Carbohydrates (Ch. 20)

General Structure and Classification

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically following the empirical formula (CH2O)n, where n > 3. They are classified based on their functional groups and carbon count.

• Monosaccharides: Simple sugars; classified as aldoses (aldehyde at C1) or ketoses (ketone at C2).

• Carbon Numbering: Named as triose (3C), tetrose (4C), pentose (5C), hexose (6C).

• Examples: Glucose (aldohexose), fructose (ketohexose), galactose (aldohexose), ribose (aldopentose), deoxyribose (aldopentose, missing O at C2).

Optical Rotation and Chirality

Chiral molecules can rotate plane-polarized light, a property measured by a polarimeter. This is due to asymmetric (chiral) centers in the molecule.

• Number of Stereoisomers: $2^n$ where n = number of chiral centers.

• D- and L- Sugars: Determined by the orientation of the -OH group on the chiral carbon furthest from the carbonyl. D-sugars (OH on right) predominate in nature.

Structural Representations

• Fischer Projections: Vertical lines = bonds into the page; horizontal lines = bonds out of the page.

• Haworth Projections: Show cyclic forms as flat rings.

Ring Formation and Anomeric Carbon

• Hemiacetal Formation: Alcohol group reacts with aldehyde/ketone group to form rings.

• Anomeric Carbon: New chiral center created during cyclization (C1 for aldoses, C2 for ketoses).

Mutarotation and Reducing Sugars

• Mutarotation: Sugar equilibrates between α and β forms via open-chain intermediate. Requires a free hemiacetal group.

• Reducing Sugars: Can be oxidized; positive in Tollen’s, Benedict’s, or Fehling’s tests. Sucrose is non-reducing.

• Ketoses: Can reduce sugars by isomerizing to aldoses under basic conditions.

Acetal Formation and Glycosidic Bonds

• Acetal Formation: Hemiacetal -OH reacts with another alcohol, forming glycosidic bonds.

• Impact: Locks the anomeric carbon, preventing mutarotation and reducing activity unless another free anomeric carbon exists.

• Catalysis: Both formation and hydrolysis require acid catalysts.

Phosphate Esters

• Phosphoesters: Formed when a sugar’s alcohol group is phosphorylated; important in metabolism.

Disaccharides

Polysaccharides and Structural Roles

• Glycosaminoglycans (GAGs): Found in connective tissues, act as lubricants and shock absorbers due to high negative charge and hydration shell.

• Cellulose: β-D-glucose monomers, β(1→4) linkages, plant cell walls.

• Chitin: N-acetyl-β-D-glucosamine monomers, β(1→4) linkages, exoskeletons and fungal cell walls.

• Starch: Amylose (linear, α(1→4)), Amylopectin (branched, α(1→4) and α(1→6)).

• Glycogen: Highly branched, α(1→4) and α(1→6), found in liver and muscles for energy storage.

Glycoproteins and Blood-Type Antigens

• Glycoproteins: Proteins with sugars attached (O-linked to Ser/Thr, N-linked to Asn); function in cell recognition, receptors, hormones.

• Blood-Type Antigens: Oligosaccharides on red blood cells; O-type (basic), A-type (extra N-acetylgalactosamine), B-type (extra galactose).

Lipids (Ch. 23)

Waxes

• Functional Group: Ester

• Structure: Long-chain fatty acid + long-chain alcohol

• Function: Protective, waterproof coatings in nature

Fatty Acids

• Structure: Long-chain carboxylic acids

• Saturated: No double bonds, straight chains, solid at room temperature

• Unsaturated: One or more double bonds, usually cis, kinked chains, liquid at room temperature

• Effect of Cis Double Bonds: Prevent tight packing, lower melting point

Essential Fatty Acids and Omega Nomenclature

Triacylglycerols (TAGs)

• Structure: Glycerol + 3 fatty acids (ester linkages)

• Function: Energy storage, insulation, protection

• Location: Adipose tissue

Fats vs. Oils

• Fats: Solid at room temperature, more saturated fatty acids

• Oils: Liquid at room temperature, more unsaturated fatty acids

Hydrogenation and Trans Fats

• Hydrogenation: Adds H2 to unsaturated bonds, making fats more saturated and solid

• Trans Fats: Formed during partial hydrogenation; trans double bonds allow tight packing, similar to saturated fats

