Effects Of Solutions On Ion Tr Essay

, Research Paper

Effects Of Solutions On Ion Transport Across Membranes

Abstract

Concentration gradients exist between two different salt molarity solutions on opposite sides of a membrane. Movement of ions across these sides can be achieved by either active or passive diffusion. In our first three experiments we used these concentration gradients to determine the characteristics of selective permeability of an artificial membrane to potassium chloride (KCl), the potential difference across the skin of a frog, Rana pipens, and the effect of inhibitors in the same frog membrane epithelium.

We measured the potential difference between the two sides of the artificial membrane with a voltmeter. These measurements were then compared with the theoretical numbers, measured using the Nernst equation. The measured results were extremely close to the values measured by the Nernst equation, but even more accurate compared to the values from the Goldman equation. These results showed that as the difference in concentrations increased, the potential across the membrane increased steadily.

Using the membrane of frog s skin, the potential difference across the membrane was also measured as well as the current with a voltmeter and a short circuit current (S.C.C.). These measurements showed that as the sodium concentration was decreased, both the potential and the current decreased as well. In addition, another experiment was conducted in order to test the effects of inhibitors and hormones on the transport through the frog membrane. Results showed that when ouabain and cyanide were added to the epithelium, potential decreased steadily, but the addition of vassopressin increased potential due to the increase of sodium transport across the membrane. Because cyanide is an ATP inhibitor and ouabain is a physical barrier of the Na/K pump, this experiment concluded that active transport in the frog skin is indeed an active process and is not possible without ATP.

Introduction

There exists a separation of electric charge across plasma membranes, known as a membrane potential, which provides an electric force that influences the movement of ions across the membrane (Vander1998). These potentials are formed due to changes from either electrical gradients, in which ions are transferred from one side of the membrane to the other by their opposite charged particle, or chemical gradients in which the differences in concentrations of solutions influence the transfer of ions across the membrane. Sodium potassium pumps cause a flow of 2 sodium ions to every 3 potassium ions, making the cell negatively charged, so a membrane potential must be created.

In the first experiment, concentrations of KCl were diluted and set in one side of an artificial membrane chamber along with a pure KCl solution in the other side. The membranes restrict the movement of either cations or anions by the presence of fixed charged groups along the water channels across the membrane. A cation that is in solution will be repelled by a channel lined with positive charges, and oppositely for an anion. Cation exchangers have fixed negative charges that permit cations to asociate or pass through it and anion exchangers have fixed positive charges and permit the passing of negative charges (Unpublished 5). Potential differences were then tested proving that an electrical gradient does indeed exist on both sides of the membrane.

Our values were compared to the Nernst equation which is:

Vm=58mV * log[C1]/[C2]

where Vm is the potential difference, and C1 and C2 are the concentration levels on each side of the membrane.

The measured results were also compared to the Goldman equation which is:

Vm=((RT/F) * ln((Pk[k]o + Pcl [Cl]I) / (Pk[K]I + Pcl[Cl]o))

Frogs must constantly use their environment in order to keep an equilibrium in their bodies. This process occurs by way of the sodium pump, which is a form of active transport because the process requires the use of energy in order to transport sodium against its concentration gradient. In order to do this, the frog must continually extract sodium from its environment to balance these losses. This is done by the active uptake of sodium by the frog s skin. By changing the concentrations of sodium on the mucosal, or outside of the frog skin, it was clear to see that as the concentration decreased, the potential did as well, therefore creating a chemical gradient across the membrane.

Membrane potentials can also occur electrochemically. Potentials can be adjusted by adding certain inhibitors or hormones with certain functions to the frog skin, and altering the usual environment of the frog skin. Cyanide ions inhibit cytochrome oxidase in electron transport, and Ouabain is a cardiac glycoside that blocks the sodium pump. Because ATP is necessary for the function of the sodium pump and functioning of frog skin equilibrium, it would be clear to see that these inhibitors would lower the potential difference across the membrane. However, when an antidiuretic hormone, vassopressin (ADH) was placed on the serosal side of the frog epithelium, the potential difference fluctuated. Such results are due to the fact that ADH is a hormone that modulates the activity in the sodium pump, therefore maintaining the sodium concentration inside the membrane necessary for equilibrium.

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