Structure and Function of Large Biological Molecules (Macromolecules)

Structure and Function of Large Biological Molecules

Introduction to Macromolecules

Large biological molecules, known as macromolecules, are essential for life and are built from smaller units called monomers. The major classes of macromolecules include carbohydrates, proteins, and nucleic acids. Lipids are also important biological molecules, but they do not form true polymers.

• Polymer: A long molecule consisting of many similar or identical building blocks linked by covalent bonds.

• Monomer: The repeating unit that serves as a building block for a polymer.

• Macromolecule: A large molecule formed by the joining of smaller molecules, usually by a condensation reaction.

Synthesis and Breakdown of Polymers

Polymers are assembled and disassembled by specific chemical reactions, often catalyzed by enzymes.

• Dehydration Reaction: Occurs when two monomers bond together through the loss of a water molecule. This process builds polymers.

• Hydrolysis: Polymers are disassembled to monomers by hydrolysis, which adds a water molecule, breaking a bond. This is the reverse of dehydration.

Enzymes are specialized macromolecules that speed up chemical reactions, such as those that make or break down polymers.

Carbohydrates

Monosaccharides and Polysaccharides

Carbohydrates serve as fuel and building material. The simplest carbohydrates are monosaccharides (simple sugars), while carbohydrate macromolecules are polysaccharides (polymers composed of many sugar building blocks).

• Monosaccharide: The simplest carbohydrate, active alone or serving as a monomer for disaccharides and polysaccharides. Example: Glucose (C6H12O6).

• Polysaccharide: A polymer of many monosaccharides, formed by dehydration reactions.

Classification of Sugars

Monosaccharides are classified by:

• The location of the carbonyl group (as aldose or ketose).

• The number of carbon atoms in the carbon skeleton (triose, pentose, hexose).

Type	Example	Structure
Aldose (Aldehyde Sugar)	Glucose	CHO at one end
Ketose (Ketone Sugar)	Fructose	C=O within chain
Triose	Glyceraldehyde	3 carbons
Pentose	Ribose	5 carbons
Hexose	Glucose, Galactose, Fructose	6 carbons

Ring Structures in Sugars

In aqueous solutions, many sugars form ring structures rather than linear skeletons.

• Glucose commonly exists in a ring form in solution.

Disaccharides and Glycosidic Linkages

A disaccharide is formed when a dehydration reaction joins two monosaccharides. The covalent bond between them is called a glycosidic linkage.

• Example: Sucrose (glucose + fructose)

Polysaccharides: Storage and Structural Roles

• Starch: A storage polysaccharide in plants, consisting of glucose monomers.

• Glycogen: A storage polysaccharide in animals, stored mainly in liver and muscle cells. Hydrolysis of glycogen releases glucose when needed.

• Cellulose: A structural polysaccharide that is a major component of the tough wall of plant cells. Like starch, cellulose is a polymer of glucose, but the glycosidic linkages differ (alpha vs. beta forms).

Polysaccharide	Function	Monomer	Linkage
Starch	Storage in plants	Glucose	Alpha (α) glycosidic
Glycogen	Storage in animals	Glucose	Alpha (α) glycosidic
Cellulose	Structure in plants	Glucose	Beta (β) glycosidic

Enzymes that digest starch by hydrolyzing α linkages cannot hydrolyze β linkages in cellulose. Cellulose in human food passes through the digestive tract as "insoluble fiber." Some microbes can digest cellulose, and many herbivores have symbiotic relationships with these microbes.

Lipids

Characteristics and Types of Lipids

Lipids are a diverse group of hydrophobic molecules that do not form true polymers. They consist mostly of hydrocarbon regions and mix poorly with water. The most biologically important lipids are fats, phospholipids, and steroids.

Fats and Fatty Acids

The major function of fats is energy storage. Fats are constructed from two types of smaller molecules: glycerol and fatty acids.

• Glycerol: A three-carbon alcohol with a hydroxyl group attached to each carbon.

