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THE SECOND LAW OF THERMODYNAMICS
The entropy of the universe is always increasing
The Second Law of Thermodynamics is the Law of Increasing Entropy. This
law states that the universe is always moving toward a greater state of disorder,
or entropy. Anything that happens spontaneously, that is, without an input of
energy, will result in molecules being more disorganized, more random, more mixed
together, and more spread out. In other words, processes always tend naturally
toward the state with the least potential energy. An easy way to visualize this
is to think of running water. Liquid water will always find the lowest spot, and
collect there.
The universe is always
becoming more disordered |
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This second law of thermodynamics explains why the molecules of gas that leak
out of a broken pipe will disperse evenly, filling a room with the smell of gas,
rather than staying together in a localized compact cloud. Similarly, you can
smell the chlorine in the air at an indoor pool because some of the chlorine molecules
escape from the water into the air. The law of increasing entropy also explains
why houses don’t spontaneously assemble from a pile of wood on the lawn,
spills don’t mop themselves up, and dust doesn’t gather itself into
a neat pile, ready to be swept up. Such processes that result in an increase of
organization (that is, a decrease in entropy) require energy input and are not
spontaneous.
Example 2: Osmotic pressure across cell membranes
Often, the concentration of a compound inside of a cell is different than the
concentration outside of the cell. Cells often use proteins called transporters
to import these compounds across the cell membrane. If a transporter for a certain
compound used the energy found in ATP hydrolysis to bring the compound in the
cell, what could you say about the relative concentrations inside and outside
the cell?
Answer
Having one concentration inside the cell and another outside the cell is a
form of order, or compartmentalization. This is therefore a state of relatively
low entropy.
If the compound had a higher concentration outside the cell than inside, then
as the compound was brought inside the cell the movement of the compound would
be toward an equilibrium state where the concentrations were equal across the
membrane. This would yield a less orderly system with increased entropy. This
process would therefore be spontaneous, and in contrast to what is observed, ATP
energy should not be required by this transporter.
Because energy is required, the situation must be reversed, and the concentration
of the compound must be higher inside the cell and outside. The process of
ATP hydrolysis must have a large increase in entropy that more than compensates
for the decrease in entropy for importing the compound into the cell.
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