Tag Archives: Glycolysis

Electron Transport Chain Video Review #1

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Video link: Khan Academy. 2009. “Electron Transport Chain” http://www.youtube.com/watch?v=mfgCcFXUZRk

After viewing this video, I gathered that after glycolysis, 10 NADH and 2 FADH2 are left.  These are used in the Electron Transport Chain. This process is used to generate ATP. In the video, he states that the NADH is indirectly responsible for 3 ATP and each FADH2 is indirectly responsible for  the generation 2 ATP molecules. This is because the electrons that are entering the ETC are at a slightly lower energy level than the ones of NADH. The oxidation of NADH (NADH—->NAD+ + H+ + 2e– ) is the 1st step of the ETC. The last step involves (2e– + H+ + ½ O2 —-> H2O) the reduction of oxygen to water; the oxidation of NADH to NAD+.

The 2e- used in oxidation then gets transported to a series of transition molecules, entering slightly lower energy states.  They are then used in the reduction of oxygen to water. When an electron goes from a higher energy state to a lower energy state, it releases energy.This energy is used to pump protons across the membrane of the cristae of the mitochondria. The oxidation and reduction processes occur in protein complexes located in the matrix of the mitochondria. When these proteins release energy, it is used to pump Hydrogen protons in particular into the outer membrane. The oxidation of NADH releases its by-product.  As a result, the outer membrane becomes more acidic than the matrix. An electric gradient/potential is the created between the outer (positive) membrane and the inner (negative) membrane.  

When this gradient forms, the hydrogen protons try to re-enter the matrix. ATP formation occurs in the cristae via the protein ATP synthase. These hydrogen ions enter the inner matrix via ATP synthase. This axle like structure on the top of the matrix as well as an extended part at the bottom allows the ions to enter via the spinning of the axle on the top. An ADP molecule and its 2 phosphate groups attach to 1 part  of the protein. The phosphate also attaches to another part of the protein. As the inner axle turns, the outer housing of the membrane due to electrical charges will squeeze the ADP and the phosphate together to form ATP. This occurs on 3 different sites simultaneously producing 3 ATP.

ATP Synthase

Summary:

Electrons are moving from the NADH and the FADH2 to essentially reduce O2. As they do this, they release energy as they go from 1 molecule to another. This energy is used to pump Hydrogen protons into the outer compartment of the mitochondria. The gradient created, makes the hydrogen protons want to enter the inner matrix. As they re-enter, this force drives the ATP Synthase “engine” which produces the ATP.

Left from Glycolysis:

10 NADH —-> 30 ATP

2 FADH2 —–> 4 ATP

Glycolysis and Krebs Cycle produces:

4 ATP molecules.

This amounts to 38 ATP molecules, from 1 molecule of Glucose.

Glycolysis breakdown.

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What is Glycolysis?

Glycolysis means the splitting of sugar. In glycolysis, glucose (a 6 carbon sugar) is split into 2 molecules of a 3-carbon sugar. Glycolysis yields 2 molecules of ATP (free energy containing molecule), 2 molecules of pyruvic acid & 2 “high energy” electron carrying molecules of NADH. Glycolysis can occur with (aerobically) or without oxygen (anaerobically). In the presence of oxygen, glycolysis is the first stage of cellular respiration. Without oxygen, glycolysis allows cells to make small amounts of ATP. This process is called fermentation.

The process of Glycolysis occurs through 10 steps.

Step 1

The enzyme hexokinase phosphorylates (adds a phosphate group to) glucose in the cell’s cytoplasm. In the process, a phosphate group from ATP is transferred to glucose producing glucose 6-phosphate.

  • Glucose (C6H12O6) + hexokinase + ATP → ADP + Glucose 6-phosphate (C6H11O6P1) 

Step 2

The enzyme phosphohexose isomerase converts glucose 6-phosphate into its isomer fructose 6-phosphate. Isomers have the same molecular formula, but the atoms of each molecule are arranged differently.

  • Glucose 6-phosphate (C6H11O6P1) + Phosphoglucoisomerase → Fructose 6-phosphate (C6H11O6P1)

Step 3

The enzyme phosphofructokinase-1 uses another ATP molecule to transfer a phosphate group to fructose 6-phosphate to form fructose 1, 6-bisphosphate.

  • Fructose 6-phosphate (C6H11O6P1) + phosphofructokinase + ATP → ADP + Fructose 1, 6-bisphosphate (C6H10O6P2) 

Step 4

The enzyme aldolase splits fructose 1, 6-bisphosphate into two sugars that are isomers of each other. These two sugars are dihydroxyacetone phosphate and glyceraldehyde phosphate.

  • Fructose 1, 6-bisphosphate (C6H10O6P2) + aldolase → Dihydroxyacetone phosphate (C3H5O3P1) + Glyceraldehyde phosphate (C3H5O3P1)

Step 5

The enzyme triose phosphate isomerase rapidly inter-converts the molecules dihydroxyacetone phosphate and glyceraldehyde phosphate. Glyceraldehyde phosphate is removed as soon as it is formed to be used in the next step of glycolysis.

