Lipids are classified as those organic compounds which do not interact with water. Most substances made of lipids will not be miscible with water. The study of the function and structure of lipids constitutes a sizable part of the study of organic compounds because they make up substances like oils, fats, wax, membranes, and energy-storing molecules in plants and animals.
Properties and Functions of Lipids
There are numerous important properties of lipids, such as:
Forming membranes
The wide variety of higher-order organisms (most plants and animals) are eukaryotic, which means that their cells and cell organelles are separated from surrounding environments by membranes. These are called biological membranes and consist of the plasma membrane and other intracellular membranes. The main building blocks of these membranes are glycerophospholipids. These are amphipathic molecules, with a head group joined to the core by a phosphate ester link, fatty acid-derived tails that link to the core with ester links, and a glycerol core.
Storage of energy
Another important function of lipids is the storage of energy. Energy is stored mostly in the form of triglycerides because they are most efficient for this function in animals and plants. There are a specialised group of cells called adipocytes that break down triglycerides. This releases energy. This constant process is controlled by the enzyme lipase. Triglycerides release energy at the rate of about 38kJ/g(9kcal/g). Compared to the 17kJ/g (4kcal/g) of carbohydrates, this is considerably higher. That is why triglycerides are preferable for storing energy. The adipose tissue of animals is where triglycerides are stored. In some animals, they even act as insulation against the cold.
Signalling
Cell signalling is communication that happens among cells and between cells and their environment. Lipids are a part of this process. Several lipids have been discovered to be cellular messengers and have roles in regulating cellular activity, growth, etc.
Other functions
Fat-soluble vitamins such as A, D, E, and K are lipids and essential nutrients needed by organisms. They are stored in the liver and fatty tissue and used in several functions in the body. Acyl-carnitine transports fatty acids to the mitochondria. Polyprenols and some derivatives transport oligosaccharides into and out of the membrane.
Metabolism
Triglycerides, sterols, and biological membrane lipids (mostly phospholipids) are the main lipids in the diets of most animals, including humans. These are metabolised in the following ways.
Biosynthesis
Surplus carbohydrates in animals are converted into triglycerides. This is done by synthesising fatty acids with the help of acetyl-CoA, followed by the esterification of the fatty acids. This process is called lipogenesis. The acids can then be converted into triglycerides. Unsaturated fatty acids are synthesised by introducing the double bond by a chemical reaction called a desaturation reaction. Triglycerides are synthesised in the endoplasmic reticulum.
Degradation
Fatty acids are broken down in the cell by a process called beta-oxidation. Most fatty acids are oxidised by a process almost identical to the reversal of the synthesis of fatty acids. Unsaturated fatty acids require additional steps which involve enzymes for degradation.
Importance of Lipids
Lipids play a wide range of roles in the functioning of organisms, such as:
- Fat in the body is stored in the form of lipids. These include triglycerides, cholesterol, and phospholipids.
- Lipids need to be included in the diet to facilitate the absorption of fat-soluble vitamins like A, D, E, and K.
- Linoleic acid is a major component of most plant-based oils and fats. These are mostly omega-6 and omega-3 fatty acids. Studies have shown them to be beneficial for the development of infants, alleviating cancer-related problems, treating cardiovascular and psychological ailments.
- Biological membranes are thought to be essential to the development of life itself. They separate the cells from their environments, cell organelles that perform specialised tasks and make metabolism and other functions possible. Consequently, the biological importance of lipids is also very high since they make up these biological membranes.
Conclusion
Since lipids play such an essential role in the body, it is very important to understand the structure and function of lipids. This understanding helps to solve several metabolism-related problems. Moreover, lipids form the building blocks of the three large food groups, namely carbohydrates, proteins, and fats. As the biological importance of lipids is immense, studying them is useful.