Structure and function of lipids in plants and animals

Lipids are the name given to a mixed group of organic compounds. The elements Hydrogen, Oxygen and Carbon are always found in Lipids. The difference between them and Carbohydrates is that they have a lower proportion of Oxygen in the molecule. Lipids are insoluble in water, but they are soluble in organic solvents, such as ether, acetone, chloroform or benzene.

There are several parts in the group of Lipids:

fats and oils,
waxes,
phospholipids,
steroids (like cholesterol, oestrogen and testosterone), and some other related compounds.

At room temperature, fats are solids and oils are liquids. Fats and oils are typically found in animals and plants. Fats are of animal origin, while oils tend to be found in plants. Fats and oils are made up of a glycerol (a type of alcohol with a hydroxyl group on each of its three carbons) and three fatty acids (an organic acid). Since there are three fatty acids attached, these are known as . A condensation reaction produces a triglyceride and water molecule when the glycerol and three fatty acids react together (see diagram below). The fatty acid molecules are insoluble in water because of their long hydrocarbon tails (the tails are referred to as hydrophobic or “water-hating”). The head of the molecule is a carboxyl group which is hydrophilic or “water-loving”.

Fatty acids are the main component of soap, where their tails are soluble in oily dirt and their heads are soluble in water to emulsify and wash away the oily dirt. However, when the head end is attached to glycerol to form a fat, that whole molecule is hydrophobic.

The digestion of triglycerides is catalysed by lipase enzymes which are found, for example, in the digestive system of animals and in germinating seeds, in the lysosomes inside cells and in the secretions of saprophytic bacteria and fungi. Triglycerides are created in fat storage depots, in such places as, in liver cells, in the adipose cells of mammals, which make up adipose connective tissue (found under the skin, between the skeletal muscles, around the kidneys and in the yellow bone marrow), in the fat body of insects and also in the food reserves and endosperm of seeds. The energy contained in triglycerides is higher than the energy in carbohydrates or proteins, nearly twice as much per gram is released, but the process requires less oxygen. In addition, more water is released on oxidation than from the oxidation of carbohydrates. This is known as metabolic water and this is important to animals living in dry climates, such as the kangaroo rat and camel. These store fat specifically for its metabolic water content. Triglycerides also form compacted food reserves, which do not upset the osmotic balance of cells.

Triglycerides are good insulators as they are poor conductors of heat. This means that any fat which is under the skin can help prevent heat loss from animals. Animals in Cold Arctic or Antarctic seas, such as whales, seals and walruses, need this fact to protect them from heat loss by building up huge deposits of blubber. Another useful thing about the blubber is that it is less dense than water; this gives the animals buoyancy which allows them to stay near the surface. To protect the mammalian kidneys from damaging knocks, a thick layer of fat also surrounds ...

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Fatty acids in oils are generally:

Smaller,
Unsaturated.

On the other hand, fatty acids in fats are generally:

Larger,
Saturated.

The term “saturated” means that there are no double bonds present between the carbon atoms in the molecule. If it does have 1 or more double bonds between the carbon atoms then it is known as an “unsaturated” acid.

Saturated fatty acids form saturated fats, when they are combined with glycerol. These fats are solid when found at normal atmospheric temperatures, and they are more common in the fats of animals. In combination with glycerol, unsaturated fatty acids form oils, which are liquid at normal atmospheric temperatures. They tend to be more characteristic of plants, particularly in oily seeds and fruits, such as castor oil seeds, sunflower seeds and coconuts. Oils can be found in some animals, though, for example the fish liver oils of cod and halibut, and in red fish such as sardines, pilchards, mackerel, salmon, tuna, eels and trout which are referred to as “oily fish”.

Olive oil is formed when glycerol combines with oleic acid (a type of unsaturated acid). Linolenic and linoleic acids (more unsaturated acids) combined with glycerol are found in linseed oil and in corn oils. They are both examples of “essential fatty acids” which are acids necessary for normal health, but in small quantities and these cannot be made by the body.

