7. Lipids and low-molecular bioregulators

Question 101. What is organic compound class of lipids?

Answer. Lipids (from Greek lipos – fat) are natural fat-similar substances which are not practically soluble in water but soluble at non-polar organic solvents.

Lipids do not belong to one organic substance class and they do not have any common structural features. In majority of cases they are esters of higher fatty acids with glycerol and other alcohols (for example, sphingosine). At lipid structure the following substances could be: phosphoric acid, amino alcohols or carbohydrates.

Question 102. Are lipids biopolymers?

Answer. Lipids are not biopolymers but in the organism they could be attached to other biopolymers, for example, proteins.

Question 103. What functions are performed by lipids in the organism?

Answer. The most important of lipid biological functions are:

1. Building (structural) – lipids take part at cellular membrane formation. Phospholipids, glycolipids and lipoproteids are in the structure of cellular membranes. Lipids take part at many biologically important compounds formation.

2. Energy function – at lipid oxidation a large amount of energy is discharged. At 1gr of fat degradation to СО₂ and Н₂О about 38,9 kJ (9,5 kcal) of energy is discharged which is twice as much as at protein and carbohydrate degradation. Lipids provide 25-30 % of energy necessary to organism.

3. Storing function – calorie-rich character and insolubility in water make fats and oils ideal components to store energy. This is important for animals which sleep at winter or migrate for a long time through the area without food supply. Plant seeds contain fat necessary to provide energy for fetus to grow.

4. Thermic regulatory function – fats are very poor at heat conducting, that is why, subcutaneous fat layer at mammals helps them keep heat.

5. Protective mechanical function – amortization of subcutaneous fat protects organs, for example, kidneys from mechanical damage.

6. Catalytic function – it is connected to fat-soluble vitamins (А, D, Е, К), which molecules have lipid basis. Vitamins as they are do not have any catalytic activity but they are in the structure of enzymes and without them enzymes could not perform their functions.

7. Source of metabolic fluid – one of fat oxidation products is water. This metabolic water is very important for creatures living at deserts. Thus, fat which is at camel hump is not the source of energy but the source of water in the first place.

8. Protection from excessive water amount and from too large water loss – fat discharge from sebaceous glands helps skin and wool being water-proof. Wax cuticle at insects and plants decreases water evaporation, as water could not cross non-dissolvable lipid layer.

9. Regulatory function – sex hormones, testosterone, progesterone and corticosteroids and also other hormones are cholesterol derivatives. Vitamin D, cholesterol derivatives play an important role at calcium and phosphorus metabolism. Bile acids take part at food digestion (fat emulsification) and also at higher carboxylic acid absorption.

10. Electric isolation – myelin which is discharged by Shwann’s cells contains 70-75% of lipids which isolate some neurons in such a way that impulse transduction is faster.

Question 104. What is the difference between saponificated and un-saponificated lipids?

Answer. Saponificated lipids contain ester bonds and they are able to be split at hydrolysis process. If hydrolysis goes on at alkaline medium then soaps are produced among reaction products. Soap is higher carboxylic acid salt. Un-saponificated lipids do not have ester bonds and are not exposed to hydrolysis.

Question 105. What lipids belong to saponificated lipids?

Answer. Saponificated lipids are subdivided into two groups: simple (unconjugated) and complex (conjugated). Unconjugated lipids are esters and are made by fatty acid residues and alcohol residues (mono-alcohols, di- or tri-alcohols), they include waxes, fats and ceramides. In the structure of conjugated lipids there are higher fatty acid residues and alcohols with substituted groups, phosphoric acid residues and mono-saccharides. Conjugated saponificated lipids include phospholipids, glycolipids and sphingolipids.

Question 106. What compounds belong to un-saponificated lipids?

Answer. Un-saponificated lipids are not esters, that is why they are not hydrolyzed at alkaline medium with fatty acid formation and they do not give soap.

