Chemistry of Carbohydrates – Comprehensive Study Notes

Occurrence and Functions of Carbohydrates

Definition
- Organic compounds with empirical formula $CnH{2n}On$ or $Cn(H2O)n$
- Better described as polyhydroxy-aldehydes (aldoses) or polyhydroxy-ketones (ketoses).
Natural origin
- Synthesised by green plants via photosynthesis (requires chlorophyll & sunlight).
- ~75 % of dry plant mass = carbohydrate; majority of non-aqueous plant matter.
- Major plant forms: cellulose (structural) & starch (storage).
  • Average human diet: ≈ 2⁄3 carbohydrate.
Biological functions
- Immediate energy
- Oxidation during metabolism → $CO2 + H2O$ with ATP production.
- Monosaccharides & disaccharides: rapid energy; starch: slower release.
- Humans lack enzyme for $\beta(1\rightarrow4)$ cellulose hydrolysis.
- Storage energy
- Principal reserves: starch (plants) & glycogen (animals).
- Hormonal control
 - High blood glucose → insulin → glycogenesis (liver glycogen).
 - Low glucose → glucagon → glycogenolysis.
- Glycogen = primary human energy store; glucose catabolism fuels activity.
- Provide carbon skeletons for synthesis of proteins, lipids, nucleic acids.
- Structural roles
- Cellulose: $\beta$ -D-glucose polymer; plant cell wall strength.
- Chitin: N-acetyl-D-glucosamine polymer; insect exoskeleton, surgical sutures.
- Ribose/deoxyribose: backbone of RNA & DNA.
- Glycolipids & glycoproteins: membrane architecture, cell recognition.
- Food science roles
1. Energy, sweetness; syrup formation.
2. Preservation at high concentration.
3. Caramelisation on heating → flavours/colours.
4. Texture: pectins & gums as thickeners/gelling agents.
5. Non-enzymatic browning (Maillard reactions) with amino acids.

Classification Schemes

By carbonyl type
- Aldose: terminal $\text{CHO}$ (e.g., glucose).
- Ketose: internal $\text{C}=\text{O}$ (e.g., fructose).
By carbon number
- Trioses (3C) → Heptoses (7C). Common examples:
- Tetroses: erythrose, threose.
- Pentoses: ribose, ribulose, xylose, lyxose.
- Hexoses: glucose, galactose, mannose, fructose, talose, etc.
- Heptose: sedoheptulose.
By size/degree of polymerisation
- Monosaccharides (single unit, 3–7 C).
- Oligosaccharides (2–10 units); disaccharides most common.
- Polysaccharides (> 10 units; may reach thousands).
- Derived carbohydrates (chemically modified e.g., sugar acids).

Key Biologically Important Monosaccharides

D-Glucose (aldohexose)
- Most abundant; “grape sugar”, “dextrose”, “blood sugar”.
- Normal blood level: $70\text{–}100\,\text{mg}/100\,\text{mL}$ .
- Six-membered pyranose dominant.
D-Fructose (ketohexose)
- Sweetest natural sugar; present in fruit & honey.
- Five-membered furanose dominant.
D-Galactose (aldohexose)
- “Milk sugar” component; part of brain tissue; blood-group determinants.
D-Ribose (aldopentose)
- Structural component of RNA, ATP, NADH, etc.

Structural Representation Methods

Fischer projections (open-chain/Kiliani)
- Vertical = carbon chain; horizontal bonds project toward observer.
- Chiral centre = intersection; top usually most oxidised carbon.
- D/L configuration determined by chiral carbon furthest from carbonyl:
  • $\text{OH}$ on right → D; left → L.
- Stereoisomer terms
- Epimers: differ at one chiral centre (non-anomeric).
- Anomers: differ only at anomeric carbon (after cyclisation).
Haworth projections (cyclic)
- Depict hemiacetal/hemiketal ring forms (pyranose = 6-member, furanose = 5-member).
- General rule: groups right (Fischer) → down (Haworth); left → up.
- $\alpha$ -anomer: anomeric $\text{OH}$ opposite $CH_2OH$ ; $\beta$ : same side.
- Mutarotation: interconversion of $\alpha$ and $\beta$ forms in solution with characteristic change in optical rotation.
Chair conformations
- 3-D low-energy depiction for pyranoses; axial/equatorial substituents analysed for steric effects.

