Functions and principal sources of lipids - class-XI

Lipids are hydrophobic (water repelling) and non-polar. Non-polar substances, for instance fats and oils cannot be dissolved in water because they have made up of only carbon-carbon and hydrogen-carbon bonds. Only polar solutes dissolve in polar solvents.

The polysaccharide is a carbohydrate that is formed by the bond formation between various monomers like glucose, fructose etc. Its examples are starch, cellulose, chitin, glycogen, etc.

Stachyose is a tetrasaccharide consisting of 2 galactose, one glucose and one fructose units. It occurs naturally in green beans, soya beans and other beans. It is less sweet than sucrose. Its molecular formula is $C _{24}H _{42}O _{21}$

Lipids are classified into simple, compound and derived lipids. 

Proteins are made of amino acids (monomers) and polysaccharides or sugars are made of monosaccharides (monomers).  Nucleic acids or DNA and RNA are made of nucleotides again monomers. However, lipids are not polymers made of smaller monomeric units. 

Lipids are compounds of carbon, hydrogen and oxygen where ratio of hydrogen and oxygen is less than that of water. Lipids are esters of fatty acids and alcohol which form emulsion with water but are soluble in organic solvents. Some of the common examples of lipids are butter, ghee, cholesterol, carotene etc. Lipids are used as energy source. Lipids provide 9.3 kcal per gram as compared to 4.5 kcal per gram by carbohydrates. Lipids have other important uses also like they lubricate hairs and feathers in animals, reduce transpiration in plants and form components of cell membranes in organisms.

A triglyceride is an ester derived from glycerol and three fatty acids. The structure consists of one glycerol molecule backbone with three fatty acid chains attached.

Unsaturated fatty acids have one or more double bonds like oleic acid (one), linoleic acid (two), linolenic acid (three), and arachidonic acid (four). An unsaturated fatty acid with one double bond is called mono unsaturated fatty acid (MUFA). Fatty acid with more than one double bond is called as polyunsaturated fatty acid (PUFA). Animals cannot synthesize fatty acids with double bonds (Linoleic, Linolenic and Arachidonic acids). Hence, these fatty acids must be obtained in diet and are thus called essential fatty acids.

Lipids are esters of fatty acids and glycerol which form emulsion with water are soluble in organic solvents, example- ghee, butter, carotene, menthol. Fatty acids are long chains of hydrocarbon having an acidic group. Chemically, lipids are diverse and may be acidic or esters but all lipids have a common property that they are hydrophobic and insoluble in water.

Lipids are hydrophobic means water-hating. They are water-insoluble. Lipids are non-polar molecules while water is a polar molecule, so there is no way for water to attach to a lipid molecule. Instead, the non-polar lipids will bind to themselves, which is why oils and fats form droplets on the surface of the water.

The average amount of protein present in muscle is 18.5%. Lipids constitute 3% in muscle

Respiration is the oxidation process in which reduced biomolecules like carbohydrates, fats, proteins etc are oxidised to release energy, which can be used for cellular processes. Fats are most reduced biomolecules and their oxidation releases maximum amount of energy. The energy released in oxidation of one gram of fat is twice the energy released in oxidation of one gram of carbohydrate. Hence, carbohydrates are used as an energy reserve biomolecules in living systems.

Phospholipids are a class of lipids that are a major component of all cell membranes as they can form lipid bilayers. Most phospholipids contain a diglyceride, a phosphate group, and a simple organic molecule such as choline; one exception to this rule is sphingomyelin, which is derived from sphingosine instead of glycerol. 

Lipids are hydrophobic means water hating. They are water insoluble. Lipids are non-polar molecules while water is polar molecule, so there is no way for water to attach to a lipid molecule. Instead, the non-polar lipids will bind to themselves, which is why oils and fats form droplets on the surface of water.

Proteins are the polymers of amino acids and have C, H, O, N and S. Carbohydrates and  nucleic acids have C, H, O, N, P and S. Lipids are chemically diverse group and mostly have long hydrocarbon chain composed of C, H and O. Thus, the correct answer is A.

Sphaerosomes are small, spherical organelles found in plant cells that are specialized for the synthesis and storage of lipids.

Cofactor is a nonprotein part of the enzyme. It can be metal ions or the organic chemical. The cofactors which are organic chemicals are known as coenzyme. These are loosely attached to the apoenzyme. The tightly bound coenzymes are prosthetic group. 

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.

a) Palmitic acid is a saturated fatty acid. Its chemical formula is CH$ _3$(CH$ _2$)$ _{14}$COOH. Meat, cheese and diary products are the main source of the palmitic acid. It was first discovered in saponified palm oil. Excess carbohydrate in the body is converted into palmitic acid. 

The use of wax used to prepare candles has changed during the past few centuries. Nowadays, standard commercial candles are made from a combination of Paraffin wax and Stearic acid. A small amount of Bee wax may or may not be added in the making.

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. 

The most widely available dietary source of omega-3 fats is oily fish, such as salmon, herring, mackerel, anchovies, menhaden, and sardines. All types of mackerel are higher-fat fish, rich in the healthful polyunsaturated fats known as long-chain omega-3s, which may help boost heart health.
So, the correct answer is option A.

The major driving force for the spontaneous formation of the lipid bilayer in biological membranes is the hydrophobic effect. Water molecules form an ordered cage around hydrophobic tails, and aggregation of these tails increases water entropy, stabilizing the bilayer.

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.

The endoplasmic reticulum (specifically the smooth endoplasmic reticulum) is the primary site for the synthesis of most membrane lipids, phospholipids, and cholesterol in eukaryotic cells.

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.

So, the correct option is 'A-(iii),B(ii),C(i),D(iv)'.