Active Transport

23/09/2010 § 1 Comment

For the second time, the class faced their final lecture before a test. Today’ s lecture was about the opposite of passive transport: active transport. Active transport is the movement of a substance against its concentration gradient, which requires energy from the cell, usually in the form of ATP. This type of transport of a substance or of molecules requires energy because instead of moving from high concentrated areas to low concentrated areas, the systems of transport are required to pump or ‘push’ the substances from their area of low concentration to an area of high concentration. The way this was explained was using the kids-in-a-closet analogy. In passive transport, I would simply have to leave the door open (if I were a channel protein and a non-gated one that let kindergarteners through), and in no time at all, all the children would be spread out either in that closet or outside and around in the classroom. From an area of high concentration, we spread out the ‘solutes’ and ‘molecules,’ in this case, the kids. In active transport, however, the process would consist of me trying to push a few kids into the closet; from an area of low resolution (the classroom), to an area of high resolution (the closet). Because the molecules or substances that we’re trying to move are energetic, always bouncing off walls or other solids and always active, it will take some energy to get kindergarteners into the closet.

The first idea of active transportation we learned in class was the sodium-potassium pump, a carrier protein. Imagining the Superman picture we were shown in class, we remember that sodium-potassium pumps (SP pumps, for short here) require the energy of one phosphate in the triphosphate part of ATP (adenosine triphosphate). Since I can’t really draw the entire scheme of the SP pump, the graph below shows how it basically works.
Figure 1: The Steps of the Sodium-Potassium Pump

The first step that happens is the binding of three sodium molecules from inside of the cell (in the beginning of the process, the SP pump’s original shape is open towards the inside of the cell, to be able to receive sodium, Na+, molecules). Then an ATP molecule comes along and gives off one of its phosphate molecules for energy. This interaction between the pump and the phosphate group cause a change in the pump’s shape. Now, the opening is not towards the inside of the cell but outside the cell. The three sodium molecules are then released to the area outside the cell, where it is more concentrated with more sodium molecules. Meanwhile, two potassium molecules sneak into the still-open opening (?) of the SP pump.

Backing up a little, we remember that there is still the phosphate group from the ATP molecule that helped make all this happen in the first place. In the first shape, the pump itself are primarily attracted to the sodium molecules. After the sodium molecules are released, however, and the new potassium molecules bind to the pump, the pump loses its affinity for particular molecules, in this case, the sodium molecules. The phosphate group is then released, therefore the pump takes back its original shape, where the opening is pointed towards the inside of the cell. (Without the phosphate group to power the change of the pump’s shape, the pump stays in its original shape).

Our bodies do this all day and we don’t even know it, which is partly why I chose this topic to explain for the essay question. The sodium-potassium pump is a daily, hourly, minutely and secondly process that we human beings perform individually and we don’t even understand it completely. (I say that because people are still discovering more about it and winning Noble Prizes in its honor.) I also chose this subject for the essay question because I understand this concept to a point where if someone gave me a pen and a piece of paper (A4), I could draw the whole process for them and explain what’s going on and maybe touch on why the sodium and potassium molecules are moving to where they’re moving. At that note, I do believe that the pump moves sodium molecules out and potassium molecules in so that there’s a charge around the membrane of the cell, as you can see in the far right of Figure 1. There is a positive sodium charge and a negative potassium charge outside the cell (and vice versa for inside. Positive potassium, negative sodium charges inside the cell.) This charge helps us think and concentrate, which makes the SP pump something all animals need or we wouldn’t be able to function at all.

The next process is relatively simple, and involves a lot of pinching into and fusing with the membrane. Endocytosis and exocytosis are ways to get substances moved into or out of a cell via transport vesicles. In endocytosis (in general, although there are specific kinds of transport that we touched upon in class), the substance fuses against the membrane and a vesicle pinches off the membrane to surround the substance, thus become able to be brought around the cell, as seen in Figure 2.

Figure 2

Figure 3

Exocytosis is just the opposite, shown in Figure 3, where there are already vesicles holding substances (maybe proteins or other molecules needed around the body) and those vesicles again fuse against the membrane (from the inside of the cell) and almost explode, releasing the substance into the blood and around our body, at least, out of the cell. The way I remember this is “endocytosis” is “IN-docytosis” and “exocytosis” is “EXIT-cytosis”.

Finally,  we learned about receptor proteins, proteins that pick up signals from signal molecules so that the cell knows what’s going on in its environment and so it knows how to respond properly. An example of signal molecules are neurotransmitters or hormones. Receptor proteins and signal molecules work in three ways: 1) They activate a channel protein and allow certain ions or substances into or out of the cell. 2) They trigger the formation of a second messenger, which are AMPs, adenosine monophosphate with one phosphate group, which acts as a signal molecule inside the cell. 3) They act like an enzyme and amplify the signal molecule’s signal, producing millions of molecules and speeding up chemical processes.

After this blog, I feel a bit more solid with my essay question and describing how the function of sodium-potassium pumps sort of rely on the shape of the pump. (Of course, in reality, they also rely fully on the energy from ATP.) This unit was interesting, maybe not as easy to follow as last unit, but there were definitely multiple concepts that I understood well (sodium-potassium pump, anyone?). I didn’t receive satisfying grades during the last test but this blog has eased some of my nerves and I now understand the relationship of function and structure in cells, organelles and around cells.

§ One Response to Active Transport

  • Dave Ferguson says:

    Kari,

    Just when I thought your posts we’re setting the standard, you’ve further improved them by adding graphics. Bravo! You certainly are making the great use of blogging!

    Good luck on your test!

    Mr. F.

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