Carbohydrates and Lipids: Fundamental Biomolecules

Carbohydrates: Structure and Function

Monosaccharides

These are simple carbohydrates, often referred to as simple sugars. They are called sugars due to their sweet taste and are carbohydrates because they contain hydrogen and oxygen in the same proportion as water. Their general formula is (CH₂O)_n, where 'n' represents the number of carbon atoms, typically ranging from 3 to 7.

Depending on whether the carbonyl group is an aldehyde or a ketone, monosaccharides are classified as aldoses or ketoses, respectively. Based on the number of carbon atoms, they are further classified as:

• Trioses (3 carbons)
• Tetroses (4 carbons)
• Pentoses (5 carbons)
• Hexoses (6 carbons)
• Heptoses (7 carbons)

Their main functions are energy storage and structural support. Regarding their energy function, energy is released when they are catabolized. Regarding their structural function, polymerized monosaccharides form molecules like cellulose and chitin, which are essential structural components.

Aldoses

A monosaccharide whose carbonyl group is an aldehyde, typically located at a primary carbon.

Ketoses

A monosaccharide whose carbonyl group is a ketone, typically located at a secondary carbon.

Disaccharides

These are molecules formed by the union of two monosaccharides through an O-glycosidic bond. These molecules are hydrolyzable, meaning they can be broken down into their constituent monosaccharides by adding water.

Polysaccharides

These are high molecular weight substances that are not crystallizable, tasteless, and poorly soluble in water. They are formed by the union of 'n' monosaccharide molecules through O-glycosidic bonds, with the separation of 'n-1' water molecules, where n > 10. They are classified into two main types:

• Storage Polysaccharides: These represent a form to store sugars without creating an osmotic problem within cells. Examples include starch and glycogen.
• Structural Polysaccharides: These carbohydrates are involved in the construction of organic structures, providing support and rigidity. Examples include cellulose and chitin.

O-Glycosidic Bond

This covalent bond forms from the reaction between two hydroxyl (-OH) groups of two monosaccharides, typically with the elimination of a water molecule.

Lipids: Essential Biomolecules

Lipids Overview

Lipids are a diverse group of organic compounds primarily composed of carbon (C), hydrogen (H), and sometimes oxygen (O). They are less dense than water, insoluble in water, but soluble in organic solvents like ether, chloroform, and benzene. They serve several crucial functions:

• Reserve Function: Stored as a significant energy reserve in organisms.
• Structural Function: Some lipids, due to their amphipathic (having both hydrophilic and hydrophobic properties) nature, organize into bilayers, forming the structural basis of cell membranes.
• Regulatory Function: Some lipids act as hormones (e.g., steroid hormones) or signaling molecules, regulating various physiological processes.

Phospholipids

Phospholipids are saponifiable lipids, meaning they can be hydrolyzed by a base. They are formed by two fatty acid molecules esterified with a glycerol molecule and a phosphate group, which may be joined to another variable molecule (e.g., choline). They are the structural basis of cell membranes because, due to their amphipathic nature, they spontaneously organize into bilayers in aqueous media.

The two hydrophobic fatty acid chains tend to separate from water and join each other through hydrophobic interactions and Van der Waals forces, forming the inner core of the bilayer. The polar (hydrophilic) heads, containing the phosphate group, are located on the outside of the bilayer, in contact with water. This arrangement forms the lipid bilayer of the cytoplasmic membrane.

Fatty Acids

Fatty acids are organic or carboxylic acids with an even number of carbon atoms, typically ranging from 1 to 30. They consist of long hydrocarbon chains with a carboxyl group (-COOH) at one end, and their geometry often results in zigzag-shaped links.

They can be classified as:

• Saturated: Possessing no double bonds in the hydrocarbon chain.
• Unsaturated: Possessing one or more double bonds in the hydrocarbon chain.

Fatty acids can be liquids or solids at ambient temperature; their melting point depends on the length of the carbon chain and the number of double bonds present. They are amphipathic molecules, having a hydrophilic carboxyl head and a hydrophobic hydrocarbon tail. Their key chemical properties include esterification with alcohols to form esters and forming salts with alkalis (saponification).