Now before we get into the second law of thermodynamics, it's first important to understand the idea behind this term called entropy. Entropy is defined as a measure of disorder, or in other words, a measure of randomness. The greater the disorder, the higher the entropy will be. Let's take a look at this image below to get a better understanding of entropy. Notice on the left-hand side we have this pool table, and the billiard balls here are very highly organized and very ordered. However, on the right-hand side, the same billiard balls are scattered throughout the entire table. They are not highly ordered. Instead, they are greatly disordered. Because of this greater disorder, the entropy is higher. This system over here is going to have higher entropy. And, of course, this system that is highly ordered is going to have low entropy. The lower the entropy, the more organized and ordered it is, whereas the higher the entropy, the more disordered and unorganized it is.

The natural tendency of reactions is to move the universe towards a state of maximum entropy or maximum disorder. The natural tendency of the universe is for things to go from a state of order to a state of disorder, a state of higher entropy. This represents the natural tendency of reactions. However, reactions can decrease the entropy of a system, essentially going backward in this direction, with an energy input. You can see that with an energy input, reactions can become more ordered. This is what life is capable of doing. Living organisms input energy to create order in their systems. But the natural tendency of the universe is to go from a state of low entropy towards a state of higher entropy. The reactions will have the tendency to move towards the state of maximum entropy or maximum disorder. This is an idea that you would get to learn a lot more about in a chemistry course, but here in our biology course, this concludes our introduction to entropy. We will apply entropy in the second law of thermodynamics, which we'll cover in our next video. So, I'll see you all there.