Life is made possible by a series of highly unlikely reactions. This chapter is an introduction as to why these reactions, so uncommon outside living organisms, occur with predictable regularity in biology. Of primary importance is the controlled storage and release of energy. Why is energy such an important concern for life? Because life is highly ordered/organized and energy is needed to maintain this organization.

The means by which cells obtain and use energy is termed metabolism. Think of a house as an example: The structure will not obtain its shape/structure without the input of significant work. Likewise, the home will tend to decay and become disorganized without the continued input of work. All forms of life follow the same rules in that their structure and anatomy require the input of significant energy and the continuation of life requires the continual expenditure of energy.

How does life obtain energy? Photoautotrophs collect solar energy and convert it to stored chemical energy (such as glucose and starch). Heterotrophs obtain such energy when they consume the organic molecules and obtain the energy stored within their covalent bonds (when animals get done removing the energy from glucose, it will be carbon dioxide and water, with fewer covalent bonds).

The paragraphs above describe two fundamental laws of our universe. The First Law of Thermodynamics states that energy cannot be created or destroyed. Application to biology: energy flows from the sun to photoautotrophs to the animals consuming the plants, constantly changing form but never being created or destroyed. So why do food pyramids exhaust their energy supplies? In each step, the amount of useful energy is reduced (e.g. the organized bonds within a glucose molecule will be converted to heat energy). This leads to the Second Law of Thermodynamics, which states that matter in the universe tends towards a state of maximum disorder, or entropy. Some think that life goes against this law but you must consider the disorder of the universe as a system, not just a small portion of the universe. A living organism does increase in organization, but at the expense of the organization of many forms of matter. Consider all the organic molecules a single animal must consume, break down and, in the process, reduce in complexity and order.

Within cells, a great number of reactions occur. Often, the reactions tend to reduce the amount of usable energy. So why don’t we run out of energy? Because cells are able to complete two types of reactions: endergonic and exergonic. Endergonic reactions add energy, usually storing the energy in covalent bonds (think of "en" as in enter, as the energy enters into the bonds). When the energy stored in the bonds is released, this is done with exergonic reactions ("ex" as in exit, which the energy is doing in such reactions).

When cells store or release energy, the most common form of "energy currency" is ATP.

Know this section! ATP is the key molecule in cell metabolism. Adenosine triphosphate contains a sugar (ribose), a nucleotide base (adenine), and a three-phosphate tail. The three phosphate groups, when connected, are not as stable as the rest of the molecule because they each have a negative charge and therefore don’t like being next to each other. When the third phosphate group is removed (yielding electrons and energy), ADP is the result. Cells can also run the reverse reaction, burning energy to reattach the third phosphate group to produce ATP from ADP, leaving the cell with a store of energy-rich molecules (ATP).

When wood (a carbohydrate, cellulose) is burned, the energy stored in its covalent bonds is released as heat energy. Cells don’t want to create only heat energy, so they don’t ‘burn’ calories in the same way as does a campfire. Cells often release the energy of a molecule by removing electrons and transferring them to an electron transport chain. The electron transport chain is confusing but just keep this in mind: Moving electrons have the ability to do work. Most of the work being done in this room is being completed by moving electrons, for both light bulbs and life.

Boring Memorization Stuff (that is going to be on the test, so you might as well learn the dull stuff): Know the term oxidation-reduction reaction. In an oxidation reaction, an electron is removed from a molecule (often removed by oxygen) and in a reduction reaction, an electron is added to a molecule (reducing the charge as an electron has a negative charge). Oxidation-reduction reactions are vital to all cells so this will not be the last chapter that the term oxidation-reduction shows up, so please memorize the two terms.

Read this section and look to define these key terms: law of conservation of mass, reversible reactions, anabolism, substrate, products, enzymes, cofactors. Most of these terms are covered in your prelab packet for the enzyme lab. The only term I will cover here is cofactor. Often, enzymes need the presence of a cofactor in order to react properly. Many vitamins act as cofactors for necessary enzymes in our body. These vitamins don’t provide us with energy but are necessary for our body to extract energy from certain food sources.

Enzymes are catalytic molecules (usually proteins, but in a few cases RNA can act as an enzyme) that speed up reactions without being used up in the process. There are four characteristics of enzymes:

1. Enzymes speed up reactions but they never make an impossible reaction possible.

2. Enzymes are not used up in the process of the reaction.

3. The same enzyme usually works for the forward and reverse reaction.

4. In general, enzymes are specific in regards to the substrates with which they interact.

How do enzymes speed up the rate of a reaction? Each enzyme has one or more active sites. Due to the shape of the enzyme, certain molecules will fit into an active site (known as the induced-fit model). When one or more molecules are in the active sites, there are four reasons a reaction may occur more quickly:

1. The active site may bring two substrates together.

2. The area of substrates that reacts may be brought closer together, speeding up a reaction.

3. The position of the substrate may increase the likelihood of hydrogen donation, increasing the reactivity of each substrate.

4. Water may be shut out of the reaction, speeding its rate.

Because enzymes function due to their shape, extreme heat can alter an enzyme’s function. Other things can also affect an enzyme’s effectiveness: extreme pH or salt concentration can interface with the active site, reducing or eliminating an enzymes affects.

Some enzymes are altered through allosteric control, in which the enzyme’s effectiveness is altered by a molecule latching onto a site other than the active site. In many cases, this is a useful form of feedback, in which extra product will bind to the orginal enzyme and reduce the production of the product.