Passive and Active Cell Transport Mechanisms

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.


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 the amount of substance available, by the velocity of kinetic motion, and by the number of openings the cell membrane (Guyton-Hall 1996). To test the simple diffusion properties, different molecular weighted membranes will be combined with different solutes to see how, and if they diffuse properly. (Hypothesis)-If the molecular weight cutoff and the size of the molecule itself are directly related to the rate of diffusion, then the maximum degree of diffusion will be achieved when the smallest molecules diffuse through the membrane with the most pores.

The next form of Passive transport is facilitated diffusion. This is similar to simple diffusion except that when a substance crosses the membrane by facilitated diffusion, it requires aid from a carrier protein. The protein serves as a channel to guide the substances through the permeable membrane. There is one other vital difference in these two transports. The rate of simple diffusion is directly linked and increases as the concentration of solute increases.

Facilitated diffusion’s rate however, cannot grow over a certain point due to the fact that the proteins cannot work fast enough to grow exponentially. In the experiment a membrane consisting of glucose-carrier proteins is formed and glucose solution is induced. (Hypothesis)-If glucose carrier proteins must go through a slow structural change to achieve complete facilitated diffusion, then the increase in concentration will only be beneficial to a certain point because the rate of diffusion will never be capable of growing over the rate that it takes for protein to make that structural change, therefore creating a maximum reachable rate of diffusion.

Osmosis was among the types of passive transport tested. Osmosis differs from the other diffusions in that it involves the universal solvent and most abundant subtance on the planet and in our bodies, water. More specifically osmosis involves the movement of water from one area of concentration to another, but not necessarily greater to less like diffusion. Osmosis is also unique in that in the happenstance a membrane is dividing water and another substance that cannot cross the membrane, and they are combined the water can cause displacement due to the fact that it can flow freely to the other side but get caught in the substance. Osmosis can also be stopped by applying a certain amount of pressure to the non-permeable substance so that the water is forced out and cannot cross the membrane. The osmotic pressure will recorded when different substances meet with water and a membrane wall. (Hypothesis)-If the osmotic pressure is directly created by the ability of a non-permeable substance to withhold water, the substances with the ability to dislocate the most water molecules will be able to create the most pressure.

The last form of passive transport is Filtration. Filtration is very particular in that it is a crucial part of the excretory process, and that it derives its energy from hydrostatic pressure, or as stated by the authors of Campbell Biology(2011), blood pressure. Filtration is conducted with Epithelial membrane and works in a somewhat simple manner. The blood pressure pushes the blood through the membrane. Large molecules are unable to pass through the membrane, causing them to become residue on the membrane, while smaller solutes pass through with ease.

These smaller filtered solutes join to form what is called a filtrate (Reece,etal. 2011). To test the capacity of filtration, Sodium Chloride (NaCl), Urea (CO(NH2)2), Glucose (C6H12O6) , and powdered charcoal, will all being ran through a filter at different molecular weight cut off levels to determine their filtration rates and the amount of membrane residue they leave. (Hypothesis)-If the amount of filtration is determined by the level of hydrostatic pressure and the size of the molecules being filtered, then the maximum filtration rate will be recorded by the smallest molecule with the highest hydrostatic pressure.

Last there is active transport. The mechanism of active transport (as stated earlier) is different from all passive transports due to the fact that it requires the use of adenosine triphosphate (ATP) to complete its cross-membrane movement. It utilizes this particular energy for the reason that this mechanism simply requires more work. This difference in work is created as a result of active transport’s opposition of the concentration gradient. Active transport can be divided into two groups, primary, or secondary. Seifter, Ratner, and Sloane(2005) define primary active transport as transport mechanism that uses adenosine triphosphate (ATP) directly to carry specific ions against on electrochemical gradient. A very common and specific example of primary active transport in our cells is the sodium-potassium pump, which exchanges three sodium ions (Na+) for two potassium ions (K+) across the plasma membrane of animal cells (Reece et al., 2011).

