Respiration: Glycolysis, Krebs Cycle, and Electron Transfer Chain

Edited by Jamie (ScienceAid Editor), Taylor (ScienceAid Editor), Sharingknowledge, Jen Moreau and 1 other


Before we look at respiration, it is useful to study where respiration happens. Below is an annotated diagram of a mitochondrion: the mitochondria are important [organelles] in all cells.


Glycolysis is a series of reactions that take place in the cell cytoplasm. It involves the oxidation of glucose into pyruvate (a 3 carbon compound), that produces (overall)ATP and reduced NAD: an enzyme that carries hydrogen. The number of carbons in each of these compounds is indicated in the green circle.

Was this helpful? Yes | No| I need help

Now, the product: pyruvate (or pyruvic acid) is actively transported into the mitochondria where the link reaction occurs. Here, the pyruvate molecule combines with coenzyme A to form the 2 carbon acetylcoenzyme A. Producing carbon dioxide and hydrogen.

Was this helpful? Yes | No| I need help

Krebs Cycle

Like the link reaction, the Krebs Cycle occurs in the matrix of the mitochondrion. The reaction is as follows:

The acetyl CoA from the link reaction combines with Oxaloacetic acid to form citric acid, this is then broken down to produce some ATP and reduced NAD and FAD (NADH and FADH).

The many hydrogen molecules carried by NAD and FAD are also used to produce ATP, by being taken to a further stage.

Electron Transfer Chain

This process takes place on the inner mitochondrial membrane which is folded to cristae. This provides a large surface area for the electron transfer chain to take place.

The carriers FAD and NAD bring the hydrogen and it separates to H+ and electrons (e-). The electrons pass from carrier to carrier and loose energy. This is used to synthesize ATP.

However, there are a lot of hydrogen ions, that unless they are removed, they'll cause a large increase in pH. Therefore, oxygen reacts with the ions to remove it and produce water. This is what the oxygen you inhale is used for (in terms of respiration).

Was this helpful? Yes | No| I need help

Questions and Answers

What are the organic molecules in the citric cycle. And what is the enzyme that breaks down from glycolysis to the citric reporting period?

I have an exam tomorrow and would like a simple review of glycolysis cycle

Molecules in Citric Cycle

  • NADH: An energy shuttle which delivers high-energy electrons to the electron transport chain where they will eventually power the production of 2 to 3 ATP molecules. When this electron shuttle is not carrying high energy electrons, meaning it has been oxidized (lost its electrons), it is left with a positive charge and is called NAD+
  • FADH2: Another energy shuttle that carries high-energy electrons to the electron transport chain, where they will ultimately drive production of 1 to 2 ATP molecules. The oxidized form of FADH2 is FAD and happens just like in NADH.
Was this helpful? Yes | No| I need help

High energy molecules: ATP: The basic energy currency of the cell. It's a form of energy that cells can use right away GTP: Similar to ATP, GTP can be easily converted to ATP in the cell.

Enzyme that breaks Pyruvates to Citric cycle Pyruvate, which is formed by the glycolytic catabolism of carbohydrates in the cytosol, after it enters mitochondrial matrix undergoes oxidative decarboxylation by a complex set of reactions catalyzed by a pyruvic dehydrogenase. The reactions catalyzed by pyruvic dehydrogenase require the participation of several coenzymes, including NAD+ and Coenzyme A.

Was this helpful? Yes | No| I need help

Glycolysis Cycle Summary

Glycolysis occurs in the cytoplasm of the cell and is present in all living organisms. Glycolysis steps are broken down below

  • In this process, glucose undergoes partial oxidation to form two molecules of pyruvic acid.
  • In plants, this glucose is derived from sucrose, which is the end product of photosynthesis, or from storage carbohydrates.
  • Sucrose is converted into glucose and fructose by the enzyme, invertase, and these two monosaccharides readily enter the glycolytic pathway.
  • Glucose and fructose are phosphorylated to give rise to glucose-6- phosphate by the activity of the enzyme hexokinase. This phosphorylated form of glucose then isomerizes to produce fructose-6- phosphate. Subsequent steps of metabolism of glucose and fructose are same.
  • In glycolysis, a chain of ten reactions, under the control of different enzymes, takes place to produce pyruvate from glucose.
  • ATP is utilized in two steps: first in the conversion of glucose into glucose 6-phosphate and second in the conversion of fructose 6-phosphate to fructose 1, 6-bisphosphate.
  • The fructose 1, 6-bisphosphate is split into dihydroxyacetone phosphate and 3-phosphoglyceraldehyde (PGAL). We find that there is one step where NADH + H+ is formed from NAD+; this is when 3-phosphoglyceraldehyde (PGAL) is converted to 1, 3-bisphosphoglycerate (BPGA). Two redox-equivalents are removed (in the form of two hydrogen atoms) from PGAL and transferred to a molecule of NAD+. PGAL is oxidized and with inorganic phosphate to get converted into BPGA.
  • The conversion of BPGA to 3-phosphoglyceric acid (PGA), is also an energy yielding process; this energy is trapped by the formation of ATP. Another ATP is synthesized during the conversion of PEP to pyruvic acid.
  • Pyruvic acid is then the key product of glycolysis.
  • There are three major ways in which different cells handle pyruvic acid produced by glycolysis. These are lactic acid fermentation, alcoholic fermentation, and aerobic respiration. Fermentation takes place under anaerobic conditions in many prokaryotes and unicellular eukaryotes. For the complete oxidation of glucose to CO2 and H2O, however, organisms adopt Krebs' cycle which is also called as aerobic respiration. This requires O2.

Cristae and matrix correct in the diagram? I thought that they were supposed to the other way around?

Are the crust and the matrix labeled correctly in that diagram? I am very confused my diagrams have them labeled the other way around and now I do not know what is correct

ScienceAid QnA. This section is not written yet. Want to join in? Click EDIT to write this answer.

Glycolysis, the link reaction, the Krebs cycle, and the electron transport chain are all part of?

1. Photosynthesis. 2. Anaerobic Respiration . 3. Aerobic Respiration. 4. Fermentation. I didn't look at the information at all yet. Sorry!

ScienceAid QnA. This section is not written yet. Want to join in? Click EDIT to write this answer.

What is Glycolysis Kreb Cycle and Electron Transport chain?

I want to know everything to be known about them, the substrate level phosphorylation, oxidation phosphorylation and amount of ATP PYRUVATE and ADP produced in each of them

ScienceAid QnA. This section is not written yet. Want to join in? Click EDIT to write this answer.

Referencing this Article

If you need to reference this article in your work, you can copy-paste the following depending on your required format:

APA (American Psychological Association)
Respiration: Glycolysis, Krebs Cycle, and Electron Transfer Chain. (2017). In ScienceAid. Retrieved Feb 27, 2024, from

MLA (Modern Language Association) "Respiration: Glycolysis, Krebs Cycle, and Electron Transfer Chain." ScienceAid, Accessed 27 Feb 2024.

Chicago / Turabian "Respiration: Glycolysis, Krebs Cycle, and Electron Transfer Chain." Accessed Feb 27, 2024.

If you have problems with any of the steps in this article, please ask a question for more help, or post in the comments section below.


ScienceAid welcomes all comments. If you do not want to be anonymous, register or log in. It is free.

Article Info

Categories : Biochemistry

Recent edits by: Jen Moreau, Sharingknowledge, Taylor (ScienceAid Editor)

Share this Article:

Thanks to all authors for creating a page that has been read 28,865 times.


Thank Our Volunteer Authors.

Would you like to give back to the community by fixing a spelling mistake? Yes | No