Lipids Structure & Functions

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  • Lipids are most often triglycerides, which are made up of 3 fatty acids and glycerol
  • Triglycerides are generated by a condensation reaction, and the bond between glycerol and the fatty acid is called an ester bond
  • Fatty acids can be saturated (single bonds between all carbons) or unsaturated, at least one double bond between carbons. This impacts whether the compound is a fat or an oil.
  • Lipids have diverse roles in the cell, including energy storage, making the cell membrane, and the production of hormones

Lipids are composed of the elements carbon, hydrogen and oxygen, similar to carbohydrates, but contain less water. In fact, lipids are insoluble in water. Fats are an example of a type of lipid. Lipids play a variety of important functions in the cells.

The most common type of lipids are called triglycerides. Triglycerides are made up of 3 fatty acid chains attached to a glycerol backbone. Fatty acids are chains of carbon atoms (between 14 and 22) with the end carbon possessing a carboxyl group (COOH). The fatty acids in the triglyceride could be the same, or could have different structures.

Glycerol has three carbons, with 3 OH molecules attached. The glycerol backbone becomes attached to the three fatty acids through a condensation reaction because three molecules of water are formed. The bond that forms between the fatty acid chain and the glycerol is called an ester bond.

Lipid structures

The structure of the fatty acids influences the structure of the lipid. In the fatty acid chains, the carbon atoms could have single bonds between them making the lipid “saturated”. This generates fats that are usually solid at room temperature.

Alternatively, if one or more of the bonds between the carbon atoms are double bonds, the lipid is said to be “unsaturated”. If there is one double bond, the triglyceride is said to be “monounsaturated”, if it has multiple double bonds, it is “polyunsaturated”. Unsaturated fatty acids are usually liquid at room temperature and are called oils.

The double bonds in unsaturated fatty acids can exist in either a cis or a trans configuration. This describes whether the hydrogen atom are on the same side (cis) or opposite sides (trans). A cis double bond generates a bend in the molecule, influencing its structure and downstream function. Trans fats are rare in nature.

Fat molecules with fully saturated tails can pack tightly against one another because the single bonds result in straight molecules. This tight packing generates fats that are solid at room temperature, for example, butter.

Unsaturated fatty acids (which in nature usually contain cis double bonds) have bent tails. This means they are not able to be tightly packed and results in oils that are liquid at room temperature.

Functions of lipids

In the human body, triglycerides are mostly stored in fat cells, called adipocytes, which form adipose tissue. Adipose tissue is primarily used as an energy store, but also helps to protect and insulate the body. Lipids have a variety of functions in the cell.

Energy storage – Triglyceride breakdown yields more energy than the breakdown of carbohydrates because the carbons are all bonded to hydrogens (and they, therefore, have a higher proportion of hydrogens relative to oxygens). This means they are electron-rich and can contribute to the production of acetyl-CoA, which is an important co-enzyme in aerobic respiration.

Biological membranes – As previously discussed, cell membranes are principally composed of a phospholipid bilayer. Phospholipids are another type of lipid, created when a phosphate group replaces one of the three fatty acid chains. Phospholipids have a hydrophobic part and a hydrophilic part. The fatty acid chains remain hydrophobic, forming the tail of the molecule, but the addition of the phosphate group to the head makes this part of the molecule hydrophilic, meaning a bilayer forms.

Hormone production – many hormones are lipid-derived, and they usually belong to a class of hormones called steroid hormones. These hormones are usually derived from cholesterol and are often structurally similar to cholesterol. Steroid hormones are important signalling molecules that can enter the cell directly through the cell membrane and influence gene expression and signalling pathways. Examples include cortisol and testosterone.

Further reading and references:

[1]. Image triglyceride

[2]. Image saturated vs unsaturated