B1.1.6 Structure of cellulose Notes

The Molecular Structure of Cellulose

1. Polysaccharide Framework
   1. Cellulose is a polymer of β-glucose monomers linked by β-1,4-glycosidic bonds.
   2. This alternating orientation (rotation of each subsequent glucose by 180°) yields straight, unbranched chains.
2. β-1,4-Glycosidic Bonds
   1. The hydroxyl (-OH) on carbon 1 of one glucose is above the ring plane, and on carbon 4 of the next glucose it is below.
   2. This arrangement forms long, linear cellulose chains rather than the coiled or branched structures seen in starch or glycogen.
3. Hydrogen Bonding
   1. Each cellulose chain exposes -OH groups at regular intervals.
   2. Adjacent chains form hydrogen bonds, creating microfibrils.
   3. Microfibrils bundle into larger fibers, lending mechanical strength and stability to the plant cell wall.

Function of Cellulose in Plant Cell Walls

1. High Tensile Strength
   1. Cellulose microfibrils confer resistance to stretching.
   2. This strength prevents cells from bursting when they absorb water (turgor pressure) and helps maintain rigidplant structures.
2. Structural Support for Growth
   1. The rigidity of cellulose permits plants to stand upright, develop stems, branches, and compete for light.
   2. Enables complex plant architectures (e.g., tall trees, sprawling vines).
3. Cross-Linking in Cell Walls
   1. Cellulose microfibrils are embedded in a matrix of hemicellulose and pectin.
   2. These cross-links add both strength and flexibility, helping cell walls adapt to environmental stresses.

Why Cellulose Isn’t an Energy Source

1. Enzymatic Resistance
   1. The β-1→4 glycosidic bonds in cellulose are difficult to hydrolyze without cellulase.
   2. Most organisms (including humans) lack this enzyme, so they cannot break down cellulose for energy.
2. Durability: This resistance ensures cellulose’s function as a structural molecule, rather than an energy reserve (like starch or glycogen).