Chem 1120 Study Guide: Carbohydrates, Lipids, and Chemical Messengers

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).

• Example: Glucose (aldohexose), Fructose (ketohexose).

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 $2^n$, 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 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 via an open-chain form.

• Requirement: Free hemiacetal group on the anomeric carbon.

• 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.

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: Both formation and hydrolysis require an acid catalyst.

Phosphate Esters

Phosphoesters form when a sugar’s alcohol group is phosphorylated, important in metabolism.

• Example: Glucose-6-phosphate in glycolysis.

Common Monosaccharides

Name	Type	Found In
Glucose	Aldohexose	Primary energy source
Galactose	Aldohexose	Lactose (milk sugar)
Fructose	Ketohexose	Fruits, honey
Ribose	Aldopentose	RNA
Deoxyribose	Aldopentose	DNA

Disaccharides

Name	Monosaccharides	Linkage	Reducing?
Sucrose	Glucose + Fructose	α(1→2)β	No
Lactose	Galactose + Glucose	β(1→4)	Yes
Maltose	Glucose + Glucose	α(1→4)	Yes

Polysaccharides and Structural Roles

Polysaccharides like glycosaminoglycans (GAGs) are vital in connective tissues, acting as lubricants and shock absorbers due to their high negative charge and hydration shell.

• Cartilage: Springy due to electrostatic repulsion and hydration.

Glycoproteins and Blood-Type Antigens

Glycoproteins are proteins with sugars attached, playing roles in cell recognition and signaling. Blood-type antigens are oligosaccharides on red blood cells.

Blood Type	Antigen Structure
O	Basic oligosaccharide foundation
A	O-antigen + N-acetylgalactosamine
B	O-antigen + galactose

Cellulose, Chitin, Starch, and Glycogen

Polymer	Monomer	Linkage	Function/Location
Cellulose	β-D-glucose	β(1→4)	Plant cell walls
Chitin	N-acetyl-β-D-glucosamine	β(1→4)	Exoskeletons, fungi
Starch (Amylose)	α-D-glucose	α(1→4)	Plants, energy storage
Starch (Amylopectin)	α-D-glucose	α(1→4), α(1→6)	Plants, branched
Glycogen	α-D-glucose	α(1→4), α(1→6)	Animals, highly branched

Lipids (Ch. 23)

Waxes

Waxes are esters formed from long-chain fatty acids and long-chain alcohols, serving as protective, hydrophobic coatings in nature.

• Example: Triacontanyl palmitate.

Fatty Acids: Structure and Types

Fatty acids are long-chain carboxylic acids, classified as saturated (no double bonds) or unsaturated (one or more double bonds).

• Saturated: Straight chains, solid at room temperature.

• Unsaturated: Cis double bonds cause kinks, liquid at room temperature.

Essential Fatty Acids and Omega Nomenclature

Essential fatty acids cannot be synthesized by humans and must be obtained from the diet. Omega-3 and omega-6 refer to the position of the first double bond from the methyl end.

Name	Carbons:Double Bonds	Omega Type	Essential?
Linoleic acid	18:2	Omega-6	Yes
Arachidonic acid	20:4	Omega-6	No (conditionally essential)
α-Linolenic acid	18:3	Omega-3	Yes

Triacylglycerols (TAGs)

TAGs are composed of one glycerol and three fatty acids, linked by ester bonds. They serve as energy storage, insulation, and protection.

• Fats: Solid, more saturated fatty acids.

• Oils: Liquid, more unsaturated fatty acids.

Hydrogenation and Trans Fats

Hydrogenation adds H2 to unsaturated fats, making them saturated and solid. Partial hydrogenation creates trans fats, which are physically similar to saturated fats.

• Trans Fats: Formed during partial hydrogenation; higher melting points.

Saponification

Saponification is the base-catalyzed hydrolysis of fats/oils, producing glycerol and soap (fatty acid salts).

