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Discuss the Structure of Plasma Membrane

    In this discuss the structure of plasma membrane post we have briefly explained about Definition, History, and ultrastructure of plasma membrane. Read on to learn more about ultrastructure of plasma membrane!

    Every cell, prokaryotic or eukaryotic, is surrounded by a thin layer of outermost boundary called the plasma membrane or cell membrane or plasma lemma. The plasma membrane is a discrete structure and is remarkably complex in its molecular organization.

    Definition

    The plasma membrane is a discrete structure and is remarkably complex in its molecular organization. It maintains the difference of the internal environment of the cell from its external environment by controlling the entrance and exit of the molecules and ions.

    It checks the loss of metabolically useful substances and encourages the release of toxic metabolic by-products of the cell. Thus, it functions as semi-permeable or selectively permeable membrane. It is about 70-100Å in thickness. In plant cells plasma lemma is further covered by cellulosic cell wall. It is an important cell organelle composed of lipids and proteins. 

    It possesses devices for attachment to other cells for cell-to-cell communications, ion pumps for controlling internal milieu of the cell, receptors for hormones and mechanisms for the production of secondary messengers that activates the cell’s physiological response.

    History

    It had been shown by Karl W. Nageli (1817-1891) that the cell membrane is semipermeable and is responsible for the osmotic and other related phenomena exhibited by living cells.

    Before 1855, he used the term zellen membrane in his early papers. The term plasma membrane was used in 1855 by him to describe the membrane as a firm protective film that is formed by out flowing cytoplasm of an injured cell when protein rich cell sap came in contact with water.

    Ultrastructure of Plasma Membrane

    1. Symmetrical Molecular Structure

    Plasma membrane

    Ultrastructure of plasma membrane: Symmetrical pattern of molecules in plasma membrane

    Plasma membrane is a tripartite structure and is made up of three layers, having total thickness of 75Å. Two di-electronic layers are there, each of 25Å thickness, enclosing a middle dielectronic layer which is also 25Å thick.

    The middle layer is a tri-molecular layer of lipids having its non-polar hydrophobic groups facing inwards, whereas polar hydrophilic groups facing outwards. The hydrophilic polar groups are covered by a protein layer which is 20 to 25Å thick. The protein chains lie at right angles to the lipids.

    2. Asymmetrical Molecular Structure

    It is also a tripartite structure having a thick inner dielectronic component of 35-40 Å, a narrow outer dielectronic component of 25Å thickness, and a central dielectronic layer (bimolecular layer of lipids) which is 30Å wide; thus total thickness comes to 90-95Å.

    Plasma membrane

    Ultrastructure of plasma membrane: Asymmetrical pattern of plasma membrane

    In different types of cells the thickness of plasma membrane varies. For example, in red blood corpuscles of rabbit, the plasma membrane is about 215 Å thick whereas, in intestinal epithelial cells it is 105 Å in thickness. Very small pores measuring about 10Å in diameter (smaller than pores of nuclear membrane) have been discovered in the membranes.

    Ultrastructure of Plasma Membrane

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