Biological Molecules: Lipids

Types and Structure of Lipids

Lipids are a diverse group of organic molecules composed mainly of carbon (C), hydrogen (H), and oxygen (O). They are essential for various biological functions and are generally insoluble in water but soluble in organic solvents.

• Triglycerides: Consist of one glycerol molecule bonded to three fatty acids. Fatty acids can be saturated (no double bonds between carbon atoms) or unsaturated (one or more double bonds).

• Phospholipids: Composed of a glycerol backbone, two fatty acids, and a phosphate group. They are key components of cell membranes.

• Steroids (e.g., cholesterol): Have a structure of four fused carbon rings. Cholesterol is important for membrane fluidity and as a precursor for steroid hormones.

Example: Olive oil is rich in unsaturated fatty acids, while butter contains more saturated fatty acids.

Common Properties of Lipids

• Insoluble in water due to nonpolar hydrocarbon chains.

• Soluble in organic solvents such as ether, chloroform, and alcohol.

• Less dense than water, causing fats and oils to float.

Functions of Lipids

• Energy Storage: Lipids are oxidized to release energy, providing more than twice the energy per gram compared to carbohydrates.

• Insulation and Protection: Fat deposits under the skin help insulate the body and protect organs from mechanical shock.

• Component of Cell Membranes: Phospholipids form the bilayer structure of cell membranes, providing fluidity and selective permeability.

• Transport: Lipids assist in the transport of fat-soluble vitamins (A, D, E, K).

• Hormone Precursors: Steroids such as cholesterol are precursors for hormones like testosterone and estrogen.

Additional info: Lipids also serve as waterproofing agents in plants and animals (e.g., waxes on leaves and feathers).

Table: Comparison of Lipid Types

Biological Molecules: Proteins

Structure and Types of Proteins

Proteins are large, complex molecules made up of carbon (C), hydrogen (H), oxygen (O), and nitrogen (N). Some proteins also contain sulfur (S). They are polymers of amino acids linked by peptide bonds.

• Amino Acids: The building blocks of proteins. There are 20 different amino acids, each with a central carbon atom, an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a variable R group (side chain).

• Peptide Bonds: Formed by a condensation reaction between the amino group of one amino acid and the carboxyl group of another, releasing water.

• Dipeptide: Two amino acids joined by a peptide bond.

• Polypeptide: A chain of many amino acids; folds into a specific three-dimensional structure to form a functional protein.

Example: Hemoglobin is a protein that transports oxygen in the blood.

Functions of Proteins

• Growth and Repair: Proteins are essential for the growth and repair of tissues.

• Enzymes: Biological catalysts that speed up chemical reactions (e.g., amylase, protease).

• Hormones: Some proteins act as chemical messengers (e.g., insulin).

• Transport: Proteins such as hemoglobin transport substances in the body.

• Antibodies: Proteins involved in immune defense.

• Structural Components: Proteins like collagen provide structural support in tissues.

• Energy Source: Proteins can be used for energy when carbohydrates and fats are insufficient.

Additional info: Some amino acids are essential and must be obtained from the diet, as the body cannot synthesize them.

Table: Comparison of Protein Functions

Key Equations and Concepts

• Formation of a Peptide Bond:

• General Structure of an Amino Acid:

• General Structure of a Triglyceride:

\[ \text{Glycerol} + 3 \times \text{Fatty acid} \rightarrow \text{Triglyceride} + 3 \text{H}_2\text{O} \]