What is 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
1. What are the main differences between glycolysis and the Krebs cycle?
The primary differences between glycolysis and the Krebs cycle relate to their location, oxygen requirement, and primary function in cellular respiration. Here’s a breakdown:
- Location: Glycolysis occurs in the cytoplasm of the cell, while the Krebs cycle takes place inside the mitochondrial matrix.
- Oxygen Requirement: Glycolysis is an anaerobic process, meaning it does not require oxygen to proceed. The Krebs cycle is strictly aerobic and only occurs when oxygen is present.
- Process Type: Glycolysis is a linear pathway of 10 steps that breaks down one glucose molecule. The Krebs cycle is a cyclic pathway of 8 steps that processes acetyl-CoA.
- Main Products: Glycolysis breaks down glucose into two molecules of pyruvic acid, with a net gain of 2 ATP and 2 NADH. The Krebs cycle oxidises acetyl-CoA to produce carbon dioxide, ATP (or GTP), NADH, and FADH₂.
2. How are glycolysis and the Krebs cycle connected in cellular respiration?
Glycolysis and the Krebs cycle are sequential and interconnected stages of aerobic respiration. The connection is the molecule pyruvic acid. After glycolysis breaks down glucose into pyruvic acid in the cytoplasm, this pyruvic acid is transported into the mitochondria. Here, it undergoes a transition reaction called oxidative decarboxylation, where it is converted into acetyl-CoA. This acetyl-CoA is the starting molecule that enters the Krebs cycle, directly linking the end of glycolysis to the beginning of the Krebs cycle.
3. Where in the cell do glycolysis and the Krebs cycle occur?
Glycolysis and the Krebs cycle occur in different compartments within a eukaryotic cell. Glycolysis, the initial stage of glucose breakdown, takes place in the cytoplasm. The subsequent stage, the Krebs cycle, occurs inside the innermost compartment of the mitochondria, known as the mitochondrial matrix. This separation is crucial for the regulation and efficiency of cellular respiration.
4. Besides their differences, what are the key similarities between glycolysis and the Krebs cycle?
Despite their distinct roles, glycolysis and the Krebs cycle share several fundamental characteristics. Both pathways are:
- Catabolic Processes: Both are part of cellular respiration, involving the breakdown of molecules to release energy.
- Enzyme-Catalysed: Each step in both glycolysis and the Krebs cycle is controlled by a specific enzyme.
- ATP Production: Both processes generate ATP through substrate-level phosphorylation, although in different amounts.
- Production of Reduced Coenzymes: Both pathways produce the reduced coenzyme NADH, which is vital for generating a larger amount of ATP in the electron transport chain.
5. Why is glycolysis considered anaerobic while the Krebs cycle is strictly aerobic?
Glycolysis is considered anaerobic because its ten enzymatic reactions do not directly use oxygen. It can occur in both the presence and absence of oxygen, making it a universal energy-releasing pathway for nearly all living organisms. In contrast, the Krebs cycle is considered aerobic because it is functionally dependent on oxygen. While the cycle itself doesn't use O₂ directly, it relies on the recycling of coenzymes NAD+ and FAD, which are regenerated only when NADH and FADH₂ donate their electrons to the electron transport chain—a process that requires oxygen as the final electron acceptor.
6. What is the net energy gain from glycolysis compared to one turn of the Krebs cycle?
The net energy gain from each process is different. For one molecule of glucose, glycolysis yields a net gain of 2 ATP molecules and 2 NADH molecules. In contrast, one turn of the Krebs cycle (which processes one acetyl-CoA molecule) yields 1 ATP (or GTP), 3 NADH, and 1 FADH₂. Since one glucose molecule produces two pyruvic acid molecules (and thus two acetyl-CoA), the total yield from the products of one glucose molecule in the Krebs cycle is 2 ATP, 6 NADH, and 2 FADH₂.
7. Is the Krebs cycle the same as the citric acid cycle?
Yes, the Krebs cycle and the citric acid cycle (TCA cycle) are two names for the same metabolic pathway. It is named the Krebs cycle after its discoverer, Sir Hans Krebs. It is called the citric acid cycle because the first molecule formed in the cycle is citrate (an ionized form of citric acid), which is created when acetyl-CoA combines with oxaloacetate. A third name, the tricarboxylic acid (TCA) cycle, is also used because citric acid is a tricarboxylic acid.
8. What is the importance of the NADH and FADH₂ produced during both glycolysis and the Krebs cycle?
The NADH and FADH₂ molecules produced are of immense importance as they are high-energy electron carriers. They do not represent the final energy currency for the cell. Instead, they transport the high-energy electrons harvested from glucose breakdown to the final stage of aerobic respiration, the Electron Transport Chain (ETC) located on the inner mitochondrial membrane. In the ETC, the energy from these electrons is used to produce a large amount of ATP through a process called oxidative phosphorylation, which is the primary source of energy for most aerobic organisms.
