Lipid

Lipid
n., plural: lipids
[ˈlɪpɪd or ˈlaɪ pɪd]
Definition: biomolecule characterized by being insoluble in water, such as fats, oils, sterols, triglycerides

A biomolecule refers to any molecule that is produced by living organisms. As such, most of them are organic molecules. The four major groups of biomolecules include amino acids and proteins, carbohydrates (saccharides), lipids, and nucleic acids.  Let us know more about lipids, their definition, characteristics, types, biological importance, and metabolism.

Lipid Definition

Characteristics of Lipids

Lipids are organic compounds. They dissolve easily in a nonpolar solvent (e.g. ether) but do not in a polar solvent (e.g water). It is usually made up of glycerol or fatty acid units, with or without other types of biomolecules. Many lipids are amphiphilic or amphipathic, meaning they have both hydrophobic and hydrophilic components.

Lipids vs. Fats

Fatty acids (including fats) are a subgroup of lipids. In particular, fat is a triglyceride (lipid) that is usually solid at room temperature. Hence, it will be inaccurate to consider the two terms synonymous.

Types of Lipids

A fatty acid is a subunit of fats, oils, and waxes. It pertains to any long chain of hydrocarbon, with a single carboxylic group at the beginning and a methyl end, and an aliphatic tail. It is produced by the breakdown of fats (usually triglycerides or phospholipids) through a process called hydrolysis. It is represented by R-COOH, where R stands for the aliphatic moiety and COOH as the carboxylic group (making the molecule an acid).

Fatty acids may be classified into: (1) unsaturated fatty acids and (2) saturated fatty acids. The unsaturated fatty acids may be further grouped into monounsaturated fatty acids and polyunsaturated fatty acids. Examples of unsaturated fats are monounsaturated fatty acids, polyunsaturated fat, omega fatty acids, etc. Saturated fatty acids are fatty acids that lack unsaturated linkages between carbon atoms. Examples include lauric acid, palmitic acid, etc.

Examples of Lipids

Here are some examples of lipids:

• Glycerols (e.g. triglycerides)

Glycerol is a type of lipid made up of glycerol linked esterically to a fatty acid. One of the most commonly known glycerolipids is a triglyceride (also referred to as triacylglycerol). The triglyceride is an energy-rich compound consisting of glycerol and three fatty acids (hence, the name).

The three hydroxyl groups of glycerol in triglyceride are all esterified. The triglyceride is a major component of animal and plant oils and fats. In plants, they are typically found in the plant cell membranes where the fatty acids are mostly unsaturated. In animals, the fatty acid component is largely saturated.

• Phospholipids

A glycerophospholipid, or simply phospholipid, is a type of lipid that is an essential component of many biological membranes, particularly the lipid bilayer of cells. It is also involved in metabolism and cell signaling. Phospholipids are amphipathic compounds in a way that the ‘head‘ is hydrophilic and the lipophilic ‘tail‘ is hydrophobic. Examples of phospholipids include phosphatidylethanolamine, phosphatidylinositol, phosphatidylserine, lecithin, plasmalogens, and sphingomyelins.

• Sphingolipids

A sphingolipid is a lipid made up of a sphingoid base (e.g., sphingosine and ceramides) backbone and sugar residue(s) linked by a glycosidic bond. Examples are cerebrosides and gangliosides.

• Sterols (e.g., cholesterol)

A sterol lipid (e.g., cholesterol) is another type of lipid that serves as an essential component of the biological membrane. Many of them act as hormones and signaling molecules.

• Prenols

A prenol lipid is a type of lipid that is synthesized from isopentenyl diphosphate and dimethylallyl diphosphate via the mevalonic acid pathway. Simple isoprenoids (e.g., carotenoids) are prenol lipids.

• Other lipids

Other types of lipids are saccharolipids (fatty acids linked to a sugar backbone) and polyketides (compounds formed by polymerization of acetyl and propionyl subunits).

Lipid Metabolism

Lipids are made available by biosynthesis or by degradation of lipid-containing food sources. Lipids are naturally produced by a process called lipogenesis. But apart from biosynthesis, lipids may also be obtained from dietary sources. Humans, for instance, need to eat essential fatty acids (e.g., omega-6-fatty acids) in order to maintain optimal health and performance.

They are essential because they cannot be synthesized naturally by the body and so must be obtained from dietary sources. (Ref. 1) Lipids, then, can be made available through the degradative process of a lipid-containing diet. In humans, the excess consumption of carbohydrates may lead to the conversion of extra carbohydrates into triglycerides. When energy is needed, the lipid is broken down to extract energy. This process is called lipolysis.

• Lipogenesis

Lipids are biosynthesized by a process called lipogenesis. Lipogenesis is the process of producing lipid or fat. In biology, lipogenesis is a biochemical process, e.g. acetyl-CoA is converted to triglyceride. It is to store biochemical energy for later metabolic use. Lipogenesis includes (1) fatty acid synthesis and (2) triglyceride synthesis.

Fatty acid synthesis occurs in the cytoplasm and is characterized by the repeated addition of two-carbon units to acetyl-CoA. In triglyceride synthesis, three fatty acids are esterified to glycerol in the endoplasmic reticulum. The cells that carry out lipogenesis are mostly adipocytes and liver cells. The liver cells, though, release triglycerides in the form of very-low-density lipoproteins (VLDL) into the bloodstream.

• Lipolysis

The process wherein lipid is broken down to extract energy is called lipolysis. Lipolysis is activated when the circulating insulin level is low whereas the circulating epinephrine is high. Fatty acids may be further degraded in the mitochondria or in the peroxisome, producing acetyl-CoA.

This metabolic process in which fatty acids are degraded resulting in the formation of acetyl-CoA is called beta-oxidation. The acetyl-CoA, in turn, may enter the citric acid cycle and degraded ultimately into CO₂ and water molecules, with the concomitant generation of ATP.

Biological Functions

The major biological functions of lipids

• For energy storage
• As a structural component of cell membrane. In biological membranes, the lipid component has a hydrophilic head that may be a glycolipid, a phospholipid, or a sterol (e.g. cholesterol) and a hydrophobic tail.
• Cell signaling

Watch this vid about lipids:

Take the Lipid Biology Quiz!

Time's up

References

• Chang, C., Ke, D., and Chen, J. (2009). Essential fatty acids and human brain. Acta Neurologica Taiwanica, 18(4). https://pubmed.ncbi.nlm.nih.gov/20329590/

‌

© Biology Online. Content provided and moderated by Biology Online Editors