Carbohydrate Metabolism Overview
Citric Acid Cycle Overview Metabolism &
Energy Summary
 Elmhurst College
Glycolysis Summary Pyruvic Acid - Crossroads  Chemistry Department
Glycolysis Reactions Glycogenesis / Glycogenolysis / Gluconeogenesis  Virtual ChemBook


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Glycolysis Summary

Introduction to 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 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

Link to: Great Animation of entire Glycolysis - John Kyrk


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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.


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Important Facts about Glycolysis (cont.):

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.

Electron Transport Diagram

 
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 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

 1 x 2 = + 2

 9 used twice

 1 x 2 = + 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. E.T.C. not used

 0

 6 used twice

 1 x 2 = + 2

 9 used twice

 1 x 2 = + 2

 NET

 2 ATP

Quiz: Starting with glycogen to make glucose-6-phosphate, how many ATP are made using aerobic glycolysis?  
 Starting with glycogen to make glucose-6-phosphate, how many ATP are made using anaerobic glycolysis?