Key differences between glycolysis and Krebs cycle in cellular respiration
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:
Glycolysis
Anaerobic Breakdown of Pyruvic acid
Krebs Cycle
Electron Transport system
Terminal oxidation and oxidative phosphorylation
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
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.
FAQs on Difference Between Glycolysis and Krebs Cycle Explained Clearly
1. What is the main difference between glycolysis and the Krebs cycle?
The main difference between glycolysis and the Krebs cycle is that glycolysis breaks down glucose into pyruvate in the cytoplasm, while the Krebs cycle further oxidizes acetyl-CoA in the mitochondria to produce energy carriers.
- Location: Glycolysis occurs in the cytoplasm; Krebs cycle occurs in the mitochondrial matrix.
- Oxygen requirement: Glycolysis can occur without oxygen; the Krebs cycle is aerobic.
- End products: Glycolysis produces pyruvate, ATP, and NADH; the Krebs cycle produces CO₂, NADH, FADH₂, and ATP (or GTP).
2. What is glycolysis?
Glycolysis is a metabolic pathway that breaks one molecule of glucose into two molecules of pyruvate to produce energy.
- Occurs in the cytoplasm of all cells.
- Does not require oxygen (anaerobic process).
- Produces 2 ATP (net), 2 NADH, and 2 pyruvate per glucose molecule.
3. What is the Krebs cycle?
The Krebs cycle is a series of enzyme-controlled reactions that oxidize acetyl-CoA to produce energy-rich molecules in the mitochondria.
- Also called the citric acid cycle or TCA cycle.
- Occurs in the mitochondrial matrix of eukaryotic cells.
- Produces NADH, FADH₂, ATP (or GTP), and releases CO₂.
4. Where do glycolysis and the Krebs cycle occur in the cell?
Glycolysis occurs in the cytoplasm, while the Krebs cycle takes place in the mitochondrial matrix.
- Glycolysis happens outside the mitochondria and does not require membrane-bound organelles.
- The Krebs cycle occurs inside mitochondria in eukaryotes and in the cytoplasm of prokaryotes.
5. Does glycolysis require oxygen like the Krebs cycle?
Glycolysis does not require oxygen, but the Krebs cycle functions only under aerobic conditions.
- Glycolysis: Anaerobic process; can occur with or without oxygen.
- Krebs cycle: Indirectly depends on oxygen because NADH and FADH₂ must be oxidized in the electron transport chain, which requires oxygen.
6. What are the products of glycolysis and the Krebs cycle?
Glycolysis produces pyruvate, ATP, and NADH, whereas the Krebs cycle produces CO₂, NADH, FADH₂, and ATP (or GTP).
- Glycolysis (per glucose): 2 pyruvate, 2 ATP (net), 2 NADH.
- Krebs cycle (per glucose): 4 CO₂, 6 NADH, 2 FADH₂, and 2 ATP (or GTP).
7. How are glycolysis and the Krebs cycle connected?
Glycolysis and the Krebs cycle are connected through the conversion of pyruvate into acetyl-CoA.
- Glycolysis produces pyruvate in the cytoplasm.
- Pyruvate enters the mitochondrion and is converted to acetyl-CoA by the link reaction (pyruvate oxidation).
- Acetyl-CoA then enters the Krebs cycle.
8. Which produces more ATP, glycolysis or the Krebs cycle?
The Krebs cycle produces more high-energy electron carriers, but glycolysis and the Krebs cycle each produce only a small amount of direct ATP.
- Glycolysis: 2 ATP (net) per glucose.
- Krebs cycle: 2 ATP (or GTP) per glucose.
- Most ATP is generated later in the electron transport chain using NADH and FADH₂.
9. Why is the Krebs cycle called the citric acid cycle?
The Krebs cycle is called the citric acid cycle because the first stable product formed is citrate (citric acid).
- Acetyl-CoA combines with oxaloacetate to form citrate.
- Citrate undergoes a series of reactions to regenerate oxaloacetate.
10. What is the importance of glycolysis and the Krebs cycle in cellular respiration?
Glycolysis and the Krebs cycle are essential stages of cellular respiration that convert glucose into usable chemical energy.
- They break down glucose step by step.
- They produce NADH and FADH₂, which carry electrons to the electron transport chain.
- They generate ATP directly through substrate-level phosphorylation.
