cell membrane structure and function in details

The Structure and Function of the Plasma Membrane for undergraduate student

1. The Structure and Function of the Plasma Membrane Chapter 4 in text book Asma Saleh Alilesh Biotechnology department. Misurata university.Libya

2. Cell membrane - cells are separated from the external world by a thin, fragile structure called the plasma membrane. This biological structure that surround the cell and separate the inside of the cell from the outside environment.

3. Function of plasma membrane 1- Compartmentalization . Membranes are continuous, unbroken sheets and, as such, inevitably enclose compartments. 2- Scaffold for biochemical activities 3- Providing a selectively permeable barrier. Membranes prevent the unrestricted exchange of molecules from one side to the other. 4- Transporting solutes . The plasma membrane contains the machinery for physically transporting substances from one side of the membrane to another, often from a region where the solute is present at low concentration into a region where that solute is present at much higher concentration. 5- Responding to external signals . The plasma membrane plays a critical role in the response of a cell to external stimuli , a process known as signal transduction. Membranes possess receptors that combine with specific molecules (or ligands) having a complementary structure.

4. 6- Intercellular interaction . Situated at the outer edge of every living cell, the plasma membrane of multicellular organisms mediates the interactions between a cell and its neighbors. 7- Energy transduction : Membranes are intimately involved in the processes by which one type of energy is converted to another type . Membranes are also involved in the transfer of chemical energy from carbohydrates and fats toATP

5. A BRIEF HISTORY OF STUDIES ON PLASMA MEMBRANE STRUCTURE Membranes have been chemically analyzed and found to be made of proteins and lipids In 1935, Hugh Davson and James Danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins.

6. In 1972, S. J. Singer and G. Nicolson proposed that the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water .

7. THE CHEMICAL COMPOSITION OF MEMBRANES Membranes are lipid–protein components which are held together in a thin sheet by noncovalent bonds. And small amount of carbohydrates. The ratio of lipid to protein in a membrane varies, depending on the type of cellular membrane (plasma vs. endoplasmic reticulum vs. Golgi), the type of organism (bacterium vs. plant vs. animal), and the type of cell. -For examp le, the inner m itochondrial membrane has a very high ratio of protein/lipid in comparison to the red blood cell plasma membrane, which is high in comparison to the membranes of the myelin sheath that form a multilayered wrapping around a nerve cell

8. Membrane Lipids Membranes contain a wide diversity of lipids, all of which are amphipathic; that is, they contain both hydrophilic and hydrophobic regions. There are three main types of membrane lipids: --- - phosphoglyceride . Sphingolipids . and cholesterol.

9. Phosphoglycerides : Most membrane lipids contain a phosphate group, which makes them phospholipids. Because most membrane phospholipids are built on a glycerol backbone, they are called phosphoglycerides . membrane glycerides are diglycerides —only two of the hydroxyl groups of the glycerol are esterified to fatty acids; the third is esterified to a hydrophilic phosphate group. membrane phosphoglycerides have an additional group linked to the phosphate, most commonly either choline (forming phosphatidylcholine,PC ),ethanolamine( formingphosphatidylethanolamine ,PE) ,serine( formingphosphatidylserine , PS), or inositol (forming phosphatidylinositol, PI) - Each of these groups is small and hydrophilic and, together with the negatively charged phosphate to which it is attached, forms ahighly water-soluble domain at one end of the molecule,called the head group.

10. In contrast, the fatty acyl chains are hydrophobic, unbranched hydrocarbons approximately 16 to 22 carbons in length .A membrane fatty acid may be fully saturated (i.e., lack double bonds), monounsaturated (i.e., possess one double bond), or polyunsaturated (i.e., possess more than one double bond). Phosphoglycerides often contain one unsaturated and one saturated fatty acyl chain.

11. Sphingolipids A less abundant class of membrane lipids. are derivatives of sphingosine , an aminoalcohol that contains a long hydrocarbon chain . Sphingolipids consist of sphingosine linked to a fatty acid by its amino group. This molecule is aceramide . The various sphingosine -based lipids have additional groups esterified to the terminal alcohol of the sphingosine : If the substitution is phosphorylcholine , the molecule is sphingomyelin , which is the only phospholipid of the membrane that is not built with a glycerol backbone. If the substitution is a carbohydrate, the molecule is a glycolipid. If the carbohydrate is a simple sugar, the glycolipid is called a cerebroside ; if it is a oligosaccharide, the glycolipid is called a ganglioside

12. The nervous system is particularly rich in glycolipids. The myelin sheath contains a high content of aparticular glycolipid, called galactocerebroside , which is formed when a galactose is added to ceramide .

