Plant cell walls: Supramolecular assemblies

The primary walls of plant cells comprise a large part of our dietary fibre. Their properties depend on the polysaccharides comprising them but also on the manner in which they are interlinked to form the three-dimensional, functional structure of the intact cell wall. This structure is far from homogeneous, but includes local adaptations to withstand local stresses at the cell corners and the margins of intercellular spaces. There is also variation in cell-wall structure at the tissue, organ and species levels. Interlinking of cell-wall polymers by covalent, ionic and hydrogen bonding varies at all these levels and affects the mechanical properties of the cell walls. Cross-linking of pectins in particular, by a variety of covalent as well as non-covalent mechanisms, determines what kind of disruption of the cell wall is necessary to bring them into solution. Solubilisation of pectins has dietary significance in its own right but also leads to an increase in the pore size of the cell wall and hence reduces the extent to which it impedes the entry of enzymes such as α-amylase and the outflow of intracellular macromolecules during the digestion process. The nutritional value of cell walls as ‘dietary fibre’ may then depend on the extent to which the cell walls remain physically intact during the digestion process.