Slide 1 Cellular respiration occurs in three stages. The first is glycolysis, the the Krebs cycle and the electron transport chain with oxidative phosphorylation.
Slide 2 Glycolysis is a complex biochmical pathway that involves ten steps and ten intermediate compounds. It is the pathway that converts glucose into pyruvate and releases energy for ATP production. For this course, we do not need to learn glycolysis in this level of detail.
Slide 3 Instead we can learn the process in this level of detail. During glycolysis glucose is broken down into 2 molecules of pyruvte or pyruvic acid (you can use either name). Glucose is a 6-carbon molecule and pyruvate is a 3-carbon molecule. Remember, it takes an input of energy to break the chemical bonds of a molecule - and glycolysis requires an input of 2 molecules of ATP to start the process. As glucose is modified through the glycolytic pathway - a total of 4 ATP molecules are generated. This means that at the end of glycolysis - there is a net gain of 2 ATP molecules. This is energy for the cell to use to do cell work. There is also a movement of electrons from the intermediates of the glycolytic pathway to two molecules of NAD+ to produce 2 molecules of NADH. We will see where this molecule gets used later. The pyruvate that is produced by this pathway is a 3-carbon compound that still has a good deal of free energy associated with it. We will see what happens to this pyruvate later.
Slide 4 There are two ways for a cell to make an ATP molecule. Either through substrate-level phosphorylation as shown here or through a process known as oxidative phosphorylation as we will see later. Phosphorylation means to add a phosphate group to something. So ADP gets phosphorylated to ATP during cellular respiration. Substrate level phosphorylation occurs when an enzyme known as a kinase transfers the phosphate group from one molecule to another. In this image we see pyruvate kinase moving a phosphate group from PEP to ADP. During glycolysis, the 4 ATP that are made, are all made by substrate-level phosphorylation.
Slide 5 Let's do some energy bookkeeping at this point. To start glycolysis - 2 ATP molecules are used to split one molecule of glucose. At the end of glycolysis, the pathway has produced 4 ATP, 2 NADH, and 2 molecules of pyruvate. In terms of energy gain for the cell - the net energy gain is 2 ATP (4 ATP minus the 2 ATP invested) and 2 NADH.
Slide 6 Pyruvate is a 3-carbon molecule that still contains a lot of free energy. At the end of glycolysis, there are 2 molecules of pyruvate. Cells have evolved to be efficient - the pyruvate will be used by the cell. All cells use the glycolysis pathway - whether we are talking about prokaryotes or eukaryotes, plants or animals - all cells use glycolysis. Because all cells use glycolysis - that means all cells use NAD+. Rememember, during glycolysis NAD+ is reduced to NADH. In order for glycolysis to keep running (and a cell to stay alive) - the cell must have a steady supply of NAD+. NAD+ is an expensive molecule for a cell to make so there is a limited supply. This means that cells must regenerate their supply of NAD+.
Slide 7 What happens to the pyruvate and the NADH depends upon whether or not oxygen is present and available for the cell to use. If oxygen is present the pyruvate goes to the next stage of cellular respiration - the Krebs cyle. It does not go into the Krebs cycle directly but is converted into a molecule known as acetyl CoA. If oxygen is present the NADH that was produced in glycolysis is oxidized to NAD+ at the electron transport chain. We will see both these events in the next tutorial.
Slide 8 If oxygen is not present - pyruvate is reduced to lactic acid or ethanol. This process is known as fermentation (not anaerobic respiration although you may have mistakenly learned this) During fermentation, NAD+ is regenerated. We will see this next.