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    3 Questions

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    1. The integrity of the plasma membrane is essential for cellular survival. Could the immune system utilize this fact to destroy foreign cells that have invaded the body? How might cells of the immune system disrupt membranes of foreign cells? (Two hints: virtually all cells can secrete proteins, and some proteins form pores in membranes.)

    2. Most cells are very small. What physical and metabolic constraints limit cell size? What problems would an enormous cell encounter? What adaptations might help a very large cell to survive?

    3. Some species of bacteria that live at the surface of sediment on the bottom of lakes are facultative anaerobes; that is, they are capable of either aerobic or anaerobic respiration. How will their metabolism change during the summer when the deep water becomes anoxic (deoxygenated)? If the bacteria continue to grow at the same rate, will glycolysis increase, decrease, or remain the same after the lake becomes anoxic? Explain why.

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    1. Immune system cells could produce specific "pore" proteins that would target foreign cells. When the "pore-forming" protein encounters a foreign cell, it would attach to its plasma membrane and form a pore right through the membrane. Now that the foreign cell has a pore in it, the cell would begin to leak. It would lose its ability to control the transport of molecules in and out. If the immune system is working well, then it would add to that foreign cell more and more "pore-creating" proteins from the immune cells. Then you've basically got a foreign cell with lots of holes punched in its membrane by these special proteins. The foreign cell would eventually die. (This actually happens.)

    2. The real issue about cell size is the surface area/volume ratio. Essentially, as a cell increases in size, its volume increases faster than its surface area. In other words, to have the largest surface area/volume ratio, the cell needs to be small. With large cells, they have a really terrible surface area/volume ratio. So what? That means that ...