Introduction to the Cell - Energy & Metabolism

Overview

Energy, which is usually discussed in another branch of science called physics, is defined as the "ability to do work." What does that mean, you ask? Well, let's discuss what the word "work" means. If you lift a heavy box, or even a light box, you've just done work. Turning on your computer is also work. Notice that in these examples, when work was done, something in the environment changed. Basically, anything that results in a change of some sort is work. Since energy is the "ability to do work", it is what allows you to cause these changes. Cells need energy to perform their processes, because all processes (like movement or reproduction) result in a change.  

Energy can come in many forms and can easily change from one form to another. For example, electricity is a type of energy. A toaster changes electrical energy to heat energy in order to toast bread. You may have also heard of solar energy: energy that comes from the sun in the form of light and can be converted into electrical energy. Another type of energy is called chemical energy. Bonds can form between atoms to form molecules. The formation of bonds requires energy, and when the bonds are broken, that energy is released. When cells break down food molecules, they release energy which can be stored and used later on for all of the cell's processes. 

In any chemical reaction, bonds are either formed or broken. Since bonds are a form of energy, all reactions result in either the absorption or release of energy. Exergonic reactions are ones that release energy; these reactions will usually occur spontaneously since they do not require energy to occur. On the other hand, endergonic reactions absorb energy to form bonds, so they do not occur spontaneously. Instead, they occur only if energy is available to be used in the reaction. 

Reactions with a negative change in free energy (G) are exergonic and proceed spontaneously.

Reactions with a positive change in G are endergonic and require an input of energy to proceed.

Metabolic energy

Cells store energy in a molecule called adenosine triphosphate (abbreviated as ATP).  ATP is a nucleotide that is used to shuttle energy to different places in the cell. Its high-energy phosphate bonds store potential energy. Hydrolysis of ATP releases energy that can be used to fuel endergonic reactions.

The adenosine molecule has three phosphate groups attached to it (triphosphate means three phosphates) which are held together by high energy bonds. If one of these bonds is broken, a great amount of energy is released which can be used in an endergonic reaction. Also, the ATP no longer has three phosphate groups; now it has only two, so it is called adenosine diphosphate (di means two). This is abbreviated as ADP. Occasionally, another phosphate bond is broken, releasing more energy and leaving the ADP with only one phosphate left. It is now AMP: adenosine monophosphate, since mono means one.

Phosphate group transfer provides energy for most cellular work.

1. ATP drives active transport by phosphorylating membrane proteins.
2. ATP drives mechanical work by phosphorylating motor proteins, such as those that move vesicles along cytoskeleton "tracks".

After work is done, the phosphate is released as inorganic phosphate (Pi).

When food is broken down, energy is released as the food molecules' bonds are broken. This energy can be used to reform the bonds between the phosphate groups, so that ATP can be recreated. The process by which ATP is synthesized is accomplished differently in anaerobic and aerobic organisms; more energy can be obtained when oxygen is present and the process proceeds aerobically. 

The main source of energy for living organisms is a sugar called glucose. In breaking down glucose, the energy in the glucose molecule's chemical bonds is released and can be harnessed by the cell to form ATP molecules. The process by which this occurs consists of several stages, including glycolysis.

ATP Adenosine triphosphate, is used in cell life forms (we know of) as an "energy currency". The more work that needs to be done the more ATP that must be spent. It is nearly a "universal molecule of energy transfer" in living things. Energy can be stored as carbohydrates or lipids but that energy (in the chemical bonds) must be transferred to ATP before it can be used in the cell.

Structure of ATP:

ATP is made of "Adenine" and "Ribose" and three phosphate groups.

The ATP Cycle

ATP can lose it terminal phosphate and in the process release the energy stored in it. This energy is then used to do work in the cell. This produces ADP and the phosphate (which itself may become part of the chemical reaction). To generate ATP again from ADP and free phosphate we must add energy back in order to create the terminal energy rich phosphate bond. The new energy rich ATP can then be reused again, etc.

