Lipids, Fats, and Biochemistry

Linoleic acid is an unsaturated fatty acid. Unsaturated fatty acids have double bonds that create kinks in the chain, preventing tight packing and keeping them liquid at room temperature.

Steroids are the organic molecule which plays an important role in the growth and development of the body. The molecular structure consists of 17 carbon which is arranged in four fused structure. It is found in plants, animals, and fungi. It is considered as an important class of lipids as they have properties similar to the triglycerides. It also plays important role in defensive mechanism of the body.

Cerebroside is an important component of muscles and nerve cell membrane. It is also known as galactosylceramide. It is a class of glycolipids which consists of N -acyl sphingosine with glucose or galactose. Its deposition causes Gaucher disease. It is synthesized in endoplasmic reticulum and membranes of Golgi apparatus. It is degraded in the lysosomes. It is also known as galactolipid.

Most common lipids are triglycerides of glycerol and fatty acids. During digestion of lipids they are broken down into constituent fatty acids and glycerols by the action of lipase enzymes.

Suberin is a type of lipid found in the cell wall of plants especially cork cells. It is a mixture of fatty acid material along with glycerol and phellonic acid. It gives rigidity and strength to cell wall in addition to imparting impermeability to cork cells.  Waxes are esters of long chain fatty acids with cetyl or mericyl alcohol. They are found in leaf surfaces of plants, bees wax, wool fat of animal furs and ear wax.  Cutin is a complex lipid formed by polymerisation of  hydroxy fatty acids with or without esterification by alcohols other than glycerol. It is found in the epidermal cell walls of plants and also as a separate layer outer to epidermis. It prevents the water loss due to transpiration. 

Lipids are storage compounds, triglycerides serve as reserve energy of the body. These are an important component of cell membranes structure in eukaryotic cells that helps in regulating membrane permeability. They serve as the source for fat-soluble vitamins like A, D, E, K. Example of lipids are fats, oils and steroids.

Many animals, plants and yeast synthesize sterols and their derivatives. Sterols are widely distributed in living organisms as lipid constituents. Cholesterol is a major sterol found as component of plasma membrane lipid bilayer in higher animals such as humans and other mammals. Cholesterol is metabolised to bile acids and steroid hormones. Intestinal microorganisms reduce a certain amount of cholesterol to coprosterol which is then excreted out as faeces. A form of coprosterol termed as 5-coprosterol has a low water solubility and therefore remains stable for many hundreds of years in the environment. Coprosterol is therefore, also used as an indicator of faecal pollution in water from different sources. Ergosterol is a sterol found in fungi and protozoa necessary for their survival and has functions similar to that of cholesterol. Lanolin is a lipid derivative secreted by the sebaceous glands of wool bearing animals. Lanolin is a wax, but it is not a true fat since it lacks glycerides.
So, the correct answer is option D.

Wax is a long chain of sterol ester. These are hydrophobic. They are insoluble in water but soluble in organic and nonpolar solvents. It consists of long alkyl chains. Hexacosyl palmitate is an ester derived from the palmitic acid and cetyl alcohol. It is white in color and forms an important constituent of tissues of stony corals. It is the main constituent of bee wax.