The Molecules of Cells: Carbohydrates, Lipids, Proteins, and Nucleic Acids

The Molecules of Cells

Introduction

Living organisms are composed of four major classes of biological macromolecules: carbohydrates, lipids, proteins, and nucleic acids. Each class has unique structures, monomers, polymers, and functions essential for life. Understanding these molecules is fundamental to biology.

Carbohydrates

Structure and Types

• Monomer: Monosaccharide (single sugar unit, e.g., glucose, fructose, galactose)

• Polymer: Polysaccharide (long chains of monosaccharides, e.g., starch, glycogen, cellulose, chitin)

Functions

• Energy Storage:

  • Animals: Glycogen

  • Plants: Starch

• Structural:

  • Cell wall in plants: Cellulose

  • Exoskeleton in insects: Chitin

Elements and Ratio

• Elements: Carbon (C), Hydrogen (H), Oxygen (O)

• Ratio: 1 : 2 : 1 (C:H:O)

Examples

• Bread, milk, plants, fruit, leafy greens, pasta

Key Terms and Structures

• Disaccharide: Two monosaccharides joined together (e.g., lactose)

• Polysaccharide: Many monosaccharides linked (e.g., starch, cellulose)

Example Equation: Formation of a Disaccharide

$\text{Monosaccharide} + \text{Monosaccharide} \rightarrow \text{Disaccharide} + \text{H}_2\text{O}$

Lipids

Structure and Types

• Monomer (Building Blocks): Glycerol + 3 fatty acid chains

• Polymer: Lipid (includes fats, oils, waxes, steroids, phospholipids)

Functions

• Energy Storage: Oils, fats

• Insulation

• Chemical Messengers: Steroids

• Cell Membranes: Phospholipid bilayer

• Waterproof Coverings: Waxes (plants)

Elements

• Mostly Carbon (C), Hydrogen (H); very little Oxygen (O)

Examples

• Oils, fats, waxes, steroids, phospholipids

Saturated vs. Unsaturated Fatty Acids

• Saturated: No double bonds in fatty acid chains

• Unsaturated: At least one double bond in fatty acid chains

Example Equation: Formation of a Triglyceride

$\text{Glycerol} + 3\ \text{Fatty Acids} \rightarrow \text{Triglyceride} + 3\ \text{H}_2\text{O}$

Proteins

Structure and Types

• Monomer: Amino acids (20 types; e.g., alanine, serine, methionine)

• Polymer: Protein (polypeptide)

Functions

• Enzymes: Speed up chemical reactions (e.g., lactase, hexokinase)

• Transport: Hemoglobin transports oxygen

• Pumps: Move molecules in and out of cells (e.g., sodium channel)

• Structure and Movement: Cytoskeleton, actin, myosin

Elements

• Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), sometimes Sulfur (S)

Examples

• Hemoglobin, lactase, hexokinase, sodium channel, actin, myosin

Key Concepts

• Shape determines function: The specific sequence and structure of amino acids determine a protein's function.

• Essential amino acids: 8 must be obtained from the diet; 12 can be synthesized by the body.

Example Equation: Peptide Bond Formation

$\text{Amino Acid}_1 + \text{Amino Acid}_2 \rightarrow \text{Dipeptide} + \text{H}_2\text{O}$

Nucleic Acids

Structure and Types

• Monomer: Nucleotide (composed of a 5-carbon sugar, phosphate group, and nitrogenous base)

• Polymer: Nucleic acid (DNA and RNA)

Functions

• Store and transmit genetic information: DNA and RNA are responsible for heredity and protein synthesis.

Elements

• Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Phosphorus (P)

Examples

• DNA, RNA

Key Components of a Nucleotide

• 5-carbon sugar (deoxyribose in DNA, ribose in RNA)

• Phosphate group

• Nitrogenous base (adenine, thymine, cytosine, guanine, uracil)

Example Equation: Nucleotide Polymerization

$\text{Nucleotide}_1 + \text{Nucleotide}_2 \rightarrow \text{Dinucleotide} + \text{H}_2\text{O}$

Summary Table: Biological Macromolecules

Macromolecule	Elements	Monomer	Polymer	Functions	Examples
Carbohydrates	C, H, O (1:2:1)	Monosaccharide	Polysaccharide	Energy storage, structure	Glucose, starch, cellulose
Lipids	Mostly C, H (very little O)	Glycerol + fatty acids	Lipid	Energy storage, insulation, cell membranes, hormones	Fats, oils, waxes, steroids
Proteins	C, H, O, N (sometimes S)	Amino acid	Polypeptide (protein)	Enzymes, structure, transport, movement	Hemoglobin, actin, myosin
Nucleic Acids	C, H, O, N, P	Nucleotide	Nucleic acid	Genetic information storage and transmission	DNA, RNA

Additional info: The structure and function of each macromolecule are determined by the specific arrangement of their monomers. For example, the sequence of amino acids in a protein determines its three-dimensional shape and function. Similarly, the type of sugar and linkage in polysaccharides determines their digestibility and role in organisms.