Learn about Complete Biology Prep in this comprehensive video by Brightstorm
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So, what are some kinds of questions that you might see on the AP exam that involve this sort of stuff. I’m gonna be hitting some slightly harder ones that you usually see towards the ends of the test, or in the essays. First up, here’s a question, you have dialysis tubing and fill it with 5 percent sodium chloride solution. When you put it into distilled water, you find that the sodium chlorine, sodium chloride is diffusing out at a rate of 20 milligrams per liter per second. What would you predict the rate would be if you repeated the experiment with 1 percent sodium chloride solution inside the tubing? Well, take a look here, what are we talking about, we got 5 percent, sodium and distilled water. Distilled water has zero, so what’s the concentration gradient there, it’s a 5 percent to 0 percent gradient, and that has a rate of 20 milligrams per liter per second. What’s the ratio here? This is 5 times less, so you just take this number, divide it by 5 and you get an answer of four milligrams per liter per second and as a little test taking tip, look at the number, you’ve seen the number 20 before, what are the common dividers, that eliminate things like three. So, just use, you can answer these questions, sometimes by knowing your math well. You look to see how the teachers try to set things up, so that the numbers are easy, coz you’re not allowed to use calculators on the AP exam. Next question, a student drops a potato core into a beaker of distilled water that was open to the air. He previously calculated that the solute component of the potato to be equal to -8 atmospheres. Describe what will happen and at what pressure will water stop moving. Alright, there’s a couple little things that they’re letting you know about here. First off, they say that it’s a beaker of distilled water, distilled water has 0 stuff, if you go back to our water potential concept, remember that the concentration of stuff, was part of the solute component and it was –i times C times R times T, well let’s see. What is C, in this case, it’s zero, so it doesn’t matter what those other numbers are, the solute component of the water on the outside is zero. This open to the air, why am I mentioning that? That’s because pressure potential is considered zero if it’s open to the air, they try to make things really easy. So the outside water potential is equal to zero, and that’s because it’s open to the air, which is a Psi P of zero and Psi S of the distilled water is zero. Inside this cell however, it’s different, we calculate it to be -8 atmospheres, so, it has a Psi of -8. So, who has got more potential to donate water? The outside, so we know that water will be moving into the cell. And so water will keep moving in until we reach equilibrium, the same amount of water in the inside and outside, but because the potato cells won't let their proteins and other things diffuse out, we can never get the same amount of water on the inside of the potato as on the outside. So what happens is that the pressure will build up until it equals the zero of the exterior. How can we do that? Well, it’s simple enough, the cell will have a Psi of -8 solute component plus +8 atmosphere of pressure. -8+8 is, carry 17, it’s zero, so there you go. Last question before we finish this off is describe how membrane protein channels and pumps make the cell membrane selectively permeable instead of simple semi-permeable. What’s going on here is remember, membrane pumps spend energy in the form of ATPs, so that means that the cell will have to spend energy to let stuff in or out. If it wants not to, then it just selects not to spend the energy and the stuff doesn’t move. With facilitate diffusion, there’s protein channels have the doors, so they can open the door, just select when things come in, you can close the door to select them not to come in. That’s different from a semi-permeable membrane like a dialysis tubing which has holes in the walls of the tubing and whatever can fi
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