• Equation: $\text{TAG} + 3\text{NaOH} \rightarrow \text{Glycerol} + 3\text{Soap}$

Soaps, Detergents, and Biodiesel

Soaps and detergents are amphipathic, forming micelles to solubilize grease. Biodiesel is produced by transesterification of TAGs with small alcohols.

• Transesterification Equation: $\text{TAG} + 3\text{Methanol} \xrightarrow{\text{Catalyst}} 3\text{Fatty Acid Methyl Esters} + \text{Glycerol}$

Membrane Lipids

Cell membranes are composed of phosphoglycerides (ester linkages) and sphingolipids (amide linkages), built from glycerol or sphingosine.

• Polar head-groups: Attach to phosphate groups.

Cholesterol and Steroids

Cholesterol modulates membrane fluidity and is a precursor for bile salts and steroid hormones.

• Bile Salts: Aid in fat digestion.

• Mineralocorticoids: Regulate salt/water balance.

• Glucocorticoids: Regulate glucose metabolism and inflammation.

• Sex Hormones: Control development and reproduction.

Fluid-Mosaic Model of Membranes

The cell membrane is a dynamic lipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• Integral Proteins: Embedded in the bilayer; may span the membrane.

• Peripheral Proteins: Loosely attached to the membrane surface.

• Glycolipids/Glycoproteins: Cell recognition and signaling.

• Receptors: Bind signal molecules to trigger cellular responses.

Membrane Transport

Transport across membranes occurs via passive (simple/facilitated diffusion) or active mechanisms (primary/secondary).

• Passive: Down concentration gradient; no energy.

• Active: Against gradient; requires energy (ATP or another gradient).

Eicosanoids

Eicosanoids are signaling molecules derived from arachidonic acid, including leukotrienes, prostaglandins, and thromboxanes.

Type	Structure
Leukotrienes	Three conjugated double bonds
Prostaglandins	Cyclopentane ring
Thromboxanes	Six-membered cyclic ether ring

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

Eicosanoid vs. Steroid Signaling

• Eicosanoids: Bind to cell surface receptors; act locally.

• Steroids: Diffuse through membrane; bind internal receptors.

Chemical Messengers and Hormones (Ch. 28)

Steroid vs. Peptide Hormones

Steroid hormones are lipophilic and act via internal receptors, while peptide/water-soluble hormones act via cell surface receptors.

• Steroid Hormones: Receptor in cytosol/nucleus; regulate gene expression.

• Peptide Hormones: Receptor on cell surface; activate second messenger systems (e.g., cAMP).

G-Protein Coupled Receptors (GPCRs) and cAMP

GPCRs activate G-proteins, which stimulate adenylate cyclase to convert ATP to cAMP, activating protein kinase A (PKA).

• Equation: $\text{ATP} \xrightarrow{\text{Adenylate Cyclase}} \text{cAMP} + \text{PP_i}$

Identifying Steroid Hormones

Steroid hormones have a characteristic four-fused-ring structure (three 6-membered, one 5-membered ring).

Functions of Major Hormones

• Bile Salts: Emulsify dietary fats.

• Mineralocorticoids: Regulate electrolyte and water balance.

• Glucocorticoids: Regulate glucose metabolism and inflammation.

• Sex Hormones: Control secondary sex characteristics and reproduction.

Acetylcholinergic Synapse and Ion Channels

Acetylcholine binds to ligand-gated ion channels, initiating a local voltage change. Acetylcholinesterase hydrolyzes acetylcholine to terminate the signal.

• Ligand-gated Channels: Open in response to chemical messengers.

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

Agonists vs. Antagonists

• Agonist: Mimics natural messenger, triggers biological response.

• Antagonist: Blocks receptor, prevents biological response.

Homeostasis and Neurotransmitters

Chemical messengers maintain internal stability (homeostasis). Neurotransmitters enable rapid communication between neurons and muscles.

• Gating: Ion channels open only under specific conditions (ligand or voltage).