Chirality & Stereochemistry

Chiral centre: carbon attached to four distinct groups.
Enantiomers: non-superimposable mirror images; identical physical properties except:
1. Rotation of plane-polarised light (optical activity).
2. Behaviour toward other chiral reagents/solvents.
Optical designations
- Dextrorotatory (+) vs levorotatory (−) determined experimentally; no direct link to D/L.
Diastereomers: stereoisomers not mirror images (e.g., glucose vs galactose).
Number of stereoisomers = $2^n$ where $n$ = chiral centres.
- Example counts:
  • D-ribose (3 centres) → $2^3 = 8$ isomers.
  • D-glucose (4 centres) → $2^4 = 16$ .
Biological preference: Natural monosaccharides mostly D; amino acids L.

Reactions of Monosaccharides

Molisch test: dehydration by conc. $H2SO4$ → furfural/5-hydroxymethylfurfural + $\alpha$ -naphthol → purple condensation product.
Reducing tests
- Benedict/Fehling (Cu $^{2+}$ → Cu $^{+}$ red ppt) & Tollens (Ag $^{+}$ → Ag mirror).
Oxidation
- Mild (e.g., $Ag^+$ ) converts aldehyde → aldonic acid (e.g., mannose → mannonic acid).
- Strong $HNO_3$ oxidises both aldehyde & primary alcohol → aldaric acid (glucaric acid from glucose).
- Ketoses (fructose) rearrange under basic conditions to aldoses → positive reducing tests.
Reduction (NaBH$4$ or Ni/H$2$): carbonyl → sugar alcohol (alditol)
- Glucose → sorbitol; xylose → xylitol.
Glycoside formation
- Alcohol + anomeric $OH$ → acetal (glycosidic bond); readily hydrolysed in acid.

Disaccharides (Oligosaccharides 2 – 10)

Disaccharide	Linkage	Components	Reducing?	Notes
Maltose	$\alpha(1\rightarrow4)$	$\alpha$ -D-Glc + $\beta$ -D-Glc	Yes	Starch breakdown product; easily digested.
Cellobiose	$\beta(1\rightarrow4)$	$\beta$ -D-Glc + $\beta$ -D-Glc	Yes	Repeating unit of cellulose; indigestible to humans.
Lactose	$\beta(1\rightarrow4)$	$\beta$ -D-Gal + $\beta$ -D-Glc	Yes	Milk sugar; lactase deficiency → intolerance.
Sucrose	$\beta(2\rightarrow1)\alpha$	$\beta$ -D-Fru + $\alpha$ -D-Glc	No	Table sugar; “head-to-head” linkage ties both anomeric carbons.
Isomaltose	$\alpha(1\rightarrow6)$	$\alpha$ -D-Glc + $\alpha$ -D-Glc	Yes	Branch point disaccharide in starch.

Polysaccharides – General Characteristics

Polymers of many monosaccharides via glycosidic bonds.
Usually tasteless; non-reducing; limited water solubility.
Categories: storage vs structural; homopolysaccharide vs heteropolysaccharide; linear vs branched.

Storage Polysaccharides

Starch (plants)
- Homopolymer of D-glucose.
- Two fractions
 a. Amylose: unbranched $\alpha(1\rightarrow4)$ ; helical; blue with $I2$ . b. Amylopectin: branched; $\alpha(1\rightarrow4)$ backbone + $\alpha(1\rightarrow6)$ every 24–30 units; red-violet with $I2$ .
- Hydrolysis
 - $\alpha$ -Amylase: random $\alpha(1\rightarrow4)$ cleavage (saliva, pancreas).
 - $\beta$ -Amylase: exo-action releasing maltose from non-reducing end until branch.
 - $\alpha(1\rightarrow6)$ -glucosidase (debranching) → complete hydrolysis.
 - Product sequence with acid + $I_2$ : starch → amylodextrin (bluish-red) → erythrodextrin (red) → achroodextrin (colourless) → maltose → glucose.
Glycogen (animals)
- Highly branched: $\alpha(1\rightarrow4)$ chains, $\alpha(1\rightarrow6)$ every 8–12 residues.
- Stored in liver & muscle; red-violet with $I_2$ .
- Metabolism: glycogenesis vs glycogenolysis regulated hormonally.
Dextran (yeast/bacteria)
- $\alpha(1\rightarrow6)$ backbone with $\alpha(1\rightarrow2/3/4/6)$ branches.
- Viscous; used in plasma expanders, photography, agriculture.