Sodium-Potassium pumps are vital to the cells survival because they serve as a regulator of these two ions in the cell. Secondary active transport differs from primary in that it indirectly uses adenosine triphosphate to complete its transport. It does this by the means of a cotransporter. A cotransporter is an intergral protein that can move more than one solute across the membrane (Seifter et al., 2005). If the ions being transported through the membrane are moving in the opposite direction, energy from one going down its concentration gradient can create enough energy to power the other. Therefore, ATP is indirectly used. Several different scenarios and factors will be changed and modified to observe triphosphate to complete its heir effect on the sodium-potassium pump. (Hypothesis)-If the pump continuously functions properly by moving potassium into the cell and sodium out, then the will recomplete this process until all the potassium is in the intracellular membrane and the sodium in the extracellular membrane, and an equilibrium is reached.


Activity 1- Stimulating Dialysis

The methods of the experiment are provided by the Human Anatomy & Physiology Laboratory Manual (2011). Initiate the exercise by selecting 5B: Cell Transport Mechanisms and Permeability and Click GO. Then watch the Cell Transport video and observe an actual dialysis experiment performed. Then click Simple Diffusion. Now place the 20 MWCO membrane in the membrane slot between the beakers. Increase the NaCl concentration to 9.00mM in the left beaker, and then dispense the right one with deionized water. Set the timer for 60 minutes. Then click Start. Observe the concentration activity a level above 0 in the NaCl level will mean diffusion is occurring. Record the diffusion data, if diffusion occurs. Repeat this experiment with all our membranes, and then repeat that experiment as a whole three times using urea, albumin, and glucose in place for NaCl. End by clicking Tools and selecting Print Data.

Activity 2- Simulating Facilitated Diffusion

Click on the Experiment menu and choose Facilitated Diffusion. Begin by setting the number of glucose carriers in the membrane to 500. Place the membrane in between the beakers. Next dispense 2.00mM of glucose concentration into the left beaker, and deionized water into the right. Set the timer for 60 minutes, then press start. Record the Glucose transport rate. Then Run the procedure again using 700 and 900 glucose carriers, and record those transport rates as well. This experiment in its entirety should then be done again with 8mM of glucose in place of 2mM. End by clicking Tools and selecting Print Data.

Activity 3-Simulating Osmotic Pressure

Begin by clicking on the Experiments Menu and then selecting osmosis. Then place the 20 MWCO membrane between the beakers. Dispense 8.00mM of NaCl solution into the left beaker, and deionized water into the right. Set the timer for 60 minutes, and then press start. Observe the pressure displays, and record the osmotic pressure in the data. Now repeat this portion of the experiment with 50, 100, and 200 MWCO membranes instead of 20. Then repeat the entire procedure using 9.00mM of albumin in place of the NaCl, and then a third time with 10.00mM of glucose. End by clicking Tools and selecting Print Data.

Activity 4- Simulating Filtration

Open the experiment by opening the Experiments Menu and selecting Filtration. Begin by placing the 20 MWCO membrane underneath the top beaker. Now dispense 5.00mg/ml of NaCl, urea, glucose, and powdered charcoal into the given beaker. Change the pressure unit on top of the beaker, if necessary, to 50mm Hg. Set the timer to 60 minutes, and press start. Observe the Filtration Rate box for any activity. A rise in the rate indicates that solutes are passing through the filtration membrane. Record the filtration rate along with the amounts of solutes present. Now move the 20 MWCO membrane to the residue analysis kit and press Start Analysis to analyze the residue left and clean the membrane. Record your data. Then repeat the experiment using g 50, 100, and 200 MWCO membranes and record that data as well. End by clicking Tools and selecting Print Data.

Activity 5- Simulating Active Transport

Open the experiment by looking in the Experiment menu and selecting Active Transport. To begin, change the number of sodium-potassium pumps and glucose carriers to 500 in the membrane builder. Click Build Membrane then place the membrane between the beakers. Then dispense 9.00mM of NaCl to the left beaker, and 6.00mM of KCl to the right. Dispense 1.00mM of ATP with the ATP dispenser. Set the timer to 60 minutes, and press start. Repeat this experiment a second time using 3.00mM of ATP instead of 1.00mM, and then a third and final time using 1.00 mM of ATP and 10.00mM of NaCl in place of the 6.00mM of KCl. End by clicking Tools and selecting Print Data.