Medical brain notes 25

  Cell Membrane   Introduction   The outer living boundary of the cell is called as the cell membrane or ‘Plasma membrane’. The term cell membrane was coined by C.J. Nageli and C. Crammer in 1855. Apart from the cell membrane, each and every organelle in the cell is also covered by membranes. The cell membrane not only limits the cell cytosol, but it has a variety of functions like membrane transport, signal transduction and neuro transmission.

  Chemical Composition   To study the chemical composition of the cell membrane, the preferred source is RBC, because they lack cell organelles and thus no contamination of other cellular organelle membranes. The membranes of the RBCs devoid of cytosol are called as ‘ghosts’.   Four major constituents are present in the cell membrane. They are (i) lipids (28 – 79%) (ii) proteins (20 – 70%). (iii) oligosaccharides (only 1 – 5%) and (iv) water (20%).   Lipids   Depending upon the tissue from which the cell membrane is isolated, the composition also differs. Nearly 80% of the myelin sheath is made up of lipids, while in liver, it constitutes only 28%.   The main lipid components of the membranes are phospholipids, cholesterol and glycolipids. The major phospholipids present are phophatidyl choline (lecithin), phophatidyl ethanolamine, phophatidyl serine and phophatidyl inositol.     Membrane lipids are amphipathic in nature and they have a head porti...

  Models proposed for the plasma membrane   1. Monolayer Model   Overton was the pioneer to postulate that plasma membrane is a thin layer of lipid. He proposed this because he found that lipid soluble substances are easily transported across the membrane.   2.  Lipid Bilayer Model   The amount of lipids present in the erythrocyte membrane was nearly twice that of its total surface area. This made Gorten and Grendel to propose that lipids in the membrane exist as bilayers. 3. Unit Membrane Model   This model was proposed by Davson and Danielli and was shaped by Robertson. Experiments showed that the surface tension of the biological membranes are lower than that of the pure lipid bilayers, suggesting the presence of proteins in them. Based upon this, Davson and Danielli proposed that proteins are smeared over the lipid bilayer. When electron microscope was invented, plasma membrane appeared as three layers. With this observation, Robertson formulated a...

  Membrane Transport   One of the vital functions of the plasma membrane is membrane transport. Such a transport is important to carry out the life processes of the cell. Hydrophobic molecules and small polar molecules rapidly diffuse in the membrane. Uncharged large polar molecules and charged molecules do not diffuse and they need proteins to get transported. Depending upon the energy required and movement of the solute for or against the concentration gradient, the transport can be classified into two, active transport and passive transport.   1.  Passive transport   Passive transport is also called as passive diffusion. In passive transport, the substances move from higher concentrations to lower concentrations generally without the help of any protein. The transport continues until the concentration of the substance becomes same on both the sides of the membrane. O 2 , CO 2  and urea can easily diffuse across the membrane.   2.  Facilitated D...

  Viscosity If two tubes, one containing water and the other containing castor oil are tilted together, the latter will flow slowly, when compared to the former. This is because of the frictional force that exists between liquid layers. This resistance for the flow of the liquid is termed as Viscosity. Viscosity is defined as the internal resistance against the free flow of a liquid to the frictional forces between the fluid layers moving over each at different velocities. Each and every liquid has its own characteristic viscosity co-efficient. The co-efficient of viscosity of a liquid is defined as the force in dynes required to maintain the streamline flow of one fluid layer of unit area over another layer of equal area separated from one another by 1 cm at a rate of 1cm/sec. Viscosity is measured in poises or millipoises.   Factors affecting viscosity   1. Density   Density and viscosity are directly proportional to each other. They are related by Stoke’s law. If ...

  Surface tension   A molecule inside the liquid mass (a) is pulled uniformly on all the sides by intermolecular forces. But a surface molecule (b) suffers a much greater intermolecular attraction towards the interior of the liquid than towards the vapour phase, because fewer molecules are present in the vapour phase. The excess of inward force on the surface layer accounts for the surface tension. Surface tension (¡) is defined as the force acting perpendicularly inwards on the surface layer of a liquid to pull its surface molecules towards the interior of the liquid mass.   1. Factors affecting surface tension   ·                Density - Macloed’s equation relates surface tension to the density of the liquid (r) and that of its vapour (r’).   ∝ ( r - r ')2.   ·                Temperature- Temperature and surface tens...

  Osmosis If a protein solution is separated by a semipermeable membrane from pure water, water tends to flow from the latter to the former. The property of the movement of solvent particles is called as osmosis. Osmosis is the net diffusion of water from the dilute solution to the concentrated solution. Osmosis is a colligative property of solution that depends on the number of molecules or ions of the solute in the solutions. Osmol units give the number of osmotically active particles per mole of a solute. Each mole of a non-ionized solute is equivalent to 1 osmol. Osmolarity of a solution is its solute concentration in osmols / litre. Osmolality of a solution is its solute concentrations in osmols/kg of the solvent. Two solutions with identical osmotic pressures are called as isoosmotic solutions. A solution having lower or higher osmotic pressure with respect to the other is called as hypo-osmotic or hyperosmotic solutions respectively. The plasma membrane is a semipermeable me...