Cellular Respiration (I) Overview

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Cellular Respiration I. Overview

Last Update: October 30, 2024

Introduction

This is Part I. of a 5 part series. You can link to the others using the list below:

Cellular Respiration in our bodies is (mainly) the oxidation of glucose into carbon dioxide and water and energy. 

It involves a series of chemical reactions that break down glucose in the presence of oxygen.

In this post we’ll provide an overview of the four stages of Cellular Respiration:

  1. Glycolysis 
  2. Pyruvate to Acetyl CoA 
  3. Krebs Cycle
  4. Oxidative Phosphorylation 

note: Some textbooks combine Stages 1 and 2

Use the video and document references in the Appendix as needed.   

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Central Metabolism

The picture below is a high level depiction of the cellular respiration pathway in the human body.

Remarkably it’s very similar in most other living organisms as well!

Glucose is the key micronutrient that our body relies on to survive (recall this is a breakdown product of carbohydrates).

For example, our brain primarily relies on glucose for energy.

“What about the protein and fat we eat?”, you might ask.

These can also be used for energy (they are also key ingredients in many anabolic reactions) and can enter this respiration pathway in different ways.

This is why the Cellular Respiration pathway is described as Central Metabolism (practically all metabolic paths for different nutrients pass through it in some way).

Picture_Central Metabolism (Cellular Respiration)

Central Metabolism Chart

Cellular respiration is (mainly) the oxidation of glucose into carbon dioxide and water and energy:
 
C6H12O6 (glucose) + 6O2(oxygen) —> 6CO2(carbon dioxide) + 6H2O(water) + Energy (is captured in ATP and released as heat)
 
You live and breath because of cellular respiration:
  • You consume, digest, and “process” food with the assistance of oxygen.
  • When you breath out, you are rejecting CO2 from your body.
Most importantly, cellular respiration produces a lot of energy that your body needs to live and grow.
 
The energy produced is in the form of chemical potential energy carried in
  • intermediate molecules (NADH and FADH2) and 
  • also carried in the ultimate desired final molecule, ATP (where about 32 molecules are made for each molecule of Glucose). 

Cellular Respiration Occurs in 4 stages.

  1. Glycolysis breaks down molecules and generates energy carrying molecules like NADH and ATP.
  2. Glucose breakdown molecules are converted to the first reactant in the Krebs cycle.
    1. This is a relatively simple stage compared to the others. 
  3. The Krebs Cycle then builds up and breaks down molecules to generate more energy carrying molecules like NADH, FADH2, and ATP.
    1. This cycle also creates building block intermediates for other molecules . 
  4. Oxidative Phosphorylation takes the energy from NADH and FADH2 and creates (a lot) more ATP.
    1. Oxidative Phosphorylation occurs due to the
      1. Electron Transport Chain (ETC) and
      2. Chemiosmosis

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Cellular Respiration is a Complicated Multi-Reaction Process

  • In Stage 1, Glycolysis is usually described as a 10 step chemical process, 
  • where the 6 carbon glucose is first destabilized (Investment Phase) and then broken down into smaller 3 carbon units called Pyruvate (Payoff Phase). 
  • Glycolysis occurs in a cell’s cytoplasm.
  • Glycolysis does not need oxygen. It is an anaerobic process. 
  • In Stage 2, Pyruvate is converted to a molecule called Acetyl CoA in the mitochondria of a cell.
  • Oxygen is required for the Pyruvate to Acetyl CoA reaction. 
  • Stage 2 and later stages require oxygen (aerobic). 
  • Acetyl CoA enters the Krebs Cycle (Stage 3) by reacting with the final intermediate chemical produced by the Krebs Cycle (Oxaloacetate)
  • The Krebs cycle goes by a few other names:
    • Citric Acid Cycle: Named after first molecule formed in the cycle. 
    • Tricarboxylic Acid (TCA) Cycle: Identifies the presence of three carboxylic acid groups in the first two intermediate molecules of the cycle.
    • Krebs Cycle: Named after Hans Krebs; one of the key biochemists who described the pathway.
  • The Krebs Cycle begins when Acetyl CoA reacts with Oxaloacetate, to form Citrate.
  • There are typically 9 intermediate chemicals described in the Krebs cycle (from the 6 carbon Citrate to the 4 carbon Oxaloacetate , back to Citrate etc.) 
  • Oxidative Phosphorylation ,Stage 4 of Cellular Respiration, is an electrochemical process that occurs in the Mitochondria of cells. 
    • Oxidative Phosphorylation occurs due to the Electron Transport Chain and Chemiosmosis. 
    • The Electron Transport Chain are a series of protein molecules that facilitate the movement of electrons donated from NADH and FADH2. 
    • The resultant electrochemical gradient allows ATP to be formed via Chemiosmosis. 
    • NADH and FADH2 are regenerated back to NAD+ and FAD and the cycle continues.

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Pyruvate to Lactic Acid in Anaerobic Conditions

We noted that Oxygen is needed for Pyruvate to react to Acetyl CoA. 

In the absence of oxygen (anaerobic conditions),

  • Pyruvate will react to Lactic Acid in a process called Metabolic Fermentation.
  • In this process, NAD+ is regenerated from NADH (Pyruvate acts as an electron acceptor)
  • This helps with NADH supply as the ETP becomes hindered in its NADH regeneration ability. 

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Cellular Respiration Energy Molecules Balance

  • Cellular Respiration produces about 32 molecules (30 – 32) of ATP for each molecule of Glucose.  
  • ATP is made directly in the process and indirectly from NADH and FADH2.
  • For each molecule of Glucose:
    • (a) Glycolysis makes 2 ATP(net) and 2 NADH. 
    • (b) The Pyruvate to Acetyl CoA reaction makes 2 NADH. 
    • (c) The Krebs Cycle makes 2 ATP, 6 NADH, and 2 FADH2.
    • (d) The ETP and Oxidative Phosphorylation make roughly 28 more ATPs (using the energy donated by NADH and FADH2).  
    • We are assuming that each NADH is responsible for making about 2.5 ATP (can vary based on efficiency of reactions)
    • We are assuming each FADH2 is making about 1.5 ATP (can vary based on efficiency of reactions). 
  • ATP will carry a lot of potential energy in its outer phosphorous-oxygen bonds , and will release this energy when the body requires it. 
  • When ATP loses a phosphate group it releases energy and become ADP, Adenosine Diphosphate.

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Conclusion

Cellular respiration is a metabolic process that occurs in the Mitochondria of cells to produce ATP, the primary energy source for cellular processes. 

In this general overview article, we described the four major chemical reaction stages that result in the production of roughly 32 molecules (30 – 32) of ATP for each molecule of Glucose.  

These stages are:

  1. Glycolysis breaks down molecules and generates energy carrying molecules like NADH and ATP.
  2. Pyruvate reacts to Acetyl CoA which enters the Krebs Cycle.     
  3. The Krebs Cycle builds up and breaks down molecules to generate more energy carrying molecules like NADH, FADH2, and ATP. 
  4. Oxidative Phosphorylation (Electron Transport Chain and Chemiosmosis) takes the energy from NADH and FADH2 and creates (a lot) more ATP.  

You can now start reading my detailed posts on each of the phases of Cellular Respiration. See the post on Glycolysis via the link below

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Appendix – Cellular Respiration References

Videos

Documents

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