The Structure and Function of Large Biological Molecules: Cell Biology

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Chapter Overview: The Structure and Function of Large Biological Molecules

This chapter explores the chemistry and biology of macromolecules, which are essential to all living organisms. The four major classes of biological macromolecules—carbohydrates, lipids, proteins, and nucleic acids—are discussed in terms of their structure, function, and the processes by which they are assembled and disassembled.

Concept 5.1: Macromolecules Are Polymers, Built from Monomers

Polymers and Monomers

Macromolecules are large molecules composed of thousands of covalently connected atoms. Most macromolecules are polymers, built from repeating units called monomers.
Polymerization is the process by which monomers are covalently bonded to form polymers.
Dehydration synthesis (or condensation reaction) is the process that joins two monomers by removing a water molecule.
Hydrolysis is the process that breaks the bond between monomers by adding a water molecule.

Example: The formation of a disaccharide from two monosaccharides via dehydration synthesis.

Equation for Dehydration Synthesis:

Equation for Hydrolysis:

Concept 5.2: Carbohydrates Serve as Fuel and Building Material

Structure and Function of Carbohydrates

Carbohydrates are sugars and polymers of sugars. Their monomers are called monosaccharides (e.g., glucose, fructose).
Monosaccharides have the general formula (e.g., glucose is ).
Two monosaccharides can join to form a disaccharide (e.g., sucrose, lactose).
Polysaccharides are polymers of many monosaccharides and serve as storage (e.g., starch, glycogen) or structural (e.g., cellulose, chitin) molecules.
All sugars contain two main functional groups: a carbonyl group (C=O) and multiple hydroxyl groups (–OH).
Sugars typically end in the suffix -ose (e.g., glucose, fructose).
A glycosidic linkage is a covalent bond formed between two monosaccharides by a dehydration reaction.

Categories of Polysaccharides

Type of Polysaccharide	Examples
Storage	Starch (plants), Glycogen (animals)
Structural	Cellulose (plants), Chitin (arthropods, fungi)

Key Carbohydrates and Their Functions

Carbohydrate	Description
Starch	Storage polysaccharide in plants; composed of α-glucose monomers with 1–4 linkages
Glycogen	Storage polysaccharide in animals; highly branched
Cellulose	Structural polysaccharide in plant cell walls; composed of β-glucose monomers
Chitin	Structural polysaccharide in exoskeletons of arthropods and cell walls of fungi

Additional info: Humans cannot digest cellulose due to the lack of enzymes that hydrolyze β-1,4-glycosidic linkages.

Concept 5.3: Lipids Are a Diverse Group of Hydrophobic Molecules

Structure and Function of Lipids

Lipids are hydrophobic molecules that include fats, phospholipids, and steroids.
Unlike other macromolecules, lipids are not true polymers.
Fats are constructed from two types of smaller molecules: glycerol and fatty acids.
A triglyceride (fat) consists of three fatty acids linked to a glycerol molecule by ester linkages.
Saturated fats have no double bonds between carbon atoms; unsaturated fats have one or more double bonds, causing kinks in the fatty acid chains.
Trans fats are unsaturated fats with trans double bonds, associated with negative health effects.
Phospholipids have two fatty acids and a phosphate group attached to glycerol; they form the bilayer of cell membranes.
Steroids are lipids with a carbon skeleton consisting of four fused rings (e.g., cholesterol).

Functions of Fats

Energy storage
Insulation
Cushioning of vital organs
Component of cell membranes (phospholipids)

Phospholipid Structure

Phospholipids have a hydrophilic "head" (phosphate group) and two hydrophobic "tails" (fatty acids).
In water, phospholipids self-assemble into bilayers, forming the basic structure of cell membranes.

Concept 5.4: Proteins Include a Diversity of Structures, Resulting in a Wide Range of Functions

Structure and Function of Proteins

Proteins are polymers of amino acids, joined by peptide bonds.
Each amino acid has a central (α) carbon, an amino group, a carboxyl group, a hydrogen atom, and a variable R group (side chain).
The sequence of amino acids determines a protein's structure and function.
There are 20 different amino acids, each with a unique R group.

Types of Proteins and Their Functions

Type of Protein	Function	Example
Enzymatic	Catalyze chemical reactions	Amylase
Structural	Support	Collagen
Storage	Store amino acids	Casein
Transport	Transport substances	Hemoglobin
Hormonal	Coordinate organismal activities	Insulin
Receptor	Response to chemical stimuli	Neurotransmitter receptors
Contractile and Motor	Movement	Actin, myosin
Defensive	Protection against disease	Antibodies

Classification of Amino Acid R Groups

Category	Common Elements
Nonpolar	Hydrocarbon side chains
Polar	Side chains with electronegative atoms (O, N, S)
Electrically charged	Acidic (negative charge) or basic (positive charge) side chains

Levels of Protein Structure

Level of Protein Structure	Explanation	Example
Primary	Sequence of amino acids in a polypeptide chain	Insulin polypeptide
Secondary	Coiling or folding of the polypeptide into α-helices and β-pleated sheets, stabilized by hydrogen bonds	α-helix in keratin, β-sheet in silk
Tertiary	Three-dimensional shape formed by interactions among R groups	Myoglobin
Quaternary	Association of multiple polypeptide chains	Hemoglobin

Protein Denaturation

Denaturation is the loss of a protein's native structure, resulting in loss of function.
Causes of denaturation include high temperature, extreme pH, and exposure to chemicals.
Denaturation is often irreversible.

Example: Sickle-cell disease is caused by a single amino acid substitution in hemoglobin, altering its structure and function.

Concept 5.5: Nucleic Acids Store, Transmit, and Help Express Hereditary Information

Structure and Function of Nucleic Acids

Nucleic acids are polymers made of monomers called nucleotides.
Each nucleotide consists of a nitrogenous base, a pentose sugar (deoxyribose in DNA, ribose in RNA), and a phosphate group.
There are two types of nucleic acids: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).
DNA stores genetic information; RNA is involved in protein synthesis and gene regulation.
DNA is double-stranded and forms a double helix; RNA is usually single-stranded.

Comparison of DNA and RNA

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

Additional info: The sequence of bases in DNA and RNA determines the sequence of amino acids in proteins, linking nucleic acids to protein synthesis.