Membranes are made of phospholipids, which are made of lipids. Lipids are the stuff of oils, fats and waxes. They’re not water soluble which makes them good for energy storage as their presence doesn’t interfere with the amount of solutes in the cell’s water (water potential) and their structure holds a lot of chemical energy (hence why fats have more than double the amount of calories when consumed compared to either carbohydrates or proteins). Their insolubility to water also comes in handy for waterproofing purposes such as the top side of plant leaves. In animals, fat can serve as insulation to conserve heat.
Unlike proteins and carbohydrates, lipids are not polymers. Lipids which store energy are triglycerides, while those which form membranes are phospholipids. Triglycerides are formed by a molecule of glycerol with three fatty acids attached. The reaction which results in triglycerides is condensation.
Glycerol (green) + 3x fatty acids (red)
The fatty acids can be simplified in drawing:
The bonds formed (C-O) are called ester bonds. Fatty acids can be saturated or unsaturated (monounsaturated; polyunsaturated). Saturated fatty acids have all their carbon (C) atoms linked to hydrogen (H) atoms, hence saturated with hydrogen. If there is a carbon atom with a double bond to its neighbour carbon atom, then it will only have one bond to a hydrogen atom, hence it is unsaturated. If there is one double bond present, the fatty acid is monounsaturated. If there are multiple double bonds present, the fatty acid is polyunsaturated.
In phospholipids one of the fatty acids is replaced by a phosphate group.
Phospholipids have a hydrophilic (water loving) head, and hydrophobic (water repelling) tails. This results in the formation of a phospholipid bilayer (double layer), which forms the basis for the plasma membrane.
These membranes are what separate cells in the body, and enable the transfer of different chemicals between cells and with their environment. Since most chemicals in cells are water soluble, lipid membranes act as good barriers that can control the movement of different substances between cells.
Only certain small molecules would be able to cross the membrane freely. Other compounds would require carriers or special channels to enable them to overcome the repelling effect between the membrane and their particular chemical state e.g. chemical charge, size, etc.
The test for lipids is the emulsion test. This test takes advantage of the property of lipids of not dissolving in water, but dissolving in ethanol (alcohol). You dissolve the sample into ethanol by shaking, then pour it into water. If milky white droplets are formed, the sample is positive for lipids.