5 - Molecular Cell Biology 32 Professor Andrew Wurmser...

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Professor Andrew Wurmser 09/11/07 Lecture 5 Sharing or distribution of lecture notes, or sharing of your subscription, is ILLEGAL and will be prosecuted. Our non-profit, student-run program depends on your individual subscription for its continued existence. These notes are copyrighted by the University of California and are for your personal use only. Sharing or copying these notes is illegal and could end note taking for this course ANNOUNCEMENTS Next week, we will try to give you some examples of test questions to guide your studying for the test. I will also give you a practice test from a previous instructor to give you an idea of the type of questions that would be asked. Discussion sections 111 and 112 have been cancelled. The other discussion sections are not too crowded. Please attend another discussion section. Do not drop the section on Telebears, just go to another section. LECTURE Review: Types of Molecular Transport. Different molecules diffuse through different mechanisms depending on their properties. For example, we discussed last time that non-polar molecules undergo a form of unaided diffusion across the plasma membrane. This process is known as passive diffusion . Examples of non- polar molecules that passively diffuse across the cell membrane are oxygen and carbon dioxide. Since carbon dioxide is usually more abundant inside the cell, it diffuses out of the cell in an attempt to equalize the concentration gradient inside relative to outside the cell. Passive diffusion is driven strictly by a concentration gradient. This means that the concentration of solutes inside and outside the cell is always trying to equalize. So if there is more of a certain solute inside the cell as compared to outside, the molecule passively diffuses out of the cell, down the concentration gradient, in an attempt to equalize the concentration levels. In passive diffusion, both the polarity and the size of the molecule have an effect on the ability of the molecule to diffuse. High polarity discourages passive, spontaneous diffusion and large molecules also do not diffuse readily. Remember that channels in the plasma membrane can be in open or closed states and that molecules travel in different directions through these channels depending on the relative concentration gradient. In the transport process known as facilitated diffusion , shuttle molecules bind solutes and transport them across the membrane. Molecules compete for binding sites within shuttle molecules. Thus, the binding sites can become saturated after a certain point. Active transport , on the other hand, transports molecules against a concentration gradient. Remember that both facilitated diffusion and passive diffusion happen in the direction of the concentration gradient. An example of primary active transport is the sodium- potassium pump. Initially, with regards to the cell, there is high sodium outside (low sodium inside) and high potassium inside (low potassium outside). The sodium-potassium
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5 - Molecular Cell Biology 32 Professor Andrew Wurmser...

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