Glycolysis A Quick Review by examville

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									GLYCOLYSIS
The most pressing need of all cells in the body is for an immediate source of energy. Some cells such as
brain cells have severely limited storage capacities for either glucose or ATP, and for this reason, the blood
must maintain a fairly constant supply of glucose. Glucose is transported into cells as needed and once
inside of the cells, the energy producing series of reactions commences. The three major carbohydrate
energy producing reactions are glycolysis, the citric acid cycle, and the electron transport chain.

The overall reaction of glycolysis which occurs in the cytoplasm is
represented simply as:

C6H12O6 + 2 NAD+ + 2 ADP + 2 P -----> 2 pyruvic acid, (CH3(C=O)COOH + 2 ATP + 2 NADH + 2 H+

The major steps of glycolysis are outlined in the graphic on the left. There are a variety of starting points for
glycolysis; although, the most usual ones start with glucose orglycogen to produce glucose-6-phosphate.
The starting points for other monosaccharides, galactose and fructose, are also shown.

Important Facts about Glycolysis:

The major steps of glycolysis are outlined in the graphic on the left. There are a variety of starting points for
glycolysis; although, the most usual ones start with glucose or glycogen to produce glucose-6-phosphate.
The starting points for other monosaccharides, galactose and fructose, are also shown.

Glycolysis - with white background for printing

There are five major important facts about glycolysis which are illustrated in the graphic.

1) Glucose Produces Two Pyruvic Acid Molecules:

Glucose with 6 carbons is split into two molecules of 3 carbons each at Step 4. As a result, Steps 5 through
10 are carried out twice per glucose molecule. Two pyruvic acid molecules are the end product of glycolysis
per mono- saccharide molecule.

2) ATP Is Initially Required:

ATP is required at Steps 1 and 3. The hydrolysis of ATP to ADP is coupled with these reactions to transfer
phosphate to the molecules at Steps 1 and 3. These reactions evidently require energy as well. You may
consider that this is a little strange if the overall objective of glycolysis is to produce energy. This energy is
used in the same way that it initially takes heat to ignite the burning of paper or other fuels - you need to
expand some energy to get it started.

3) ATP is Produced:

Reactions 6 and 9 are coupled with the formation of ATP. To be exact, 2 ATP are produced at step 6
(remember that the reaction occurs twice) and 2 more ATP are produced at Step 9. The net production of
"visible" ATP is: 4 ATP.
Steps 1 and 3 = - 2ATP
Steps 6 and 9 = + 4 ATP
Net "visible" ATP produced = 2.

4) Fate of NADH + H+:

Reaction 5 is an oxidation where NAD+ removes 2 hydrogens and 2 electrons to produce NADH and H+.
Since this reaction occurs twice, 2 NAD+ coenzymes are used.

If the cell is operating under aerobic conditions (presence of oxygen), then NADH must be reoxidized to
NAD+ by the electron transport chain. This presents a problem since glycolysis occurs in the cytoplasm
while the respiratory chain is in the mitochondria which has membrane that is not permeable to NADH.
This problem is solved by using glycerol phosphate as a "shuttle." - see graphic on the left. The hydrogens
and electrons are transferred from NADH to glycerol phosphate which can diffuse through the membrane
into the mitochondria. Inside the mitochondria, glycerol phosphate reacts with FAD coenzyme in enzyme
complex 2 in the electron transport chain to make dihydroxyacetone phosphate which in turn diffuses back
to the cytoplasm to complete the cycle.

As a result of the the indirect connection to the electron transport at FAD, only 2 ATP are made per NAD
used in step 5. If step 5 is used twice per glucose, then a total of 4 ATP are made in this manner.

If the cell is anaerobic (absence of oxygen), the NADH product of reaction 5 is used as a reducing agent to
reduce pyruvic acid to lactic acid at step 10. This results in the regeneration of NAD + which returns for use
in reaction 5.
ATP Summary for Glycolysis:

Starting with glucose (six carbons) how many ATP are made using aerobic glycolysis? E.T.C = electron
transport chain

                               Step                             ATP (used -) (produced +)
                                 1                                          -1
                                 3                                          -1
                   5 - NADH to E.T.C to FAD = 2
                        step 5 used twice                               2 x 2 = +4
                           6 used twice                                 1x2=+2
                           9 used twice                                 1x2=+2
                               NET                                        6 ATP

Starting with glucose (six carbons) how many ATP are made using anaerobic glycolysis? E.T.C = electron
transport chain

                               Step                             ATP (used -) (produced +)
                                 1                                          -1
                                 3                                          -1
              5 - NADH to pyruvic acid to lactic acid.
                                                                            0
                         E.T.C. not used
                          6 used twice                                  1x2=+2
                          9 used twice                                  1x2=+2
                               NET                                       2 ATP




Glycolysis literally means "sp
								
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