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Glucose Moleculre in Anaerobic and Aerobic Respiratory Pathways

Paper Type: Free Essay Subject: Biology
Wordcount: 2174 words Published: 8th Feb 2020

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This essay will explain the fate of a Glucose molecule as it is processed in the various anaerobic and aerobic respiratory pathways. Also, how the energy is metabolised in the glycolysis cycle, Krebs cycle and the electron transport chain.


Cellular respiration is the process that oxidises energy contained in organic molecules into simpler molecules. This energy is made available for active process of cells, which are essential for life. Activities carried out by the cell are dependent on chemical reactions, this ultimately requires more energy. Glucose is disassembled over several chemical reactions and the released energy is stored within APT molecules. Both oxidation of glucose and organisms’ cellular activities are provided by the energy store adenosine triphosphate (ATP) (Boyle 2008).

Figure 1: Left the structure of Adenosine Triphosphate (ATP), right the structure of ADP

The two types of respiration are anaerobic and aerobic respiration. Anaerobic respiration produces less energy and does not break down food molecules completely into carbon and can occur without oxygen present.

Anaerobic chemical equations;

Animals- C6H12O6 + 2 ADP  2C3H6O3 + 2 ATP

Yeast- C6H12O6 + 2 ADP  2C2H5OH + 2CO2 + 2 ATP

Whereas Aerobic respiration is a complex process that requires oxygen to breakdown food molecules in a series of steps to release more energy. The released energy is used to produce ATP from ADP and inorganic phosphate (Aldridge 1996).

Aerobic chemical equations;

C6H12O6 + 6O2 + 38 ADP  6CO2 + 6H20 + 38 ATP

(Simpkins and William 1987)

The breakdown of Glucose in respiration

Glucose broken down in respiration is derived from the hydrolysis of polysaccharides for example starch and glycogen. There are four processes of aerobic respiration of glucose; glycolysis, link reaction, Krebs cycle and electron chain transport. In anaerobic conditions glucose acts as an energy rich substrate that is oxidised to ethanol or lactic acid and carbon dioxide. Lactic acid is produced in anaerobic respiration of animals and bacteria. Ethanol and carbon dioxide are created by higher plant cells following the absence of oxygen. These two processes both undergo the common step of glycolysis. 

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1 Glycolysis

Glycolysis is the breaking down of glucose and the important oxidation steps. Weather it is anaerobic or aerobic conditions. Glycolysis occurs within the cytosol of the cytoplasm. Glycolysis uses two ATP molecules to increase the energy level of glucose molecules by adding phosphate groups to create fructose 1,6-diphosphate. The Fructose 1,6-diphosphate is separated to produce two molecules of triose phosphate (3C sugar containing a phosphate group) called 3-phosphoglyceraldehyde (PGAL). Which is then oxidised to pyruvic acid by removing hydrogen from the triose phosphate molecules, catalysed by dehydrogenase enzymes. The hydrogen is given to coenzyme nicotinamide adenine dinucleotide (NAD+) and reduced to NADH. The oxidation of PGAL molecules creates 2 additional ATP molecules to be synthesized from 2 ADP molecules and 2 molecules of pyruvate. Pyruvic acid, combined with other organic acids in the cell, appears as the anion of a salt, thus it is known as pyruvate. The fate of pyruvate depends on the organism and if oxygen is available or not. Lactic acid, the product of anaerobic respiration the NADH produced in glycolysis donates its hydrogen to pyruvic acid and lactic acid is formed. Lactate fermentation happens in animals that respire anaerobically. Furthermore, alcoholic fermentation takes place with oxygen absent in plant cells and yeast. Pyruvic acid is separated into ethanal and carbon dioxide. Ethanal is reduced to ethanol by hydrogen from NADH. The NADH+ is produced again acting as a hydrogen acceptor when more PGAL is oxidised.

Figure 5: The fate of pyruvic acid in the absence of oxygen

Figure 4: The glycolysis pathway self-produced (Khan Academy 2015)

Figure 3: Table summary of glycolysis

Molecules required

Molecules produced

1 glucose molecule

2 pyruvate molecules

2 ATP molecules

2 ADP + 2P

4 ATP molecules (net ATP=2)

2 NAD+ molecule

2 NADH molecules

If oxygen is present these two pyruvate molecules enter the link reaction.

