Respiration is a process that occurs in all living beings in which oxygen is utilised and carbon dioxide is released from the body. The mechanism of cellular respiration involves the following mechanism:
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Glycolysis
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Anaerobic Breakdown of Pyruvic acid
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Krebs Cycle
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Electron Transport system
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Terminal oxidation and oxidative phosphorylation
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Pentose phosphate pathway
Here, in the article, let us discuss the difference between the Krebs Cycle and glycolysis but first let us take a look at what each of these terms means.
Glycolysis – It is an anaerobic process in which a molecule of glucose is converted into two molecules of pyruvic acid. It takes place in the cytoplasm
Krebs Cycle – It is an aerobic process that takes place in the mitochondria that involves the oxidation of pyruvic acid into water and carbon dioxide.
Given below in a tabular column are the differences between glycolysis and Krebs Cycle.
Differences Between Glycolysis and Krebs Cycle
|
Glycolysis |
Krebs Cycle |
|
It is the first step in respiration in which glucose is broken down into two molecules of pyruvate. |
Krebs Cycle is the second step of respiration in which it degrades pyruvate into inorganic substances (water and carbon dioxide). |
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Occurs inside the cytoplasm. |
Occurs inside the mitochondria. |
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No carbon dioxide evolved. |
Carbon dioxide evolved. |
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One molecule of glucose liberates 4 ATP molecules through substrate-level phosphorylation. |
Two acetyl residues liberate two ATP and GTP molecules through substrate-level phosphorylation. |
|
Oxygen is not required for glycolysis. |
Oxygen is required for Krebs Cycle. |
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Occurs as a linear sequence. |
Occurs as a cyclic sequence. |
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Consumes 2 molecules of ATP for initial phosphorylation of substance molecules. |
Doesn’t consume ATP. |
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Two molecules of ATP and two molecules of NADH gained for every molecule of glucose broken down. |
Six molecules of NADH and two molecules of FADH2 for every acetyl-CoA oxidised. |
Both glycolysis and Krebs are enzymes medicated and are under constant regulation based on the energy requirements of cells/organisms. The rates of these processes vary under various conditions such as the well-fed state, fasting state, exercised state, and starvation state.
The Krebs cycle or Citric acid cycle is a series of enzyme catalysed reactions occurring in the mitochondrial matrix, where acetyl-CoA is oxidised to form carbon dioxide and coenzymes are reduced, which generate ATP in the electron transport chain.
Krebs cycle was named after Hans Krebs, who postulated the detailed cycle. He was awarded the Nobel prize in 1953 for his contribution.
It is a series of eight-step processes, where the acetyl group of acetyl-CoA is oxidised to form two molecules of CO2 and in the process, one ATP is produced. Reduced high-energy compounds, NADH, and FADH2 are also produced.
Two molecules of acetyl-CoA are produced from each glucose molecule, so two turns of the Krebs cycle are required which yields four CO2, six NADH, two FADH2, and two ATPs.
Krebs cycle can be defined as an eight-step process occurring in the mitochondrial matrix. Acetyl CoA, derived from carbohydrates, proteins, and fats, is completely oxidised to release carbon dioxide. In the form of ATP, the energy released is stored. The eight steps involved are as follows:
Step 1: The first step is the condensation of acetyl CoA with oxaloacetate (4C) to form citrate (6C), coenzyme A is released. The reaction is catalysed by citrate synthase.
Step 2: Citrate is turned to its isomer, isocitrate. The enzyme aconitase catalyses this reaction.
Step 3: Isocitrate undergoes dehydrogenation and decarboxylation to form 𝝰-ketoglutarate (5C). A molecule of CO2 is released. Isocitrate dehydrogenase catalyzes the reaction. It is an NADh-dependent enzyme. NAD+ is converted to NADH.
Step 4: 𝝰-ketoglutarate (5C) undergoes oxidative decarboxylation to form succinyl CoA (4C). The reaction is catalyzed by 𝝰-ketoglutarate dehydrogenase enzyme complex. One molecule of CO2 is released and NAD+ is converted to NADH.
Step 5: Succinyl CoA is converted to succinate by the enzyme succinyl CoA synthetase. This is coupled with substrate-level phosphorylation of GDP to form GTP. GTP transfers its phosphate to ADP forming ATP.
Step 6: Succinate is oxidised to fumarate by the enzyme succinate dehydrogenase. In the process, FAD is converted to FADH2.
Step 7: Fumarate gets converted to malate by the addition of one H2O. The enzyme catalysing this reaction is fumarase.
Step 8: Malate is dehydrogenated to form oxaloacetate, which combines with another molecule of acetyl CoA and starts the new cycle. Hydrogens removed get transferred to NAD+ forming NADH. Malate dehydrogenase catalyses the reaction.