The energy to create ATP from ADP + P comes ultimately from the Sun via photosynthesis -----> extra energy stored as glucose. Glucose itself can thus be used as a "fuel" to create more ATP.

Enzymes

Enzymes are very special types of proteins for all living things; they are called organic catalysts. The first part makes sense, since enzymes are proteins, and proteins are organic, but let's make clear what it means to be a catalyst. A catalyst is any substance that speeds up the rate of a chemical reaction but is itself not affected once the reaction is completed. Enzymes are used in organisms to increase the rate of the chemical reactions that are necessary for life. Life depends on chemical reactions that occur within cells and organisms. Without enzymes to catalyze these reactions, most would proceed at a rate far too slow for life to exist. Most enzymes are proteins that catalyze very specific reactions – therefore there are literally millions of different types of enzymes in the biological world.
 

For an enzyme to catalyze a reaction, it must join with one or more of the molecules in the reaction; the molecules that an enzyme attaches to are called substrates, and when they join they form an enzyme-substrate complex. However, enzymes are able to attach only to certain substrates, a fact which is explained by the structure of an enzyme. Enzymes are proteins which have folded onto themselves several times to create a complex, three-dimensional structure. Each enzyme has an area called the active site where the substrate will join, but, like a lock and a key, only certain substrates will fit into the active site.
 

Unfortunately, it's not as simple as just a lock and a key. Sometimes, a substrate doesn't fit exactly into the active site of an enzyme, but the match is fairly close. In these cases, the enzyme is induced (persuaded) to change the shape of its active site slightly so that the substrate fits. Enzyme shape can be affected by a number of different factors, including temperature and pH. Many enzymes must also have other types of molecules bound to them in order to be active.

Coenzymes are organic molecules which are not proteins like enzymes but still play a role in reactions catalyzed by enzymes. In cells, coenzymes frequently serve as electron acceptors; they bond with electrons released by chemical reactions in the cell. Coenzymes are especially important for the process of cell respiration. 

There are a number of influences than can change the efficiency of enzyme. We will discuss the four most important factors: inhibitors, allosteric factors, pH, and temperature.

 

There are two types of inhibitors: competitive inhibitors and noncompetitive inhibitors, and their names give a good indication of what they actually do. Competitive inhibitors have a similar structure to the enzyme's substrate, so they can "compete" with the substrate for the active site of an enzyme. Often the enzyme will bond not to its substrate but to the competitive inhibitor, blocking the substrate from the active site and causing the formation of enzyme-substrate complexes to occur at a slower rate. Noncompetitive inhibitors, on the other hand, do not attach to the active site and block the enzyme-substrate complex from forming. Instead, they react with portions of the active site, which results in the changing of its shape. Once the active site's shape is changed, it can no longer attach to the substrate.
  

Some enzymes have special areas other than the active site. These special areas are sometimes called regulatory sites. Any molecule that attaches to the regulatory site is called an allosteric factor. Allosteric inhibitors join with the regulatory site and change the shape of the entire enzyme (including the active site), thus preventing it from binding with the substrate. However, not all allosteric factors are detrimental. Some join with the regulatory site and actually bring the enzyme and its active site into the proper shape so that it can successfully form enzyme-substrate complexes.

 
pH also plays an important role in affecting the rate of an enzyme's activity. Remember that pH is a measure of how acidic or basic a solution is; that is, how many H+ or OH- ions there are. These ions are charged, and charged molecules tend to pull on other molecules. So, if too many ions are present, the enzyme may be denatured (twisted and pulled so out of shape that it can no longer function). However, this is not to say that all enzymes work best when the pH is neutral. Some enzymes actually work best in acidic or basic environments, but these characteristics are particular to the enzyme.

The final factor that influences an enzyme's efficiency is temperature. To a certain extent, a high temperature increases the rate of an enzyme's activity, because at high temperatures, molecules move around faster, so an enzyme is likely to come in contact with a substrate very quickly. However, at too high temperatures, the enzyme can become denatured and lose all function. Low temperatures slow the rate of formation of the enzyme-substrate complex because the molecules move at slower speeds and so do not come in contact with one another as frequently.