Macromolecules II: Structure and Function of Biological Macromolecules

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Biological Macromolecules

Overview of Macromolecules

Biological macromolecules are large, complex molecules essential for life. They are typically polymers, constructed from smaller subunits called monomers. The four major classes of biological macromolecules are carbohydrates, proteins, nucleic acids, and lipids.

Carbohydrates: Serve as energy sources and structural components.
Proteins: Function as enzymes, structural elements, and signaling molecules.
Nucleic Acids: Store and transmit genetic information.
Lipids: Provide energy storage, form cell membranes, and act as signaling molecules.

Summary Table: Major Biological Macromolecules

Type	Examples	Functions	Monomer (Building Block)
Carbohydrates	Starch, glycogen (polysaccharides)	Energy storage, cell surface marker, cell signaling	Simple sugars (glucose, galactose)
Proteins	Hemoglobin, enzymes, collagen	Catalyze reactions, physical structure, cell signaling	Amino acids (glycine, cysteine, etc.)
Nucleic Acids	DNA, RNA	Store genetic information, gene expression	Nucleotides (A, C, G, T/U)
Lipids	Triacylglycerol, cholesterol	Energy storage, cell membranes, hormones	Fatty acids and glycerol

Carbohydrates

Monosaccharides

Monosaccharides are the simplest carbohydrates, often referred to as simple sugars. They serve as the monomers for more complex carbohydrates.

General formula: , where n = 3–6.
Contain both a carbonyl group (C=O) and multiple hydroxyl groups (–OH).
Classified by the number of carbon atoms:
- Triose (3C): e.g., Glyceraldehyde
- Pentose (5C): e.g., Ribose
- Hexose (6C): e.g., Glucose, Galactose, Fructose
Can be classified as aldoses (aldehyde group) or ketoses (ketone group).

Example: The structure of glyceraldehyde (a triose) is:

Monosaccharides: Linear vs. Ring Forms

Although monosaccharides are often drawn in their linear form, in aqueous solutions they predominantly exist in a ring (cyclic) form. The conversion between linear and ring forms involves the carbonyl group reacting with a hydroxyl group on the same molecule.

Each carbon atom in the sugar is numbered, starting from the end nearest the carbonyl group.
Ring formation creates two isomers: alpha (α) and beta (β) forms, differing in the orientation of the hydroxyl group on the anomeric carbon.

Example: Glucose can exist as both α-glucose and β-glucose, which differ in the position of the –OH group on carbon 1.

Disaccharides and Glycosidic Bonds

Disaccharides are formed when two monosaccharides are joined by a dehydration reaction, resulting in a glycosidic bond and the release of a water molecule.

Maltose: glucose + glucose
Sucrose: glucose + fructose
Lactose: glucose + galactose

Equation for dehydration synthesis:

Polysaccharides: Structure and Function

Polysaccharides are long polymers of monosaccharides linked by glycosidic bonds. They serve as energy storage or structural materials.

Starch: Polymer of α-glucose, made by plants for energy storage. Easily digested by animals.
Glycogen: Highly branched polymer of α-glucose, made by animals for energy storage.
Cellulose: Polymer of β-glucose, forms plant cell walls. Not digestible by most animals due to the β-linkages.
Chitin: Polymer of a nitrogen-containing derivative of glucose, found in fungal cell walls and arthropod exoskeletons.

Comparison of Storage and Structural Polysaccharides:

Polysaccharide	Monomer	Linkage	Function	Source
Starch	α-glucose	α(1→4), α(1→6) branches	Energy storage	Plants
Glycogen	α-glucose	Highly branched α(1→4), α(1→6)	Energy storage	Animals
Cellulose	β-glucose	β(1→4)	Structural (cell wall)	Plants
Chitin	Modified glucose (N-acetylglucosamine)	β(1→4)	Structural (exoskeleton, cell wall)	Fungi, Arthropods

Lipids

Overview of Lipids

Lipids are a diverse group of hydrophobic molecules that are not true polymers. They are characterized by their insolubility in water and play key roles in energy storage, membrane structure, and signaling.

Major types: Fats (triglycerides), phospholipids, and steroids.

Fats (Triglycerides)

Fats are composed of one glycerol molecule and three fatty acids, joined by ester linkages through dehydration synthesis.

Glycerol: A three-carbon alcohol with hydroxyl groups.
Fatty acids: Long hydrocarbon chains (usually 16–18 carbons) with a terminal carboxyl group.
Function as long-term energy storage molecules.

Equation for fat synthesis:

Saturated vs. Unsaturated Fats

Saturated fats: No double bonds in fatty acid chains; straight chains allow tight packing, solid at room temperature (e.g., butter).
Unsaturated fats: One or more double bonds; kinks prevent tight packing, liquid at room temperature (e.g., oils).
Trans fats: Produced by hydrogenation of unsaturated fats; associated with negative health effects.

Phospholipids

Phospholipids consist of a glycerol backbone, two fatty acids, and a phosphate group. They are amphipathic, with a hydrophilic (polar) head and hydrophobic (non-polar) tails.

Major component of cell membranes, forming a bilayer with hydrophobic tails facing inward and hydrophilic heads facing outward.

Steroids

Steroids are lipids with a structure of four fused carbon rings. Cholesterol is the main steroid in animals, serving as a membrane component and precursor to steroid hormones (e.g., estrogen, testosterone).

Review: Key Features of Macromolecules

All biological macromolecules are essential for life and have unique structures and functions.
Carbohydrates and proteins are polymers made from monomers (sugars and amino acids, respectively).
Lipids are not true polymers but are large, hydrophobic molecules with diverse roles.
Nucleic acids (not detailed here) store and transmit genetic information.

Example Application: Understanding the structure of macromolecules is fundamental for studying metabolism, genetics, and cell biology.