Membrane Permeability

Topics: Diffusion, Osmosis, Molecular diffusion Pages: 7 (2133 words) Published: March 20, 2013
The Neurophysiology of Nerve Impulses and Effects of Inhibitory Chemicals on their Action Potentials

Aferdita Sabani
Biol 2401.C5L
Dr. Endley
March 20, 2013

Cell structure and function can be defined in many aspects but one the most important characteristic is that it is enclosed within a cell membrane called a plasma membrane. The plasma membrane is by-layer composed of lipids and embedded proteins. This membrane is semi-permeable due to its hydrophobic and hydrophilic regions. At the boundary of every cell the plasma membrane functions as a selective barrier that allows nutrients to be brought in and/or removed from inside the cell. The cells permeability and transport mechanisms allow for this occurrence and it is vital for a functional and healthy cell. Transport through the plasma membrane occurs in two basic ways: passive and active processes. The passive transport process is driven by the concentration or pressure differences between the interior and exterior environment of the cell. According to Kenyan college biology department, “Simple diffusion is when a small non-polar molecule passes through a lipid bilayer. It is classified as a means of passive transport. In simple diffusion, a hydrophobic molecule can move into the hydrophobic region of the membrane without getting rejected”. Particles diffuse passively through small pores within the plasma membrane and they also move from an environment of high concentration towards an environment with lower concentration. Osmosis is a type of diffusion when it comes to water transport. Both diffusion and osmoses move substances down their concentration gradient. Facilitated diffusion is also passive transport, but does not involve the simple movement through pores and lipid dissolving. In this case a carrier protein in the membrane is introduced to facilitate the transport of substances down their concentration gradient. Active transport is not passive because energy in the form of cellular ATP is required to drive the substances across the membrane, therefor the cell must spend some of its energy to get through or move against the concentration gradient. In one type of active transport the substance gets across the membrane by forming a substrate –enzyme complex where the substance is picked up by a carrier protein and are then able to move into cell. This combination is lipid and large so energy is needed to defy opposing forces. According to Pearson/biology, “Active transport uses energy to move a solute "uphill" against its gradient, whereas in facilitated diffusion, a solute moves down its concentration gradient and no energy input is required.” If an experiment was conducted where the conditions of transfer were manipulated by adding in larger membrane pores, increasing protein carriers, increasing pressure and adding higher levels of ATP for active transport the rates of transfer will increase providing an optimal level of reactions.

Experimental Methods and Materials
In conducting this experiment the materials needed were a computer the PhysioEX 8.0 C D and the Anatomy and Physiology Lab Manual because this was a computer simulated experiment. Activity One: Simple Diffusion

Two beakers were placed next to each other and joined by a membrane holder. Four membranes were used and each possessed a different molecular weight cut off (MWCO) consisting of 20, 50, 100, and 200 MWCO; and were tested using NaCl, Urea, Albumin, and Glucose solutions. First, the 20 MWCO membrane was placed in the membrane holder between the beakers and the first solute studied was NaCl. A 9mM concentrated solution was dispersed into the left beaker and the right beaker was filled with deionized water. This transfer was allowed 60 minutes. At the end of this time lapse the results were recorded (see result section of the report). The 20 MWCO membrane was removed and each beaker was flushed for the next run. A membrane with the 50 MWCO was...
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