The Molecules of Cells: Structure and Function of Biological Molecules

The Molecules of Cells

Introduction

Cells are composed of a variety of molecules that are essential for life. These molecules include carbohydrates, lipids, proteins, and nucleic acids. Understanding their structure and function is fundamental to the study of biology.

Hydrocarbons and Carbon Skeletons

Properties of Carbon

• Carbon can form four covalent bonds, allowing for a diversity of stable organic compounds.

• Carbon's ability to bond with other atoms forms the basis for large and diverse organic molecules.

• Examples: Methane (CH4), Ethane (C2H6), Propane (C3H8).

Hydrocarbons

• Composed only of carbon and hydrogen.

• Form the backbone of most organic molecules.

• Hydrocarbons are hydrophobic (do not mix well with water).

Carbon Skeletons

• Can be straight, branched, or arranged in rings.

• May include double bonds, which can vary in location.

• Example: Butane (C4H10) and 2-methylpropane (C4H10) are isomers.

Isomers

• Compounds with the same molecular formula but different structures.

• Structural isomers differ in the covalent arrangements of their atoms.

• Example: Butane and 2-methylpropane.

Functional Groups and Biological Molecules

Functional Groups

• Groups of atoms attached to the carbon skeleton that participate in chemical reactions.

• Common functional groups: hydroxyl (-OH), carbonyl (>C=O), carboxyl (-COOH), amino (-NH2), phosphate (-PO4), methyl (-CH3).

Chemical Group	Example
Hydroxyl	Ethanol
Carbonyl	Acetone
Carboxyl	Acetic acid
Amino	Glycine
Phosphate	Glycerol phosphate
Methyl	5-Methyl cytosine

Monomers and Polymers

• Monomers are small molecules that can join to form polymers.

• Polymers are formed by dehydration reactions (removal of water).

• Polymers are broken down by hydrolysis (addition of water).

Classes of Biological Molecules

• Carbohydrates

• Lipids

• Proteins

• Nucleic acids

With the exception of lipids, these are formed through polymerization.

Carbohydrates

Structure and Function

• Carbohydrates range from small sugar molecules (monomers) to large polysaccharides.

• General formula: $C_{n}(H_{2}O)_{n}$

• Monosaccharides: glucose, galactose, fructose (isomers).

• In aqueous solutions, monosaccharides often form ring structures.

Disaccharides

• Formed by linking two monosaccharides via a dehydration reaction.

• Examples: sucrose (glucose + fructose), lactose (glucose + galactose).

Polysaccharides

• Long chains of sugar units.

• Can be structural (cellulose, chitin) or storage (starch, glycogen).

• Cellulose is found in plant cell walls; chitin is found in insect exoskeletons and fungal cell walls.

Lipids

Structure and Types

• Lipids are hydrophobic compounds composed largely of carbon and hydrogen.

• Main types: fats, phospholipids, steroids.

• Fats are composed of glycerol and fatty acids.

Saturated vs. Unsaturated Fats

• Saturated fats: no double bonds between carbon atoms; solid at room temperature.

• Unsaturated fats: one or more double bonds; liquid at room temperature.

• Trans fats are unsaturated fats that have been hydrogenated, associated with health risks.

Phospholipids

• Major components of cell membranes.

• Composed of two fatty acids, a glycerol, and a phosphate group.

Steroids

• Characterized by a carbon skeleton with four fused rings.

• Examples: cholesterol, hormones such as testosterone and estradiol.

Health Implications

• High intake of saturated and trans fats is associated with increased risk of heart disease.

• Cholesterol is essential for cell membrane structure but high levels can be harmful.

Summary Table: Types of Biological Molecules

Class	Monomer	Polymer	Function
Carbohydrates	Monosaccharide	Polysaccharide	Energy storage, structure
Lipids	Glycerol, fatty acids	Triglyceride, phospholipid, steroid	Energy storage, membranes, hormones
Proteins	Amino acid	Polypeptide	Enzymes, structure, transport
Nucleic acids	Nucleotide	DNA, RNA	Genetic information

Additional info: Proteins and nucleic acids are not detailed in these slides but are essential classes of biological macromolecules. Their monomers and functions are included for completeness.