13. Cholesterol Present in certain animal cell may contain up to 50% of lipids of membrane. Cholesterol is absent from the plasma membranes of most plant and all bacterial cells.

14. Feature of lipid bilayer the flexibility of the lipid bilayer, membranes are deformable and their overall shape can change, as occurs during locomotion or cell division . The lipid bilayer is thought to facilitate the regulated fusion or budding of membranes. Another important feature of the lipid bilayer is its ability to self-assemble, which can be demonstrated more easily within a test tube than a living cell. If, for example, a small amount of phosphatidylcholine is dispersed in an aqueous solution, the phospholipid molecules assemble spontaneously to form the walls of fluid-filled spherical vesicles,called liposomes.

15. The lipid bilayer consists of two distinct leaflets that have a distinctly different lipid composition(Asymmetric).

16. Phospholipids in the plasma membrane can move within the bilayer Most of the lipids, and some proteins, drift laterally Rarely does a molecule flip-flop transversely across the membrane The Fluidity of Membranes:

17. As temperatures cool, membranes switch from a fluid state to a solid state The temperature at which a membrane solidifies depends on the types of lipids Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids Membranes must be fluid to work properly; they are usually about as fluid as salad oil The steroid cholesterol has different effects on membrane fluidity at different temperatures At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids At cool temperatures, it maintains fluidity by preventing tight packing

18. holesterol is also a key determinant of membrane fluidity: at high temperatures,  cholesterol acts to stabilize the cell membrane  and increase its melting point; while at low temperatures, it inserts into phospholipids and prevents them from interfering with each other to avoid aggregation

19. Membrane Carbohydrates Depending on the species and cell type, the carbohyrate content of the plasma membrane ranges between 2 and 10 percent by weight. More than 90 percent of the membrane’s carbohydrate is covalently linked to proteins to form glycoproteins; the remaining carbohydrate is covalently linked to lipids to form glycolipids. The carbohydrate of glycoproteins is present as short, branched hydrophilic oligosaccharides, typically having fewer than about 15 different sugars per chain. These carbohydrate project outside of the membrane and play an important role in mediating the interactions of a cell with its environment and sorting of membrane proteins to different cellular compartments.

20. THE STRUCTURE AND FUNCTIONS OF MEMBRANE PROTEINS Protein of membrane distributed asymmetrically. Depending on the cell type and the particular organelle within that cell, a membrane may contain hundreds of different proteins . Membrane proteins can be grouped into three distinct classes depending on their relationship to the lipid bilayer: 1-integral proteins 2- peripheral proteins 3- lipid- anchored proteins

21. A- Integral proteins: Penetrate the lipid bilayer ( transmembrane protein). Amphipathic Some have only 1 membrane spanning segment or multispanning . Acts as: Receptors or channels, enzyme. they not fixed , they able to move laterally within the membrane.

22. B- Peripheral proteins: Located entirely outside of the lipid bilayer, on either the cytoplasmic or extracelular side. Attached to integral membrane proteins, or associated with peripheral regions of the lipid bilayer. noncovalently bonded (electrostatic bonds) Acts as enzymes, factors that transmit signal and mechanical support for membrane.

23. C- Lipid anchored proteins : Located the lipid bilayer, on either the cytoplasmic or extracelular side, but are covalently bonded to lipids . Numerous proteins present on the external face of the plasma membrane are bound to the membrane by a small, complex oligosaccharide linked to a molecule of phosphatidylinositol that is embedded in the outer leaflet of the lipid bilayer which called GPI- anchored protein Acts as enzymes, receptors and cell adhesion proteins . - Another group of proteins present on the cytoplasmic side of the plasma membrane is anchored to the membrane by one or more long hydrocarbon chains embedded in the inner leaflet of the lipid bilayer. ( like Ras protein)