Saponification

• Reaction: Hydrolysis of fats/oils with base (NaOH)

• Products: Glycerol + fatty acid salts (soap)

Soaps and Detergents

• Amphipathic: Hydrophobic tails dissolve grease, hydrophilic heads interact with water, forming micelles

Biodiesel Production

• Transesterification: TAGs react with small alcohols (e.g., methanol) and catalyst to produce fatty acid methyl esters (biodiesel)

Membrane Lipids

• Phosphoglycerides: Built from glycerol, ester linkages

• Sphingolipids: Built from sphingosine, amide linkages

• Polar Head Groups: Attach to phosphate groups

Cholesterol and Steroids

• Cholesterol: Modulates membrane fluidity, precursor for bile salts and steroid hormones

• Functions: Membrane structure, digestion (bile salts), hormone synthesis

Hormones Derived from Steroids

• Bile Salts: Aid in fat digestion

• Mineralocorticoids: Regulate salt/water balance (e.g., Aldosterone)

• Glucocorticoids: Regulate glucose metabolism and inflammation (e.g., Cortisol)

• Sex Hormones: Development and reproduction (e.g., Testosterone, Estrogen)

Fluid-Mosaic Model of Membranes

• Structure: Lipid bilayer with proteins, cholesterol, carbohydrates floating within or on it

• Integral Proteins: Embedded in bilayer, often transmembrane

• Peripheral Proteins: Loosely attached to membrane surface

• Glycolipids/Glycoproteins: Carbohydrate chains attached, function in cell recognition

• Receptors: Integral proteins with specific binding sites for signaling molecules

Transport Across Membranes

• Passive Transport: No energy required; includes simple diffusion (small, nonpolar molecules) and facilitated diffusion (via channels/carriers)

• Active Transport: Requires energy; primary (direct ATP use, e.g., Na+/K+ pump), secondary (uses gradient of one molecule to drive another)

Eicosanoids

• Precursor: All derived from arachidonic acid (20:4)

• NSAIDs: Aspirin and ibuprofen inhibit cyclooxygenase (COX), blocking prostaglandin synthesis

• Receptor Location: Eicosanoids act via cell surface receptors; steroids act via internal (cytosolic/nuclear) receptors

Chemical Messengers and Hormones (Ch. 28)

Steroid vs. Peptide/Water-Soluble Hormones

• Steroid Hormones: Lipophilic, cross cell membrane, bind to cytosolic/nuclear receptors, regulate gene expression

• Peptide/Water-Soluble Hormones: Hydrophilic, bind to cell surface receptors, activate second messenger systems (e.g., cAMP)

G-Protein-Coupled Receptors (GPCRs) and cAMP

• Mechanism: Hormone binds to GPCR, activates G-protein (GDP to GTP), G-protein activates adenylate cyclase

• cAMP Formation: Adenylate cyclase converts ATP to cAMP

• Cellular Response: cAMP activates protein kinase A (PKA), which phosphorylates proteins

Identifying Steroid Hormones

• Structure: Four-fused-ring steroid nucleus (three 6-membered, one 5-membered ring)

Functions of Hormones

• Bile Salts: Emulsify dietary fats

• Mineralocorticoids: Regulate electrolyte and water balance

• Glucocorticoids: Regulate glucose metabolism, anti-inflammatory

• Sex Hormones: Control secondary sex characteristics and reproduction

Acetylcholinergic Synapse and Ion Channels

• Acetylcholine: Released into synapse, binds to ligand-gated receptor

• Esterase Reaction: Acetylcholinesterase hydrolyzes acetylcholine to choline and acetate

• Ligand-Gated Channels: Open in response to chemical messenger

• Voltage-Gated Channels: Open in response to changes in membrane potential, propagate action potential

Agonists vs. Antagonists

• Agonist: Binds to receptor, mimics natural messenger, triggers response

• Antagonist: Binds to receptor, blocks natural messenger, prevents response

Homeostasis and Neurotransmitters

• Homeostasis: Chemical messengers maintain stable internal environment

• Neurotransmitters: Fast communication between neurons or neurons and muscles

• Gating: Ion channels are gated, opening only under specific conditions

Additional info: For stereoisomer calculations, use $2^n$ where n is the number of chiral centers. For mutarotation, only sugars with a free hemiacetal group can undergo this process. For membrane transport, primary active transport uses ATP directly, while secondary uses the gradient established by primary transport.