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: Asymmetric (chiral) centers interact with light.

• 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 D/L designation is based on the orientation of the -OH group on the chiral carbon furthest from the carbonyl.

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

• L-sugars: -OH on the left.

Ring Formation and Anomeric Carbon

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

• Aldoses: Anomeric carbon is C1.

• Ketoses: Anomeric carbon is C2.

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; RNA component.

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

Reducing Properties and Tests

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

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

Acetal Formation and Glycosidic Bonds

Acetals form when the hemiacetal -OH reacts with another alcohol, creating 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 tissues.

• GAGs: Lubricants in joints; shock absorbers in cartilage due to high negative charge and hydration.

• Cartilage: Springy due to electrostatic repulsion and water retention.

Glycoproteins

• Definition: Proteins with attached sugars via glycosidic bonds.

• Types: O-linked (Ser/Thr), N-linked (Asn).

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

Blood-Type Antigens (A, B, O)

• Function: Act as cell 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: Waterproofing and protection in plants and animals

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 (liquid oils)

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

• 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 double bonds, making fats more saturated and solid

• Trans Fats: Formed during partial hydrogenation; pack tightly, similar to saturated fats

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

• Steroids: Four-fused-ring structure; includes hormones and bile salts

Functions of Cholesterol and Steroid Derivatives

• Membranes: Maintains fluidity

• Digestion: Precursor to bile salts

• Hormones: Precursor to steroid hormones

Bile Salts, Mineralocorticoids, Glucocorticoids, Sex Hormones

• 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: Control development and reproduction (e.g., Testosterone, Estrogen)

Fluid-Mosaic Model of Membranes

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

• Integral Proteins: Embedded in bilayer; often transmembrane

• Peripheral Proteins: Loosely attached to membrane surface

• Glycolipids/Glycoproteins: Carbohydrate chains attached; cell recognition

• Receptors: Integral proteins for signal transduction

Membrane Transport

• Passive Transport: Down concentration gradient; no energy

• Simple Diffusion: Small, nonpolar molecules

• Facilitated Diffusion: Channel or carrier proteins

• Active Transport: Against gradient; requires energy

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

• Secondary Active Transport: Uses gradient of one molecule to drive another

Eicosanoids

• Precursor: Arachidonic acid (20:4)

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

Eicosanoid vs. Steroid Signaling

• Eicosanoids: Bind to cell surface (transmembrane) receptors; act locally

• Steroids: Diffuse through membrane; bind to internal (cytosolic/nuclear) receptors

Chemical Messengers and Hormones (Ch. 28)

Steroid vs. Peptide/Water-Soluble Hormones

• Steroid Hormones: Lipophilic; receptors in cytosol/nucleus; regulate gene expression

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

G-Protein-Coupled Receptors (GPCRs) and cAMP

• Hormone binds to GPCR → G-protein activated (GDP replaced by GTP) → activates adenylate cyclase → converts ATP to cAMP

• cAMP activates protein kinase A (PKA), leading to cellular response

Identifying Steroid Hormones

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

Functions of Bile Salts and Steroid Hormones

• 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 ligand-gated receptor

• Esterase Reaction: Acetylcholinesterase hydrolyzes acetylcholine to choline + acetate

Ligand-Gated vs. Voltage-Gated Channels

• Ligand-Gated: Open in response to chemical messenger (e.g., acetylcholine)

• Voltage-Gated: Open in response to changes in membrane potential; propagate action potentials

Agonists vs. Antagonists

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

• Antagonist: Binds to 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 (fatty acids) and Figures referenced are not included; essential fatty acids to memorize: linoleic acid (omega-6), α-linolenic acid (omega-3), arachidonic acid (omega-6).