Lipids: The Molecular Builders of Life

Lipids are compounds that occur frequently in nature. As a component of plant, animal, and microbial membranes, they can be found in a diverse range of things, from egg yolk to the human nervous system (Campbell et al., 2016).

The definition of a lipid is based on solubility. Lipids are marginally soluble in water but readily soluble in organic solvents such as chloroform and acetone. Lipids are a broad group of naturally-occurring molecules that includes fats, waxes, sterols, fat-soluble vitamins, monoglycerides, diglycerides, phospholipids, and others (Fahy et al., 2009).

A lipid is more than just a component of our diet. It moves and stores energy, transmits signals, absorbs vitamins, produces hormones, and serves as the structural component of our cell membranes. (Subramaniam et al., 2011). Classified according to their chemical nature, lipids fall into two main categories. The first group consists of polymers of fatty acids that contain a long, non-polar hydrocarbon chain with a small polar region containing oxygen, such as fatty acids, triacylglycerols, sphingolipids, phosphoacylglycerols, and glycolipids (Figure 1). The second group consists of steroids, structurally defined as fused ring compounds (two rings that have two atoms and one bond in common), an important example of this group includes cholesterol.

Figure 1: The classification of lipids

Understanding The Chemistry of Lipids

Fatty acids are organic compounds, consisting of aliphatic chains (an organic compound containing carbon and hydrogen joined together in straight chains, branched or unbranched) with a carboxylic acid group (a carbon that is bonded to an oxygen and a hydroxyl group). The carboxyl group is hydrophilic (polar) and the hydrocarbon chain is hydrophobic (non-polar). Living systems generally contain even-numbered, unbranched fatty acids (Figure 2).

Unsaturated fatty acids are characteristically identified as those with carbon-carbon double bonds, whereas the presence of single-carbon bonds indicates saturation and are called saturated fatty acids. Fatty acids are rarely found unbound, rather forming part of many commonly occurring lipids (Fahy et al., 2009).

Triglycerides are tri-esters consisting of a glycerol molecule bound to three fatty acid molecules (Figure 2). A large percentage of human and animal body fats are composed of triglycerides, as are the fats found in vegetables. Phosphoacylglycerols are glycerol-based phospholipids. They are the main component of biological membranes. Waxes are complex mixtures of esters, composed of long-chain carboxylic acids and long-chain alcohols. In addition to serving as protective coatings for both plants and animals, they are also used as food additives. A sphingolipid does not contain glycerol, but it does contain sphingosine, a long-chain amino alcohol (Vance & Vance, 2002). In both plants and animals, the nervous system is among the most abundant sources of these compounds (Merrill Jr & Sandhoff, 2002).

Glycolipids are lipids with a carbohydrate attached by a glycosidic (covalent) bond (Figure 2). They maintain the stability of the cell membrane and facilitate cellular recognition, which is crucial to the immune response and in the formation of tissues as cells connect to each other (Hölzl & Dörmann, 2007). Steroids, as well as being a form of lipid, have a fused ring system consisting of three six-membered rings and one five-member ring. There are many important steroids including reproductive hormones and probably the most common known steroid, cholesterol (Campbell et al., 2016). Figure 2 below indicates the structural differences of lipids (Figure 2).

Figure 2: A comparison of the structural differences of lipids (Magtanong et al., 2016)

The Vital Role of Lipids in Nutrition

We use the terms animal fats and plant oils because of the solid and fluid nature of these groups of lipids. The triglyceride molecule is the basis of all oils and fats. However, what distinguishes “oil” from “fat” is the composition of the fatty acid chains within the oil or fat. This could either be saturated or unsaturated, with unsaturated fatty acids further divided into mono- and polyunsaturated fatty acids. The saturation of the fatty acids depends on the amount of double or single bonds present. When there are only single bonds present we refer to the fatty acid as saturated, in the case of unsaturation one or more double bond is present. This difference is far more important than the length of the fatty acid chain, which as well affects the melting points of the fats. An exception to this, butter, contains a high proportion of short-chain fatty acids, leading to that “melt in your mouth” effect. Membranes need to maintain a certain degree of fluidity to be functional. As a result, unsaturated fats are distributed differently throughout the body. Warm-blooded mammals have more saturated fats in their internal organ membranes than in their skin tissues, enabling the membranes to remain solid at higher temperatures. Plant oils alone contain different proportions of saturated fats. Table 1 shows the distribution of fatty acids in commonly used oils and fats (Campbell et al., 2016).

