The key difference between Krebs cycle and glycolysis is that Krebs cycle, which takes place in mitochondria, is the second stage of cellular respiration, while glycolysis, which takes place in the cytoplasm, is the first stage of cellular respiration.
Krebs’s cycle and glycolysis are two main stages of cellular respiration that produces energy in cells. Both processes occur in different cellular locations. Moreover, they use different enzymatic reactions in order to convert different starting materials into different products. Furthermore, these two processes create different amounts of ATP. In aerobic respiration, Krebs cycle follows glycolysis. But in anaerobic respiration, glycolysis takes place alone.
CONTENTS
1. Overview and Key Difference
2. What is Krebs Cycle
3. What is Glycolysis
4. Similarities Between Krebs Cycle and Glycolysis
5. Side by Side Comparison – Krebs Cycle vs Glycolysis in Tabular Form
6. Summary
What is Krebs Cycle?
Krebs cycle, also known as the citric acid cycle, is one of the three stages of cellular respiration. This occurs in the mitochondrion. This organelle is present only in eukaryotes. This is the second step of glucose catabolism in eukaryotes and does not occur in prokaryotes like bacteria. Krebs’s cycle uses the product of glycolysis; pyruvic acid as the starting material, but it cannot directly enter into Krebs’s cycle. Thus, pyruvic acid molecules transform into Acetyl Co-A, releasing CO2. This conversion releases some energy, which is sufficient to convert NAD to NADH.
Figure 01: Krebs Cycle
Inside the mitochondrion, oxaloacetic acid (4 Carbon molecule) captures acetyl Co-A (2 Carbon molecule) and makes citric acid (6 C molecule). This substrate then undergoes a series of enzyme-driven reactions and gets converted again to oxaloacetic acid – the starting material. This is why we call it a cycle. Many of the steps of the Krebs cycle release high energy electrons that can reduce NAD to NADH2. FAD also acts as an electron acceptor and become FADH2. This cycle also forms ATP. If we consider the overall result of the Krebs cycle, a glucose molecule (6C) entering the Krebs’s cycle produces 2 ATP molecules, 10 NADH2, 2 FADH2, and 6 CO2.
What is Glycolysis?
Glycolysis is the cellular process which breaks a glucose molecule into two pyruvic acid molecules. Unlike the Krebs cycle, this process is common to animals, plants and microorganisms. This takes place in the cytoplasm and consists of multi-steps. Although 4 ATP molecules are produced per glucose, it uses 2 ATP molecules during the intermediate steps. Therefore, the net ATP production of glycolysis is 2. In addition, it also produces 2 NADH2 molecules. If the pyruvic acid molecules do not enter Krebs’s cycle, it undergoes fermentation and results in ethanol in plants and lactic acid in animals.
Glycolysis does not need the presence of oxygen. Therefore, glycolysis can happen in anaerobic environments. However, when the glycolysis takes place in anaerobic environments, its efficiency is low compared to aerobic respiration.
What are the Similarities Between Krebs Cycle and Glycolysis?
- Krebs cycle and glycolysis are two processes of cellular respiration.
- Both processes produce energy in the form of ATP and NADH2.
- They take place inside the cells.
- Both processes have many reactions.
- These processes occur only in living organisms.
- Different enzymes catalyze both these processes.
- In bacteria, both these processes occur in the cytoplasm.
What is the Difference Between Krebs Cycle and Glycolysis?
Krebs cycle is the second stage of aerobic respiration while glycolysis is the initial stage of both aerobic and anaerobic respiration. This is the key difference between Krebs cycle and glycolysis. Furthermore, Krebs cycle takes place in the mitochondria while glycolysis takes place in the cytoplasm. Thus, this is another difference between Krebs cycle and glycolysis. Moreover, Krebs cycle is a cyclic process while glycolysis is a linear process.
In addition, glycolysis consumes ATP while Krebs cycle does not consume ATP. Another difference between Krebs cycle and glycolysis is that Krebs cycle occurs only in eukaryotes while glycolysis occurs in both prokaryotes and eukaryotes.
The below infographic summarizes the difference between Krebs cycle and glycolysis.
Summary – Krebs Cycle vs Glycolysis
Krebs cycle and glycolysis are two major processes of cellular respiration. But, glycolysis can occur in both aerobic and anaerobic conditions. Krebs cycle occurs only in the presence of oxygen. Furthermore, glycolysis is the first stage while Krebs cycle is the second stage of aerobic respiration. Moreover, glycolysis occurs in the cytoplasm while Krebs cycle occurs in the mitochondria matrix. In addition, glycolysis is a linear process while Krebs cycle is a cyclic process. This summarizes the difference between Krebs cycle and glycolysis.
Reference:
1. “Glycolysis.” Khan Academy, Available here.
2. “Boundless Microbiology.” Lumen, Available here.
Image Courtesy:
1. “Citric acid cycle with aconitate 2” By Narayanese, WikiUserPedia, YassineMrabet, TotoBaggins – http://biocyc.org/META/NEW-IMAGE?type=PATHWAY&object=TCA. Image adapted from :Image:Citric acid cycle noi.svg|(uploaded to Commons by wadester16) (CC BY-SA 3.0) via Commons Wikimedia
2. “Glycolysis” By WYassineMrabetTalk✉This W3C-unspecified vector image was created with Inkscape. – Own work (CC BY-SA 3.0) via Commons Wikimedia