a steroid framework. The structures of important steroids like testosterone, estrogen and cortisol, to name a few, have the basic framework shown in green below.

In cholesterol, that basic framework is augmented with the hydrophobic tail common to all lipids.

This multipurpose use of a common framework in biochemistry is very common. Proteins and nucleic acids are polymers of repeating units, and the DNA nucleotide bases are also used as transporters of energy in cells, just to name a couple.


Trans-membrane proteins

The cells of more complicated organisms, eukaryotes, have membranes that are selectively permeable to most substances. That means no chemical substance can just cross the membrane boundary freely.

Instead eukaryotic cells have a wide variety of trans-membrane (literally "across the membrane") proteins that form specific channels for specific substances. For example, the potassium (K)-channel protein is shown schematically below.

The alpha helices of the K-channel protein are hydrophobic, so being buried in among the

hydrophobic tails of the phospholipid bilayer is a perfect environment for them. The parts of any trans-membrane protein that stick out on either side of the membrane surface tend to be hydrophilic.

The K-channel protein forms a perfect tube that is selective for the size and charge of K+ ions, allowing the cell to keep just the right concentration of K+ around.

In the proteins notes you can see a representation of the aquaporin molecule, a trans-membrane channel that regulates the flow of water ("aqua" + "pore") into a cell.




Lipids as electrical insulators

Just as one more example of the importance of lipids to higher organisms, consider neurons, the long cells that form the wiring (nerves) of the body and brain. Electrical signals, in the form of a wave of ion movement across membranes, carry nervous impulses between the brain & spinal chord and the other control, motor and sensory neurons. These signals would not propagate at the high speeds needed if those "wires" – the long axons of the neurons weren't electrically insulated from their surroundings.

The insulation of neurons is in the form of special cells called Schwann cells that produce a lipid mixture called myelin. Myelin consists of several lipids, but mainly the galactoceramide molecule, shown below, and cholesterol.



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