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).

Optical Rotation and Chirality

Many carbohydrates are chiral and can rotate plane-polarized light, a property known as optical rotation.

• Cause: Interaction of light with asymmetric (chiral) centers.

• Measurement: Measured using a polarimeter.

• Stereoisomer Calculation: Number of possible stereoisomers is , where n = number of chiral centers.

Fischer and Haworth Projections

Carbohydrate structures are depicted using Fischer (linear) and Haworth (cyclic) projections.

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

• Haworth Projection: Shows cyclic form as a flat ring.

D- and L- Sugars

The configuration of the -OH group on the chiral carbon furthest from the carbonyl determines D- or L- form.

• D-sugars: -OH on the right; predominant in nature.

• L-sugars: -OH on the left.

Ring Formation and Anomeric Carbon

Monosaccharides cyclize to form rings via hemiacetal formation, creating a new chiral center called the anomeric carbon.

• Anomeric Carbon: C1 in aldoses, C2 in ketoses.

Mutarotation and Reducing Sugars

Mutarotation is the interconversion between α and β anomers in aqueous solution.

• α anomer: -OH on anomeric carbon is down (trans to CH2OH).

• β anomer: -OH is up (cis to CH2OH).

• Requirement: Free hemiacetal group allows mutarotation and reducing activity.

Key Monosaccharides

• Glucose: Aldohexose; main cellular energy source.

• Galactose: Aldohexose; component of lactose.

• Fructose: Ketohexose; found in fruits/honey.

• Ribose: Aldopentose; in RNA.

• Deoxyribose: Aldopentose; in DNA (lacks O at C2).

Reducing Properties and Tests

• Reducing Sugars: Can be oxidized; positive in Tollen’s, Benedict’s, or Fehling’s tests.

• Ketoses: Can act as reducing sugars after isomerization to aldoses under basic conditions.

Acetal Formation and Glycosidic Bonds

Acetal formation occurs when the 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.

• Catalyst: Acid catalyst required for both formation and hydrolysis.

Phosphate Esters

• Phosphoesters: Formed by phosphorylation of sugar alcohol groups; important in metabolism.

Disaccharides

Disaccharides are formed by glycosidic bonds between two monosaccharides.

Polysaccharides and Glycosaminoglycans (GAGs)

Polysaccharides serve structural and mechanical roles in connective tissues.

• Lubricants: GAGs in synovial fluid reduce friction.

• Shock Absorbers: GAGs withstand compressive forces; rehydrate quickly.

• Cartilage: Springy due to electrostatic repulsion and hydration shell.

Glycoproteins

• Definition: Proteins with sugars attached via glycosidic bonds.

• O-linked: Sugar binds to Serine/Threonine (O atom).

• N-linked: Sugar binds to Asparagine (N atom).

• Function: Cell recognition, receptors, hormones, structural matrix.

Blood-Type Antigens (A, B, O)

• Function: Act as ID tags; immune system recognizes unfamiliar patterns.

Cellulose and Chitin

Starch and Glycogen

• Starch (plants):

  • Amylose: Linear, α(1→4) linkages, helical shape.

  • Amylopectin: Branched, α(1→4) main chain, α(1→6) branches every 25-30 residues.

• Glycogen (animals):

  • Highly branched, α(1→4) main chain, α(1→6) branches every 8-12 residues.

  • Short-term energy storage; found in liver and muscles.

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; cis double bonds cause kinks, lower melting point

Omega Fatty Acids

• Omega-3 (ω−3): First double bond at third carbon from methyl end (e.g., α-linolenic acid)

• Omega-6 (ω−6): First double bond at sixth carbon (e.g., linoleic acid, arachidonic acid)

Triacylglycerols (TAGs)

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

• Function: Energy storage, insulation, organ protection

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

• Trans Fats: Formed during partial hydrogenation; pack tightly, higher melting point

Saponification

• Definition: 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; form micelles

Transesterification and Biodiesel

• Transesterification: TAGs + small alcohol (e.g., methanol) + catalyst → biodiesel (fatty acid methyl esters)

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:

  • Membranes: fluidity regulation

  • Digestion: bile salt precursor

  • Hormones: steroid hormone precursor

Bile Salts and Hormones

• Bile Salts: Aid in fat digestion

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

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

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

Fluid-Mosaic Model of Membranes

• Structure: Fluid lipid bilayer with mosaic of proteins, cholesterol, carbohydrates

• Integral Proteins: Embedded in bilayer; often transmembrane

• Peripheral Proteins: Loosely attached to membrane surface

• Glycolipids/Glycoproteins: Cell recognition, signaling

• Receptors: Bind signal molecules, trigger cellular responses

Membrane Transport

• Passive Transport: Down concentration gradient; no energy

  • Simple Diffusion: Small, nonpolar molecules

  • Facilitated Diffusion: Channel/carrier proteins

• Active Transport: Against gradient; requires energy

  • Primary: Direct ATP use (e.g., Na+/K+ pump)

  • Secondary: Uses gradient of one molecule to drive another

Eicosanoids

• Precursor: Arachidonic acid (20:4)

• NSAIDs (Aspirin/Ibuprofen): Inhibit cyclooxygenase (COX), blocking prostaglandin production

Eicosanoid vs. Steroid Signaling

Chemical Messengers and Hormones (Ch. 28)

Steroid vs. Peptide Hormones

G-Protein-Coupled Receptors (GPCRs) and cAMP

• Mechanism: Hormone binds to GPCR → G-protein activated (GDP to GTP) → activates Adenylate Cyclase → converts ATP to cAMP

• cAMP: Activates protein kinase A (PKA), phosphorylates proteins, triggers cellular response

Identifying Steroid Hormones

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

Bile Salts and Hormones Functions

• Bile Salts: Emulsify dietary fats

• Mineralocorticoids: Regulate electrolyte/water balance

• Glucocorticoids: Regulate glucose metabolism, anti-inflammatory

• Sex Hormones: Control secondary sex characteristics, reproduction

Acetylcholinergic Synapse and Ion Channels

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

• Esterase Reaction: Acetylcholinesterase hydrolyzes acetylcholine to choline and acetate

• Ligand-gated Channels: Open in response to chemical messenger; initiate local voltage change

• Voltage-gated Channels: Open in response to membrane potential changes; propagate action potential

Agonists vs. Antagonists

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

• Antagonist: Binds receptor, blocks 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 open only under specific conditions (ligand or voltage)

Additional info: Table 23.1 and Figure references are not included due to lack of original data; key essential fatty acids and membrane lipid structures are summarized based on standard textbook knowledge.