Biological Molecules: Cell Composition and Properties

Introduction to Biological Molecules

Cells are composed of a variety of biological molecules, each with specific chemical properties that determine their function and behavior in living organisms. The diversity of life is rooted in the molecular diversity of these compounds, which are primarily based on carbon.

• Biological molecules include carbohydrates, lipids, proteins, and nucleic acids.

• Each group has unique monomers (building blocks) and functional groups that predict their chemical properties.

• Complex biological molecules are formed by linking monomers into polymers.

• Polymers can be broken down into monomers by hydrolysis reactions.

Carbon Compounds and Functional Groups

Overview of Carbon Compounds

Carbon is the foundational element in organic molecules due to its ability to form four covalent bonds, resulting in a vast array of complex structures.

• Organic compounds are molecules containing carbon and are essential for life.

• Major classes of biological molecules: carbohydrates, lipids, proteins, and nucleic acids.

• Large biological molecules are called macromolecules.

Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules.

• Common functional groups include hydroxyl, carbonyl, carboxyl, amino, sulfhydryl, phosphate, and methyl groups.

• Functional groups determine the polarity, reactivity, and solubility of organic molecules.

• Example: The carboxyl group (-COOH) is acidic and found in amino acids and fatty acids.

Macromolecules: Monomers and Polymers

Formation and Breakdown of Polymers

Macromolecules are formed by joining monomers through dehydration reactions and are broken down by hydrolysis.

• Dehydration reaction: Removes a water molecule to form a covalent bond between monomers.

• Hydrolysis: Adds a water molecule to break a covalent bond, separating monomers.

• Enzymes facilitate both synthesis and breakdown of polymers.

Carbohydrates

Structure and Function

Carbohydrates serve as energy sources and structural materials in cells. They include sugars and their polymers.

• Monosaccharides: Simple sugars (e.g., glucose, fructose) with the general formula .

• Disaccharides: Formed by joining two monosaccharides via a glycosidic linkage (e.g., sucrose).

• Polysaccharides: Long chains of monosaccharides; serve storage (starch, glycogen) and structural (cellulose, chitin) roles.

Example: Starch is a storage polysaccharide in plants, while glycogen serves this role in animals.

Lipids

Types and Properties

Lipids are hydrophobic molecules that include fats, phospholipids, and steroids. They do not form true polymers.

• Fats (triglycerides): Composed of glycerol and three fatty acids. Used for energy storage.

• Saturated fatty acids: No double bonds; solid at room temperature.

• Unsaturated fatty acids: One or more double bonds; liquid at room temperature.

• Phospholipids: Two fatty acids and a phosphate group attached to glycerol; major component of cell membranes.

• Steroids: Lipids with a carbon skeleton of four fused rings (e.g., cholesterol).

Example: Phospholipids form bilayers in cell membranes due to their hydrophilic heads and hydrophobic tails.

Proteins

Structure and Function

Proteins are polymers of amino acids and perform a wide range of functions, including catalysis, transport, and structural support.

• Amino acids: Organic molecules with amino and carboxyl groups; 20 types differ by their R group.

• Polypeptides: Chains of amino acids linked by peptide bonds.

• Protein structure: Four levels—primary (sequence), secondary (alpha helix, beta sheet), tertiary (3D folding), quaternary (multiple polypeptides).

• Protein function depends on its shape, which can be altered by changes in temperature, pH, or other environmental factors (denaturation).

Example: Enzymes are proteins that catalyze biochemical reactions.

Nucleic Acids

DNA and RNA Structure and Function

Nucleic acids store and transmit genetic information. DNA and RNA are polymers of nucleotides.

• Nucleotide: Consists of a nitrogenous base, a pentose sugar, and one or more phosphate groups.

• DNA: Double helix with complementary base pairing (A-T, G-C); stores genetic information.

• RNA: Single-stranded; involved in protein synthesis and gene regulation.

Example: Messenger RNA (mRNA) carries genetic instructions from DNA to ribosomes for protein synthesis.

Genomics and Proteomics

Modern Biological Inquiry

Genomics and proteomics are fields that analyze large sets of genes and proteins to understand biological function and evolution.

• Genomics: Study of whole genomes, including sequencing and analysis.

• Proteomics: Study of the entire set of proteins expressed by a genome.

• These approaches enable rapid advances in medical science, evolutionary biology, and conservation.

Example: The Human Genome Project sequenced the entire human genome, revolutionizing biology and medicine.

Summary Table: Major Classes of Biological Molecules

Key Equations and Concepts

• General formula for monosaccharides:

• Peptide bond formation:

• Phosphodiester linkage in nucleic acids:

Additional info: Some explanations and examples have been expanded for clarity and completeness.