A cell is the basic unit of life. Cells are broadly divided into two categories viz. prokaryotic and eukaryotic cells. The latter being slightly advanced in comparison to the former owing to the presence of properly compartmentalized membrane-bound organelles. The survival of cells depends on the nutrition derived from the external environment. However, the nutrients have to be transported across the cell envelope. Apart from this, the envelope also provides structural integrity to the cell. In prokaryotes, it protects the cell from the internal turgor pressure caused due to a high concentration of macromolecules inside the cell. Let us study the composition of the cell envelope in the coming lines.
What is a Cell Envelope?
Cell envelope is a combination of the cell membrane, cell wall, and an outer membrane if it is present. Usually, this envelope is a characteristic of prokaryotes like bacteria. Hence, the composition would be cell membrane, bacterial cell wall and bacterial outer membrane (made of glycocalyx). Although the three layers have distinct functions, they act together as a protective unit for the bacterial cells. Based on the differences in the envelopes and the manner in which they respond to Gram’s staining procedure, bacterial cells are termed as Gram-positive if they take up the stain and as Gram-negative if they don’t. Thickness and composition of glycocalyx vary among bacterial species. In certain bacteria like pneumonia-causing Streptococcus pneumoniae, it is present as a loose sheath known as a slime layer. In others, it is thicker and sturdy known as capsule as found in Pseudomonas aeruginosin.
The bacterial cell wall not only maintains the cell shape and prevents it from bursting or collapsing but, as observed in the case of motile bacteria, it forms filamentous extensions called flagella. The bacterial flagellum has three parts – filament, hook, and basal body, with filament being the longest part extending from the cell surface to the outside. Besides flagella, pilus and fimbriae are the other surface structures in a bacteria that do not have motility functions. The cell membrane is a semi-permeable membrane that regulates the transport of substances in and out of the cell. A special structure known as mesosome is formed by an extension of plasma membrane into the cell wall. These extensions are usually in the form of vesicles, tubules, and lamellae. They help the cell for various functions like a synthesis of a cell wall, DNA replication, and distribution of daughter cells, respiration, secretions, etc.
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