Chemistry Study Guide: Carbohydrates, Lipids, and Chemical Messengers

Carbohydrates

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 the number of carbon atoms.

• 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 $2^n$, where n is the number of chiral centers.

Fischer and Haworth Projections

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

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

• Haworth: 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 (C1 for aldoses, C2 for ketoses).

• Mutarotation: Interconversion between α and β anomers in aqueous solution.

• Reducing Sugars: Sugars with a free hemiacetal group can act as reducing agents.

Common Monosaccharides

• Glucose: Aldohexose; main energy source.

• Galactose: Aldohexose; component of lactose.

• Fructose: Ketohexose; found in fruits/honey.

• Ribose: Aldopentose; in RNA.

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

Reducing Properties and Tests

• Reducing sugars: Give positive results in Tollen’s, Benedict’s, or Fehling’s tests.

• Ketoses: Can isomerize to aldoses under basic conditions and act as reducing sugars.

Acetal Formation and Glycosidic Bonds

When the hemiacetal -OH reacts with another alcohol, an acetal (glycosidic bond) forms, locking the anomeric carbon.

• Impact: Prevents mutarotation and reduces sugar activity unless another free anomeric carbon exists.

• Catalysis: Formation and hydrolysis require an acid catalyst.

Phosphate Esters

Phosphorylation of a sugar’s alcohol group forms phosphate esters, important in metabolism.

Disaccharides

Disaccharides are formed by glycosidic bonds between two monosaccharides.

Disaccharide	Monosaccharides	Linkage	Reducing?
Sucrose	Glucose + Fructose	α(1→2) (anomeric C1 of glucose to anomeric C2 of fructose)	No
Lactose	Galactose + Glucose	β(1→4) (C1 of galactose to C4 of glucose)	Yes
Maltose	Glucose + Glucose	α(1→4) (C1 of first glucose to C4 of second)	Yes

Polysaccharides and Glycosaminoglycans

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

Glycoproteins

Proteins with attached sugars via glycosidic bonds, important for cell recognition and signaling.

• O-linked: Sugar binds to Serine or Threonine.

• N-linked: Sugar binds to Asparagine.

Blood-Type Antigens

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

Structural Polysaccharides

• 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.

Storage Polysaccharides

• 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; found in liver and muscles.

Lipids

Waxes

Waxes are esters formed from long-chain fatty acids and long-chain alcohols, serving as protective, hydrophobic 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, lowering melting point (liquid oils).

Essential Fatty Acids and Omega Classification

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

Additional info: Essential fatty acids must be obtained from the diet.

Triacylglycerols (TAGs)

• Structure: Glycerol + 3 fatty acids via ester linkages.

• Function: Energy storage, insulation, 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: Addition of H2 to unsaturated bonds, converting oils to fats.

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

Saponification

Hydrolysis of fats/oils with base produces glycerol and fatty acid salts (soap).

• Reaction: Saponification

• Products: Glycerol + soap

Soaps and Detergents

Amphipathic molecules; hydrophobic tails dissolve grease, hydrophilic heads interact with water, forming micelles.

Biodiesel Production

Transesterification of TAGs with small alcohols (e.g., methanol) and catalyst produces 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.

• Steroid Structure: Four fused rings (three 6-membered, one 5-membered).

Functions of Cholesterol and Steroid Hormones

• Membranes: Modulates 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).

Cell Membranes and Fluid-Mosaic Model

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: Lipids with carbohydrate chains; cell recognition.

• Glycoproteins: Proteins with carbohydrate chains; ID tags.

• Receptors: Integral proteins for signal transduction.

Membrane Transport

• Passive Transport: No energy required; includes simple diffusion (direct through bilayer) and facilitated diffusion (via channels/carriers).

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

Eicosanoids

• Leukotrienes: Three conjugated double bonds.

• Prostaglandins: Cyclopentane ring.

• Thromboxanes: Six-membered cyclic ether ring.

• Precursor: Arachidonic acid (20:4).

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

Eicosanoid vs. Steroid Signaling

Type	Solubility	Membrane Action	Receptor Location	Precursor
Eicosanoids	Local/short-lived	Bind outside membrane	Transmembrane (cell surface)	Arachidonic acid
Steroids	Lipophilic	Diffuse through membrane	Internal (cytosolic/nuclear)	Cholesterol

Chemical Messengers and Hormones

Steroid vs. Peptide/Water-Soluble Hormones

• Steroid Hormones: Lipophilic; receptors in cytosol or nucleus; regulate gene expression by binding to DNA hormone response elements.

• 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 on cell surface.

• G-protein exchanges GDP for GTP, activates adenylate cyclase.

• Adenylate cyclase converts ATP to cAMP.

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

Identifying Steroid Hormones

• Look for 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 anti-inflammatory responses.

• 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, terminating 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

• 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 binding or voltage change).

Additional info: These notes cover key concepts from chapters 20, 23, and 28, focusing on carbohydrates, lipids, and chemical messengers relevant to biochemistry and cell biology.