Table 1: The distribution of saturated and unsaturated fatty acids in a tablespoon (14g) of different oils (Campbell et al., 2016)

Diets high in saturated fats are associated with cardiovascular disease, so eating more unsaturated fats may reduce the risk of heart disease and stroke (Briggs et al., 2017). Having a high ratio of unsaturated to saturated fatty acids makes canola oil an attractive dietary choice. It has been known since the 1960s that foods rich in polyunsaturated fatty acids are healthier (Forouhi et al., 2018). While olive oil and canola are popular cooking oils, they are not appealing for spreading on bread or toast. Due to this, companies began marketing butter substitutes based on unsaturated fatty acids, the aim being that these would also have butter-like characteristics, such as being solid at room temperature. This was achieved by partially hydrogenating the double bonds in unsaturated fatty acids.

Ironically, butter substitutes were created from polyunsaturated oils by removing their double bonds, thereby making them more saturated (Figure 3). In addition, many of the soft spreads that are marked as being healthy (sunflower oil spread and canola oil spread) may indeed pose new health risks. In the hydrogenation process, some double bonds are converted to the trans form. It has now been shown that trans fatty acids increase low-density lipoprotein (LDL) cholesterol levels and lower your good high-density lipoprotein (HDL) cholesterol levels, which correlate with heart disease (Oteng & Kersten, 2020). As a consequence, trans fatty acids have similar effects to saturated fatty acids. In recent years, novel and improved substitutes have been marketed that claim to contain no trans fats.

Figure 3: Full and partial hydrogenation of unsaturated fatty acids (Temkov & Mureșan, 2021)

Lipids and Multiple Sclerosis

Nerve tissues throughout the body, such as those in the brain and spinal cord, are covered with myelin, an insulating layer. It is made up of protein and fatty acid substances and has a particularly high content of sphingomyelins. It consists of many layers of plasma membranes that have been wrapped around the nerve cell (Figure 4). Myelin is essentially an all-lipid bilayer with a small number of embedded proteins, unlike many other membranes. As a result of its structure, which consists of segments and nodes, nerve impulses are transmitted rapidly from node to node. It is believed that the loss of myelin results in nerve impulses slowing down and eventually ceasing. Multiple sclerosis (MS) results from the gradual destruction of the myelin sheath by sclerotic plaques, which appear to be autoimmune. A few epidemiologists have raised concerns about the involvement of viral infections in the onset of these plaques. Furthermore, in the progression of the disease, periods of active destruction of myelin are interspersed with periods of no destruction. People affected by MS experience weakness, lack of coordination, and speech and vision impairments (Campbell et al., 2016).

Figure 4: A healthy neuron with an intact myelin sheath is depicted on the left and an unhealthy neuron affected by MS is depicted on the right

The Connection between Lipids and Our Vision

Vitamin A (also known as retinol) is a lipid-soluble vitamin. Vitamin A is formed through beta-carotene. Beta-carotene is abundant in carrots but is also found in other vegetables, often those that are yellow in colour. Beta-carotene is converted into Vitamin A when an organism needs it. A derivative of vitamin A plays a crucial role in vision when it is bound to the protein called opsin. Cone cells in the retina produce several types of opsin, which are responsible for vision in bright light and for colour perception. Rod cells are responsible for vision in dim light, they contain only one type of opsin. Vitamin A has an alcohol group that is enzymatically oxidised to an aldehyde group forming retinal, the derivative that plays a crucial role in vision. The behaviour of the retina is influenced by two forms of isomers around its double bonds, namely the cis and trans isomers (Campbell et al., 2016).

Rhodopsin is a visual receptor in the retina. When light strikes rhodopsin, the cis double bond is isomerized to a trans double bond, this is the primary chemical reaction that takes place in vision (Figure 5). During this reaction, an electrical impulse is sent to the brain for processing as a visual stimulus (Ignatov & Mosin, 2014). Vitamin A deficiency can have drastic consequences as would be predicted from its importance in vision, which could lead to night blindness or even total blindness in children. On the contrary, having too much vitamin A can also have negative effects, such as bone fragility. Adipose tissue can accumulate excessive amounts of lipid-soluble vitamins since these compounds are not excreted as easily as water-soluble substances.

