Lipids

Lipids are biological molecules that contain the elements carbon, hydrogen, and oxygen. These are the same elements found in carbohydrates, but unlike carbohydrates, lipids have much less oxygen.

Lipids are non-polar molecules or uncharged and therefore are insoluble in water. They will dissolve in organic solvents such as ethanol. Due to the fact that they are non-polar and not soluble in water, they are described as being hydrophobic.

Lipids are made up of two molecules, fatty acids, and glycerol, and they do not form polymers. There are many types of lipids, but triglycerides (fats and oils) and phospholipids (in cell membranes) are the two key types you need to learn. All lipids are made up of glycerol molecules and fatty acids.

Contains C, H, O but not much O.

Your answer should include: L / Lipids
Explanation: Lipids

Mostly insoluble in water.

Your answer should include: L / Lipids
Explanation: Lipids

Contains C, H, O.

Your answer should include: B / Both
Explanation: Both

Mostly insoluble in water.

Your answer should include: L / Lipids
Explanation: Lipids

Common examples include fats and oils.

Your answer should include: L / Lipids
Explanation: Lipids

Monomers are not sweet to the taste when dissolved in water.

Your answer should include: C / Carbohydrates
Explanation: Carbohydrates

Quick Quiz - for each statement below type if it is to do with lipids, carbohydrates, or both. You can answer L for Lipids, C for Carbohydrates, and B for Both.

Fatty Acids

Fatty Acids are long chains of carbon and hydrogen atoms with a carboxyl group at one end (COOH). This hydrocarbon tail can vary in length. Fatty Acids can be either saturated or unsaturated, and this refers to whether there are any double bonds between the carbon atoms.

Saturated Fatty Acids have no double bonds between the carbon atoms. Unsaturated Fatty Acids have at least one double bond between carbon atoms. If they have one double bond, they are described as being monounsaturated. If there are many double bonds, it is polyunsaturated.

Saturated Fatty Acids are holding as many H atoms as possible, due to the lack of double bonds - hence the name saturated. This results in a relatively straight shape, so molecules can be tightly packed in parallel. This tight structure results in these lipids being solid, or fats.

Unsaturated fatty acid chains kink where the double bonds are, and are therefore far less straight. This means the lipid molecules can be as tightly packed and thus in a liquid state, oils.

Triglycerides

Triglycerides are made up of one glycerol molecule and three (tri) fatty acids. These fatty acids are each bonded onto the glycerol by a condensation reaction. The condensation reaction occurs between the carboxyl group (COOH) of the fatty acid and the hydroxyl group (OH) of the glycerol. What molecule is removed in condensation reactions? How many of this molecule will be removed in the creation of a triglyceride? The bond that forms between the glycerol and carboxyl group of the fatty acids is an ester bond.

How the triglyceride structure results in its properties

It is an energy storage. Due to the large ratio of energy-storing carbon-hydrogen bonds compared to the number of carbon atoms, a lot of energy is stored in the molecule. Due to the high ratio of hydrogen to oxygen atoms, they can act as a metabolic water source. This is because triglycerides can release water if they are oxidized. This is essential for animals in the desert, such as camels. As lipids are large, hydrophobic molecules, they are insoluble in water. This means they will not affect water potentials and osmosis. Lipids are relatively low in mass. This means a lot can be stored in an animal without it increasing the mass and preventing movement.

Phospholipids

Phospholipids are made up of a glycerol molecule, two fatty acid chains, and a phosphate group (attached to the glycerol). The two fatty acids also bond to the glycerol via two condensation reactions, resulting in two ester bonds.

How the phospholipid structure results in its properties

The phosphate molecule, described as the hydrophilic ‘head’ of a phospholipid, can interact with water as it is charged. Due to the phosphate being charged, it repels other fats. The fatty acid chain is not charged. It is known as the hydrophobic ‘tail’ and it repels water but will mix with fats. Due to these two regions on a phospholipid that act differently, it is classified as a polar molecule.

The impact this has is that if phospholipids are in water, they will move to a position where the heads are exposed to water and the tails are not and explains many of the properties stated below.

This behavior of the tails moving away from water results in the formation of a phospholipid bilayer membrane structure, which forms the plasma membrane around cells. The hydrophilic nature of the phosphate head enables the surface of the plasma membrane to stay in place. The phospholipid bilayer arrangement enables carbohydrates to attach and form important receptors on the membrane (glycolipids).

Ethanol Emulsion Test

The emulsion test is how to check for the presence of lipids.

A few drops of the sample are added to ethanol. This is shaken to dissolve the sample in ethanol. (You must say dissolve to get this mark).

Then, distilled water is added. If a cloudy white, like milk, precipitate forms, then a lipid is present.