Carbohydrates (Part III)

Monosaccharide:
- A carbohydrate consisting of a single sugar unit.
Disaccharide:
- A carbohydrate consisting of two covalently joined monosaccharide units.
Polysaccharide (Glycan):
- A linear or branched polymer of monosaccharide units linked by glycosidic bonds.
Carbohydrate:
- A sugar (monosaccharide), one of its dimers (disaccharide), or polymers (polysaccharide).

Natural carbohydrates are primarily found as polymers.
Types of Polysaccharides:
- Homopolysaccharides: Composed of one type of monosaccharide unit.
- Heteropolysaccharides: Contain more than one type of sugar.
- Linear and Branched forms.
Molecular Weight: Polysaccharides do not have a defined molecular weight, differing from proteins and DNA which use templates for synthesis.

Composition: may consist of 1, 2, or several monosaccharides in straight or branched chains of varying lengths.
Homopolysaccharides Examples: Starch, glycogen, collagen, chitin.
Heteropolysaccharides Examples: Agarose, peptidoglycans, glycosaminoglycans.

Structure: Mixture of two homopolysaccharides of D-glucose:
- Amylose: Unbranched polymer with (α1 → 4) linked residues. Molecular weight varies from few thousands to over 1 million.
- Amylopectin: Branched structure similar to glycogen; branch-points with (α1 → 6) linkers occur every 24–30 residues. Molecular weight can reach up to 200 million.
Hydrated: Starch is heavily hydrated due to a high number of exposed -OH groups.
Hydrolysis: Hydrolyzed by α-amylases.

Structure: Branched homopolysaccharide of glucose:
- Glucose monomers form chains linked by (α1 → 4).
- Branch-points occur every 8–12 residues via (α1 → 6) linkers, making it more compact than starch.
- Molecular weight reaches several millions.
Function: Main storage polysaccharide in animals;
- Found abundantly in the liver and skeletal muscle.
- Contains ‘n’+1 non-reducing ends and 1 reducing end.
- Glucose units are removed from the non-reducing ends by enzymes (glycosidases), hydrolyzing simultaneous branches, effectively converting the polymer to monosaccharides.

Description: Bacterial and yeast polysaccharides.
- Poly-D-glucose forms chains linked by (α1 → 6).
- Branch-points include (α1 → 3), and some contain (α1 → 2) and (α1 → 4) linkers.
- Rich in dental plaque on teeth and commercially sold as synthetic dextrans (chemically cross-linked, forming insoluble materials with varying porosities).

Structure: Unbranched homopolysaccharide of glucose:
- D-glucose forms chains linked by (β1 → 4) (10,000 – 15,000 residues).
- Hydrogen bonds formed between adjacent monomers and between chains.
- Results in a tough, water-insoluble structure; cellulose is the most abundant polysaccharide in nature, primarily found in cotton and wood.
Metabolism:
- Cellulose is difficult to act on due to fibrous structure and water-insolubility, common in cell walls of plants.
- Cellulases secreted by fungi, bacteria, and protozoa facilitate its breakdown, allowing them to utilize wood as a glucose source.
- Most animals cannot digest cellulose because they lack the enzyme to hydrolyze (β1 → 4) linkages.
- Ruminants and termites are exceptions, existing symbiotically with cellulase-producing microorganisms.
- Cellulases are promising for biomass fermentation into biofuels.

Wood Fungi:
- Utilize cellulases to hydrolyze the (β1 → 4) glycosidic bonds of cellulose, converting wood into metabolizable glucose.
Ruminants:
- Only vertebrates that can use cellulose as a nutrient source. In the rumen, they have bacteria and protists that secrete cellulase.
Example: Trichonympha.

Structure: Linear homopolysaccharide of N-acetylglucosamine (NAG):
- NAG monomers linked by (β1 → 4) form chains.
- Creates extended fibers akin to cellulose (hydroxyl group at C-2 replaced with an acetylated amino group).
- Features: tough, flexible, water-insoluble, cannot be digested by vertebrates.
- Found in the cell walls of mushrooms and exoskeletons of insects, spiders, crabs, and lobsters (making it the 2nd most abundant polysaccharide).

Agar: Complex mixture of heteropolysaccharides containing modified galactose units, serving as a cell wall component in certain seaweeds.
Agarose: One component of agar with minimal charged groups, forming gels used in laboratories for growing bacteria and separating DNA by electrophoresis.

Description: Linear polymers with repeating disaccharide units, unique to animals and bacteria, containing sulfate groups.
Role in Tissues: Extracellular matrix (ECM) in multicellular tissues holds cells together and facilitates nutrient and oxygen diffusion.
- Specialized ECM: Basement membranes comprising collagens, laminin, and heteropolysaccharides (glycosaminoglycans).

Energy Storage: Starch and glycogen serve as stored fuel in plants and animals, respectively.
Structural Polysaccharides: Includes cellulose, chitin, and dextran, providing strength and rigidity to cell walls and exoskeletons.
3D Folding: Homopolysaccharides can form helical structures (starch and glycogen) with interchain hydrogen bonding, while cellulose and chitin have long straight strands interacting with neighboring strands.
Strength of Cell Walls: Heteropolysaccharides like peptidoglycan and agar strengthen bacterial and algae cell walls.
Glycosaminoglycans: Provide extracellular support, often have high negative charge density, aiding in diffusion.

Monosaccharides: Chemistry of sugars, carbonyl groups, and chiral centers.
Forms of Monosaccharides: Open-chain and ring forms, including hemiacetals and hemiketals.
Structures and Nomenclature: Important for understanding monosaccharides.
Disaccharides: Structures and properties, including the glycosidic bond.
Biological Functions: Importance of linear and branched homo- and heteropolysaccharides in nature.

Which homopolysaccharide is an energy store in plants?
Which homopolysaccharide is an energy store in animals?
Which of these are polymers of α-glucose? (Starch, Glycogen, Cellulose, Chitin)
Name a polymer of β-glucose.
Name a homopolysaccharide which only has unbranched chains?
True or False: Agarose is a homopolysaccharide.
True or False: Glycosaminoglycans are polymers with repeating disaccharide units?