Carbohydrates and Lipids: Structure, Properties, and Functions

Carbohydrates: Bonds and Structures

O-glycosidic bond:

• Maltose: Known as malt sugar, it is a product of starch or glycogen hydrolysis. It consists of two molecules of α-D-glucose linked by an easily hydrolyzable monocarbonyl bond and possesses reducing properties.
• Lactose: The sugar found in mammalian milk, formed by the union of β-D-galactose and α-D-glucose. It has a free reducing end.
• Sucrose: Common table sugar extracted from sugar cane and beet. It involves a linkage between glucose and fructose and is non-reductive.
• Cellobiose: Does not exist in a free state in nature; it results from the hydrolysis of cellulose. It has reducing properties and is hydrolyzed with difficulty.

Cellulose

Cellulose is a linear polymer of β-D-glucose molecules. The glucose chains are arranged in parallel, stabilized by both intra-chain and inter-chain hydrogen bonds, providing high structural strength. Cellulose can only be fully hydrolyzed to glucose by specific enzymes called cellulases.

Polysaccharides

Polysaccharides accumulate as insoluble granules in the cytoplasm, preventing an increase in osmotic pressure that would occur if free glucose molecules were stored.

• Starch: Composed of amylose (helical α-D-glucose chains) and amylopectin. Found in roots, tubers, and seeds, it is hydrolyzed by amylases to yield glucose and maltose.
• Glycogen: The primary energy reserve in animal cells. Its structure is similar to amylopectin. It is stored as granules in the liver and skeletal muscle, where it is easily hydrolyzed to release glucose when needed.

Unit 4: Lipids

Lipids are organic compounds composed of carbon, hydrogen, and oxygen, and occasionally phosphorus and sulfur. They contain a significantly lower proportion of oxygen relative to carbon and hydrogen.

Physical Properties

Lipids are oily substances, sparingly soluble in water but soluble in nonpolar organic solvents, which are used to extract them from cells and tissues.

Biological Functions

• Structural: They are major components of all cell membranes.
• Energetic: Triglycerides serve as efficient energy reserves.

Classification

• Saponifiable: Contain fatty acids and can form soaps.
• Unsaponifiable: Do not contain fatty acids and do not undergo saponification (e.g., terpenes, steroids, and prostaglandins).

Fatty Acids

These are monocarboxylic acids with an even number of carbon atoms, typically ranging between 10 and 22.

• Saturated: Contain no double bonds and are usually solid at room temperature.
• Unsaturated: Contain one or more double bonds in the carbon chain. Polyunsaturated fatty acids are not synthesized by mammals and are considered essential nutrients, sometimes referred to as Vitamin F.