Selective permeability means that the cell membrane has some control over what can cross it, so that only certain molecules either enter or leave the cell.  Molecules can cross the plasma membrane in three main ways.
1. Passive Transport. Go there!
2. Active Transport. Go there!

    The manner by which molecules cross the plasma membrane depends on whether the molecules are small or large, and on whether the molecules are already concentrated either inside or outside of the cell. If a molecule is tiny enough to fit through a special protein channel in the plasma membrane, it will be able to move across the membrane (either by passive transport or active transport). If the molecule is too large to fit through any tiny pore in a channel, then it will have to enter or exit the cell by moving in a vesicle. So, size of the molecule alone will distinguish whether the molecule will cross by transport or within a vesicle.  To summarize:

    So what is the difference between passive and active transport? The difference has to do with whether the movement of the molecules can occur without additional energy (as is the case with diffusion and osmosis, both forms of passive transport).  If no energy needs to be expended to get the molecules across the membrane, then we are talking about passive transport. If extra energy needs to be applied, then we are talking about active transport.  You will need to learn about diffusion and osmosis (both methods for PASSIVE transport) in order to know whether a molecule needs energy or not in order to cross.  Those are explained below.

Another web page that has some nice information about passive and active transport (if you don't understand what I have written) is: http://www.ndsu.nodak.edu:80/instruct/gerst/z120/mtrans.htm .

Let's now explore each item in more detail.

1. Passive Transport

    Passive transport is the way that small molecules can cross the membrane without any additional energy being expended. These molecules could be water molecules, or they could be molecules other than water (like ions, oxygen gas, and monomers).

    You already know that some molecules other than water will diffuse without any additional energy:  think about what happens after you put a drop of food coloring in a glass of water.  Even if you don't stir, the food coloring will spread all over the glass of water.  It certainly will NOT just stay in a little drop-like spot within the glass!  This is an example of diffusion.  But it is not too meaningful to us in biology.  Let's consider another example...

    If you put a bit of cauliflower in a glass of colored water, the cauliflower will turn colored.  What has happened here?  The dye molecules have moved across cell membranes into the cells of the cauliflower.  This notion of molecules diffusing across membranes (to get into, or out of, cells) is really important in biology.  This will be important after we eat, for example.  All of the cells of our body will need to receive the nutrients from our meal.  The nutrients from our meal circulate in our blood.  How are they going to get into our cells?  Diffusion!

    Osmosis is how water itself moves.  And water has ways to move across a membrane.  Again, it is important that water be able to get where it needs to go in our bodies.

    Because both diffusion and osmosis can cause molecules to move without additional energy, both of these can be the basis for how molecules will move across a membrane. Yet, when we talk about molecules diffusing across a membrane, we have two categories of diffusion to consider. This is because some molecules can easily cross the cell membrane, right through its lipid component, while others have to go through channels.  I'll remind you of this in the next unit.

    This then leads us to understand that there are three main ways that molecules can move by passive transport:

1. Simple Diffusion-- (your book just calls this diffusion, but I added the word simple to make sure you didn't confuse it with facilitated diffusion).  This is how small molecules cross the plasma membrane (examples: oxygen gas and steroids) without the need for any channels.
2. Facilitated Diffusion-- this is for small molecules that cross the plasma membrane through channels (examples: ions and monomers).
3. Osmosis-- this is how water crosses the membrane.

Below are some details about diffusion and osmosis:

Diffusion: Molecules (other than water) will move by diffusion as long as there is a concentration gradient. Do you understand what a concentration gradient is? It exists when a particular type of molecule (like food coloring dye molecules) is not spread out in an even concentration, but exists in a higher concentration at some point. If we are talking about molecules moving across the plasma membrane, then we would just compare the solutions inside and outside of the cell for their concentrations of the molecule in question. For example, if glucose (a monosaccharide) is in equal concentrations inside and outside of the cell, there is no glucose concentration gradient and glucose will not move. But, if glucose is in a higher concentration outside of the cell than inside of the cell, the glucose concentration gradient is from outside to inside the cell, and glucose will move into the cell. There is no need to expend any energy in order for glucose to enter the cell. The concentration gradient for the movement of molecules other than water is always from high concentration to low concentration.

Osmosis: Water will move by osmosis as long as there is an osmotic gradient. In order to understand an osmotic gradient, you have to evaluate how much solute is in each solution you are comparing. If the cytoplasmic solution has an equal concentration of solute molecules as the extracellular solution, the cell is isotonic to the extracellular solution, there is no osmotic gradient, and water will not move in any one particular direction. However, if one solution contains more solute molecules than the other, the one with more solute molecules is called hypertonic, and the one with less solute molecules is called hypotonic. The osmotic gradient for the movement of water is always from hypotonic to hypertonic.

    However, the movement of water may have devastating effects on a cell! Think about it... if water leaves a cell, it will shrivel, and if water enters the cell, it will swell (maybe even to the point of explosion, called lysis). This is diagrammed in Figure 3.24 of your book.  You have seen the word "lysis" before in a different form... remember?  You saw it in "lysosome," meaning a body that breaks.

    It should make sense that if enough water floods into a cell, it could cause the cell to explode. It should also make sense that the cell explodes because as the water enters, it exerts pressure on the cell membrane until the plasma membrane can no longer withstand the pressure. People call the pressure that water can exert osmotic pressure.  I will show you an example of this in lab.

    Osmosis will be a very important concept for you to thoroughly understand when we cover the urinary system in A & P II.   You may as well get working on it now!

2. Active Transport

    Active transport requires energy to occur.  There are two possible methods of active transport, and they are:

1. Active transport using pumps
2. Active transport using vesicles

Let's go through each of these options below.

Active Transport using Pumps

    Again, this is how small molecules cross the membrane. But for these molecules to move, they need energy. Why? Because they cannot move by diffusion! That is because active transport moves molecules against the concentration gradient. What does that mean? Active transport pushes molecules from where they are in low concentration to where they are in high concentration; that means that it builds up a high concentration of the molecule.

    In order for active transport to work, there have to be special channels in the membrane that use the energy (ATP) to push the molecules through the membrane. These special channels are called pumps.  An example of a pump that is essential for all cells is the sodium/potassium pump.  An animation of it in action can be seen at http://www.youtube.com/watch?v=awz6lIss3hQ&feature=related .  Another important pump is the calcium pump, and here's a link to a simple calcium pump animation:  http://www.bio.davidson.edu/courses/Bio111/SERCAanimation.html .

Active Transport using Vesicles

    Now you have already seen that small molecules will move across the membrane through transport (either passive or active). But what about larger molecules? They will move using a vesicle to cross the membrane. They will either move into the cell in a vesicle, called endocytosis, or they will exit the cell in a vesicle, called exocytosis. You know that -cyto means cell, and if you put that together with endo- (meaning in or into) or with exo- (meaning out, like in exit), then you have a process for getting material into the cell or out of the cell.

Here are some links on endocytosis and exocytosis for further reference if my notes below are not sufficient.

You saw above in the section on membrane composition that the plasma membrane is an oily film.  This film can pinch off a piece when needed.  The piece that comes off is a rounded circle of membrane called a vesicle.  The film can also fuse with another vesicle and get larger.  Both of these are shown in the diagram here.  In each line, the first drawing is the "before" image and the last drawing is the "after" image.  The one in the middle, between the arrows, is the "during" image.  The arrows are meant to indicate time passing.

Endocytosis could be represented by the second line of drawings, while exocytosis could be represented by the last line of the drawings.  Notice that in either case, material crosses the membrane (either from outside to inside or vice versa).

Endocytosis has been further classified according to exactly how large those large molecules are. If the molecules are really, really large, then the type of endocytosis that occurs is called phagocytosis (and the vesicle is really big). If the molecules are just too large to cross the membrane via transport, a size where the molecules can dissolve into the solution so that it doesn't look like there are molecules in the solution, then it is called pinocytosis (and the vesicles are really small).

The other two parts of the diagram (the first and third) are not how endo- and exo-cytosis occur, but are included to show you other ways that membranes fuse and pinch.  When a vesicle pinches off of the rER to head toward the Golgi apparatus, it happens as shown in the first drawing.  When a vesicle from the rER fuses with the Golgi, it happens as shown in the third drawing.

© 2011 STCC Foundation Press
written by Dawn A. Tamarkin, Ph.D.