Chapter 8, Problem 1RE

RECALL Proteins, nucleic acids, and carbohydrates are grouped by common structural features found within their group. What is the basis for grouping substances as lipids?

Interpretation Introduction

Interpretation:

The basis for grouping substances such as lipids is to be defined.

Concept Introduction:

Proteins, carbohydrates and nucleic acids are the polymers of amino acid residues, sugar residues and nucleotides, respectively.

Saturated hydrocarbons contain only single-bonded carbon atoms, and unsaturated hydrocarbons contain multiple double or triple bonds.

Electronegativity is the property of an atom to pull electrons towards itself.

Answer to Problem 1RE

Solution:

Lipids are grouped only on the basis of solubility and non-polar nature of their molecules, whereas, some lipids do not share any structural similarity, like, groups if proteins, carbohydrates and nucleic acids.

Explanation of Solution

The amino acid residues are linked together by peptide bonds and polymerize to form a peptide chain, which eventually gives rise to a protein structure. The sugar molecules like glucose are linked together by glycosidic linkages to form a polymer of carbohydrate. DNA (deoxyribonucleic acid) is a polymer of deoxynucleotides, like, adenine, guanine, cytosine, and thymine. Carbohydrates have carbon, hydrogen, and oxygen in their structures. Proteins have amino and carboxyl groups in their structures. The nucleic acids have phosphate, sugar residue and a nitrogenous base in their structures. It suggests that all these molecules are grouped on the basis of some common groups or atoms that are constant in their structure.

The lipids are grouped on the basis of their hydrophobic nature, that is, the fear of water. The lipids are not soluble in water because of the presence of a hydrocarbon chain which is non-polar in nature. Polar molecules are those molecules that contain opposite charges on their molecules, for example, water. In water (H2O), the oxygen atom carries a partial negative charge and the hydrogen carries a partial positive charge. It is because oxygen is more electronegative than hydrogen, and therefore, tends to pull the electrons towards itself.

Lipids, sometimes, do not share a common structure. For example, the lipid compounds that carry long hydrophobic hydrocarbon chains and polar head groups are classified as fatty acids, sphingolipids, glycolipids, triacylglycerol, and others. The lipids that contain fused ring structures are grouped as steroids, and cholesterol is the most common type of steroid.

Conclusion

The lipids are grouped on the basis of solubility and non-polar nature. Also, some lipids are not structurally related.