Biological Macromolecules: Structure, Function, and Diversity

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Carbon: The Framework of Biological Molecules

Introduction to Carbon in Biology

Carbon is the fundamental element in biological molecules, forming the backbone of macromolecules essential for life. Its unique chemical properties allow for the formation of diverse and complex structures.

Carbon atoms can form up to four covalent bonds, enabling the construction of large, stable molecules.
Common elements bonded to carbon include hydrogen (H), oxygen (O), nitrogen (N), sulfur (S), and phosphorus (P).
Hydrocarbons are molecules consisting only of carbon and hydrogen; they are typically nonpolar and hydrophobic.

Functional Groups

Role and Properties of Functional Groups

Functional groups are specific clusters of atoms within molecules that confer distinct chemical properties and reactivity. They are critical in determining the behavior and function of biological molecules.

Functional groups attach to carbon-hydrogen cores and retain their chemical properties wherever they occur.
They influence the overall behavior of molecules, including their solubility, acidity, and participation in chemical reactions.

Examples of Functional Groups

Functional Group	Structure Formula	Example	Found In
Hydroxyl	-OH	Ethanol	Carbohydrates, alcohols
Carbonyl	-C=O	Acetaldehyde	Carbohydrates
Carboxyl	-COOH	Amino acids	Proteins
Amino	-NH2	Cysteine	Proteins
Sulfhydryl	-SH	Cysteine	Proteins
Phosphate	-PO4	Glycerol phosphate	Nucleic acids
Methyl	-CH3	Methylated compounds	Various biomolecules

Isomers

Types and Importance of Isomers

Isomers are molecules with the same molecular or empirical formula but different structures or spatial arrangements. They play a crucial role in biological specificity and function.

Structural isomers: Differ in the arrangement of the carbon skeleton.
Stereoisomers: Differ in the spatial arrangement of groups attached to the carbon skeleton.
Enantiomers: A type of stereoisomer; molecules that are mirror images of each other, often due to a chiral center (carbon with four unique groups attached).
Examples include D-sugars and L-amino acids, which are critical in metabolism and protein synthesis.

Carbohydrates: Energy Storage and Structural Molecules

Overview and Functions

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically in a 1:2:1 ratio. They serve as energy sources and structural components in cells.

Monosaccharides: Simple sugars (e.g., glucose, fructose, galactose).
Disaccharides: Two monosaccharides linked by dehydration synthesis (e.g., sucrose, lactose, maltose).
Polysaccharides: Long chains of monosaccharides; used for energy storage (starch in plants, glycogen in animals) and structural support (cellulose in plants, chitin in fungi and arthropods).

Key Reactions

Dehydration synthesis: Formation of large molecules by the removal of water; joins monomers to form polymers.
Hydrolysis: Breakdown of large molecules by the addition of water; splits polymers into monomers.

Example: Glucose

Glucose () is a primary energy source for cells.
Structural isomers: Fructose and galactose.
Stereoisomers: D-glucose and L-glucose.

Nucleic Acids: Information Molecules

Structure and Function

Nucleic acids store and transmit genetic information. They are polymers made of nucleotide monomers, each consisting of a sugar, phosphate group, and nitrogenous base.

DNA (Deoxyribonucleic acid): Contains deoxyribose sugar; bases include adenine (A), guanine (G), cytosine (C), and thymine (T).
RNA (Ribonucleic acid): Contains ribose sugar; bases include adenine (A), guanine (G), cytosine (C), and uracil (U).
Nucleotides are joined by phosphodiester bonds.
DNA is typically double-stranded (double helix), while RNA is single-stranded.

Base Pairing Rules

A pairs with T (or U in RNA), G pairs with C.

Important Nucleotides

ATP (Adenosine triphosphate): Primary energy currency of the cell.
NAD+ (Nicotinamide adenine dinucleotide) and FAD (Flavin adenine dinucleotide): Electron carriers in cellular reactions.

Proteins: Molecules with Diverse Structures and Functions

Structure and Levels of Organization

Proteins are polymers of amino acids, performing a wide range of functions in cells. Their structure determines their function.

Amino acids: Monomers with a central carbon, amino group, carboxyl group, hydrogen, and variable R group.
Peptide bonds: Link amino acids via dehydration synthesis.

Levels of Protein Structure

Primary structure: Sequence of amino acids.
Secondary structure: Local folding (α-helix, β-sheet) due to hydrogen bonding.
Tertiary structure: Overall 3D shape of a single polypeptide chain.
Quaternary structure: Arrangement of multiple polypeptide chains (subunits).

Protein Functions

Enzyme catalysis
Defense
Transport
Support
Motion
Regulation
Storage

Protein Folding and Chaperones

Chaperone proteins assist in proper folding of newly synthesized proteins.
Deficiencies in chaperones can lead to diseases (e.g., cystic fibrosis).

Denaturation

Proteins lose structure and function due to changes in temperature, pH, or ionic concentration.

Lipids: Hydrophobic Molecules

Structure and Diversity

Lipids are a diverse group of hydrophobic molecules, characterized by a high proportion of nonpolar C–H bonds. They are insoluble in water and serve as energy storage, structural components, and signaling molecules.

Triglycerides: Composed of one glycerol and three fatty acids; used for energy storage.
Saturated fatty acids: No double bonds; higher melting point, typically animal origin.
Unsaturated fatty acids: One or more double bonds; lower melting point, typically plant origin.
Phospholipids: Composed of glycerol, two fatty acids (nonpolar tails), and a phosphate group (polar head); major component of biological membranes.
Steroids: Four fused carbon rings; include cholesterol and hormones.

Phospholipid Bilayer

Phospholipids arrange in bilayers, with hydrophilic heads facing outward and hydrophobic tails inward, forming the basis of cell membranes.

Example Table: Saturated vs. Unsaturated Fatty Acids

Type	Bonding	Melting Point	Origin
Saturated	No double bonds	High	Animal
Unsaturated	One or more double bonds	Low	Plant

Additional info:

Modern biochemistry studies biological molecules outside the context of living cells, using systems such as cell-free fermentation to analyze biochemical reactions.
Cell-free fermentation demonstrates that enzymes can function outside of cells, allowing for the study of biochemical pathways in vitro.