Biomolecules: Cellulose, Chitin, and Starch

Cellulose

• Primary structural component of plant cell walls.
• Accounts for over half of the carbon in the biosphere.
• Linear polymer of up to 15,000 D-glucose residues linked by β(1→4) glycosidic bonds.
• Fibers consist of ~40 parallel, extended glycan chains.
• Tightly packed, fully extended conformation due to β(1→4) linkages and interchain hydrogen bonds.
• Exceptional strength and water insolubility despite hydrophilicity.
• Linked with other polysaccharides and lignin, hindering biofuel conversion.
• Hydrolyzed by cellulases in herbivores and termites.

Chitin

• Structural component of invertebrate exoskeletons and fungal cell walls.
• Second most abundant biomolecule after cellulose.
• Homopolymer of β(1→4) linked N-acetyl-d-glucosamine residues.
• Differs from cellulose by an acetamido group replacing each C2-OH group.
• Similar structure to cellulose based on X-ray analysis.

Starch

• Plant's principal energy reserve.
• Mixture of α-amylose and amylopectin.
• Deposited in chloroplasts as insoluble granules.

α-Amylose

• Linear polymer of several thousand glucose residues linked by α(1→4) bonds.
• Irregularly aggregating helical coiled (left-handed helix) conformation due to α-glycosidic bonds.

Amylopectin

• Primarily α(1→4) linked glucose residues with α(1→6) branch points every 24-30 residues.
• Contains up to 10⁶ glucose residues.
• Large molecules.

Starch Digestion

• Reducing sugar with one reducing end.
• Digestion begins in the mouth with salivary amylase.
• Pancreatic amylase degrades starch to oligosaccharides in the small intestine.
• α-glucosidase and debranching enzyme further hydrolyze to monosaccharides for absorption.

Glycogen

• Animal storage polysaccharide.
• Most prevalent in skeletal muscle and liver as cytoplasmic granules.
• Structure resembles amylopectin but is more branched (every 8-14 glucose residues).
• Degraded by glycogen phosphorylase (cleaves α(1→4) bonds).
• Highly branched structure allows rapid glucose mobilization.
• α(1→6) branches cleaved by glycogen debranching enzyme.

Polysaccharides (Glycans)

• Monosaccharides linked by glycosidic bonds.
• Homopolysaccharides: one type of monosaccharide.
• Heteropolysaccharides: more than one type of monosaccharide.
• Form branched and linear polymers.
• Hydrolyzed by exo- and endoglycosidases.

Lactose

• Reducing disaccharide found naturally only in milk.
• Composed of galactose and glucose (O-β-D-galactopyranosyl-(1→4)-D-glucopyranose).
• Free anomeric carbon on glucose.

Sucrose

• Most abundant disaccharide.
• Major carbohydrate transport form in plants.
• Composed of glucose and fructose (O-α-D-glucopyranosyl-(1→2)-β-D-fructofuranoside).
• Non-reducing sugar due to participation of both anomeric carbons in glycosidic bond.

Artificial Sweeteners

• Added to foods and beverages for sweetness without calories.
• Examples: saccharin, aspartame, acesulfame.
• Varying levels of sweetness relative to sucrose.
• Potential health concerns and limitations (e.g., saccharin, aspartame).

This quiz covers the structural and functional aspects of cellulose, chitin, and starch. Explore the primary components of plant walls, invertebrate exoskeletons, and the energy reserves of plants. Test your knowledge on their compositions and significance in the biosphere.