The Molecules of Cells: Structure and Function of Biological Macromolecules

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The Molecules of Cells

Overview

This study guide covers the fundamental concepts of biological molecules, focusing on their structure, diversity, and function in living cells. The main topics include the chemistry of carbon, functional groups, the four classes of biological macromolecules, and the structure and function of carbohydrates, lipids, proteins, and nucleic acids.

Chemical Structure of Large Biomolecules

The Importance of Carbon in Biological Molecules

Carbon is the backbone of all organic molecules due to its ability to form four covalent bonds with other atoms, including itself.
This versatility allows carbon to form chains, branched molecules, and rings, leading to a vast diversity of molecular structures.
Hydrocarbons are organic molecules consisting only of carbon and hydrogen. They are nonpolar and hydrophobic, often found in fats and other biological molecules.
The arrangement of carbon atoms forms the carbon skeleton of organic molecules, which can vary in length, branching, and ring structure.
Example: Methane (CH4), ethane (C2H6), and cyclohexane (C6H12) illustrate different carbon skeletons.

Functional Groups

Functional groups are specific groups of atoms attached to the carbon skeleton that participate in chemical reactions and determine the properties of organic molecules.
Common functional groups include hydroxyl (-OH), carbonyl (>C=O), carboxyl (-COOH), amino (-NH2), phosphate (-PO4), and methyl (-CH3).
Most functional groups (except methyl) are polar and increase the solubility of molecules in water.
The arrangement and type of functional groups impact molecular shape and function.
Example: The difference between testosterone and estradiol (sex hormones) is due to the arrangement of functional groups.

The Four Classes of Biological Macromolecules

Cells are composed of four major classes of large biological molecules, each with unique structures and functions:

Carbohydrates
Lipids
Proteins
Nucleic Acids

Polymers and Monomers

Most biological macromolecules (except lipids) are polymers, long chains made by linking together smaller units called monomers.
Polymers are formed by dehydration reactions (removal of water to form a bond) and broken down by hydrolysis (addition of water to break a bond).
Example: Proteins are polymers of amino acids; nucleic acids are polymers of nucleotides.

Carbohydrates

Structure and Function

Carbohydrates are sugars and their polymers, serving as energy sources and structural materials.
Monosaccharides are the simplest carbohydrates (e.g., glucose, fructose), typically with the formula (CH2O)n.
Disaccharides are formed by joining two monosaccharides via a dehydration reaction (e.g., sucrose, lactose, maltose).
Polysaccharides are long chains of monosaccharides linked by glycosidic bonds. They serve as storage (starch, glycogen) or structural (cellulose) molecules.

Comparison of Major Polysaccharides

Polysaccharide	Function	Monomer	Structure	Occurrence
Starch	Energy storage	Glucose	Unbranched or slightly branched	Plants
Glycogen	Energy storage	Glucose	Highly branched	Animals
Cellulose	Structural	Glucose	Unbranched, forms fibers via hydrogen bonds	Plants (cell walls)

Lipids

Structure and Types

Lipids are hydrophobic molecules, not true polymers, and include fats, phospholipids, and steroids.
Fats (Triglycerides): Composed of glycerol and three fatty acids linked by ester bonds. Function as energy storage molecules.
Fatty acids can be saturated (no double bonds, solid at room temperature, animal fats) or unsaturated (one or more double bonds, liquid at room temperature, plant oils).
Phospholipids: Similar to fats but with a phosphate group replacing one fatty acid. They have hydrophilic heads and hydrophobic tails, forming the basis of cell membranes.
Steroids: Lipids with four fused carbon rings. Cholesterol is a common steroid and precursor to other steroids such as hormones.

Proteins

Structure and Function

Proteins are polymers of amino acids, essential for structure, function, and regulation of the body's tissues and organs.
Amino acids have a central carbon, amino group, carboxyl group, hydrogen atom, and a variable R group.
Amino acids are linked by peptide bonds formed via dehydration reactions.
Proteins have four levels of structure:
- Primary: Sequence of amino acids.
- Secondary: Local folding into alpha-helices and beta-pleated sheets, stabilized by hydrogen bonds.
- Tertiary: Overall 3D shape, stabilized by interactions between R groups (hydrogen bonds, ionic bonds, disulfide bridges).
- Quaternary: Association of multiple polypeptide chains (subunits).
Denaturation is the loss of a protein's native structure due to changes in temperature, pH, or other environmental factors, resulting in loss of function.
Functions of proteins: Enzymes, structural support, transport, defense (antibodies), signaling, movement (muscle contraction), and storage.

Nucleic Acids

Structure and Function

Nucleic acids (DNA and RNA) store and transmit genetic information.
Nucleotides are the monomers of nucleic acids, each consisting of a phosphate group, a pentose sugar (deoxyribose in DNA, ribose in RNA), and a nitrogenous base.
DNA contains adenine (A), thymine (T), guanine (G), and cytosine (C). RNA contains uracil (U) instead of thymine.
Nucleotides are joined by phosphodiester bonds, forming a sugar-phosphate backbone.
DNA is double-stranded, forming a double helix with complementary base pairing (A with T, G with C). RNA is usually single-stranded.

Comparison of DNA and RNA

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

The Central Dogma of Molecular Biology

Genetic information flows from DNA to RNA to protein.
Transcription: DNA is transcribed into messenger RNA (mRNA).
Translation: mRNA is translated by ribosomes to synthesize proteins.
The sequence of nucleotides in DNA determines the sequence of amino acids in proteins, which in turn determines protein structure and function.

Summary Table: Four Classes of Biological Macromolecules

Class	Monomer	Polymer	Function	Example
Carbohydrates	Monosaccharide	Polysaccharide	Energy storage, structure	Starch, cellulose
Lipids	Fatty acid, glycerol	Not true polymers	Energy storage, membranes, hormones	Fats, phospholipids, steroids
Proteins	Amino acid	Polypeptide	Catalysis, structure, transport, defense	Enzymes, antibodies
Nucleic Acids	Nucleotide	Polynucleotide	Genetic information storage and transfer	DNA, RNA

Key Equations and Concepts

Dehydration Reaction: Formation of a covalent bond with the removal of a water molecule.
Hydrolysis: Breaking of a covalent bond by the addition of water.
Base Pairing in DNA:

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard biology curriculum.