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Plasma Membrane, The biological membrane, which is present in both eukaryotic and prokaryotic cell. It is also, called as cell membrane as it is works as a barrier between the inner and outer surface of a cell., In animal cells, the plasma membrane is present in the outer most layer of the cell and in plant, cell it is present just beneath the cell wall., , Plasma Membrane Definition, Plasma membrane can be defined as a biological membrane or an outer membrane of a cell,, which is composed of two layers of phospholipids and embedded with proteins. It is a thin semi, permeable membrane layer, which surrounds the cytoplasm and other constituents of the cell., Structure of Plasma Membrane, , Plasma Membrane, Structure, 1. It is the boundary, which, separates the living cell, from their non-living, surroundings., 2. It is the phospholipids, bilayer., 3. Plasma membrane is an, amphipathic, which, contains both, hydrophilic heads and hydrophobic tails., 4. It is a fluid mosaic of lipids, proteins and carbohydrate., 5. It is lipid bilayer, which contains -two layers of phospholipids, phosphate head is polar, (water loving), fatty acid tails non-polar (water fearing) and the proteins embedded in, membrane., , Dr.C.V.Narasimha murthy. Reader in Zoology,JBDC zoology notes-2016

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Components of Plasma Membrane, The main components of plasma membrane include:, 1. Proteins like glycoprotein, which are used for cell recognition and act as receptors and, antigens., 2. Proteins like glycolipids are attached to phospholipids along with the sugar chains., 3. Lipids with short chain of carbohydrates are attached on the extracellular side of the, membrane., 4. Phospholipid Bilayer - which are made up of phosphates and lipids? They create a, partially permeable membrane, which allows only certain substances to diffuse through, the membrane., 5. Cholesterol – it maintains the fluidity of cell surface membrane., , THE FLUID MOSAIC MODEL, The fluid mosaic model was first proposed by S. J. Singer and G. L. Nicolson in 1972 to describe, the structure of cell membranes (Singer and Nicolson 1972). In this now-accepted theory about, cell structure, Phospholipid molecules, each with one hydrophobic, and one hydrophilic end,, make up most of the membrane. The hydrophilic heads form the inner and outer surfaces the, membrane and the hydrophobic tails, which are repelled by the water within and outside the cell,, are sandwiched in between (see figure). This is known as the lipid bilayer., , Dr.C.V.Narasimha murthy. Reader in Zoology,JBDC zoology notes-2016

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This arrangement is fluid not solid, because the various functional macromolecules embedded in, the Phospholipid matrix can move about the surface of the cell. Because of this fluidity such, membranes are often called plasma membranes (one meaning of the word plasma is a complex, fluid)., The model is called mosaic because it proposes that the membrane is made up of many different, parts, including proteins, carbohydrates, and lipids (see figure). This composite structure allows, the membrane to perform multiple functions. For example, certain embedded proteins may act as, channels allowing particular molecules to pass through the membrane. Others may serve as, labels allowing recognition of the cell. Still others may act as sensors that detect various features, of the ambient environment of, the cell, , Proteins in Plasma, Membrane, In plasma membrane, a protein, helps in providing the support, and shape to the cell. There are, three types of proteins in plasma, membrane, which includes:, 1. Cell membrane receptor, proteins- It helps in, communication of a cell, with their external, environment with the, help of hormones,, neurotransmitters and, other signaling, molecules., 2. Transport proteins - It, helps in transporting molecules across cell membranes through facilitated diffusion. For, example: globular proteins., 3. Glycoprotein - It helps in cell to cell communications and molecule transport across the, membrane., , Function of Plasma Membrane, 1. It separates the contents of the cell from its outside environment and it regulates what, enters and exits the cell., 2. Plasma membrane plays a vital role in protecting the integrity of the interior of the cell by, allowing only selected substances into the cell and keeping other substances out., Dr.C.V.Narasimha murthy. Reader in Zoology,JBDC zoology notes-2016

