Lipids

Each type of macromolecule has a different function. In looking at the different macromolecules, some generalizations can be made. These are the main characteristics of macromolecules in nature:

• Nucleic acid—this molecule makes genetic information or participates in its transfer to make proteins. • Carbohydrate—these will be energy sources but will also form receptors, cell walls, and exoskeletons, and are involved as a food source. • Peptides or proteins—these can make enzymes and will be involved in cell structures, receptors, and in nutrient transport. • Lipids—these will store energy and make up membrane structures, pigments, and insulation in the organism.

LIPIDS

The two main atoms in lipids are carbon and hydrogen but lipids can also have nitrogen, oxygen, phosphorus, and sulfur in them. They provide an energy source for cells, help to store carbon atoms, and are the main structural component of the cell membrane. There are different types of lipid molecules you might encounter in your study of microbiology.

Fatty acids are common lipids. They contain long hydrocarbon chains that are hydrophobic because they do not like water and are not soluble in water. Fatty acids that do not have any double bonds but are considered saturated with hydrogen atoms are saturated fatty acids. Fatty acids with one or more double bonds are called unsaturated. There are monounsaturated fatty acids with one double bond and polyunsaturated fatty acids with more than one double bond. Cis-fatty acids are shaped differently than trans fatty acids. Figure 30 shows the different fatty acids:

Figure 30.

A triglyceride or triacylglycerol is what the main storage form of lipids are found in. Body fat and the fat in sebaceous or oil glands of the skin are triglycerides. They consist of three fatty acid side chains attached to a three-carbon molecule called glycerol. Between the glycerol and fatty acids are aldehyde linkages. Figure 31 shows a triglyceride molecule:

Figure 31.

While triglycerides are simple types of lipids, there are lipids that are more complex. Phospholipids have a polar phosphate group attached to them that make up most of the cell membrane. Another complex lipid is a glycolipid, which has some type of carbohydrate attached to it.

Lipids are largely hydrophobic, meaning that they do not dissolve well in water. For things like phospholipids, which have a polar end, the polar phosphate group will interact with water but the hydrocarbon chains will cluster together away from water. This explains why cell membranes are lipid bilayers that have a hydrocarbon core and a polar exposed surface. Those molecules that have hydrophobic and hydrophilic components are called amphipathic.

Lipids can form micelles, which are spherical and contain a completely hydrophobic core along with a hydrophilic outer component. Micelles, by nature of their structure, are mainly seen in phospholipids because of their amphipathic nature. Larger than micelles are lipid bilayers, which are hydrophilic on the inside of the sphere and

hydrophilic on the outside of the sphere. The hydrophobic part is between the two layers. Figure 32 shows a micelle formed by a phospholipid:

Figure 32.

Isoprenoids are lipids that are branched. Their name comes from the fact that they come from the isoprene molecule. They are used to make pigments and pharmaceuticals like beta carotene and capsaicin. Fragrances are also isoprenoids and longer chain isoprenoids are used to make waxes. Sebum in humans is a wax in the skin; this wax is used as food by certain skin bacteria.

Steroids are ringed lipids found to be part of cell membranes. Most steroids in cells are sterols, which are steroids that contain a hydroxyl group. These are largely hydrophobic but the hydroxyl OH groups or alcohol groups are hydrophilic. Cholesterol is the most common sterol in animal tissues; it allows for strengthening of the cell membrane. Prokaryotes don’t make cholesterol but they do make hopanoids, which are similar. Fungi do not make cholesterol either but will make ergosterol.