How many NADH are produced by glycolysis?

a. 2

b. 5

c. 1

d. 4

e. 3

Diagram of aerobic cellular respiration
Diagram of aerobic cellular respiration

The correct answer is A. 2

Cellular respiration is the mechanism by which cells use sugar to form energy in the form of ATP. The ATP is then used to provide the energy for other cellular reactions and cell transport mechanisms that take place in the cell.

There is both anaerobic respiration (when oxygen is absent) and aerobic respiration (when oxygen is present) processes that occur in living eukaryotic cells.

The first set of all reactions, glycolysis, occurs regardless of the presence of oxygen. These reactions take place in the cytoplasm of the cell and result in the formation of pyruvate and two NADH molecules.

If oxygen is present the pyruvate enters the Kreb’s cycle which occurs in the matrix of the mitochondrion. This cycle produces six NADH, two FADH, and two ATP molecules.

The NADH and FADH carry electrons into the final stage, the electron transport chain. These electrons provide energy to drive the establishment of a proton gradient across the cristae.

The movement of protons through channels lined with ATP synthase generates ATP.  A total of 32 ATP molecules are produced during the electron transport chain reactions.

Cellular respiration

The goal of cell respiration is to produce energy in the form of ATP by breaking down a glucose molecule in a series of reactions.

Cellular respiration in eukaryotic cells can be aerobic and for a short while, anaerobic. Anaerobic respiration occurs when no oxygen is present and it includes glycolysis and fermentation reactions.

Aerobic respiration occurs in the presence of oxygen and includes three stages: glycolysis, the Kreb’s cycle and the electron transport chain.

The first stage, glycolysis, occurs in the cytoplasm while the other two stages take place in the mitochondrion of the cell.

The mitochondrion consists of an outer and an inner membrane. The inner membrane is folded into projections called cristae, which increase the area for the reactions of the electron transport chain.

Glycolysis

The glycolysis reactions always occur in the cytoplasm of a cell, and they occur even if no oxygen is present. The process involves several enzyme-catalyzed reactions in which a glucose molecule is modified to form different intermediates.

Some energy is used and formed during glycolysis. Adenosine triphosphate (ATP) carries a great deal of energy in the phosphate bonds of the molecule. Thus, when needed a phosphate bond of the molecule can be broken and energy released.

This energy is then used for the reaction in which the glucose is transformed into a glucose 6-phosphate molecule. Several other intermediates are formed and eventually the molecule glyceraldehyde-3-phosphate is produced, which is converted to pyruvate.

A small amount of energy is also generated in the cell by reduction reactions of ADP and NAD+ to produce ATP and NADH. By the end of glycolysis, four molecules of ATP are formed and two molecules of NADH are produced.

Kreb’s cycle

The second set of reactions of aerobic respiration takes place in the matrix of the mitochondrion. It is important to note, these reactions are known as the Kreb’s cycle and only take place if oxygen is present.

Pyruvate that was formed in glycolysis is actively transported across the mitochondrial membrane to enter the cycle. This molecule then loses electrons in an oxidation reaction, resulting in the formation of two acetyl groups.

While this reaction is occurring, NAD+ undergoes a reduction reaction in which it gains electrons to form NADH.

An intermediate 6-carbon molecule known as citrate is produced by a reaction in which oxaloacetate and an acetyl group combine. At the same time, carbons are stripped off the molecules and released as carbon dioxide gas.

The breaking of bonds of the various carbohydrate intermediate molecules is used to form ATP from ADP and an inorganic phosphate. More NAD+ is reduced to form NADH and another molecule, FAD+ is reduced to produce FADH.

These molecules will be used to carry electrons into the final stage of aerobic respiration. At the end of the Kreb’s cycle, two ATP, six NADH, and two FADH are formed.

Electron transport chain

The NADH and FADH enter the electron transport chain system which occurs in the cristae (inner membrane) of the mitochondrion.

An oxygen molecule is used to drive the reactions and “pull” the electrons down a chain of molecules. This is possible because oxygen is a very electronegative molecule.

Electrons from the two carrier molecules, the FADH and NADH are passed from one molecule to another and in the process, they give the energy to help pump protons across the membrane.

The idea is to establish a proton gradient in which there is a difference in the concentration of protons on either side of the cristae.

ATP synthesis

With the proton pumps in action, it causes a large number of protons to accumulate in the intermembrane space of the cristae. The proton concentration in the matrix is thus lower than in this space when the gradient is established.

This causes the protons to passively diffuse back through the membrane and into the matrix along their concentration gradient from high to low.

Although this is a passive transport process, the protons only move back across the membrane through special integral protein channels.

These proteins are lined with ATP synthase which is the enzyme needed to form ATP. As protons move through a channel they activate the reaction in which ADP is reduced to form ATP.

A large number of ATP molecules are synthesized in this fashion, and by the end of the electron transport chain, a total of 32 ATP are formed. The final electron acceptor in the chain is oxygen, which accepts electrons to form water.

The process is known as chemiosmotic phosphorylation since it involves the movement of molecules across a membrane which then phosphorylates ADP.

It is largely due to the electron transport chain that large numbers of ATP are produced during the process of aerobic cellular respiration.

References

  1. PH Raven, RF Evert, SE Eichhorn (1987). Biology of Plants. Worth Publishers.
  2. C Rye, R Wise, V Jurukovski, J DeSaix, J Choi, and Y Avissar. (2017). Biology. Rice University.
  3. RL Dorit, WF Walker, RD Barnes (1991). Zoology. Saunders College Publishing.
  4. Editors of Encyclopedia Britannica (2019). Glycolysis. Retrieved from Britannica.com.
  5. Editors of Encyclopedia Britannica (2019). Cellular respiration. Retrieved from Britannica.com.

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