Biological Macromolecules: Structure, Function, and Properties

Biological Macromolecules

Overview of Macromolecules

Biological macromolecules are large, complex molecules essential for life, including carbohydrates, lipids, proteins, and nucleic acids. Each type has unique structures and functions, contributing to the structure and metabolism of cells.

• Carbohydrates: Serve as energy sources and structural materials. Examples include glucose, starch, cellulose, and glycogen.

• Lipids: Hydrophobic molecules such as fats, oils, phospholipids, and steroids. They store energy, form cell membranes, and act as signaling molecules.

• Proteins: Polymers of amino acids with diverse functions, including catalysis (enzymes), structure, transport, and signaling.

• Nucleic Acids: DNA and RNA, which store and transmit genetic information.

Monomers and Polymers

Macromolecules are formed by linking smaller units called monomers into long chains called polymers through dehydration synthesis (condensation reactions), which release water molecules. The reverse process, hydrolysis, breaks polymers into monomers by adding water.

• Dehydration Synthesis: Joins monomers by removing a water molecule.

• Hydrolysis: Breaks polymers into monomers by adding water.

• Example: The formation of a peptide bond between two amino acids releases one water molecule.

Carbohydrates

Carbohydrates are composed of monosaccharides (simple sugars) and serve as energy sources and structural components.

• Monosaccharides: Single sugar units (e.g., glucose, fructose).

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

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

• Storage Polysaccharides: Starch (plants), glycogen (animals).

• Structural Polysaccharides: Cellulose (plant cell walls), chitin (arthropod exoskeletons, fungal cell walls).

Lipids

Lipids are hydrophobic molecules, including fats, phospholipids, and steroids. They are not true polymers.

• Fats (Triglycerides): Composed of glycerol and three fatty acids. Used for long-term energy storage.

• Saturated Fatty Acids: No double bonds; solid at room temperature (e.g., butter).

• Unsaturated Fatty Acids: One or more double bonds; liquid at room temperature (e.g., oils).

• Phospholipids: Major component of cell membranes; have hydrophilic heads and hydrophobic tails.

• Steroids: Lipids with a four-ring structure (e.g., cholesterol, hormones like testosterone and estradiol).

Type	Structure	Function
Fat (Triglyceride)	Glycerol + 3 fatty acids	Energy storage, insulation
Phospholipid	Glycerol + 2 fatty acids + phosphate group	Cell membrane structure
Steroid	Four fused rings	Hormones, membrane fluidity

Proteins

Proteins are polymers of amino acids linked by peptide bonds. Their structure determines their function.

• Primary Structure: Sequence of amino acids.

• Secondary Structure: Alpha helices and beta-pleated sheets formed by hydrogen bonding.

• Tertiary Structure: 3D folding due to side chain interactions.

• Quaternary Structure: Association of multiple polypeptide chains.

• Denaturation: Loss of protein structure (and function) due to heat, pH changes, or chemicals, which disrupt hydrogen bonds and other interactions.

• Enzymes: Proteins that catalyze biochemical reactions.

Nucleic Acids

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

• Nucleotide: Consists of a phosphate group, a five-carbon sugar (deoxyribose in DNA, ribose in RNA), and a nitrogenous base.

• DNA: Double helix, bases are adenine (A), thymine (T), cytosine (C), guanine (G).

• RNA: Single-stranded, bases are adenine (A), uracil (U), cytosine (C), guanine (G).

• Base Pairing: A-T (DNA), A-U (RNA), C-G.

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Strands	Double	Single
Bases	A, T, C, G	A, U, C, G
Function	Genetic information storage	Protein synthesis, gene regulation

Key Processes and Concepts

• Dehydration Synthesis: $\text{Monomer}_1 + \text{Monomer}_2 \rightarrow \text{Polymer} + \text{H}_2\text{O}$

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

• Peptide Bond Formation: $\text{Amino acid}_1 + \text{Amino acid}_2 \rightarrow \text{Dipeptide} + \text{H}_2\text{O}$

• Denaturation: Disruption of protein structure due to environmental changes.

• Enzyme Specificity: Enzymes act only on specific substrates due to their unique active sites.

Examples and Applications

• Cellulose: Structural polysaccharide in plant cell walls; indigestible by humans but digestible by some animals due to symbiotic microorganisms.

• Chitin: Structural polysaccharide in arthropod exoskeletons and fungal cell walls.

• Starch: Storage polysaccharide in plants; digestible by humans.

• Glycogen: Storage polysaccharide in animals, especially in liver and muscle cells.

• Cholesterol: Steroid important for membrane fluidity and as a precursor for hormones; high levels are a risk factor for heart disease.

Comparisons and Classifications

Macromolecule	Monomer	Bond Type	Function
Carbohydrate	Monosaccharide	Glycosidic bond	Energy, structure
Lipid	Glycerol, fatty acids	Ester bond	Energy storage, membranes
Protein	Amino acid	Peptide bond	Catalysis, structure, transport
Nucleic Acid	Nucleotide	Phosphodiester bond	Genetic information

Additional info:

• During the polymerization of amino acids to form a polypeptide, one water molecule is released per peptide bond formed.

• Phospholipids are amphipathic, having both hydrophilic (phosphate head) and hydrophobic (fatty acid tails) regions, which is critical for membrane formation.

• Denaturation is often irreversible and results in loss of protein function.

• Enzymes such as amylase can break down starch but not cellulose due to differences in glycosidic bond orientation.