In this chapter we will cover the following main topics:
Before we embark on a discussion of cellular respiration, I think that a review of where we have been is in order. So far we have explored the ways in which chemical reactions result in molecules. We then investigated the macromolecules necessary for life (proteins, carbohydrates, lipids and nucleic acids). We saw how these molecules are highly organized inside of membrane-bound units called cells. We studied two types of cells, prokaryotes and eukaryotes, and learned that eukaryotes have complex membrane-bound organelles missing in prokaryotes. The ways in which cells take in materials and export them were studied next, and we learned about passive and active transport, including osmosis. Since cells are highly organized they are constantly fighting against the second law of thermodynamics, which says that everything is tending to randomness and disorder (i.e. entropy). This means that cells must be regularly supplied with energy or they will die and decay. The source of energy for life on the surface of the earth is light from the sun. This energy is trapped by green plants by the process of photosynthesis, whereby light energy is converted to chemical energy and stored in molecules such as glucose. Green plants use these molecules they have manufactured to provide them with ATP to do the work of their cells. Other organisms, such as animals, obtain their energy-rich molecules directly or indirectly from plants.
We will now learn how cells take energy-rich molecules (e.g. glucose) and break them down to release the energy stored in them. This energy is used to phosphorylate ADP to ATP. The ATP is used to do the work of the cells. As with photosynthesis, the process has several stages. We will begin with the breakdown of glucose to pyruvate. This process is called glycolysis.
Glycolysis is the process in which glucose is broken down into two molecules of pyruvate (pyruvic acid). It occurs in the cytoplasm of the cell and is an anaerobic (without oxygen) process. As we did in photosynthesis, we will simplify the process and look only at the carbon atoms, ATP and NAD. Remember, glycolysis is actually much more complicated than this. Keep in mind that glucose is being broken down to extract as much stored energy as possible. Remember, the second law of thermodynamics assures that some energy will be lost in the process. The energy that is captured will be stored in molecules of ATP.
There are two major parts to glycolysis. The first part is shown in light blue in the diagram. What happens is quite a surprise! Two molecules of ATP are used instead of being formed. This seems to be going against what we set out to do. You've heard the phrase, "you have to spend money to make money". Well, that is what is happening here. It is necessary to spend energy to extract the energy stored in glucose. The two molecules of ATP are used to push one molecule of glucose up the energy hill. Two molecules of G3P result.
In the second stage of glycolysis, shown in green, the 2G3P are broken down into two molecules of pyruvate. In this stage several events occur that are important. First, 2NAD are reduced to 2NADH. Second, 2ADP are phosphorylated to 2ATP. This step pays back the two ATP that were used in the first stage to take glucose up to 2G3P. Phosphorylation of ADP to ATP without chemiosmosis is called a substrate-level phosphorylation. Third, another 2ADP are phosphorylated to 2ATP. Finally, 2 pyruvate molecules are synthesized. There is a net of 2 ATP from glycolysis. (2 ATP used, then 4 ATP synthesized = net of 2 ATP.)
Let's look at glycolysis in terms of what goes in and what comes out. This way of viewing things should be familiar to you by now.
|Glucose (C6)||2 pyruvate 2(C3)|
|a net of 2ATP|
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© copyright June B. Steinberg, 2000