Thursday, 7 January 2016

Frequently Asked Cell Biology Interview Questions and Answers

61. What do facilitated diffusion and active transport have in common? What are the differences between them?
Facilitated diffusion can be confused with active transport because in both processes there is participation of membrane proteins.
In active transport however the transported substance moves against its concentration gradient and with energy spending. Facilitated diffusion is a passive transport in favor of the concentration gradient and it does not require energy.
Cell Membrane Review - Image Diversity: active transport

62. Which are the molecules that make possible active transport through membranes?
Active transport is made by specific membrane proteins. These proteins are called “pumps” because they “pump” the moving substance through the membrane using energy from ATP molecules.

63. How does the sodium-potassium pump present in the cell membrane work? What is the importance of this protein for the cell?
The sodium-potassium pump is the transport protein that maintains the concentration gradient of these ions between the intra and the extracellular spaces. This protein is phosphorylated in each pumping cycle and then it pumps three sodium ions outside the cell and puts two potassium ions inwards. The phosphorylation is made by the binding of a phosphate donated by one ATP molecule that then is converted into ADP (adenosine diphosphate).
The job of the sodium-potassium pump, also known as sodium-potassium ATPase, is fundamental to keep the characteristic negative electric charge in the intracellular side of the membrane of the resting cell and to create adequate conditions of sodium and potassium concentrations inside and outside the cell to maintain the cellular metabolism.
Cell Membrane Review - Image Diversity: sodium-potassium pump

64. What is mass transportation across the cell membrane?
Mass transportation is the entrance or the exiting of substances in or from the cell engulfed by portions of membrane. The fusion of internal substance-containing membranous vesicles with the cell membrane is called exocytosis. The entrance of substances in the cell after they have been engulfed by projections of the membrane is called endocytosis.

65. What are the two main types of endocytosis?
Endocytosis is the entrance of materia in the cell engulfed by portions of the cell membrane.
Endocytosis can be classified as pinocytosis or phagocytosis. In pinocytosis small particles on the external surface of the membrane stimulate the invagination of the membrane inwards and vesicles full of that particles then detach from the membrane and enter the cytoplasm. In phagocytosis bigger particles on the external surface of the membrane induce the projection of pseudopods outwards enclosing the particles; the vesicle then detachs from the membrane and enter the cytoplasm receiving the name phagosome.
Cell Membrane Review - Image Diversity: pynocitosis phagocytosis

66. How does the plant cell wall react when it is placed under hypotonic medium?
The plant cell wall (the covering of the cell external to the cell membrane) is made of cellulose, a polymer of glucose.
When the cell is put under hypotonic medium it absorbs too much water through osmosis. In that situation the cell wall pressure acts to compensate the osmotic pressure thus forbiding excessive increase of the cellular volume and the cell lysis.

67. What is meant by suction force of the plant cell? Does the suction force facilitate or make difficult the entrance of water in the cell?
The suction force (SF) is the osmotic pressure of the plant cell vacuole, i.e., of the vacuolar internal solution.
Since the vacuolar solution is hypertonic in comparison to cytosol it attracts water then increasing the cytosol concentration. With the osmotic action of the vacuole the cytosol becomes hypertonic in relation to the exterior and more water enters the cell.

68. What is the wall resistance of plant cells? Does this resistance facilitate or make it difficult the entrance of water in the cell?
Wall resistance, or turgor pressure (TP), is the pressure made by the distension of the plant cell wall in opposition to the increase of the cell volume. The wall resistance works against the entrance of water in the cell, i.e., it acts forcing the exiting of water and compensating the entrance of the solvent by osmosis.

69. What does the formula DPD = SF – TP mean?
DPD is the abbreviation of diffusion pressure deficit, SF (suction force) is the vacuolar osmotic pressure and TP is the turgor pressure.
The difference between SF and TP determines whether water tends or not to enter the cell. If SF > TP, DPD > 0 and water tends to enter the cell by osmosis. If TP > SF, DPD < 0 and water cannot enter the cell by osmosis.

70. What are the values of DPD for plant cells under hypertonic, isotonic and hypotonic media?
In plant cells under hypertonic medium there is loss of water for the exterior, SF > 0 (the vacuolar pressure is high because it is concentrated) and TP = 0 (there is no distension of the cell wall since the cellular volume is reduced) so DPD = SF. These cells are called plasmolysed cells, situation characterized by the retraction of the cell membrane that detach from the cell wall.
In plant cells under isotonic medium there is no increase of the internal water volume, SF > 0 and TP = 0 (since the cell wall is not distended). The cell membrane slightly touches the cell wall and in this situation the cell is called flaccid cell.
In plant cells under hypotonic medium there is tendency of water to enter, SF = TP (since the osmotic pressure is totally compensated by the distension of the cell wall) and DPD = 0. The cell that expanded itself to this point is called turgid cell.
Cell Membrane Review - Image Diversity: plasmolysed cell flaccid cell turgid cell

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