Biological Macromolecules: Structure and Function

Biological Molecules

Introduction to Biological Macromolecules

Biological macromolecules are large, complex molecules essential for life, composed of thousands of covalently connected atoms. These molecules are typically polymers, constructed from repeating subunits called monomers. The four major classes of biological macromolecules are carbohydrates, lipids, proteins, and nucleic acids.

• Macromolecule: A large molecule formed by the polymerization of smaller subunits.

• Monomer: A small molecule that can join with other similar molecules to form a polymer.

• Polymer: A macromolecule consisting of many similar or identical monomers linked together.

Polymer Synthesis and Breakdown

Polymers are synthesized and broken down by specific chemical reactions involving water.

• Dehydration Reaction: Two monomers are covalently bonded together with the removal of a water molecule, forming a polymer.

• Hydrolysis Reaction: Polymers are disassembled into monomers by the addition of a water molecule, breaking the covalent bond.

Example: Formation and breakdown of starch in plants involves dehydration and hydrolysis reactions, respectively.

Polymerization Reactions

• Dehydration: $\text{Monomer}_1 + \text{Monomer}_2 \rightarrow \text{Polymer} + H_2O$

• Hydrolysis: $\text{Polymer} + H_2O \rightarrow \text{Monomer}_1 + \text{Monomer}_2$

Macromolecule #1: Carbohydrates

Monosaccharides and Polysaccharides

Carbohydrates are sugars and their polymers. The simplest carbohydrates are monosaccharides, such as glucose, galactose, and fructose. These can be joined to form disaccharides and polysaccharides.

• Monosaccharide: Single sugar molecule (e.g., glucose, galactose, fructose).

• Disaccharide: Two monosaccharides joined by a glycosidic bond (e.g., sucrose).

• Polysaccharide: Long chains of monosaccharides (e.g., starch, glycogen, cellulose).

Hexoses: 6-Carbon Sugars

Hexoses have the formula $C_6H_{12}O_6$ and include:

Monosaccharide	Structure
Glucose	Linear and ring forms; main energy source for cells
Galactose	Similar to glucose; differs in arrangement of -OH groups
Fructose	Structural isomer of glucose; found in fruits

Structural Forms of Monosaccharides

• Monosaccharides can exist in linear or ring forms in aqueous solutions.

• Ring structures are more common in biological systems.

Major Polysaccharides

Polysaccharide	Function	Structure
Starch	Main fuel storage in plant cells	1-4 linkage of α-glucose monomers
Glycogen	Main fuel storage in animal cells	Highly branched polymer of glucose
Cellulose	Major component of plant cell walls	1-4 linkage of β-glucose monomers

Example: Starch is stored in plant plastids, while glycogen is stored in animal liver and muscle cells.

Macromolecule #2: Lipids

Structure and Types of Lipids

Lipids are hydrophobic macromolecules primarily composed of carbon and hydrogen. They include fats, phospholipids, and steroids.

• Fat: Constructed from glycerol and fatty acids; used for energy storage.

• Phospholipid: Composed of a glycerol backbone, two fatty acid tails, and a phosphate group; major component of cell membranes.

• Steroid: Characterized by a carbon skeleton consisting of four fused rings; includes cholesterol and hormones.

Saturated vs. Unsaturated Fats

Type	Structure	Properties
Saturated Fat	No double bonds in fatty acid tails	Solid at room temperature; found in animal fats
Unsaturated Fat	One or more double bonds; causes bending	Liquid at room temperature; found in plant oils

Phospholipids and Membrane Structure

• Phospholipids have hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• They form bilayers in aqueous environments, which are the basis of biological membranes.

Steroids

• Cholesterol: Important component of animal cell membranes; precursor for steroid hormones.

Macromolecule #3: Proteins

Structure and Function of Proteins

Proteins are polymers of amino acids, folded into specific three-dimensional structures that determine their function. Each amino acid contains a central carbon, an amino group, a carboxyl group, and a variable side chain (R group).

• Polypeptide: Unbranched polymer of amino acids.

• Protein: One or more polypeptides folded into a functional shape.

Levels of Protein Structure

Level	Description
Primary	Unique sequence of amino acids
Secondary	Coils and folds (α-helix, β-pleated sheet) stabilized by hydrogen bonds
Tertiary	Overall 3D shape determined by interactions among side chains (hydrophobic interactions, disulfide bridges, ionic bonds)
Quaternary	Association of multiple polypeptide chains

Protein Folding and Chaperonins

• Chaperonins are protein molecules that assist in the proper folding of other proteins.

• Correct folding is essential for protein function; misfolded proteins can lead to disease.

Protein Functions

Type	Function	Example
Enzymatic	Selective acceleration of chemical reactions	Digestive enzymes
Defensive	Protection against disease	Antibodies
Storage	Storage of amino acids	Casein in milk
Transport	Transport of substances	Hemoglobin
Hormonal	Coordination of organismal activities	Insulin
Receptor	Response to chemical stimuli	Cell membrane receptors
Contractile and Motor	Movement	Actin and myosin in muscles
Structural	Support	Collagen, keratin

Macromolecule #4: Nucleic Acids

Structure and Function of Nucleic Acids

Nucleic acids are polymers (polynucleotides) made of nucleotide monomers. Each nucleotide consists of a nitrogenous base, a pentose sugar, and one or more phosphate groups.

• DNA (Deoxyribonucleic Acid): Stores genetic information; double-stranded helix.

• RNA (Ribonucleic Acid): Involved in protein synthesis; single-stranded.

Central Dogma of Molecular Biology

• DNA is transcribed into messenger RNA (mRNA) in the nucleus.

• mRNA is translated into protein in the cytoplasm at the ribosome.

Genetic Information and Evolution

• DNA sequences are passed from parents to offspring, determining traits.

• Related species have more similar DNA sequences than unrelated species.

Example: Comparison of human and mouse chromosomes reveals evolutionary relationships.

Additional info: Nucleic acids are essential for the storage, transmission, and expression of genetic information in all living organisms.