Lipids, Fats, and Biochemistry

Lipid bilayer of the cell membrane contains hydrophilic head groups facing outward and hydrophobic tails facing each other. The transport of phospholipids and lipid-linked oligosaccharides is difficult to achieve due to this bipolar nature. The transmembrane lipid transporter proteins called flippases transport lipids across cell membrane through transverse diffusion by flipping their orientation. Floppases transport in reverse direction. In mammals, this affects various processes such as blood coagulation, immune recognition, and programmed cell death.Most protein molecules do not flipflop due to their extensive polar regions, which are unfavorable in the hydrophobic core of the membrane.

The tails of the lipids are made up of fatty acids. These are non-polar hydrophobic so they tend to arrange away from the cytoplasmic face as well as the extracellular face. So, they avoid aqueous medium by arranging themselves towards the inner part of the cell membrane.

Phosphoinositides make up a small fraction of cellular phospholipids, They play a fundamental role in controlling membrane-cytosol interfaces, regulation of membrane traffic and the permeability and transport functions of membranes. They are found in brain tissue in animals and soybean seeds. Thus the correct answer is option B.

Ketone bodies are byproducts in the metabolism of fat. During starvation or fasting conditions the liver synthesizes glucose through a process called gluconeogenesis. Gluconeogenesis is a metabolic pathway that results in the generation of glucose from fat. During gluconeogenesis, the ketone bodies are formed as byproducts where the product is glucose. The Kerone bodies consist of acetoacetic acid, acetone and beta-hydroxybutyric acid.

Statins are products of fermentation by yeast Monascus purpureus which resemble mevalovate and are competitive inhibitors of $\beta - hydroxy-\beta - methylglutaryl$ or HMG CoA reductase.This inhibits cholesterol synthesis. Statins are, therefore, used in lowering blood cholesterol, e.g., lovastatin, pravastatin, simvastatin. 

• The exoskeleton of animals within the phylum Arthropoda mainly consists of a coating called the cuticle.
•  On the exterior surface of the cuticle is a thin, waxy layer called the epicuticle. 
• The epicuticle is formed of three layers; the inner layer is the cuticulin, which is made from lipoproteins. Hence, Epicuticle in exoskeleton is made up of Lipoprotein
So, the correct answer is 'Lipoprotein'.

Sterols are classified as lipids because they are hydrophobic and soluble in organic solvents. Cholesterol is the most well-known sterol and serves as a fundamental component of cell membranes.

Cholesterol is a precursor for bile salts, Vitamin D, and steroid hormones. Insulin is a protein hormone, not a steroid, and is not derived from cholesterol.

Fatty acids are components of the phospholipids that make up the bulk of the lipid bilayers of all cellular membranes. It is synthesized by cytosolic acetyl-CoA  through carboxylation by acetyl-CoA carboxylase into malonyl CoA. This conversion occurs primarily in the liver, adipose tissue and lactating mammary glands, where the fatty acids are combined with glycerol to form triglycerides

Cholesterol helps to keep the membrane stable. This stability is maintained  as: At high temperatures, cholesterol reduces membrane fluidity by restraining the movement of phospholipids. At low temperatures, cholesterol gives solidification of the membrane because it prevents the close packing of phospholipids.

Cholesterol content is minimum in vegetable oils as plant have ergosterol as the major sterol present in them. Cholesterol is present in animals and 20 % of total cholesterol is sythesised in liver.