Waxes are the esters of fatty acids with long chain alcohols such as cetyl alcohol (CH16H33OH). They are rigid and form protective water-resistant coverings on biological surfaces, for example, the cuticle of arthropod exoskeletons, leaf surfaces, fruits, seeds, mammalian skin, fur, feathers. Beeswax is used to construct honeycombs of bees. Lanolin is a waxy substance that coats mammalian hair.

Phospholipids are made from glycerol, two fatty acids, and (in place of the third fatty acid) a with some other molecule attached to its other end. The phosphate group end of each molecule is hydrophilic because of the oxygen’s and all of their pairs of unshared electrons; they ionise and become soluble in water. The hydrocarbon tails, however, are hydrophobic, non-polar and therefore remain insoluble in water (see diagram below). This means that phospholipids are soluble in both water and oil. Our cell membranes are made mostly of phospholipids arranged in a double layer (bilayer) with the tails from both layers “inside” (facing toward each other) and the heads facing “out” (toward the watery environment) on both surfaces. (See diagram below.)

The fact that the layer is dynamic (fluid), i.e. it can move sideways and exchange places in their own row, means that the membrane can shut itself if it is punctured. This also allows substances to be passed in and out of the cell through either phagocytosis (also called cell eating) or by the process of pinocytosis (known as cell drinking). This allows the protein parts of the membrane to move about easily. The fat-soluble substances associate with the fat-soluble portions of phospholipids and they can cross the membrane, while the water-soluble substances are restricted of transport by the hydrophobic tails. They therefore have to be left to the protein carriers to transport them.

Cholesterol (a steroid) is also found in some cell membranes, between the phospholipid molecules. When it is present, it reduces the fluidity of the membrane. Another substance that is found in the membrane is glycolipids, these are complexes of lipid and polysaccharides. They are normally found at the outer surface of cell membranes as the glycocalyx. Its involvement in membranes is in the communications with other cells, particularly during growth and development. Viruses and bacteria also use these sites for infection.

Lipids are very important to a good diet because of their many roles within our body. Excess intake of lipids, however, can lead to problems with our bodies. Any triglycerides which are additional are stored in our bodies as fat in the adipose tissue and this can cause people to become overweight and obese. This can lead to heart problems as the strain of having to carry around extra weight. This is also not helped by the extra pumping needed to make blood flow through all the capillaries in the surplus adipose tissue. Excess triglycerides, phospholipids, some lipoproteins and cholesterol may be deposited in and the cells on the inside of blood vessels, forming plaques or atheromas. This can inhibit blood flow, making the heart work much harder and therefore causing more strain on it.

To get cholesterol into our bodies, we can take it in through our diets or it can be made from saturated fats, (which are found in high abundance in beef, pork, butter, whole milk, eggs, cheese, palm oil and coconut oil). This is why the intake of saturated fats should be prevented, especially in the bodies of people with high cholesterol levels. Unsaturated fats are better to have in our diets, because they help to lower our cholesterol levels in the blood.

Lipoproteins are the combination of proteins and water soluble lipids. These act as a means of carrying lipids, including cholesterol, around in our blood. There are two main categories of lipoproteins distinguished by how compact/dense they are:

The first type is Low Density Lipoproteins or LDLs. They are bad because they transport cholesterol into the blood system and lay it inside the cells and on the walls of arteries. This increases the cholesterol levels in the blood and therefore increases the risk of coronary heart disease.
The second type is High Density Lipoproteins or HDLs. They are better because they transport cholesterol out of the system into the liver, where it can be excreted. Therefore, they decrease the cholesterol levels and help the body stay healthier. A good source of HDLs is Red (oily) fish.

By taking in unsaturated fatty acids known as omega-3 fatty acids, the risk of getting heart disease is lowered, because the cholesterol levels will decrease. Omega-3 fatty acids are found in oily (red) fish and in shellfish, these acids should be included in a good healthy diet, as will keep the body healthier.

By Nayan Patel