Figure 5: Photocycle scheme of rhodopsin (Ignatov & Mosin, 2014)

Conclusion

To conclude, lipids play a crucial role in our overall health and well-being. A variety of physiological processes depend on these essential fats, including providing a concentrated source of energy and supporting cell structure. Moreover, lipids play a vital role in absorbing fat-soluble vitamins and transporting them, which is essential for maintaining optimal vision. Educating ourselves about lipids in nutrition empowers us to make informed dietary decisions that include healthy fats. In addition to supporting our overall health, lipids can enhance our vision and improve our quality of life.

Bibliographic references

Briggs, M. A., Petersen, K. S., & Kris-Etherton, P. M. (2017). Saturated fatty acids and cardiovascular disease: replacements for saturated fat to reduce cardiovascular risk. Healthcare (Basel, Switzerland), 5(2), 29. https://doi.org/10.3390/healthcare5020029

Campbell, M. K., Farrell, S. O., & McDougal, O. M. (2016). Biochemistry. Cengage Learning.

Fahy, E., Subramaniam, S., Murphy, R. C., Nishijima, M., Raetz, C. R., Shimizu, T., Spener, F., van Meer, G., Wakelam, M. J., & Dennis, E. A. (2009). Update of the LIPID MAPS comprehensive classification system for lipids. Journal of lipid research, 50, S9-S14.

Forouhi, N. G., Krauss, R. M., Taubes, G., & Willett, W. (2018). Dietary fat and cardiometabolic health: evidence, controversies, and consensus for guidance. British Medical Journal, 361.

Hölzl, G., & Dörmann, P. (2007). Structure and function of glycoglycerolipids in plants and bacteria. Progress in lipid research, 46(5), 225-243.

Ignatov, I., & Mosin, O. (2014). Studying of phototransformation of light signal by photoreceptor pigments-rhodopsin, iodopsin and bacteriorhodopsin. Nanotechnology Research and Practice(2), 80-95.

Magtanong, L., Ko, P., & Dixon, S. (2016). Emerging roles for lipids in non-apoptotic cell death. Cell Death & Differentiation, 23(7), 1099-1109.

Merrill Jr, A. H., & Sandhoff, K. (2002). Sphingolipids: Metabolism and cell signaling. In New comprehensive biochemistry (Vol. 36, pp. 373-407). Elsevier.

Oteng, A.-B., & Kersten, S. (2020). Mechanisms of action of trans fatty acids. Advances in Nutrition, 11(3), 697-708.

Subramaniam, S., Fahy, E., Gupta, S., Sud, M., Byrnes, R. W., Cotter, D., Dinasarapu, A. R., & Maurya, M. R. (2011). Bioinformatics and systems biology of the lipidome. Chemical Reviews, 111(10), 6452-6490.

Temkov, M., & Mureșan, V. (2021). Tailoring the structure of lipids, oleogels and fat replacers by different approaches for solving the trans-fat issue—A review. Foods, 10(6), 1376.

Vance, D. E., & Vance, J. E. (2002). Biochemistry of lipids, lipoproteins and membranes (Vol. 36). Elsevier.

Visual sources

Cover image: Retrieved June 18, 2023. https://www.canva.com/photos/MADmTILXv0k--fat-tissue-coloured-scanning-electron-micrograph-sem-of-a-sample-of-fat-tissue/

Figure 2: Retrieved June 22, 2023. Magtanong, L., Ko, P., & Dixon, S. (2016). Emerging roles for lipids in non-apoptotic cell death. Cell Death & Differentiation, 23(7), 1099-1109.

Figure 3: Retrieved June 22, 2023. Temkov, M., & Mureșan, V. (2021). Tailoring the structure of lipids, oleogels and fat replacers by different approaches for solving the trans-fat issue—A review. Foods, 10(6), 1376.

Figure 4: Retrieved June 20, 2023. https://www.canva.com/photos/MADUZAmSAIg-multiple-sclerosis-ms-heatlhy-neuron-with-intact-myelin-sheath-neuron-afffected-by-ms-right-/

Figure 5: Retrieved June 20, 2023. Ignatov, I., & Mosin, O. (2014). Studying of phototransformation of light signal by photoreceptor pigments-rhodopsin, iodopsin and bacteriorhodopsin. Nanotechnology Research and Practice(2), 80-95.

Table 1: Retrieved June 22, 2023. Campbell, M. K., Farrell, S. O., & McDougal, O. M. (2016). Biochemistry. Cengage Learning.