Polysaccharides — Cellulose and Plant Cell Walls

Cellulose is the single most abundant biological polymer on Earth. It forms the structural framework of plant cell walls and provides the mechanical strength that allows plants to grow upright and resist turgor pressure. This lesson covers OCR specification point 2.1.2 (b)(iv): the structure of cellulose and its relationship to its structural function.

1. Overview

Cellulose is a polysaccharide made of β-glucose monomers joined by 1,4-glycosidic bonds. Despite sharing the same monomer composition as starch and glycogen (all C₆H₁₂O₆), cellulose has an entirely different structure and function because of the β configuration.

Key Definition — Cellulose: A structural polysaccharide of plant cell walls, consisting of unbranched chains of β-1,4-linked glucose molecules held together by extensive hydrogen bonding.

2. Why β-Glucose Creates a Straight Chain

In α-glucose, the –OH on C1 points below the ring, on the same side as the –H on C4 of the next residue. This allows α-glucose chains to coil easily into helices.

In β-glucose, the –OH on C1 points above the ring. For two β-glucose residues to form a 1,4-glycosidic bond while keeping the ring oxygens in their natural positions, every alternate β-glucose must be flipped by 180°.

β-glucose chain (every alternate residue flipped 180°):

      β-glu   β-glu(flipped)   β-glu   β-glu(flipped)
        |         |              |         |
     1,4-bond  1,4-bond       1,4-bond  1,4-bond

The consequence is that the chain is straight and unbranched, with the –OH groups projecting alternately above and below the chain. This allows extensive hydrogen bonding between adjacent parallel chains.

3. From Chains to Microfibrils to Cell Walls

Cellulose molecules associate in a hierarchical structure:

graph TD
  A[β-glucose monomer] --> B[Cellulose chain<br/>up to 15,000 glucose]
  B --> C[Microfibril<br/>~60–70 parallel chains<br/>held by H-bonds]
  C --> D[Macrofibril<br/>bundles of microfibrils]
  D --> E[Plant cell wall<br/>macrofibrils embedded in matrix<br/>of hemicellulose and pectin]

Cellulose chains — each may contain 1,000–15,000 β-glucose units.
Microfibrils — around 60–70 parallel cellulose chains are cross-linked by hydrogen bonds between the –OH groups. These hydrogen bonds are individually weak but collectively extremely strong. Microfibrils are about 10 nm wide.
Macrofibrils — many microfibrils bundle together to form macrofibrils.
Cell wall — macrofibrils are embedded in a matrix of other polysaccharides (hemicellulose, pectin) and sometimes lignin (in secondary cell walls), creating a composite material analogous to reinforced concrete.

4. Structure-Function Relationships

Cellulose's structure gives it properties ideally suited to its role as the main structural material in plant cell walls:

4.1 High Tensile Strength

Each microfibril can withstand enormous tensile (pulling) forces because:

The 1,4-glycosidic bonds along the chain are strong covalent bonds.
Thousands of hydrogen bonds hold parallel chains together in the microfibril.
Individually weak hydrogen bonds combine additively to create enormous overall strength.

This tensile strength is what allows plant cells to resist the pressure generated by the vacuole pushing against the cell wall (turgor pressure). Turgor keeps non-woody plants upright — loss of turgor causes wilting.

4.2 Mechanical Support for the Plant

The arrangement of microfibrils in different directions in successive layers of the cell wall gives isotropic strength, allowing plants to resist forces from many directions. This is essential for:

Support of stems and leaves against gravity and wind.
Containment of root pressure.
Maintaining shape during growth.

4.3 Permeability

The spaces between microfibrils in the cell wall are large enough to allow water, dissolved ions, amino acids and sugars to pass through freely. Cellulose cell walls are therefore freely permeable — they do not control what enters or leaves the cell. That is the role of the underlying plasma membrane.

Lesson 5: Polysaccharides — Cellulose and Plant Cell Walls