1.3.2.2. Lipids

Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E, and K),monoglycerides, diglycerides, triglycerides, phospholipids, and others. The main biological functions of lipids include energy storage, as structural components of cell membranes, and as important signaling molecules.

They are compounds of C, H and O. Occasionally also N and P.

CLASSIFICATION

FATTY ACIDS

In chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long aliphatic tail (chain), which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from triglycerides or phospholipids. When they are not attached to other molecules, they are known as “free” fatty acids. Fatty acids are important sources of fuel because, when metabolized, they yield large quantities of ATP. Many cell types can use either glucose or fatty acids for this purpose. In particular, heart and skeletal muscle prefer fatty acids. The brain cannot use fatty acids as a source of fuel; it relies on glucose or ketone bodies.

TRIGLYCERIDES

A triglyceride (TG, triacylglycerol, TAG, or triacylglyceride) is an ester derived from glycerol and three fatty acids.

Triglycerides are the main constituents of vegetable oil (typically more unsaturated (with double links inside the structure of the fatty acids) and animal fats (typically more saturated (without double links). In humans, triglycerides are a mechanism for storing unused calories, and their high concentration in blood correlates with the consumption of starchy and other high carbohydrate foods. Triglycerides are a major component of human skin oils.

PHOSPHOLIPIDS

Phospholipids are a class of lipids that are a major component of all cell membranes as they can form lipid bilayers.  The structure of the phospholipid molecule generally consists of hydrophobic tails and a hydrophilic head. It is usually found with cholesterol molecules which are found in-between the spaces of the phospholipid.

STEROL LIPIDS

Sterol lipids, such as cholesterol and its derivatives, are an important component of membrane lipids, along with the phospholipids and sphingomyelins. The steroids, all derived from the same fused four-ring core structure, have different biological roles as hormones and signaling molecules (the estrogen family, the androgens such as testosterone and androsterone, the progestogens and the vitamin D.  Other examples of sterols are the bile acids and their conjugates, which in mammals are oxidized derivatives of cholesterol and are synthesized in the liver. The plant equivalents are the phytosterols, such as β-sitosterol, stigmasterol, and brassicasterol; the latter compound is also used as a biomarker for algal growth. The predominant sterol in fungal cell membranes is ergosterol.

BIOLOGICAL FUNCTIONS

• Cells membranes. Eukaryotic cells are compartmentalized into membrane-bound organelles that carry out different biological functions. The Phospholipids  are the main structural component of biological membranes, such as the cellular plasma membrane and the intracellular membranes of organelles. While glycerophospholipids are the major component of biological membranes, other non-glyceride lipid components such as sphingomyelin and sterols. A biological membrane is a form of lipid bilayer. The formation of lipid bilayers is an energetically preferred process when the glycerophospholipids described above are in an aqueous environment.
• Energy storage. Triglycerides, stored in adipose tissue, are a major form of energy storage both in animals and plants. The complete oxidation of fatty acids provides high caloric content, about 9 kcal/g, compared with 4 kcal/g for the breakdown of carbohydrates and proteins

• Signaling. In recent years, evidence has emerged showing that lipid signaling is a vital part of the cell signaling.

• Other functions.
  • The “fat-soluble” vitamins (A, D, E and K) are essential nutrients stored in the liver and fatty tissues, with a diverse range of functions.

• Acyl-carnitines are involved in the transport and metabolism of fatty acids in and out of mitochondria.

• Polyprenols and their phosphorylated derivatives also play important transport roles, in this case the transport of oligosaccharides across membranes.

• Lipids also form the basis of steroid hormones.

NUTRITION AND HEALTH

Most of the fat found in food is in the form of triglycerides, cholesterol, and phospholipids. Some dietary fat is necessary to facilitate absorption of fat-soluble vitamins (A, D, E, and K) and carotenoids. Humans and other mammals have a dietary requirement for certain essential fatty acids, such as linoleic acid (an omega-6 fatty acid) and alpha-linolenic acid (an omega-3 fatty acid) because they cannot be synthesized from simple precursors in the diet. Both of these fatty acids are 18-carbon polyunsaturated fatty acids differing in the number and position of the double bonds. Most vegetable oils are rich in linoleic acid (safflower, sunflower, and corn oils). Alpha-linolenic acid is found in the green leaves of plants, and in selected seeds, nuts, and legumes (in particular flax, rapeseed, walnut, and soy). Fish oils are particularly rich in the longer-chain omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). A large number of studies have shown positive health benefits associated with consumption of omega-3 fatty acids on infant development, cancer, cardiovascular diseases, and various mental illnesses, such as depression, attention-deficit hyperactivity disorder, and dementia.

In contrast, it is now well-established that consumption of trans fats, such as those present in partially hydrogenated vegetable oils (palm oil, for example), are a risk factor for cardiovascular disease. The trans fats are present in industrial bakery, and chips.

A few studies have suggested that total dietary fat intake is linked to an increased risk of obesity and diabetes.