Lec 3: Carbon and the Molecules of Life: Structure, Isomerism, and Biological Macromolecules

Carbon: The Foundation of Biological Molecules

Versatility of Carbon in Biological Systems

Carbon is a unique element that forms the backbone of most biological molecules due to its ability to create stable covalent bonds with many other elements. This versatility allows for the formation of a wide variety of complex organic compounds essential for life.

• Covalent Bonding: Carbon can form up to four covalent bonds, enabling the construction of large and diverse molecules.

• Backbone of Biomolecules: Carbon is central to the structure of proteins, nucleic acids, carbohydrates, lipids, and sterols.

• Isomerism: Carbon-based molecules can exist as structural isomers, cis-trans isomers, and enantiomers, each with distinct biological roles.

Simple Carbon Compounds: Structure and Bonding

Single and Double Bonds in Organic Molecules

Simple organic molecules containing carbon can have single or double bonds, influencing their chemical properties and three-dimensional shapes.

Molecule	Molecular Formula	Structural Formula	Ball-and-Stick Model	Space-Filling Model
Methane	CH4	H–C–H	Ball-and-stick representation	Space-filling representation
Ethane	C2H6	H–C–C–H	Ball-and-stick representation	Space-filling representation
Ethene (ethylene)	C2H4	H2C=CH2	Ball-and-stick representation	Space-filling representation

Isomerism in Organic Molecules

Cis-Trans Isomers

Cis-trans isomers (geometric isomers) arise from the restricted rotation around double bonds, resulting in different spatial arrangements of atoms.

• Cis Isomer: Substituents are on the same side of the double bond.

• Trans Isomer: Substituents are on opposite sides of the double bond.

Example:

• Cis isomer: The two Xs are on the same side of the C=C bond.

• Trans isomer: The two Xs are on opposite sides of the C=C bond.

Enantiomers (Mirror Image Isomers)

Enantiomers are pairs of molecules that are non-superimposable mirror images of each other, often referred to as left-handed (L) and right-handed (D) forms.

• Chirality: A molecule is chiral if it cannot be superimposed on its mirror image.

• Biological Activity: Enantiomers can have dramatically different biological effects.

Example:

• L-dopa: Biologically active enantiomer used in the treatment of Parkinson's disease.

• D-dopa: Biologically inactive enantiomer.

Functional Groups in Organic Molecules

Common Chemical Groups and Their Properties

Functional groups are specific groups of atoms within molecules that confer characteristic chemical properties and reactivity.

Chemical Group	Group Properties	Examples
Hydroxyl (–OH)	Alcohol	Ethanol
Carbonyl (–C=O)	Ketone, Aldehyde	Acetone, Propanal
Carboxyl (–COOH)	Carboxylic acid	Acetic acid
Amino (–NH2)	Amine	Glycine
Sulfhydryl (–SH)	Thiol	Cysteine
Phosphate (–OPO32–)	Organic phosphate	Glycerol phosphate
Methyl (–CH3)	Methylated compound	5-Methylcytosine

Structural Modifications and Biological Function

Sterols: Estradiol and Testosterone

Small covalent modifications to a common carbon skeleton can result in molecules with very different biological functions.

• Estradiol: A primary female sex hormone.

• Testosterone: A primary male sex hormone.

• Both share a sterol backbone but differ in functional groups, leading to distinct physiological effects.

Carbohydrates: Monosaccharides, Disaccharides, and Polysaccharides

Roles and Structures of Sugars

Carbohydrates are essential biomolecules that serve as energy sources, structural components, and signaling molecules.

• Monosaccharides: Simple sugars like glucose; provide energy and carbon skeletons for biosynthesis.

• Disaccharides and Polysaccharides: Serve as storage forms (e.g., starch, glycogen) and structural materials (e.g., cellulose in plants).

• Glycosylation: Oligosaccharides covalently bound to lipids and proteins play roles in cell recognition and immune response.

Monosaccharide Forms: Linear and Ring Structures

Monosaccharides exist in equilibrium between linear and ring forms in aqueous solutions.

• Ring Formation: The carbonyl group reacts with a hydroxyl group to form a cyclic structure.

• Example: Glucose can cyclize to form alpha or beta ring structures.

Polysaccharide Linkages and Functions

Storage and Structural Polysaccharides

Polysaccharides are long chains of monosaccharide units linked by glycosidic bonds, with different linkages conferring distinct properties.

• Starch: Storage polysaccharide in plants; consists of alpha-1,4 glycosidic linkages.

• Cellulose: Structural polysaccharide in plant cell walls; consists of beta-1,4 glycosidic linkages.

Lipids: Structure and Biological Roles

Fatty Acids, Triglycerides, and Membranes

Lipids are hydrophobic molecules that serve as energy stores, membrane components, and signaling molecules.

• Fatty Acids: Long hydrocarbon chains; can be saturated (no double bonds) or unsaturated (one or more double bonds).

• Triglycerides: Three fatty acids attached to a glycerol backbone via ester bonds; main form of stored energy in animals.

• Phospholipids: Major component of biological membranes; consist of two fatty acids, a glycerol, and a phosphate group.

• Sterols: Cholesterol and its derivatives; modulate membrane fluidity and serve as precursors for hormones.

Formation of Ester Bonds

Attachment of fatty acids to glycerol occurs via dehydration synthesis, forming ester bonds and releasing water.

Biological Membranes and Lipid Diversity

Phospholipids and Cholesterol in Membranes

Biological membranes are composed of a phospholipid bilayer with embedded proteins and sterols, providing structural integrity and regulating cellular processes.

• Phospholipid Bilayer: Hydrophilic heads face outward, hydrophobic tails face inward.

• Cholesterol: Intercalates within the bilayer, affecting membrane fluidity and serving as a precursor for steroid hormones.

Clinical Relevance: Lipids and Disease

Atherosclerosis and Lipoproteins

Excess cholesterol and lipid accumulation in blood vessels can lead to atherosclerotic plaque formation, increasing the risk of cardiovascular disease.

• Apolipoproteins: Proteins that transport lipids in the blood.

• Cholesteryl Esters: Storage form of cholesterol in cells.

• Plaque Formation: Lipid deposits in vessel walls can obstruct blood flow and cause thrombosis.

Additional info: The study notes expand on the brief points in the source material to provide a comprehensive overview suitable for General Biology students, including definitions, examples, and clinical relevance.