1. What is a membrane? Membrane is any delicate sheet that separates one region from another blocking or permitting (selectively or completely) the passage of substances. The skin, for example, can be considered a membrane that separates the exterior from the interior of the body; cellophane, used in chemical laboratories to separate solutions, acts as a membrane too.
2. Concerning their permeability how are membranes classified? Membranes can be classified as impermeable, permeable, semipermeable or selectively permeable. An impermeable membrane is that through which no substance can pass. Semipermeable membranes are those that let only solvents, like water, to pass through it. Permeable membranes are those that let solvent and solutes, like ions and molecules, to pass across it. There are also selectively permeablemembranes, i.e., membranes that besides allowing the passage of solvent, let only some specific solutes to pass while blocking others.
3. What is diffusion? Diffusion is the spreading of substance molecules from a region where the substance is more concentrated to another region where it is less concentrated. For example, during the boiling of water in a kitchen gaseous water particles tend to uniformly spread in the air by diffusion.
4. What is meant by concentration gradient? Is it correct to refer to “concentration gradient of water”? Concentration gradient is the difference of concentration of a substance between two regions. Concentration is a term used to designate the quantity of a solute divided by the total quantity of the solution. Since water in general is the solvent in this situation it is not correct to refer to “concentration of water” in a given solution.
5. What is the difference between osmosis and diffusion? Osmosis is the phenomenon of movement of solvent particles (in general, water) from a region of lower solute concentration to a region of higher solute concentration. Diffusion, on the other hand, is the movement of solutes from a region of higher solute concentration to a region of lower solute concentration.
6. What is osmotic pressure? Osmotic pressure is the pressure created in an aqueous solution by a region of lower solute concentration upon a region of higher solute concentration forcing the passage of water from that to this more concentrated region.
7.Can solutions with the same concentration of different solutes have different osmotic pressures? The osmotic pressure of a solution does not depend on the nature of the solute, it depends only on the quantity of molecules (particles) in relation to the total solution volume. Solutions with same concentration of particles even containing different solutes exert the same osmotic pressure.
8. How are solutions classified according to their comparative tonicity? Comparative to another, a solution can be hypotonic (or hyposmotic), isotonic (or isosmotic) or hypertonic (or hyperosmotic). When a solution is less concentrated than another the adjective hypotonic is given and the more concentrated is called hypertonic. When two compared solutions have the same concentration both receive the adjective isotonic. So this classification makes sense only for comparison of solutions.
9.Concerning permeability what type of membrane is the cell membrane?The cell membrane is a selectively permeable membrane, i.e., it allows the passage of water and some selected solutes.
10.What are the basic constituents of the cell membrane? The cell membrane is formed of lipids, proteins and carbohydrates. The membrane lipids are phospholipids, a special type of lipid to which one extremity a phosphate group is bound thus assigning electrical charge to this region of the molecule. Since phospholipids have one electrically charged extremity and a long neutral organic chain they can organize themselves in two layers of associated molecules: the hydrophilic portion (polar) of each layer faces outwards in contact with water (a polar molecule too) of the extracellular and the intracellular space and the hydrophobic chains (non polar) face inwards isolated from the water.
Because this type of membrane is made of two phospolipid layers it is also called a bilipid membrane. Membrane proteins are embedded and dispersed in the compact bilipid structure. Carbohydrates appear in the outer surface of the membrane associated to some of those proteins under the form of glycoproteins or bound to phospholipids forming glycolipids. The membrane carbohydrates form the glycocalix of the membrane.
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