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3. It also serves as a base of attachment for the cytoskeleton in some organisms and the cell, wall in others. Thus the cell membrane supports the cell and helps in maintaining, the shape of the cell., 4. The cell membrane is primarily composed of proteins and lipids. While lipids help to, give membranes their flexibility and proteins monitor and maintain the cell's chemical, climate and assist in the transfer of molecules across the membrane., 5. The lipid bilayer is semi-permeable, which allows only selected molecules to diffuse, across the membrane., , Characteristics of Plasma Membrane, Below you could see characteristics of plasma membrane, 1. The plasma membrane (cell membrane) is made of two layers of phospholipids., 2. The plasma membrane has many proteins embedded in it., 3. The plasma membrane regulates the entry and exit of the cell. Many molecules cross the, cell membrane by diffusion and osmosis., 4. The fundamental structure of the membrane is Phospholipid bilayer and it forms a stable, barrier between two aqueous compartments., 5. The proteins present in the plasma membrane, act as pumps, channels, receptors,, enzymes or structural components ., , TRANSPORT MECHANISMS IN CELLMEMBRANE, SIMPLE DIFFUSION, In simple diffusion, small noncharged molecules or lipid soluble molecules pass, between the phospholipids to enter or leave the cell, moving from areas of high, concentration to areas of low concentration (they move down their concentration, gradient). Oxygen and carbon dioxide and most lipids enter and leave cells by simple, diffusion., OSMOSIS, Osmosis is a type of simple diffusion in which water molecules diffuse through a selectively, permeable membrane from areas of high water concentration to areas of lower water, concentration. (Note that the more particles dissolved in a solution, the less water there is in it, so, osmosis is sometimes described as the diffusion of water from areas of low solute concentration, to areas of high solute concentration)., , Dr.C.V.Narasimha murthy. Reader in Zoology,JBDC zoology notes-2016

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Hypertonic and hypotonic. A hypertonic solution has more solutes and less, water than a hypotonic solution. During osmosis, water moves from the, hypotonic solution (more water, less solutes) to the hypertonic solution (less, water, more solutes)., FACILITATED DIFFUSION, In facilitated diffusion, substances move into or out of cells down their, concentration gradient through protein channels in the cell membrane. Simple, diffusion and facilitated diffusion are similar in that both, involve movement down the concentration gradient. The, difference is how the substance gets through the cell, membrane. In simple diffusion, the substance passes, between the phospholipids; in facilitated diffusion there, are a specialized membrane channels. Charged or polar, molecules that cannot fit between the phospholipids, generally enter and leave cells through facilitated, diffusion., ACTIVE TRANSPORT, Primary active transport., It is also possible to move substances across membranes against their concentration gradient, (from areas of low concentration to areas of high concentration). Since this is an energetically, unfavorable reaction, energy is needed for this movement. The source of energy is the, breakdown of ATP. If the energy of ATP is directly used to pump molecules against their, concentration gradient, the transport is called primary active transport., , Secondary active transport., In some cases, the use of ATP may be indirect. For example, if a cell uses ATP to pump out Na+, and then uses the Na+ concentration gradient to bring in glucose, the transport of glucose would, be an example of secondary active transport., , Dr.C.V.Narasimha murthy. Reader in Zoology,JBDC zoology notes-2016

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Symport:, Symport is where two molecules or ions are transported in the same direction across a, membrane. If a molecule is accompanied by another molecule so that both molecules at the same, time together are transported into or out of the cell through a protein channel, then the condition, is called symport., Antiport, Antiport is where two molecules or ions are transported in opposite directions across a, membrane., Uniport, If only a molecule is transported across the membrane into or out of the cell through a protein, channel, then it is called uniport membrane transport., ENDOCYTOSIS AND EXOCYTOSIS: MOVEMENT OF LARGE PARTICLES, It is possible for large molecules to enter a cell by a process called endocytosis, where a small, piece of the cell membrane wraps around the particle and is brought into the cell. If the particle, is solid, endocytosis is also called phagocytosis. If fluid droplets are taken in, the process is, called pinocytosis. The opposite of endocytosis is exocytosis. Cells use exocytosis to secrete, molecules too large to pass through the cell membrane by any other mechanism., , Dr.C.V.Narasimha murthy. Reader in Zoology,JBDC zoology notes-2016