Membrane Permeability

Introduction

Cell structure and function can be defined in many aspects but one of the most important characteristics is that it is enclosed within a cell membrane called a plasma membrane. The plasma membrane is a 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 cell 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 the 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 the energy in the form of cellular ATP is required to drive the substances across the membrane, therefore the cell must spend some f 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 the cell. This combination is a 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.

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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 for 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 placed between the beakers and the steps performed above were repeated using the 9 mM NaCl solution for 60 min. and then repeated again for the 100 and 200 MWCO, as described by the A & P Lab Manual by Marieb and Mitchell. The next solutions tested were Albumin, Urea, and Glucose. All were placed into the left beaker independently and the tests were run exactly like that for NaCl.

Activity Two: Facilitated Diffusion In this experiment the set-up of the two beakers and membrane holders was used again. Only NaCl and Glucose solutes were used and membranes with 500, 700, and 900 glucose carrier proteins The 500 membrane was placed between the beakers, and the glucose solution with a concentration of 2. 00mM was delivered to the left beaker. The right beaker was filled with deionized water. The timer was set for 60 minutes. When the time was up the data was recorded and the beakers were flushed to set up for the next run. The same steps were repeated using the 2. 00 mM glucose solution with the 700 and 900 carrier protein membranes, separately for 60 minutes. The last run of this transport mechanism was done by increasing the 2. 00mM to 8. 00mM glucose concentration. This experiment was done the same way as above for each of the 500, 700, and 900 carrier protein membranes for 60 min. respectively. Activity 3: Osmotic Pressure In this experiment pressure readers were added in order to measure osmotic pressure change and were placed on top of the two beakers. A 20 MWCO membrane was placed between the beakers and a NaCl concentration of 8mM was put into the left beaker. Deionized water was placed in the right beaker. Time was set at 60 minutes. The pressure steps were repeated with the 50, 100, and 200 MWCO membranes Activity 4: Active Transport This experiment resembled the osmosis experiment except that an ATP dispenser was substituted for the pressure meters on top of the beakers. In this experiment, it was assumed that the left beaker was the inside of the cell and the right beaker was the extracellular space.

The membrane used had 500 glucose carrier proteins and 500 sodium-potassium pumps. The membrane was placed between the beakers and a NaCl concentration of 9. 00mM was delivered into the left beaker and a KCl concentration of 6mM was dispensed into the right beaker. The ATP was the changing variable in this experiment. 1mM of ATP was dispensed and the transfer was observed for 60 min. It was observed when no ATPmM was applied and finally when 3mM ATP was applied. Upon observing the results for all of the solutes with the 20 MWCO membrane between the left beaker and the artificial external environment of deionized water in the right beaker no diffusion occurred, because the pores were not large enough for them to pass through. An observation that is important to note is that even the small ions of NaCl did not diffuse here, so it is obvious that the other molecules would also not diffuse. At 50 MWCO the pores were just large enough for the dissociated NaCl ions to get through but the threshold stopped there because Urea, Albumin, and Glucose molecules in the solute were too large. Observations of the diffusion of the solutes with the 100 MWCO membrane showed that all but albumin and Glucose passed, so urea size was now compatible with the size of this pore. Finally, when the 200 MWCO membrane was introduced everything except Glucose got through because it is a very large molecule that cannot diffuse simply. It must be facilitated.

Activity 2: Facilitated Diffusion In the facilitated diffusion of Glucose the parameters that were introduced were the number of carrier proteins available for transport in the membrane. According to the results, when there was a 2. 00mM concentration of Glucose in the left beaker there was evidence of diffusion based on the measured rate of diffusion in mM/min. As the number of carrier proteins increased by 200 between 500 and 900 the rate between 0.0008 to 0. 0012mM/ min also increased by 0. 0002 min into the beaker. When 8. 00mM of Glucose was placed in the left beaker with the same carrier protein-membrane criteria of 500, 700, and 800 the rate increased. The rate was actually faster than that of the 2. 00 mM concentration. As the concentration of glucose raised the demand for the protein attachment increased so more carrier proteins got involved, while previously some were just hanging out because there was less glucose to transfer.

Activity 3: Osmosis In this experiment the study was based on the transfer of water across a membrane. Osmosis of water tends to balance out concentrations, so it will flow to an area of higher solute concentration. Water flowing to a more concentrated solution will usually increase in volume but in this closed system for the experiment, the focus was on the increase of pressure. The solutes were confined to their area by a semi-permeable membrane based on the pores of the membrane and the size of the molecules in the solute. With 8mM of NaCl with a 20 MWCO membrane the pressure, reading was 272 mmHg because the salt was not able to pass through the membrane, but the water diffused to the salt side so there was pressure causing an unequal balance, but with the membranes of 50, 100 and 200 MWCO there was no pressure because the membrane became permeable to the salt allowing an equilibrium between he beakers, therefore no pressure. In the case of Albumin, the water diffused building up pressure until there was no more water left to diffuse so pressure remained constant at all MWCOs. The same occurred with Glucose until the membrane was replaced with the 200 MWCO membrane. Glucose was able to diffuse thus resulting in equilibrium in both beakers. Pressure will rise until equilibrium is obtained.

Activity 4: Active Transport

The experiment showed that at 1 ATP the reaction took place at a very slow rate and not completely. Without ATP the transfer didn’t take place at all. When 3 ATPs were added transfer took place quickly and almost completely. The more ATP introduced to the cell, the faster and more complete the transport will occur which is very important for the transport of glucose since it is a substrate for the production of more ATP.

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