Primary cell wall
plural: primary cell walls
[ˈpɹaɪməɹi sɛl wɔːl]
The layer of the plant cell wall that forms prior to the deposition of the second cell wall, and characterized by being thin, flexible and extensible layer of the cell wall composed of cellulose, pectin and hemicellulose
A plant cell is a membrane-bound structure characterized by the presence of cell wall, plastids (especially chloroplasts) and large vacuole (apart from the other cytoplasmic structures and organelles (e.g. nucleus, mitochondria, endoplasmic reticulum, and Golgi apparatus). Similar to an animal cell, the plant cell has a cell membrane that surrounds the protoplasm and separates it from the outside environment. However, the plant cell differs from an animal cell in having another layer on top of the cell membrane called the cell wall. The presence of cell wall, though, is not exclusive to plants. Other organisms that have cell walls are algae, fungi, and most prokaryotes. The plant cell wall is different from the cell wall of these organisms in terms of structure and composition.
In plant cells, the cell wall is a tough, rigid structure that may consist of the primary cell wall which is generally a thin, flexible and extensible layer composed of cellulose, pectin and hemicellulose, and a secondary cell wall which is a thick layer rich in lignin that strengthens and waterproofs the wall and is formed inside the primary cell wall that has stopped increasing in surface area when the cell is fully grown. In between the primary walls is a middle lamella which is a pectin-rich intercellular material that glues the adjacent cells together. The cell wall is very essential in plants as it helps resist osmotic pressure. In bacteria, the cell wall is composed of peptidoglycan, and is essential to the survival of many bacteria. Bacteria have been classified into Gram-positive and Gram-negative based on the structure of the cell wall. Gram-positive bacteria possess thick cell wall consisting of many layers of peptidoglycan and teichoic acids. Gram-negative bacteria have relatively thin cell wall consisting of few layers of peptidoglycan. In archaea, the cell wall is characteristically lacking the peptidoglycan (except for a group of methanogens) and is composed of glycoprotein S-layers, pseudopeptidoglycan, or polysaccharides. In fungi, the cell wall is composed of chitin and other polysaccharides whereas in algae, it is made up of glycoproteins and polysaccharides, and in certain algal species it may be composed of silicic acid.
Primary cell wall features
In plants, there are three parts of cell walls: (1) primary cell wall, (2) secondary cell wall, and (3) middle lamella. The primary cell wall is a thin, flexible and extensible layer of the cell wall composed of cellulose, pectin, and hemicellulose (especially, xyloglucan). The primary cell wall contains more pectin than the secondary cell wall. Secondary cell wall forms beneath the primary cell wall. The middle lamella is the cell wall layer above the primary cell wall. Thus, the primary cell wall is found in between the middle lamella and the secondary cell wall of the plant cell wall.
Primary cell wall expands by acid growth mechanism. Acid growth refers to the growth of a plant cell through plant cell wall elongation (or expansion) at low pH. In particular, plant hormone auxin is prompted by the low acidic condition to initiate the cell wall genes to produce expansins. Expansins are proteins that loosen the connections between cellulose microfibrils within the cell wall. This causes the cell volume to increase by turgor and osmosis, thus, increases the extensibility of the cell wall.
The cell wall protects the plant cell from mechanical stress. In plant cells, the cell wall provides strength, rigidity, and protection, especially from osmotic lysis. The primary cell wall is the part or layer of cell wall in which cell growth is permitted. By acid growth mechanism, the primary cell wall may also function by increasing the extensibility of the cell wall. The primary cell wall of the plant cuticle, i.e. the protective film covering on the epidermis of leaves and young shoots of plants, is usually impregnated with cutin and wax. Thus, the primary cell wall, in this regard, helps form the permeability barrier of the plant cuticle.
- Braidwood, L., Breuer, C., & Sugimoto, K. (January 2014). "My body is a cage: mechanisms and modulation of plant cell growth". The New Phytologist. 201 (2): 388–402.
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