cell wall

Typical plant and animal cell. Plant and animal cells share many structures, such as ribosomes, mitochondria, and chromosomes, but they also have notable differences: plant cells have chloroplasts, a large vacuole, and a cellulose cell wall. Animal cells do not have a rigid cell wall but have an outside cell membrane only.

Tough outer surface of the cell in plants. It is constructed from a mesh of cellulose and is very strong and only very slightly elastic so that it protects the cell and holds it in shape. Most living cells are turgid (swollen with water; see turgor). Water is absorbed by osmosis causing the cell to expand and develop an internal hydrostatic pressure (wall pressure) that acts against the cellulose wall. The result of this turgor pressure is to give the cell, and therefore the plant, rigidity. Plants, or sections of plants, that are not woody are particularly reliant on this form of support.

The cellulose in cell walls plays a vital role in global nutrition. No vertebrate is able to produce cellulase, the enzyme necessary for the breakdown of cellulose into sugar. However, most mammalian herbivores rely on cellulose, using secretions from micro-organisms living in the gut to break it down. Humans cannot digest the cellulose of the cell walls; they possess neither the correct gut micro-organisms nor the necessary grinding teeth. However, cellulose still forms a necessary part of the human diet as fibre (roughage). Because it is not broken down it acts as a signal that the gut needs to contract to move the contents on.

The cell walls of bacteria and fungi are not made of cellulose, but are just as strong. Some antibiotics kill bacteria by weakening the cell wall. Penicillins work in this way.