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Biological membrane

Shaikh Shohidul Islam
Shaikh Shohidul Islam

The document discusses biological membranes. It describes how membranes form closed compartments that separate cells and organelles. Membranes are composed of lipids and proteins. They are selectively permeable and allow for material exchange through transporters, channels, and endocytosis/exocytosis. Membranes also localize enzymes and are involved in cellular signaling and energy transduction.

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Biological membrane By SHAIKH SHOHIDUL ISLAM Lecturer Dept. of Biochemistry and Molecular Biology Gono Bishwabidyalay
Plasma membrane Membranes are highly viscous, plastic structures. Plasma membranes form closed compartments around cellular protoplasm to separate one cell from another and thus permit cellular individuality. It separates and protects the cell from the external hostile environment. Besides being a protective barrier, plasma membrane provides a connecting system between the cell and its environment. The subcellular organelles such as nucleus, mitochondria, lysosomes are also surrounded by membranes.
It forms closed compartment around cellular protoplasm to define cell boundaries. It has selective permeability for various substances and ions provided by transporters & ion channel. It also exchanges material with the ECF by exocytosis & endocytosis and with adjacent cell through gap junction. It also plays a key role in cell-cell interactions & in trans-membrane signalling. It provides site of energy transduction; such as in photosynthesis & oxidative phosphorylation. It localizes enzymes & acts as integral element in excitation-contraction coupling. Functions of the plasma membrane:
Chemical composition Lipids and proteins together constitute the major components of membranes. Some of the proteins and lipids, however, may have oligosaccharides, covalently attached to them. The sugar containing sequences of these glycoproteins and glycolipids also play a role in determining the identity of cells. (a) Membrane Lipids: The major lipids in the cell membrane are phospholipids, cholesterol and glycolipid, Cholesterol is abundant in the plasma membrane of mammals. Plant membranes have little or cholesterol. Phytosterols are present in plant membranes. It is absent in cell membranes of mo of the prokaryotes. However, all the membranes have phospholipids.
membrane-spanning sections. A typical membrane-spanning segment consists of 20-25 hydrophobic amino acids arranged in an alpha helix, although not all transmembrane proteins fit this model. Some integral membrane proteins form a channel that allows ions or other small molecules to pass, as shown below. Peripheral membrane proteins are found on the outside and inside surfaces of membranes, attached either to integral proteins or to phospholipids. Unlike integral membrane proteins, peripheral membrane proteins do not stick into the hydrophobic core of the membrane, and they tend to be more loosely attached.
(c) Carbohydrates/ the glycocalyx The surface of the cell is covered by a carbohydrate coat known as the glycocalyx, which is formed by the oligosaccharides of glycolipids and transmembrane glycoproteins.
1)Part of the role of the glycocalyx is to protect the cell surface. 2)In addition, oligosaccharides of the glycocalyx serve as markers for a variety of cell-cell interactions. 3)The oligosaccharides exposed on the cell surface provide a set of markers that help identify the distinct cell types of multicellular organisms. these proteins are usually glycosylated, as are the peripheral membrane proteins bound to the external face of plasma membrane. Functions of glycocalyx:
Biological membrane

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Biological membrane

  • 1. Biological membrane By SHAIKH SHOHIDUL ISLAM Lecturer Dept. of Biochemistry and Molecular Biology Gono Bishwabidyalay
  • 2. Plasma membrane Membranes are highly viscous, plastic structures. Plasma membranes form closed compartments around cellular protoplasm to separate one cell from another and thus permit cellular individuality. It separates and protects the cell from the external hostile environment. Besides being a protective barrier, plasma membrane provides a connecting system between the cell and its environment. The subcellular organelles such as nucleus, mitochondria, lysosomes are also surrounded by membranes.
  • 3. It forms closed compartment around cellular protoplasm to define cell boundaries. It has selective permeability for various substances and ions provided by transporters & ion channel. It also exchanges material with the ECF by exocytosis & endocytosis and with adjacent cell through gap junction. It also plays a key role in cell-cell interactions & in trans-membrane signalling. It provides site of energy transduction; such as in photosynthesis & oxidative phosphorylation. It localizes enzymes & acts as integral element in excitation-contraction coupling. Functions of the plasma membrane:
  • 4. Chemical composition Lipids and proteins together constitute the major components of membranes. Some of the proteins and lipids, however, may have oligosaccharides, covalently attached to them. The sugar containing sequences of these glycoproteins and glycolipids also play a role in determining the identity of cells. (a) Membrane Lipids: The major lipids in the cell membrane are phospholipids, cholesterol and glycolipid, Cholesterol is abundant in the plasma membrane of mammals. Plant membranes have little or cholesterol. Phytosterols are present in plant membranes. It is absent in cell membranes of mo of the prokaryotes. However, all the membranes have phospholipids.
  • 5. membrane-spanning sections. A typical membrane-spanning segment consists of 20-25 hydrophobic amino acids arranged in an alpha helix, although not all transmembrane proteins fit this model. Some integral membrane proteins form a channel that allows ions or other small molecules to pass, as shown below. Peripheral membrane proteins are found on the outside and inside surfaces of membranes, attached either to integral proteins or to phospholipids. Unlike integral membrane proteins, peripheral membrane proteins do not stick into the hydrophobic core of the membrane, and they tend to be more loosely attached.
  • 6. (c) Carbohydrates/ the glycocalyx The surface of the cell is covered by a carbohydrate coat known as the glycocalyx, which is formed by the oligosaccharides of glycolipids and transmembrane glycoproteins.
  • 7. 1)Part of the role of the glycocalyx is to protect the cell surface. 2)In addition, oligosaccharides of the glycocalyx serve as markers for a variety of cell-cell interactions. 3)The oligosaccharides exposed on the cell surface provide a set of markers that help identify the distinct cell types of multicellular organisms. these proteins are usually glycosylated, as are the peripheral membrane proteins bound to the external face of plasma membrane. Functions of glycocalyx: