Passive Transport

20/09/2010 § 1 Comment

In our second-to-last lecture before the unit test, the class learned about passive transport in cells. This type of transport lacks the need to use any energy from the cell and depends mainly on the random movement of each molecule or substance. Substances and molecules that move and transport themselves (without energy from the cell) do so to move from a high concentrated area to a low concentrated area. In the textbook, the whole act of moving from high to low, which in scientific terms is called diffusion, was described as numerous never-ending bouncy balls in two rooms connected by an open doorway. Where there are more balls (in Room A, say), because they just keep bouncing, it’s more likely that some balls will eventually move to Room B and later find an equal number of balls in each room. In biology, this describes molecules and substances trying to find homeostasis within a cell. Passive transport and the transportation of substances and molecules happen so that the individual cell finds a balance within it. It tries to find some sort of equilibrium.

In class, we started looking through osmosis and the types of transportation water performs to get around the cell; into or out of one. As review, osmosis is the diffusion of water through selectively permeable membranes. (The phrase “selectively permeable,” I had to look up, means that a membrane allows only certain liquids, gases or substances go through it. In osmosis, we can basically imagine a U-Tube (not YouTube.) which is basically a glass tube that’s been curved into a perfect U. In the curve of the U, we also have to imagine a selectively permeable membrane that only lets molecules smaller than glucose to go through it.

So now, in the U-Tube, there’s a membrane, there’s water in it and we pour a little bit of glucose on one opening (side B) and even less glucose on the other opening (side A). There is then less glucose particles in side A and more glucose in side B. Now, remember that the membrane prevents any of the glucose from moving so the glucose particles themselves can’t move to find equilibrium in the U-Tube. Instead, to find homeostasis, the water moves from a hypotonic environment (side A) to a hypertonic environment. Personally, I think that this happens so that the glucose molecules can be more spread out and more even in the U-Tube. All in all, that might be the general idea of diffusion and osmosis; finding that equilibrium.

Just for the sake of restating the main idea of the previous paragraph: The osmotic potential is greater than gravity, so in a U-Tube, the water (since there is a membrane and certain substances can’t get through), from the hypotonic solution moves into the hypertonic solution to even out the balance of oxygen molecules. Another example of this is in our very own bodies. When human beings exercise, we sweat, and the water that we release (a characteristic we learned last unit) takes the heat and drains us of our H20. When we lose this water, our bodies have more solutes and less water, becoming hypertonic. To find homeostasis in our bodies, we drink water, consume water and dump water over our heads to gain back the water we lost, to find an isotonic state. (When the body, in this example, has found a balance and is neither hypertonic or hypotonic.)

Substances and particles that cells are exposed to also can diffuse through ion channels. These channels are basically transport proteins that allow certain ions and certain substances through. These channels are scattered all over the membrane of any cell and maintain what kind of things go into the cell. Channels help transport molecules and are little holes. Sometimes they have gates that only open when encountering the right stimuli. (For example, the gate could be triggered by an atom, a smaller molecule or an ion, etc.) Of course, when the gate opens, the molecules with the appropriate size are able to enter the cell.

Carrier proteins, are another form of transportation (that don’t include energy from the cell). They are proteins that only specific substances can bind to. After ‘binding’ together, the carrier proteins are like a revolving door. When the molecule attaches to the carrier protein in shape A, the molecule and the carrier protein interact. After the interaction, the protein is no longer attracted to the molecule and shape-shifts into shape B, allowing the molecule to leave on the other side; whether into the cell or the other side of the cell.

To clear things up a little, molecules try to find homeostasis not because they just want to but because of the frequency of their collisions. More collisions in a space obviously have more concentration. From there, molecules then just automatically bounce around until they’re all eventually spread out evenly throughout a certain space. Diffusion moves down the concentration gradient, which is the difference in concentration of a substance across a space.

In terms of my essay question, (Within the context of cellular structure and function, describe and explain how form relates to function) I think I can explain form’s relation to function by using membranes and phospholipids and how they form protective walls. Or, I could explain everything I’d just explained, except recollect the drawings and sketches of transportation I put in my notebook into the essay question and label and describe what’s going on in the sketches. (Since I’m not very much of an artist, it’ll be easier to just sketch the drawing and explain from there all of the labels.) I’m planning on practicing describing equilibrium and diffusion tomorrow and also practicing my sketches to make sure I have the concept down to a pat. Or at least, as much “to a pat” as I can.

§ One Response to Passive Transport

  • Dave Ferguson says:


    As usual, an outstanding post. It’s always a pleasure to read your writing. I especially like the way you’ve added color, italics and bold text for emphasis. And best of all is your application of knowledge to the essay question.

    Mr. F.

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