Carbohydrates: Structures, Reactions & Biological Roles

Condensation vs. Hydrolysis Reactions

• Condensation (dehydration synthesis)
– Joins monomers → polymers.
– Releases a molecule of water ( $H2O$ ) per bond formed. • Hydrolysis – Opposite process: splits polymers → monomers. – Requires the input of water; the $H^+$ and $OH^-$ from $H2O$ cap the newly-formed ends.
– Central to digestion/absorption of macromolecules (carbohydrates, proteins, lipids, nucleic acids).
– Overall mnemonic: Condensation = make & lose water; Hydrolysis = break & add water.

Enzyme-Specific Hydrolytic Examples

• Polysaccharide → monosaccharide
– Glycosidic bond broken by the enzyme amylase (e.g., starch → glucose).
• Polypeptide → amino acids
– Peptide bond broken by proteases.
• Nucleic acid → nucleotides
– Phosphodiester bond hydrolysed by nucleases.

Introduction to Carbohydrates

• Found in pasta, bread, potatoes, rice, honey, fruit, etc.
• Composed of carbon, hydrogen, oxygen in the general ratio $CnH{2n}O_n$ .
• One of the four major biological macromolecules.

Classification of Carbohydrates

• Monosaccharides (“one sugar”) – simplest units; e.g., glucose, fructose, galactose, ribose.
• Disaccharides (“two sugars”) – two monosaccharides joined; e.g.,
– Sucrose = glucose + fructose (table sugar)
– Lactose = glucose + galactose (milk sugar)
– Maltose = glucose + glucose (malt sugar)
• Oligosaccharides – $3{-}9$ linked sugars (e.g., galacto-oligosaccharides in soybeans).
• Polysaccharides – $≥10$ linked sugars; e.g., starch, glycogen, cellulose, dietary fibre.

Functional Roles of Carbohydrates

• Primary, quick energy source—substrate for cellular respiration (need glucose + $O_2$ ).
• Structural components – cellulose in plant cell walls; chitin in fungi/arthropods.
• Transportable/soluble – glucose dissolves in blood plasma & xylem/phloem sap.
• Protein-sparing – adequate carbs prevent catabolism of body proteins for energy.

Monosaccharides in Detail

• General formula again $CnH{2n}O_n$ with $n=3{-}7$ carbons.
• Pentoses (5C): ribose (RNA), deoxyribose (DNA).
• Hexoses (6C): glucose, fructose – main metabolic fuels.

Key Properties of Glucose

• Highly soluble – many hydroxyl (-OH) groups allow H-bonding with water.
• Stable ring form under physiological pH/temperature → safe storage/transport.
• High energy yield – aerobic oxidation of one molecule nets $36{-}38\,ATP$ .

Polysaccharides: Structure ↔ Function

• Formed by repeated condensation of α-glucose monomers; linkage = 1→4 and/or 1→6 α-glycosidic bonds.

Amylose

• Unbranched helix; only α-1→4 bonds.
• Fewer accessible ends → slower enzymatic attack → slow, sustained energy release.

Amylopectin

• Branched; α-1→4 main chain + α-1→6 branches every $25{-}30$ residues.
• Faster mobilisation than amylose because amylase can act on many branch ends simultaneously.

Glycogen (animal storage)

• Highly branched (branch every $8{-}12$ residues).
• Stored in liver & muscle; maximal surface area → extremely rapid glucose release during exercise/flight-or-fight.

α- vs. β-Glucose Isomers

• Difference = orientation of hydroxyl on carbon-1.
– α-glucose: $OH$ below ring.
– β-glucose: $OH$ above ring.
• Consequences:
– α-glucose builds starch & glycogen.
– β-glucose builds cellulose (different bonding geometry).

Cellulose (Plant Structural Polymer)

• Long, straight chains of β-1→4 linked glucose; every alternate monomer rotated $180^{\circ}$ .
• Chains H-bond laterally → microfibrils → fibres → plant cell wall.
• Key properties:
– High tensile strength – resists turgor pressure.
– Slight elasticity – flexibility without rupture.
– Insoluble & indigestible to most animals (lack cellulase) – anti-herbivory benefit.

Functional Consequences

• Prevents osmotic lysis of plant cells, maintains upright stems for photosynthesis.
• Contributes to dietary fibre in human nutrition – passes undigested.

Summary of Video Content (Applied Nutrition Context)

• Carbs subdivided into simple vs. complex based on chain length.
• Digestion: salivary/pancreatic amylase → brush-border enzymes (lactase, sucrase, maltase).
• Absorbed monosaccharides enter bloodstream; insulin promotes uptake & glycogenesis.
• Recommended intake: $45{-}65\%$ total kcal as carbohydrates (e.g., $1100$ kcal of a $2000$ kcal diet).
– Fibre target: $28\,g$ (≈ $56$ kcal ≈ $3\%$ of diet).
– Added sugars < $10\%$ of kcal (WHO/USDA).
• Starch vs. fibre vs. added sugar: impact on glycaemic response & nutrient density.

Glycoproteins

• Definition: proteins covalently attached to carbohydrate chains (glycans).
• Embedded in outer leaflet of cell membrane; functions in signalling, adhesion, immune recognition.

Immune Example

• Major Histocompatibility Complex (MHC) on leukocytes – presents antigenic peptides; self vs. non-self discrimination.

Blood Type Example

• All RBCs share base H-antigen glycoprotein.
– Type A: N-acetylgalactosamine added.
– Type B: galactose added.
– Type AB: both sugars.
– Type O: no additional sugar.
• Transfusion rules:
– Mismatch → antibodies bind foreign glycoprotein → agglutination, potential fatality.
– Type O = universal donor; Type AB = universal recipient.

Energetic Pros & Cons of Carbohydrates

• Advantages
– Rapid ATP production, usable anaerobically & aerobically.
– Water-soluble transport; stored as glycogen for quick mobilisation.
• Disadvantages
– Lower energy density ( $4\,\text{kcal g}^{-1}$ ) vs. lipids ( $9\,\text{kcal g}^{-1}$ ).
– Glycogen hydrated (binds water) → heavier; limited long-term storage compared with fat.
– Stores deplete faster than adipose triglycerides.

These bullet-point notes capture all major & minor details, key definitions, structural nuances, enzymatic examples, nutritional data, metabolic pathways, and real-world implications discussed in the transcript and accompanying video.