Study Guide

Energy Flow and Enzymes - The Second Law of Thermodynamics

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The Second Law of Thermodynamics

You may have figured out that no good standoff, either between biology and physics or you and your folks on Saturday night, can happen without a good amount of energy. What is this biological standoff we speak of? Only one of the most fundamental laws of physics, referred to as the second law of thermodynamics. Don't you feel silly? Why would we discuss the first law of thermodynamics, first? Making sense is overrated.

The second law of thermodynamics states that the degree of disorder is always increasing in the universe. The amount of disorder in a system can be quantified, and this measurement is called entropy. You will often see our new friend entropy expressed as the letter S. Don't be fooled; it's still entropy. As you might imagine, creating order in a universe that is always headed toward greater disorder requires a good deal of energy. The universe has made her decision, and there is no talking her out of it; at this point, there isn't much to do but stay strong and keep going.

Now you know why life comes down to an everlasting standoff between order and disorder. Life strives to create ordered structures while the disorder-loving universe rains on his parade as much as possible. Not cool, Universe. Not. Cool. (Not to be a Debbie Downer, either, but the universe generally wins these standoffs when all is said and done. We'll let you know if the outcome is ever in Life's favor.)

What does it mean that the amount of disorder is increasing? Let’s think about your…biology binder. The three-ring binder keeps all the pages in the correct order (or it would, if you ever put anything in it). The second law of thermodynamics as applied here means that if you accidentally drop your binder, and the rings pop open, all of the pages will be mixed together. Obviously.

Think about why this is. According to the biology teacher and the class, there is only one correct order for the pages to be in, and many more ways for the dropped pieces of paper to land on the ground. Putting the papers back in the right order takes energy.

We can also think of the second law of thermodynamics in terms of organizing your bedroom. Your room naturally tends toward disorder (pretty crazy that we knew that, right?), or room arrangements with greater entropy, and keeping it all tidy and clean takes a lot of work. The next time that someone, anyone for that matter, tells you to clean your room, tell her that you are letting the second law of thermodynamics run its course. It's best to be one with Nature, after all.

This person that rudely commented on the state of your belongings will probably be quick to realize that your excuse for not cleaning your room is a poor one (no offense). Unfortunately for you, the second law of thermodynamics applies to the whole universe, and your room is one small component of a ginormous universe. Therefore, you can clean your room, and as long as the entire universe is still headed toward disorder, all is right in the world. Sorry to burst your bubble.

Actually, it comes down to basic math. All the disorder in the rest of the universe is to compensate for the little bit of order in your newly immaculate and well-kept room. Your room is what we call an open system, which means that energy, entropy, and even you and your stuff can pass freely from the walls of your room to the rest of the universe. Nice try locking the door.

On the other hand, a closed system can exchange energy, but not matter (matter = anything that has a mass and occupies space), with its surroundings. An isolated system cannot exchange anything with its surroundings. If your room were an isolated system, you might be excused from cleaning, but you would also be grounded. For life. Physics is not cutting you much of a deal here, is he?

Here is a generic picture of a system. The dotted line is the determining factor. Can the system exchange everything? If so, it's an open system. Everything but matter? Then it's a closed system. Nothing? Well, in that case, we're looking at an isolated system.

This might blow your mind, but having a clean room in a disordered universe is similar to the cells in your body. Your body is practically the living definition of organization. First, you have your organs, which are composed of cells, which in turn are organized into compartments like the nucleus, mitochondria, endoplasmic reticulum…and on and on. You might wonder how on Earth your body could possibly become ordered when the disorder of the universe is always increasing, right? The answer is that a cell takes energy from its environment to generate order. In turn, the cell releases energy as heat to the environment, giving it what it wants and making it more disordered.

You might also be wondering why adding heat to the environment makes it more disordered. Heat is basically the most disordered form of energy. When it is released to the environment, it speeds up the random movement of atoms and molecules in its path. This randomness is an excellent example of increasing entropy.

Let's contemplate the opposite scenario, where the temperature of a system decreases because heat flows out of a system. In this case, atoms and molecules move more slowly until they can’t move at all. This is in fact a more ordered state of being, and therefore, the entropy of the system has decreased. Adding more heat to the system by increasing the temperature will increase the disorder of this system once again.

Brain Snack

One way that the cell creates order is through the generation of a membrane potential (read: the concentration of hydrogen ions across the membrane). A cell has a membrane potential of about 300 millivolts (mV). This amount may not seem like a lot to you, but at the small scale of a cell, that potential accomplish do a lot.

Energy Flow and Enzymes - The Second Law of Thermodynamics Study Group

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