Cell Transport Mechanisms and Permeability

Topics: Diffusion, Cell, Osmosis Pages: 14 (5029 words) Published: February 8, 2013
Zack Johnson
Human Anatomy & Physiology Lab 2101
Instructor: Alan Byboth, M.S.

The cell is invaluable building block of all biological life on this planet, and one of its most important and unique characteristics is its ability to be selectively permeable with its plasma membrane. This outer membrane’s sophisticated mechanisms of transport through its bilayer are vital in maintaining homeostasis in the cell and the entire body. To further understand these mechanisms, which can be further described as passive and active transport, five experiments were conducted. These tests were done over simple diffusion, facilitated diffusion, osmosis, filtration, and active transport by changing and observing different variables and how they affect transport through the membrane. Obtained was the understanding of the changing of the molecular weight cut off in a membrane, and how it does not play a part directly in changing the rate of diffusion, but instead determined if diffusion was accomplishable at all. Knowledge on the size of molecules in relation to how they behave through these mechanisms was also learned. The smaller solutes tend to be consistently more successful in all forms of transport. To understand the cell transport is to, to an extent, understand the cell and indirectly the human race. Introduction-

All living cells possess the ability to be selectively permeable. In other words they are able to control what substances and molecules enter and leave the cell. This monitoring, however, is in many ways much more complex than a simple wall and gate concept, but in some forms it does share likeness to it. All of this inward and outward traffic is controlled by the Plasma Membrane. The Plasma Membrane is the outermost layer of the cell, and consists mostly of a phospholipid bilayer, but also houses some proteins and other substances to aid in the crossing through the membrane. The movement, or transport, of substances and molecules through the plasma membrane can be done in many different fashions. In what form the molecules or ions make this cross from extracellular to intracellular and back, though, depends entirely on that molecule or ion itself, and how it will react with the cell membrane. There are two main types of transportation mechanisms, Passive, and Active. Guyton and Hall (1996) define Passive transport as the random molecular movement of substances molecule by molecule either through intermolecular spaces in the membrane or in combination with a carrier protein. Active transport, however, is a different in that it involves the movement of molecular substances with carrier proteins against the concentration gradient. Thus, needing more energy, energy provide by Adenosine Triphosphate, to complete its progression. The concentration gradient referred to in these prior statements simply pertains to the concept that when a highly concentrated or populated area is met with more space, the occupants of that once smaller area tend to spread out evenly within the new larger total space, thus lowering the concentration. Passive and Active transport are not the simplest form of movement through the selectively permeable bilayer. They consist of several smaller and more detailed processes. Passive transport can be stated in a whole by its subunits. These subunits include simple diffusion, facilitated diffusion, osmosis, and filtration. Firstly there is simple diffusion. Simple diffusion is exactly what the name portrays it to be. Molecules or ions that use simple diffusion to move in and out of the do not require any assistance other than the kinetic energy which empowers it to complete its cross membrane transport. This is achievable primarily due to the fact that the substances that use simple diffusion are so small that they simply fit through microscopic gaps in the membrane with ease. The rate of diffusion is determined by...

Cited: Guyton, A.C., Hall, J.E., 1996. Transport of Ions and Molecules through the Cell Membrane. Textbook of Medical Physiology. 43-54
Reece, J.B., Urry, L.A., Cain, M.L., Wasserman, S.A., Minorsky, P.V., Jackson, R.B., 2011. Membrane Structure and Function. Campbell Biology. 131-138, 960
Seifter, J., Ratner, A., Sloane, D., 2005. Membrane Transport. Concepts of Medical Physiology. 16-19
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