Thursday, 7 January 2016

Latest Cell Biology Interview Questions and Answers

41. 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 solvent, 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 still selectively permeable membranes, i.e., membranes that besides allowing the passage of solvent let only some specific solutes to pass blocking others.

42. What is diffusion?
Diffusion is the spreading of substance molecules from a region where the substance is more concentrated to other 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.

43. 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.

44. 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, in the other hand, is the movement of solutes from a region of higher solute concentration to a region of lower solute concentration.
One can consider osmosis as movement of water (solvent) and diffusion as movement of solutes, both concentration gradient-driven.

45. What is osmotic pressure?
Osmotic pressure is the pressure created in a 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. The intensity of the osmotic pressure (in units of pressure) is equal to the pressure that is necessary to apply in the solution to prevent its dilution by the entering of water by osmosis.
It is possible to apply in the solution another pressure in the contrary way to the osmotic pressure, like the hydrostatic pressure of the liquid or the atmospheric pressure. In plant cells, for example, the rigid cell wall makes opposite pressure against the tendency of water to enter when the cell is put under a hypotonic environment. Microscopically, the pressure contrary to the osmotic pressure does not forbid water to pass through a semipermeable membrane but it creates a compensatory flux of water in the opposite way.

46. Can solutions with 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 same osmotic pressure.
Even when the solution contains a mixture of different solutes its osmotic pressure depends only on its total particle concentration regardless the nature of the solutes.

47. How are solutions classified according to their comparative tonicity?
Comparatively to other a solution can be hypotonic (or hyposmotic), isotonic (or isosmotic) or hypertonic (or hyperosmotic).
When a solution is less concentrated than other the adjective hypotonic is given and the more concentrated is called hypertonic. When two compared solutions have same concentration both receives the adjective isotonic. So this classification makes sense only for comparison of solutions.

48. 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.
Cell Membrane Review - Image Diversity: cell membrane

49. 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 electric charge to this region of the molecule. Since phospholipids have one electric 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) faces inwards isolated from the water. Because this type of membrane is made of two phospolipid layers it is also called 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 glycolipidis. The membrane carbohydrates form the glycocalix of the membrane.
This description (with further explanations) is kown as the fluid mosaic model about the structure of the cell membrane.
Cell Membrane Review - Image Diversity: phospholipid bilayer membrane proteins glycocalyx

50. What are the respective functions of phospholipids, proteins and carbohydrates of the cell membrane?
Membrane phospholipids have structural function they form the bilipid membrane that constitutes the cell membrane itself.
Membrane proteins have several specialized functions. Some of them are channels for substances to pass through the membrane, others are receptors and signalers of information, others are enzymes, others are cell identifiers (cellular labels) and there are still those that participate in the adhesion complexes between cells or between the internal surface of the membrane and the cytosketeleton.
Membrane carbohydrates, associated to proteins or to lipids, are found in the outer surface of the cell membrane and they have in general labeling functions for recognition of the cell by other cells and substances (for example, they differentiate red blood cells in relation to the ABO blood group system), immune modulation functions, pathogen sensitization functions, etc.

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