• Fatty acid: Consists of a carboxyl group attached to a long carbon skeleton.

• Three fatty acids are joined to glycerol by an ester linkage, creating a triacylglycerol (triglyceride).

Saturated vs. Unsaturated Fatty Acids

Type	Structure	Physical State	Source
Saturated	No double bonds, maximum H atoms	Solid at room temperature	Animal fats
Unsaturated	One or more double bonds	Liquid at room temperature	Plant and fish fats

Cis and trans fatty acids refer to the configuration around the double bond. Cis fatty acids have hydrogens on the same side, causing a bend; trans fatty acids have hydrogens on opposite sides, resulting in a straighter chain.

Phospholipids

Phospholipids are major components of cell membranes. In a phospholipid, two fatty acids and a phosphate group are attached to glycerol. The fatty acid tails are hydrophobic, while the phosphate group forms a hydrophilic head.

• Phospholipids self-assemble into bilayers in water, forming the boundary between the cell and its environment.

Steroids

Steroids are lipids characterized by a carbon skeleton consisting of four fused rings. Cholesterol is a type of steroid found in animal cell membranes and is a precursor for other steroids.

Proteins

Functions of Proteins

Proteins perform a wide range of functions in cells, including catalyzing chemical reactions, defense, storage, transport, cellular communication, movement, and structural support.

• Enzymatic proteins: Accelerate chemical reactions (e.g., digestive enzymes).

• Transport proteins: Transport substances (e.g., hemoglobin transports oxygen).

• Receptor proteins: Respond to chemical stimuli (e.g., nerve cell receptors).

• Contractile and motor proteins: Responsible for movement (e.g., actin and myosin in muscles).

Structure of Proteins

Proteins are polymers built from amino acids. The bond between amino acids is a peptide bond. A protein consists of one or more polypeptides folded into a specific three-dimensional structure.

• Amino acid: Organic molecule with amino and carboxyl groups.

• Polypeptide: Polymer of amino acids.

• Protein: Biologically functional molecule consisting of one or more polypeptides.

Levels of Protein Structure

Level	Description
Primary	Sequence of amino acids
Secondary	Coils and folds (α helix, β pleated sheet)
Tertiary	Overall 3D shape due to interactions among R groups
Quaternary	Association of multiple polypeptides

Protein structure is determined by inherited genetic information and can be affected by environmental factors such as pH and temperature. Loss of native structure is called denaturation, which can lead to loss of function and is associated with diseases like Alzheimer's and Parkinson's.

Nucleic Acids

DNA and RNA

Nucleic acids store, transmit, and help express hereditary information. The two types are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

• Gene: A unit of inheritance, made of DNA.

• DNA directs synthesis of messenger RNA (mRNA), which controls protein synthesis (gene expression).

• The flow of genetic information: DNA → RNA → Protein

Structure of Nucleic Acids

• Nucleotide: Monomer of nucleic acids, consisting of a nitrogenous base, a pentose sugar, and one or more phosphate groups.

• Nucleoside: Portion of a nucleotide without the phosphate group.

• Nitrogenous bases: Pyrimidines (C, T, U) and Purines (A, G)

• DNA contains deoxyribose; RNA contains ribose.

• Nucleotides are linked by phosphodiester bonds, forming a sugar-phosphate backbone.

• DNA strands run antiparallel and are held together by complementary base pairing (A-T, G-C).

Type	Sugar	Bases	Structure
DNA	Deoxyribose	A, T, G, C	Double helix
RNA	Ribose	A, U, G, C	Single strand

Complementary base pairing allows DNA to be copied during cell division and enables RNA to carry genetic information for protein synthesis.

*Additional info: Protein folding is stabilized by hydrogen bonds, ionic bonds, hydrophobic interactions, van der Waals forces, and sometimes disulfide bridges. Environmental changes can cause denaturation, affecting protein function. Misfolded proteins are implicated in several neurodegenerative diseases.*