Many un-saponificated lipids are:

– low-molecular regulators (thromboxanes, leukotrienes, prostaglandins, prostacyclins);

– vitamins (all fat-soluble vitamins);

– hormones (steroid sex hormones, glucocorticoids and mineralocorticoids);

– plant hormones (hibberellins, abscisic acid, ethylene);

– pigments (carotine, licopene);

– smelling substances (geraniol, geranial, menthol, mircen);

– pheromones (citral, grandisol).

Question 107. What alcohol residue is at plant oils?

Answer. Plant oils belong to un-conjugated saponificated lipids, to be more particular they belong to liquid fats. In the structure of liquid fats (as well as in the structure of solid fats) there are tri-alcohol (glycerol) residues.

Question 108. What carboxylic acid residues are at plant oils?

A nswer. In the structure of plant oils there are mainly glycerol and higher carboxylic unsaturated acid esters. Most widely-spread acids are oleic acid С₁₇Н₃₃СООН, linoleic acid C₁₇H₃₁COOH and linolenic acid C₁₇H₂₉COOH. All these acids have 18 carbon atoms, at oleic acid radical there is one double bond (in the middle of the molecule), at linoleic acid radical there are two double bonds separated by –СН₂– group, at the linolenic acid radical there are three double bonds separated by methylene groups (–СН₂–).

Question 109. What carboxylic acid residues are in solid fat structure?

A nswer. At solid fat structure there are mainly esters of glycerol and higher fatty saturated acids. Most widely-spread acids are palmitic acid C₁₅H₃₁COOH and stearic acid С₁₇Н₃₅СООН:

Question 110. What configuration in relation to multiple bond is typical of unsaturated acids which are in the structure of natural fats?

Answer. At plant oil structure there are mainly higher carboxylic unsaturated acid residues containing one or several double bonds at radical at cis-configuration. Hydrocarbon radical at an acid has curved and shortened form, molecule has large volume and at crystal formation it is packed more loosely than trans-isomers. It determines difference in melting temperature and aggregate conditions of acids. Thus, oleic acid at room temperature is liquid and its trans-isomer, elaidic acid, is solid.

Cis-configuration of unsaturated acids is less stable than trans-configuration. This fact makes food fats more exposed to catabolism, it is easier to digest them.

Question 111. Write down tri-oleyl-glycerol formula.

A nswer. Tri-oleyl-glycerol is ester of glycerol and three oleic acid molecules:

At the scheme thick letters stand for glycerol residue.

Question 112. Write down 1-oleyl-2-stearyl-3-palmityl-glycerol.

Answer. 1-Oleyl-2-stearyl-3-palmityl-glycerol is ester of glycerol and three different acid molecules: oleic, stearic and palmitic acids. Carbon atoms at glycerol fragment are enumerated starting from the top down, it means that oleic acid residue should be located at the top, lower stearic acid residue is placed and palmitic acid residue should be at the bottom.

Question 113. What reagent transforms liquid fats into solid fats? Write down reaction scheme.

Answer. At liquid fat tri-acyl-glycerols there are mainly unsaturated acid residues, in the structure of solid fats there are mainly saturated acid residues. Thus, liquid fats could be transformed into solid fats due to reduction reaction by hydrogen with metal catalysts. Reaction scheme with tri-oleyl-glycerol has the following form:

Question 114. What reagents could give soap?

Answer. Soaps are higher fatty acid salts. They are formed at neutralization reaction between higher carboxylic acid and base or alkaline hydrolysis reaction of fats (saponification reaction).

СН₃(СН₂)₇СН=СН(СН₂)₇СООН + NaOH 

 СН₃(СН₂)₇СН=СН(СН₂)₇СООNa + H₂O

СН₂–О–СО–(CH₂)₇–СН=СН–(СН₂)₇–CH₃

СН – О–СО–(CH₂)₁₆–CH₃ + 3 NaOH 

СН₂–О–СО–(CH₂)₁₄–CH₃

1-oleyl-2-stearyl-3-palmityl-glycerol

+

+

+

+

Question 115. How many benzene rings are there at cholesterol molecule?

A nswer. To answer this question it is necessary to write cholesterol molecule and analyze its structure.

It is evident that at cholesterol molecule there are three six-membered rings, two of them are saturated and one is unsaturated, but there are no aromatic benzene rings.