Is energy essential for the transport of substances in and out of the cell?
Answers
Answer:
Cell membranes act as barriers to most, but not all, molecules. They are thus differentially (or semi-) permeable. 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 sources for this movement. 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).
Bulk flow is the overall movement of a fluid. This moves liquid throughout a multicellular individual. 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 (10 ft x 10 ft x10 ft) 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.
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), there is available energy 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.
Answer:
Passive transport requires no energy from the cell. Examples include the diffusion of oxygen and carbon dioxide, osmosis of water, and facilitated diffusion. Image from W.H. Freeman and Sinauer Associates, used by permission. Active transport requires the cell to spend energy, usually in the form of ATP.
Explanation:
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