Lipids Review in 11 Questions and Answers

2. What is the molecular structure of glycerol? To which organic functional group does this substance belong?

Glycerol is a linear chain of three carbon atoms; the central carbon atom is bound to one hydroxyl radical and to one hydrogen atom and the two other carbon atoms at the ends are bound to a hydroxyl radical and to two hydrogen atoms. Spatial position side of the hydroxyl radicals is the same.

• Lipids Review - Image Diversity: glycerol molecule

3. What is the composition of triglycerides?

Triglycerides, which are fats or oils, are made up of three molecules of fatty acids bound to one molecule of glycerol. Hydroxyls of each one of the three fatty acids and each hydrogen atom of the hydroxyls of the glycerol bind to form three molecules of water that are released.

• Lipids Review - Image Diversity: triglyceride molecule

4. What are phospholipids?

Phospholipids are molecules made up of one molecule of glycerol bound to two long molecules of fatty acids and to one phosphate group. Therefore, phospholipids are amphipathic molecules, meaning that they have a non-polar portion, due to the long fatty acid chains, and a polar portion, due to the phosphate group.

Phospholipids are the main component of cell membranes. Sphingomyelin, the substance that forms the myelin sheath of axons in the nervous system, is a phospholipid.

• Lipids Review - Image Diversity: phospholipid molecule
  

5. What are steroids? What are some examples of steroids with a biological function?

Steroids are lipids that consist of an angular combination of four carbon rings, three of which are made of six carbon atoms and one of which, located at the end, made of five carbon atoms in the extremity. The bond between each ring and the adjacent ring is made through by the sharing of two adjacent carbon atoms which belong to both rings.

Bile salts, cholesterol, the sexual hormones estrogen, progesterone and testosterone, corticosteroids and pro-vitamin D are examples of steroids.

• Lipids Review - Image Diversity: steroid structure

6. What are hydrophobic molecules (or hydrophobic molecular regions)? What are hydrophilic molecules? How can they be described in relation to their polarity?

Hydrophobic molecules are molecules with little or no propensity to dissolve in water (hydro = water, phobia = fear). Hydrophilic molecules are those that have a large propensity to dissolve in water (philia = friendship).

Water is a polar substance. Remembering the rule that “equal dissolves equal”, it is easy to conclude that hydrophobic substances are non-polar molecules whereas hydrophilic molecules are polar molecules.

7. Are organic solvents such as benzene and ethers polar or non-polar substances?

Benzene and ethers are molecules without electrically charged portions and therefore are non-polar substances.

8. How are lipids classified according to solubility?

Fats and oils are hydrophobic molecules, meaning that they are non-polar and insoluble in water. Lipids in general are molecules with a large non-polar extension, making them soluble in non-polar solvents, such as benzene, ether and chloroform.

There exist some amphipathic lipids, which are lipids with both a hydrophilic portion, which gives them the property of water-solubility, as well as a hydrophobic portion, which is non-polar. 

9. What is meant by saturation or unsaturation when referring to oils and fats?

When a triglyceride is saturated, it means that in its molecule the carbon chain is bound to hydrogen molecules in its maximum capacity, meaning that there are no double or triple bonds between carbon atoms. Saturated molecules are generally solid fats at room temperature.

Unsaturated triglyceride molecules are those in which there are double or triple bonds between carbon atoms and, as a result, they are less hydrogenated than the saturated ones . In general, these unsaturated molecules are oils, and are liquid at room temperature.

The terms saturated and unsaturated refer to the saturation of the carbonic chain by hydrogen atoms.

• Lipids Review - Image Diversity: saturated fatty acid unsaturated fatty acid

10. Why do fats have thermal insulation properties?

Triglycerides are poor heat conductors and, in addition, they form thick layers of fatty tissue when accumulated in an organism. That is why they are good thermal insulators.

In animals that live in cold climates, such as polar bears, seals and whales, adipose tissue helps the maintenance of internal body temperature.

11. How are lipids used as an energy source by the body?

Carbohydrates are the main energy source for aerobic cell respiration. When such substances are absent or deficient, the body can use lipid stores since fats (like proteins) can be broken down into acetyl-CoA to feed the Krebs cycle (a stage of aerobic cellular respiration).