2 Link reaction

This reaction links glycolysis and the Krebs cycle. Link reaction takes place with oxygen present. The pyruvate (3C molecule produced by glycolysis) enters the mitochondrial matrix from the cytosol. The Pyruvate is oxidised into 2 carbon compound called acetyl coenzyme A (acetyl CoA), that releases a by-product of carbon dioxide. Two molecules of NADH are also produced. The pyruvate loses a pair of hydrogen atoms by donating them to the NAD, synthesizing ATP which is then synthesized into ETC. Acetyl CoA features coenzyme A, which is an important intermediate in respiration that disperses to Krebs cycle site. As a result, hydrogen and carbon dioxide are removed from the pyruvate.

Figure 6: Table summary of the link reaction

Molecules required

Molecules produced

2 Pyruvate

2 NAD+

1 x O2

2 Acetyl-coA

2 CO2



3 Krebs cycle

This is the main central phase of the aerobic respiration of glucose that occurs in the matrix of the mitochondria. Combining two Acetyl coenzyme A with a four-carbon forms a six-carbon compound named Citrate. Citrate is gradually broken down and converted to oxaloacetate though a series of reactions of oxidative decarboxylation. Carbon atoms are removed and combined with 2 oxygen atoms forming carbon dioxide.  The dehydrogenation reaction removes pairs of hydrogen atoms and joins them to NAD+ or FAD to create NADH and FADH2, which leads to the production of ATP formed from ADP. Redox reactions are exergonic meaning they release enough energy to produce one molecule of ATP for each turn of the cycle. Two ATP molecules are produced in the Krebs cycle from each molecule of glucose that enters glycolysis. This occurs as one molecule of glucose gives two molecules of Acetyl-CoA, which doubles the ATP production of glycolysis. Hydrogen released during the Krebs cycle delivers the reducing power to create more ATP for the electron transport system. The Four-carbon compound is regenerated, allowing the cycle to commence again.

Figure 8: the Krebs cycle self-produced (Biovision 2018)

Figure 7: Table summary of the Krebs cycle

Molecules required

Molecules produced

2 Acetyl- CoA

2 CO2



6 NAD+





Electron transport chain

This reaction occurs in aerobic conditions within the cristae of the mitochondria inner membrane and is the final state of respiration. A large amount of ATP molecules is produced. Initially, hydrogen atoms are carried by coenzymes NAD and FAD. Their electrons are distributed along the chains of the carrier molecule. This process is the electron transport system. The chain has electron carrier molecules, that firstly accept an electron and then lose it, NADH and FADH2 donate electrons when hydrogen is given to the next carrier.

Consequently, coenzymes FAD and NAD become oxidised by losing its electron and hydrogen. The following carrier is reduced by gaining the electrons and hydrogen. Hydrogen separates to produce protons and electrons at a point in the electron transport system. The electrons lose energy at these transfers, which is used to aid transport of hydrogen ions across the inner mitochondrial membrane. This results in a high concentration of hydrogen ions of the outer mitochondrial space and low concentration within the inner mitochondrial space. Hydrogen diffuses down the electron chemical gradient through ATPase enzyme. Sufficient energy is released allowing free inorganic phosphate molecules to be added to ADP, converting it to ATP. This is named oxidation phosphorylation, the model of ATP synthesis using released energy from the redox reactions.

The final product of the electron transport system are low energy hydrogen ions and electrons which combine with oxygen to produce water. If oxygen is not present the pathway cannot be completed which results in intermediate compounds of glucose to accumulate and the cell loses its ability to produce the sufficient ATP it requires.

Figure 10: the electron transport chain self-produced (Boyle et al 2008)


Figure 9: In total, per 1 glucose molecules

Molecules required

Molecules produced

10 NADH molecules

10 NAD+ molecules

2 FADH2 molecules

2 FAD molecules

3 O2 molecules

6 H2O

34 ADP

34 ATP + heat


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