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  3. Cell membranes act as barriers to most, but not all, molecules. Development of a cell membrane that could allow some materials t

Cell membranes act as barriers to most, but not all, molecules. Development of a cell membrane that could allow some materials t

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问题     Cell membranes act as barriers to most, but not all, molecules. Development of a cell membrane that could allow some materials to pass while constraining the movement of other molecules was a major step in the evolution of the cell. Cell membranes are differentially permeable barriers separating the inner cellular environment from the outer cellular(or external)environment.
    Water potential is the tendency of water to move from an area of higher concentration to one of lower concentration. Energy exists in two forms: potential and kinetic. Water molecules move according to differences in potential energy between where they are and where they are going. Gravity and pressure are two enabling forces for this movement. These forces also operate in the hydrologic(water)cycle. Remember in the hydrologic cycle that water runs downhill(likewise it falls from the sky, to get into the sky it must be acted on by the sun and evaporated, thus needing energy input to power the cycle).
    Diffusion is the net movement of a substance(liquid or gas)from an area of higher concentration to one of lower concentration. You are on a large(10ft×10ft×10ft)elevator. An obnoxious individual with a lit cigar gets on at the third floor with the cigar still burning. You are also unfortunate enough to be in a very tall building and the person says "Hey we’re both going to the 62nd floor!" Disliking smoke you move to the farthest corner you can. Eventually you are unable to escape the smoke! An example of diffusion in action. Nearer the source the concentration of a given substance increases. You probably experience this in class when someone arrives freshly doused in perfume or cologne, especially the cheap stuff.
    Since the molecules of any substance(solid, liquid, or gas)are in motion when that substance is above absolute zero(0 degrees Kelvin or -273 degrees C), energy is available for movement of the molecules from a higher potential state to a lower potential state, just as in the case of the water discussed above. The majority of the molecules move from higher to lower concentration, although there will be some that move from low to high. The overall(or net)movement is thus from high to low concentration. Eventually, if no energy is input into the system the molecules will reach a state of equilibrium where they will be distributed equally throughout the system.
    The cell membrane functions as a semi-permeable barrier, allowing a very few molecules across it while fencing the majority of organically produced chemicals inside the cell. Electron microscopic examinations of cell membranes have led to the development of the lipid bilayer model(also referred to as the fluid-mosaic model). The most common molecule in the model is the phospholipid, which has a polar(hydrophilic)head and two nonpolar(hydrophobic)tails. These phospholipids are aligned tail to tail so the nonpolar areas form a hydrophobic region between the hydrophilic heads on the inner and outer surfaces of the membrane. This layering is termed a bilayer since an electron microscopic technique known as freeze-fracturing is able to split the bilayer.
This passage is mainly concerned with______.
选项 A、bilayers
B、cell membranes
C、cellular environment
D、phospholipid
答案B
解析 目的题。本文主要讨论了细胞膜的功能。虽然第二段谈的是水势，第三段谈的是扩散运动，但是这都是为了说明细胞膜的功能而作的理论铺垫。
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