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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.
Middle lamella features
In plants, there are three parts of cell walls: (1) primary cell wall, (2) secondary cell wall, and (3) middle lamella. The middle lamella is the part of cell wall that is the outermost layer in between cells. It is rich in pectin that cements the primary cell walls of adjacent cells together. It renders stability, and form plasmodesmata between cells. It is also the first layer which is deposited at the time of cytokinesis, particularly from the cell plate that divides the two cells during cell division. The middle lamella contains calcium and magnesium pectates.
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 middle lamella is the part of the cell wall that binds the adjacent cells together. It is also involved in cell to cell communication by forming plasmodesmata between cells.
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