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This topic is about free-energy change. We won't be talking about all of this concept.
When you are reading the textbook, focus on what I cover in the recorded lecture. You
are free to read more, but you will be tested on this part.
We will be covering free energy, stability, and equilibrium and how this relates to metabolism.
Molecules and mixtures tend to move from a state of high potential energy to one of lower
potential energy. To use the diving from the platform example, molecules don't tend to
hang out on the top of the platform. Their friends down below make fun of them until
they jump. Well, molecules don't really tease each other, but it can help you understand
what is going on. Diffusion is an example of moving from a state
of high potential energy to one of lower potential energy. If I dropped a sugar cube into a glass
of water, the sugar molecules begin to dissolve as they form hydrogen bonds with the water.
I told you we'd need the information from Chapter 2 -- 4! The sugar and water molecules,
both in a liquid state, are bumping into each other randomly. The sugar molecules eventually
mix equally with the water molecules. We call this process diffusion and we say the well-mixed
solution is at equilibrium. It can't go back to having more sugar in one area than another.
There is energy in this movement from a concentrated state to a well-mixed state. This energy can
be used by the cell. Molecules that have a greater potential energy
stored in their chemical bonds tend to be less stable. They will spontaneously break
apart, releasing energy as they do. Once the molecule has broken into two or more parts,
the parts have less energy than starting molecule, or reactant.
It's like the guy on the diving platform. Once the molecule breaks its covalent bonds
and "falls" into molecules with less energy in their bonds, these resulting molecules,
or products, have less potential energy to do work for the cell.
We often use graphs to show this release of energy or the storing of energy. We measure
the amount of free energy on this axis and the progress of the reaction, or you can think
of it as "time", on this axis. In this graph, the starting reactants have more energy stored
in their bonds than the products do afterwards. Here is another example to help you understand
this rather abstract concept. Let's say this is my bank account before a spending spree.
This is my bank account after the spending spree. After the spending spree, I now have
less free money to do pay bills that I had before the spending spree. Bringing it back
to molecules, the free energy released is kind of like money. Once the money is spent,
or the energy released, you can't get it back. We call this reaction, one that releases energy,
an exergonic reaction. To go back to metabolism, catabolic reactions are also exergonic reactions.
In this graph, the reactants have less energy than the products. The products have more
potential energy than reactants. To use my money example, after my spending spree, I
have to go to work to put more money in my checking or savings account. Afterwards, I
have more potential to pay bills and buy stuff. Going back to the molecules, I have to use
energy to raise the molecules to a higher state of potential energy, kind of like me
working away at my job. These reactions are called endergonic reactions. To connect this
to term to metabolism, anabolic reactions are also endergonic reactions.
Whew, that was a tough concept. Don't worry if you don't understand it at first. We will
be talking about these concepts with specific examples, which always helps. Also, the in
class activities should help. Make sure to write down anything you want to ask me about
in the lecture question and answer session.