The cell wall

• Plant cells are more regular in appearance than animal cells because each cell is bounded by a cell wall
• The cell wall is an important feature that gives plants their strength and support
• The cell wall is mostly made of insoluble cellulose
• The plant cell wall is usually freely permeable to everything dissolved in water. However suburin is added to the cell wall in cork tissues and lignin is added to the cell wall structure in wood. These compounds reduce the permeability of the cell wall so that water and dissolved substances cannot pass through it
• The plant cell wall is made of several layers. The middle lamella is the first layer and is made when a plant cell divides into two new cells. It is mostly made of pectin, a polysaccharide that acts like glue and holds the cell walls of adjacent plant cells together. Pectin has lots of negatively charged carboxyl (-COOH) groups and these combine with positive ions to make calcium pectate that bonds with the cellulose on the other side.
• The cellulose microfibrils ans the matrix build up on both sides of the middle lamella. To begin with, these walls are very flexible with the cellulose microfibres all arranged in a similar direction. They are called primary cell walls
• Secondary thickening of the cell wall may occur as the plant ages. A secondary cell wall builds up, with the cellulose microfibrils laid densely at different angles. This makes it much more rigid. Hemicelluloses help to harden it further.
• In some plants, particularly wooden plants, lignin is added to the cell walls to produce wood, making the structure even more rigid. Within the structure of a plant, there are many long cells with cell walls that have been lignified. These are called plant fibres, and people use them in many ways including clothing, building material, ropes and paper
• Cellulose is the main component of a plant cell walls. Cellulose is a complex carbohydrate made of long chains of glucose joined by glycosidic bonds. In cellulose, the glucoses are beta-glucoses, ans they are held together by 1, 4-glycosidic bonds
• In cellulose, one beta glucose has to be inverted so the bonding can take place. Hydrogen bonds can be made between the positively charged hydrogen atoms of the hydroxyl groups and the partially negatively charged oxygen groups. This is called cross-linking and holds chains together
• Many hydrogen bonds are made, giving cellulose considerable strength. Cellulose molecules do not coil or spiral - they remain as very strong, straight chains.
• Most animals do not contain enzymes to digest cellulose. It is cellulose in plant food that acts as fibre in the human diet,
• cellulose fibres are deposited in layers which are held together by a matrix of hemicelluloses, and other short-chain carbohydrate. The combination of cellulose microfibrils in the flexible matrix makes a composite material, combining the properties of both these materials in the plant cell wall. The cells are turgid, giving strength to plants and to support them in a vertical position
• In primary cell walls, and in the cell walls which do nit have lignin in them, materials are exchanges through plasmodesmata. They seem to be produced when cells divide. The interconnected cytoplasm is called the symplast.
• The cell walls are thinned in the region of the plasmodesmata. When secondary thickening takes place, hemicellulose and lignin are deposited in the cell wall making it thicker. This process doesn’t happen in areas around the plasmodesmata, producing pits. Pits allow water to move between xylem vessels