Cell Wall

The cell wall is the outermost, protective covering of a cell, and it was, historically, the very first cell structure ever seen - Robert Hooke was looking at cork cell walls, not living cells, when he coined the word 'cell' in 1665. A cell wall is present in bacteria, fungi and plants, but it is completely absent in animal cells, which is one of the clearest and most consistently used ways to tell a plant cell from an animal cell at a glance. Far from being a passive, dead casing, the wall is an actively growing structure, and its exact chemical make-up differs sharply between the groups that have one: bacterial walls are built of peptidoglycan; fungal walls of chitin and a fungal form of cellulose; algal walls of cellulose together with galactans and mannans; and plant walls of a more elaborate mixture including cellulose, hemicellulose, pectin, lignin, cutin, suberin and even silica. In a typical plant cell, the wall itself is not a single uniform layer but has three distinct regions - laid down in a defined order as the cell matures, from the outermost middle lamella through the primary wall to the innermost secondary wall (which is further divided into three sub-layers, S1, S2 and S3, each with cellulose microfibrils oriented differently to add mechanical strength). Neighbouring plant cells are not fully sealed off from one another either: channels called plasmodesmata run straight through the wall, letting the protoplasm of adjacent cells communicate, and at certain points along the wall - where the secondary layer is laid down unevenly but the primary wall and middle lamella remain continuous - thin regions called pits form (matched by a corresponding pit on the neighbouring cell), each with a pit chamber and a porous pit membrane that keeps that connection permeable. Functionally, the wall does a great deal of work for the cell: it gives th …