General Biology: Structure and Function of Biological Macromolecules

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Carbon and the Molecular Diversity of Life

Formation of Bonds with Carbon

Carbon is a versatile element that forms the backbone of most biological molecules due to its ability to form four covalent bonds with other atoms.

Valence electrons: Carbon has four valence electrons, allowing it to bond with a variety of atoms.
Bonding: Can form single, double, or triple covalent bonds, primarily with hydrogen (H), oxygen (O), nitrogen (N), and other carbons.
Complex molecules: Enables the formation of large, complex organic molecules.

Molecular Diversity from Carbon Skeleton Variation

Variation in carbon skeletons leads to molecular diversity in organic compounds.

Carbon chains: Can be branched, linear, or form rings; may have single, double, or triple bonds.
Hydrocarbons: Organic molecules composed only of carbon and hydrogen; hydrophobic and can release energy during reactions.
Functional groups: Chemical groups attached to carbon skeletons that affect molecular function and participate in chemical reactions.

Table: Major Elements in Organic Molecules

Element	Role
Carbon (C)	Backbone of organic molecules
Hydrogen (H)	Completes valence of carbon
Oxygen (O)	Participates in functional groups
Nitrogen (N)	Found in amino acids, nucleic acids

ATP: The Energy Currency of the Cell

Structure and Function of ATP

Adenosine triphosphate (ATP) is the primary energy source for cellular processes.

Structure: Organic molecule adenosine attached to three phosphate groups.
Function: Stores energy that can be released by hydrolysis and used by the cell.

Equation:

Organic Compounds and Macromolecules

Types of Organic Compounds

Organic compounds contain carbon and hydrogen and serve as the foundation for biological macromolecules.

Carbohydrates
Lipids
Proteins
Nucleic acids

Macromolecules: Polymers Built from Monomers

Macromolecules are large molecules made of repeating units called monomers, linked by covalent bonds.

Polymerization: Formation of polymers by dehydration reactions (loss of water).
Hydrolysis: Breakdown of polymers into monomers by addition of water.
Exception: Lipids do not form true polymers.

Equation for Dehydration Synthesis:

Carbohydrates: Fuel and Building Material

Classification and Structure

Carbohydrates are classified based on the number of carbon atoms and their structure.

Monosaccharides: Simple sugars (e.g., glucose), major energy source for cells.
Disaccharides: Two monosaccharides linked by glycosidic bonds (e.g., sucrose, lactose).
Polysaccharides: Long chains of monosaccharides; function as energy storage (starch, glycogen) or structural components (cellulose).

Table: Types of Carbohydrates

Type	Example	Function
Monosaccharide	Glucose	Energy source
Disaccharide	Sucrose	Transport sugar in plants
Polysaccharide	Starch	Energy storage in plants
Polysaccharide	Cellulose	Structural support in plants

Lipids: Hydrophobic Molecules

Types and Functions

Lipids are a diverse group of hydrophobic molecules, including fats, phospholipids, and steroids.

Fats: Composed of glycerol and three fatty acids; function in energy storage.
Saturated fatty acids: No double bonds; solid at room temperature; animal source.
Unsaturated fatty acids: One or more double bonds; liquid at room temperature; plant source.
Phospholipids: Major component of cell membranes; amphipathic (hydrophilic head, hydrophobic tail).
Steroids: Lipids with four fused rings; cholesterol is a key example, maintaining membrane fluidity.

Table: Comparison of Saturated and Unsaturated Fatty Acids

Type	Bond Type	Source	Physical State
Saturated	Single bonds	Animal	Solid
Unsaturated	Double bonds	Plant	Liquid

Proteins: Diversity of Structure and Function

Structure of Proteins

Proteins are polymers of amino acids and are the most abundant macromolecules in cells, performing a wide range of functions.

Amino acids: 20 types, each with a central carbon, amino group, carboxyl group, hydrogen atom, and variable R group.
Polypeptides: Chains of amino acids linked by peptide bonds.
Protein structure: Determined by sequence and interactions of amino acids.

Levels of Protein Structure

Primary (1°): Linear sequence of amino acids.
Secondary (2°): Folding into α-helices and β-pleated sheets via hydrogen bonds.
Tertiary (3°): Further folding due to interactions among R groups (hydrogen bonds, hydrophobic interactions, disulfide bridges).
Quaternary (4°): Association of multiple polypeptide chains.

Protein Function

Enzymatic proteins: Catalyze biochemical reactions.
Transport proteins: Move substances across membranes.
Structural proteins: Provide support (e.g., collagen).
Defense proteins: Immune response.
Signaling proteins: Cellular communication.

Factors Affecting Protein Structure

Temperature
pH
Salt concentration
All can disrupt bonds and interactions, leading to denaturation.

Nucleic Acids: Store, Transmit, and Express Hereditary Information

Types and Roles of Nucleic Acids

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

DNA (Deoxyribonucleic acid): Stores genetic information; directs synthesis of messenger RNA (mRNA) and proteins.
RNA (Ribonucleic acid): Involved in protein synthesis; can be single-stranded or form complementary base pairs.

Components of Nucleic Acids

Nucleotide: Composed of a nitrogenous base, pentose sugar, and phosphate group.
Nitrogenous bases: Adenine (A), Guanine (G), Cytosine (C), Thymine (T, DNA only), Uracil (U, RNA only).

Table: Nitrogenous Bases in DNA and RNA

Base	DNA	RNA
Adenine (A)	Yes	Yes
Guanine (G)	Yes	Yes
Cytosine (C)	Yes	Yes
Thymine (T)	Yes	No
Uracil (U)	No	Yes

Structure of DNA and RNA Molecules

DNA: Double helix with two antiparallel strands; complementary base pairing (A-T, G-C).
RNA: Usually single-stranded; base pairing (A-U, G-C) possible.
Phosphodiester linkage: Bonds between nucleotides forming the sugar-phosphate backbone.
Directionality: 5' end (phosphate group) and 3' end (hydroxyl group).

Equation for Phosphodiester Bond Formation:

Summary Table: Major Biological Macromolecules

Macromolecule	Monomer	Bond Type	Function
Carbohydrate	Monosaccharide	Glycosidic	Energy, structure
Lipid	Fatty acid, glycerol	Ester	Energy, membranes
Protein	Amino acid	Peptide	Catalysis, structure, transport
Nucleic acid	Nucleotide	Phosphodiester	Genetic information

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