Carbohydrates

Overview of Carbohydrates

Carbohydrates are one of the four major types of organic molecules found in living organisms. They serve as energy sources, structural components, and are involved in cell recognition processes. Carbohydrates are composed of sugar units called saccharides.

• Monosaccharides: Simple sugars, such as glucose, that contain a single sugar unit.

• Disaccharides: Composed of two monosaccharide units joined by a glycosidic bond (e.g., sucrose).

• Polysaccharides: Long chains of monosaccharide units; examples include starch, glycogen, cellulose, and chitin.

General formula for glucose:

Cellular energy conversion:

Monosaccharides

Monosaccharides are the simplest carbohydrates and serve as building blocks for more complex sugars.

• Structure: Contain a carbonyl group and several hydroxyl groups.

• Isomerization: Monosaccharides can exist in linear or ring forms in solution.

• Examples: Glucose, fructose, galactose.

Polysaccharides: Storage and Structural Roles

Polysaccharides are large carbohydrate molecules formed by the polymerization of monosaccharides. They serve as energy storage and structural materials in cells.

• Storage Polysaccharides: Starch (plants) and glycogen (animals) are used for energy storage.

• Structural Polysaccharides: Cellulose (plants) and chitin (fungi and animals) provide structural support.

Cellulose: The major structural carbohydrate in plant cell walls. It is insoluble and indigestible by most animals.

Chitin: Found in the exoskeletons of arthropods and cell walls of fungi. Contains amine groups that differentiate it from other polysaccharides.

Lipids

Overview of Lipids

Lipids are a diverse group of macromolecules that serve as long-term energy storage, structural components of cell membranes, and signaling molecules. Lipids are insoluble in water but soluble in nonpolar solvents.

• Types of lipids: Fats (triglycerides), phospholipids, steroids (e.g., cholesterol).

• Functions: Energy storage, insulation, cell membrane structure, signaling.

Fats (Triglycerides)

Fats, also known as triglycerides, are composed of one glycerol molecule and three fatty acid chains. They are formed by dehydration synthesis reactions.

• Structure: Glycerol backbone attached to three fatty acids.

• Formation: Three dehydration synthesis reactions join the fatty acids to glycerol.

• Fatty acids: Typically 16-18 carbons long.

Saturated vs. Unsaturated Fats

The physical properties of fats depend on the saturation of their fatty acid chains.

• Saturated fats: No double bonds; molecules pack closely, solid at room temperature (e.g., butterfat).

• Unsaturated fats: One or more double bonds; molecules have kinks, liquid at room temperature (e.g., olive oil).

• Trans fats: Unsaturated fats artificially saturated; solid at room temperature, associated with health risks.

Phospholipids

Phospholipids are major components of cell membranes. They consist of a glycerol backbone, two fatty acid tails, and a phosphate group.

• Structure: Hydrophilic (polar) phosphate head and hydrophobic (nonpolar) fatty acid tails.

• Amphipathic nature: The molecule has both hydrophilic and hydrophobic regions, allowing it to form bilayers in water.

• Function: Form the structural basis of cell membranes and serve as a barrier between the cell and its environment.

Steroids

Steroids are lipids characterized by a four-ring structure. Cholesterol is a common steroid found in cell membranes and serves as a precursor for steroid hormones.

• Cholesterol: Maintains membrane fluidity and is a precursor for hormones such as estrogen and testosterone.

Summary Table: Carbohydrates vs. Lipids

Additional info:

• Dietary fiber is composed of indigestible polysaccharides such as cellulose.

• Phospholipids form the bilayer structure of cell membranes due to their amphipathic nature.

• Trans fats are associated with increased risk of cardiovascular disease.