  • Dihydroxyacetone phosphate (C3H5O3P1) → Glyceraldehyde phosphate (C3H5O3P1) 

Net result for steps 4 and 5: Fructose 1, 6-bisphosphate (C6H10O6P2) ↔ 2 molecules of Glyceraldehyde phosphate (C3H5O3P1)

Step 6

The enzyme Glyceraldehyde 3-phosphate dehydrogenase serves two functions in this step. First the enzyme transfers a hydrogen (H–) from glyceraldehyde phosphate to the oxidizing agent nicotinamide adenine dinucleotide (NAD+) to form NADH. Next triose phosphate dehydrogenase adds a phosphate (P) from the cytosol to the oxidized glyceraldehyde phosphate to form 1, 3-bisphosphoglycerate. This occurs for both molecules of glyceraldehyde phosphate produced in Step 5.

  • Triose phosphate dehydrogenase + 2 H– + 2 NAD+ → 2 NADH + 2 H+
  • Triose phosphate dehydrogenase + 2 P + 2 glyceraldehyde phosphate (C3H5O3P1) → 2 molecules of 1,3-bisphosphoglycerate (C3H4O4P2) 

Step 7

The enzyme phosphoglycerate kinase transfers a P from 1,3-bisphosphoglycerate to a molecule of ADP to form ATP. This happens for each molecule of 1,3-bisphosphoglycerate. The process yields two 3-phosphoglycerate molecules and two ATP molecules.

  • 2 molecules of 1,3-bisphoshoglycerate (C3H4O4P2) + phosphoglycerate kinase + 2 ADP → 2 molecules of 3-phosphoglycerate (C3H5O4P1) + 2 ATP 

Step 8

The enzyme phosphoglycerate mutase relocates the P from 3-phosphoglycerate from the third carbon to the second carbon to form 2-phosphoglycerate.

  • 2 molecules of 3-Phosphoglycerate (C3H5O4P1) + phosphoglyceromutase → 2 molecules of 2-Phosphoglycerate (C3H5O4P1)

Step 9

The enzyme enolase removes a molecule of water from 2-phosphoglycerate to form phosphoenolpyruvic acid (PEP). This happens for each molecule of 2-phosphoglycerate.

  • 2 molecules of 2-Phosphoglycerate (C3H5O4P1) + enolase → 2 molecules of phosphoenolpyruvic acid (PEP) (C3H3O3P1) 

Step 10

The enzyme pyruvate kinase transfers a P from PEP to ADP to form pyruvic acid and ATP. This happens for each molecule of PEP. This reaction yields 2 molecules of pyruvic acid and 2 ATP molecules.

  • 2 molecules of PEP (C3H3O3P1) + pyruvate kinase + 2 ADP → 2 molecules of pyruvic acid (C3H4O3) + 2 ATP

In summary, a single glucose molecule in glycolysis produces a total of 2 molecules of pyruvic acid, 2 molecules of ATP, 2 molecules of NADH and 2 molecules of water. 

Although 2 ATP molecules are used in steps 1-3, 2 ATP molecules are generated in step 7 and 2 more in step 10. This gives a total of 4 ATP molecules produced. If you subtract the 2 ATP molecules used in steps 1-3 from the 4 generated at the end of step 10, you end up with a net total of 2 ATP molecules produced.

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Multiple choice questions on Glycolysis.

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Select the correct multiple answer by using ONE of the keys A, B, C, D or E below.

A) 1,2,3 are correct

B) 1 & 3 are correct

C) Only 4 is correct

D) 1 & 2 are correct

E) All are correct

1) Glycolytic pathway regulation involves:

1) Mg2+ as a cofactor.

2) Feedback, Product, Inhibition by ATP.

3) Allosteric simulation by ADP.

4) Allosteric inhibition by ATP.

5) All of the above.

2) Which of these enzymes regulates the glycolytic steps?

1)Phosphofructokinase.

2) Hexkinase.

3) Pyruvate Kinase.

4) Lactase.

3)Fructose-2,6 Bisphosphate:

1) Activates Phosphofructokinase.

2) Activates Glyceraldehyde-3-Phosphate.

3) Inhibits Fructose-1,6 Bisphosphate.

4) Inhibits Fructose-6-Phosphate.

4) Glucose from the breakdown of Glycogen is acquired from:

1) The muscles by phosphorolysis.

2) The liver by phosphorolysis.

3) The muscles by hydrolysis.

4) Feeder pathways.

5) Glycogen has:

1) α-1,4 linkages.

2) α-1,6 linkages.

3) α-1,4 and β-1,6 linkages.

4) α-1,4 and α-1,6 linkages.

5) α-1,6 and β-1,6 linkages.