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4 Biology Problems

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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?

4. 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.

5. Often, prokaryotic cells exist as simple unicellular organisms, but in some species, prokaryotic cells can grow together in colonies or filaments. In addition, some species, such as Cynaobacteria or Myxobacteria, demonstrate intercellular communicate , or might even produce specialized cells and structures. However, only eukaryotic cells form the bodies of multicellular organisms with complex internal specialization. Develop one or two hypotheses explaining why only eukaryotic cells are found in multicellular organisms.

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Solution Preview

1) Immune cells are present for the purpose of destroying any foreign cells that may enter the body, like disease-causing bacteria or those that infect cuts in the skin. One common method of fighting these invaders is to secrete proteins that can create channels in the cell membrane of invaders while recognizing self cells and leaving them intact. These channels or holes can disrupt the membrane, and, if there are enough of the holes, make it impossible for the cell to remain intact. This is because the holes allow water to enter the cell in such quantity that the cell swells and eventually bursts open - due to osmotic pressure. You should be able to explain this in a bit more detail in terms of osmosis. Any text will have a clear explanation of osmosis if you need to review this process.

2) The main physical constraint on cell size is the surface to volume ratio. This is due to metabolic ...

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