Structural Polysaccharides

Cellulose
- $\beta(1\rightarrow4)$ D-glucose; > 300 units; insoluble; main plant cell-wall component; cotton ≈ 100 % cellulose.
Pectin (polygalacturonic acid)
- Complex heteropolysaccharide of plant primary walls & middle lamella.
- Commercial gelling agent in jams; soluble fibre → lowers cholesterol & slows glucose uptake; fermented to SCFA (probiotic).
Hemicellulose
- Amorphous heteropolymers (e.g., xylan, glucomannan); weaker than cellulose.
Chitin
- $\beta(1\rightarrow4)$ N-acetyl-D-glucosamine; arthropod exoskeleton, fungal walls; biomedical applications.

Acidic/Derived Heteropolysaccharides (Glycosaminoglycans)

Hyaluronic acid: repeating $\beta(1\rightarrow3)$ D-glucuronic acid–N-acetyl-D-glucosamine; connective tissue, vitreous humour, skin repair.
Chondroitin sulfate: similar to hyaluronan but with N-acetyl-D-galactosamine ± sulfate at C-4/6; cartilage resilience; dietary supplement for osteoarthritis.
Peptidoglycan: bacterial cell-wall mesh of NAG–NAM $\beta(1\rightarrow4)$ with peptide cross-links.
Heparin: highly sulfated acidic polymer stored in mast cells; anticoagulant via antithrombin III.

Enzymatic & Chemical Reactions of Starch / Polysaccharides

Hydrolytic enzymes: $\alpha$ -, $\beta$ -amylase, debranching enzyme; commercial diastase = mix.
Limit dextrin: residual branched core after exhaustive amylase action (amylose produces none).
Industrial inversion: sucrose + $H^+$ → glucose + fructose (“invert sugar”).

Dietary Considerations

Carbohydrates should supply ≈ 60 % of caloric intake.
- Regional staples: corn (S. America), rice (Asia), root crops (Africa), wheat/potato (N. America).
Nutritionists classify
- Simple carbs: mono- & disaccharides (“sugars”); ~20 % of US energy.
- Complex carbs: polysaccharides (starch, fibre); not sweet.
Glycemic effect
- Measures rate & magnitude of blood glucose rise and return to baseline.
- Glycemic Index (GI) established for comparing foods; rapid-digestion foods have higher GI.

Glycolipids & Glycoproteins (Cell Recognition)

Carbohydrates covalently attached to lipids/proteins function as “molecular signatures”.
Determine ABO blood groups (galactose differentiation), immune recognition, hormone action.

Key Numerical / Statistical References

Blood glucose normal range: 70\text{–}100\,\text{mg·dL}^{-1}.
Cotton fibre: ~100 % cellulose; sugar cane juice ≈ 20 % sucrose; sugar beet ≈ 17 %.
Amylopectin branching frequency: every 24–30 glucose; glycogen: every 8–12.

Ethical / Practical Implications & Applications

Lactose intolerance: enzyme deficiency → gastrointestinal distress; drives development of lactose-free products.
Use of chitin-based biodegradable sutures reduces need for suture removal.
Heparin essential medication but contamination scandals underline need for rigorous biochemical quality control.
GI concept guides dietary planning for diabetes & metabolic syndrome.

Summary Flow (Concept Map)

Basic chemistry → structure → stereochemistry → reactivity.
Simple sugars cyclise → anomers → mutarotation → glycosidic bonding → complex carbohydrates.
Structure (linkage type & branching) dictates digestibility, biological storage vs rigidity.
Chemical tests (Molisch, Benedict) leverage functional groups; enzymology exploits specific linkages.
Physiological & nutritional roles connect molecular properties to health outcomes.