Carbohydrate Catabolism Part 4


In this video we are going to discuss
what happens to pyruvate following glycolysis. Cellular respiration is
defined as the complete oxidation of a glucose molecule to yield energy in the
form of ATP. Recall from the Carbohydrate Catabolism Part 2 video, that oxidation
is seen as hydrogen ions are stripped from the glucose molecule resulting in
the formation of reduced electron carriers. The reduced electron carriers
that were produced in glycolysis included two molecules of NADH as
hydrogen atoms were removed from glyceraldehyde-3-phosphate. So at the end of glycolysis only two of the 12 hydrogen’s have been removed and
transferred to electron carriers. There are hydrogen atoms remaining on the two
pyruvate sugar molecules. In the next steps of cellular respiration the goal
will be to finish removing the remaining hydrogen ions, in order to complete the
oxidation of the molecules. Cellular respiration is said to occur in phases
some text books and videos describe the process in three phases glycolysis, the
citric acid cycle, and the electron transport chain. Before we proceed to the
next step of cellular respiration let’s look at a couple of images. The image on
the left of the screen is showing the last of the steps of glycolysis. The
image on the right shows the citric acid cycle. The starting compound for the
citric acid cycle, acety-CoA is shown in better detail in the enlarged image
at the bottom on the right of the screen. Looking at the two images on the screen
I want you to count how many carbon atoms are present in a single molecule
of pyruvate and a single molecule of acetyl-CoA. Do not misinterpret the CoA
as being a carbon molecule. It is crossed out in the image to assist you. Now that you’ve had time to look at the
number of carbons in pyruvic acid and acetyl-CoA, let’s talk about why pyruvic
acid will not directly enter into the citric acid cycle. As you learned pyruvate
has three carbon atom. The starting molecule acetyl-CoA has only two carbon
atoms. Acetyl-CoA will combine with the last molecule of the citric acid cycle
oxaloacetic acid to produce the first product citric acid. The cycle is
named after this first product. Look closely at the number of carbon atoms in
oxaloacetic acid and citric acid. You should see four carbon atoms in the
oxaloacetic acid and six carbon atoms in the citric acid. So in order
to run this cycle the molecule that will enter from glycolysis must be only two
carbons. So there has to be an extra step in the cellular respiration process
between glycolysis and the citric acid cycle that will convert the three carbon
pyruvate to the two carbon acetyl-CoA needed as a substrate in the citric acid
cycle. This extra step is known as the intermediate or preparatory step. In the
intermediate step pyruvate undergoes a decarboxylation,
meaning that a carbon dioxide is removed from the molecule carbon dioxide is a
waste product. The remaining molecule is acetate. The
acetate molecule then undergoes a redox reaction in which hydrogen is removed
from acetate and passed to the electron carrier NAD and Coenzyme A is bound to the acetate, resulting in the production of the reduced electron carrier NADH and a
molecule of acetyl-CoA. It is the acetyl-CoA that will then enter the citric
acid cycle remember two molecules of pyruvate were produced in glycolysis,
therefore for each glucose molecule the preparatory step must occur twice. you

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