Cellular Respiration (level 1/chapter 6 - honors/chapter 9)

 

Introductory terms

 

Oxidation - the loss of electrons.

Reduction - to gain electrons.

Redox reactions - any reaction that involves the loss and gain of electrons.

Redox pathway - series of electronic transfers between proteins e.g. the electron transport chain.

Electron transport chain - series of proteins that relay electrons in the process of ATP fromation. Located in the inner membrane of a mitochondrion.

Cytochromes - metal containing proteins of the electron transport chain that relay electrons.

Proton pumps - molecular metaphor for the process of transfering protons from the inner matrix of the mitochondrion, across the inner membrane of the mitochondrion to the intermembrane space.

Oxygen is the final electron acceptor in the electron transport chain.

1/2 O2 + 2e- +2 H+ --> H2O

 

 

As electrons "fall" down the electron transport chain they generate an electrochemical gradient by "pumping" protons from the matrix to the intermembrane space.

The electron transport chain acts as a series of three proton pumps that delivers protons from the matrix to the inner membrane space.

Chemiosmosis - the process that uses electrons to generate a proton gradient, which can channel its energy to drive the ATP synthase.

Protons are pumped onto one side of the inner membrane. The positively charged protons repel each other and return down their gradient to the matrix. The selectively permeable inner membrane of the mitochondrin will only allow the protons to return through a protein channel. This energy release (electronic current) drives an ATP synthase complex that makes ATP from ADP and Pi.

 

Mitochondrial Electron Transport  

 

Dehydrogenases - transfer hydrogen atoms to the electon carrier molecules

NAD +

NADH

FAD

FADH2

 

2H = 2e- + 2H+

 

electon carrier molecule - a dinucleotide that acts as an electron shuttle molecule

 

NADH and FADH2 shuttle their electrons to the electron transport chain where a cell can convert each NADH to 3 ATP and each FADH2 to 2 ATP.

 

 

 
 

Substrate-level phosphorylation - to make ATP in accord with an enzyme dependent reaction.

Oxidative phosphorylation - to make ATP by the process of chemiosmosis.

Proton motive force (pmf) is the Delta G or driving force of ATP synthesis.

Proton gradient - an electrochemical gradient made up of protons.

Electochemical gradient - gradient of a charged substance.

ATP synthase - an enzyme complex situated in the inner membrane of the mitochondrion. It is the molecular machinery that can capture "positive current" (protons returning to the matrix) as the chemical energy stored in ATP.

Biochemical reactions of cellular respiration include:

Glycolysis - Ancient anerobic process that occurs in the cytosol of all cells.

The oxidation of pyruvic acid - produes CO2 and NADH, as well as, acetyl CoA.

Krebs cycle - pathway worked out by Sir Hans Krebs.

(see summary below)

The oxidation of pyruvic acid -

Acetyl Coenzyme A - electron rich two carbon acetyl group is processed in the Krebs cycle.

CoA - hands off the acetyl group to the Krebs cycle. A resulting six carbon citric acid gives up many of its electrons in a series of eight reactions that produces CO2 and reduced electron carries like NADH and FADH2, as well as 1 substrate level ATP.
 

Flowchart for the Complete Oxidation of Glucose

 

Step 1. Glycolysis (cytosol)
Glucose > 2 Pyruvic Acid + 2 NADH + 2 ATP

yield 2 NADH + 2 ATP/ glucose

 

Step 2. Oxidation of Pyruvic Acid [(mitochondrion) site of aerobic respiration]

Pyruvic Acid --> Acetyl CoA + CO2 + NADH

yield 2 NADH/glucose

 

Step 3. Krebs Cycle - (matrix of mitochondrion)

Acetyl CoA + four carbon molecule > 8 reactions

to yield 1 ATP + 3 NADH + 1 FADH2 + 2 CO2

yield 2 ATP + 6 NADH + 2 FADH2/glucose

 

 

Oxidative Phosphorylation - the process by which the energy captured by the electron carriers (NADH and FADH2) is used to make ATP.

Terminal Electron Transport (inner membrane of the mitochondrion)

  • Electron carriers in the matrix of the mitochondrion release their high energy captured electron to the cytochromes of the electron transport chain.
  • Oxygen is the final electron acceptor.

 

Chemiosmotic coupling - actual mechanism

As electrons drop down the transport chain, protons are pumped from the matrix into the intermembrane space.

potential energy is created by

  • proton gradient across the membrane
    • difference in protons/difference in electric charge
  • protons can only return through a specialized channel of the ATP synthase

Oxidative Phosphorylation - the process by which the energy captured by the electron carriers (NADH and FADH2) is used to make ATP. Includes the electron transport chain and chemiosmotic coupling mechanisms. Produces the bulk of ATP molecules associated with the complete oxidation of glucose. Some ATP is produced at the substrate level, however, most ATP is produced by oxidative phosphorylation.

Terminal Electron Transport (inner membrane of the mitochondrion)

  • System of proteins (cytochromes) that relays high energy electrons from coenzyme electron carriers (NADH and FADH2) through a series of oxidation reduction reactions.
  • Oxygen is the final electron acceptor of the electron transport chain. Oxygen is reduced to water.

 

2 H+ + 2 e- + 1/2 O2 --> H2O

 

The "downhill" flow of electrons powers the proton pumps which pass protons from the matrix of the mitochondrion to the inner membrane space.

Proton Pump - active transport mechanism to move H+ to the inner membrane space of the mitochondrion. Creates the electrochemical gradient.

 

The Electrochemical Gradient - two components

1) proton gradient - provides potential energy

2) electrical energy - high density of + electrical charge.

 

This energy drives the ATP synthase complex - a protein complex found in the inner membrane of the mitochondrion that provides a channel for the back flow of protons to the matrix. protons moving through this channel provide the actual chemiosmotic coupling mechanism whereby the transport of protons (both pumps and channel) are coupled with the (chemical) synthesis of ATP from ADP and P.

Relevant sites

Cellular Respiration: The Big Picture 

Mitochondrial Electron Transport

Cellular Respiration and Fermentation

Muscle Soreness and Weightlifting

Use of Metabolic Poisons to Study Cellular Respiration

Quiz yourself

 

1. What are the products of the oxidation of pyruvic acid?

Oxidation of Pyruvic Acid - Oxidation of pyruvate - transition reaction

  • This reaction links glycolysis to the Krebs cycle.
  • Takes place in the mitochondria.
  • Pyruvic acid oxidized to acetyl group and one CO2; acetyl group transferred to coenzyme A (CoA).  

How many times does this reaction occur per glucose molecule?

What is the energy yield of this reaction?

 

2. Where does this occur?

 

3. Write a complete and balanced equation for the oxidation of pyruvic acid.

 

Oxidative Respiration - Mechanism

 

4. What is the relationship of the Krebs cycle and the electron transport chain?

 

5. What are electron carrier molecules?

 

6. Where does the Krebs cycle occur?

  

7. Where are the proteins found that make up the electron transport chain?

 

8. What are these proteins called?

Cytochromes, which also are utilized in energy transfer inphotosynthesis, are electron transferring proteins that act in sequence to transfer electrons to O2. The prosthetic group in all cytochromes is heme, which undergoes reversible FeII-FeIII oxidation.

 

9. What is the final electron acceptor in the electron transport chain?

  

10. Write a complete and balanced chemical equation to describe this event.

 

Check Your Answers