The Pentose Phosphate Pathway (PPP) and the Citric Acid Cycle (also known as the Krebs cycle or TCA cycle) are two interrelated metabolic pathways that are crucial for cellular biosynthesis and energy production.
A crucial metabolic mechanism that takes place in the mitochondria of eukaryotic cells and the cytoplasm of prokaryotic cells is the citric acid cycle, sometimes referred to as the Krebs cycle or the tricarboxylic acid (TCA) cycle. The process through which cells produce energy from glucose and other organic molecules in the presence of oxygen is known as aerobic respiration, and it is a crucial part of that process.
The basic goal of the citric acid cycle is to generate ATP (adenosine triphosphate), the major source of energy for cells, by extracting high-energy electrons through a series of chemical processes. As a waste product from the cycle, carbon dioxide is also produced.
The electron transport chain and glycolysis, which breaks down glucose, are two more metabolic pathways that are closely related to the citric acid cycle. In the electron transport chain, where the energy is progressively released and used to
pump protons across the mitochondrial membrane to create a proton gradient, the NADH and FADH2 produced during the citric acid cycle feed their electrons. After that, ATP synthase uses this gradient to produce ATP.
The pentose phosphate route (PPP), sometimes called the phosphogluconate pathway or the hexose monophosphate shunt, is a chain of metabolic processes that takes place in the cytoplasm of cells. It performs a variety of crucial tasks inside the cell, including making ribose-5-phosphate, a precursor to nucleotide synthesis, and producing reducing equivalents in the form of NADPH (nicotinamide adenine dinucleotide phosphate).
The PPP is controlled by the cellās needs for ribose-5-phosphate and NADPH. The oxidative phase can be upregulated by increased NADPH demand, which is frequently seen in procedures like fatty acid synthesis and the detoxification of reactive oxygen species. On the other hand, increased activity in the non-oxidative phase might result from a requirement for ribose-5-phosphate for nucleotide synthesis.
The liver, fatty tissue, and actively dividing cells are examples of tissues with high biosynthetic and detoxifying demands that benefit greatly from the PPP. Due to its activities, it is an important pathway for supporting anabolic processes, supplying precursors for nucleotide synthesis, and preserving cellular redox equilibrium.
Here are 36 differences between the citric acid cycle (Krebs cycle) and the pentose phosphate pathway (PPP):
|
S.No. |
Aspect |
Citric Acid Cycle (Krebs Cycle) |
Pentose Phosphate Pathway (PPP) |
|
1 |
Purpose |
Energy production through the oxidation of acetyl-CoA. |
Production of NADPH and ribose-5-phosphate for biosynthesis. |
|
2 |
Location |
Occurs in the mitochondria. |
Occurs in the cytoplasm and can also involve the mitochondria. |
|
3 |
Substrates |
Receives acetyl-CoA from glycolysis. |
Utilizes glucose-6-phosphate (G6P) primarily. |
|
4 |
NADH and NADPH production |
Produces NADH as an energy carrier. |
Produces NADPH as a reducing agent. |
|
5 |
ATP production |
Generates ATP through substrate-level phosphorylation. |
Generates little ATP, mostly through glycolysis. |
|
6 |
Carbon source |
Derives carbon from acetyl-CoA. |
Derives carbon from G6P. |
|
7 |
Role in biosynthesis |
Provides precursors for biosynthesis of amino acids, nucleotides, and heme. |
Supplies NADPH for biosynthetic reactions. |
|
8 |
Enzymes involved |
Involves enzymes like citrate synthase, isocitrate dehydrogenase, and succinate dehydrogenase. |
Involves enzymes like glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase. |
|
9 |
Carbon dioxide production |
Produces carbon dioxide as a byproduct. |
Does not produce carbon dioxide. |
|
10 |
Role in the electron transport chain |
Feeds electrons into the electron transport chain for ATP synthesis. |
Not directly connected to the electron transport chain. |
|
11 |
Role in gluconeogenesis |
Not directly involved in gluconeogenesis. |
Supplies ribose-5-phosphate for gluconeogenesis. |
|
12 |
ATP generation location |
Most ATP is generated in the mitochondria. |
Little ATP generation occurs in the pathway. |
|
13 |
Net ATP production |
Produces ATP during the cycle. |
Generally consumes ATP due to glucose oxidation. |
|
14 |
Involvement of oxygen |
Requires oxygen as itās part of aerobic respiration. |
Can operate in the presence or absence of oxygen. |
|
15 |
Regulatory control |
Regulated mainly by substrate availability and product inhibition. |
Regulated mainly by the demand for NADPH and ribose-5-phosphate. |
|
16 |
Role in reactive oxygen species (ROS) |
Generates ROS as a byproduct. |
Can help reduce oxidative stress by producing NADPH. |
|
17 |
Number of phases |
Consists of eight enzymatic reactions. |
Consists of two phases: oxidative and non-oxidative. |
|
18 |
Role in red blood cells (RBCs) |
Not involved in red blood cell metabolism. |
Essential for red blood cell metabolism. |
|
19 |
Intermediates |
Produces intermediates like citrate, isocitrate, Ī±-ketoglutarate, and oxaloacetate. |
Produces intermediates like ribose-5-phosphate and erythrose-4-phosphate. |
|
20 |
Role in DNA synthesis |
Does not directly contribute to DNA synthesis. |
Provides ribose-5-phosphate, a precursor for DNA synthesis. |
|
21 |
Role in nucleotide synthesis |
Provides intermediates for nucleotide synthesis. |
Supplies ribose-5-phosphate for nucleotide synthesis. |
|
22 |
Role in lipid synthesis |
Provides intermediates for fatty acid synthesis. |
Not directly involved in lipid synthesis. |
|
23 |
Role in heme synthesis |
Provides intermediates for heme synthesis. |
Not directly involved in heme synthesis. |
|
24 |
Main product |
Produces ATP, NADH, and FADH2 as energy-rich products. |
Produces NADPH and ribose-5-phosphate as primary products. |
|
25 |
Key reactions |
Involves reactions like citrate formation, isocitrate dehydrogenase, and succinyl-CoA synthesis. |
Includes oxidative reactions catalyzed by glucose-6-phosphate dehydrogenase. |
|
26 |
Role in cancer metabolism |
Dysregulation can contribute to tumor growth. |
Dysregulation can affect cancer cell survival. |
|
27 |
Role in the TCA cycle |
The central pathway of the TCA cycle. |
Not part of the TCA cycle. |
|
28 |
Role in nitrogen metabolism |
Not directly involved in nitrogen metabolism. |
Involved in the production of NADPH for nitrogen fixation. |
|
29 |
Role in energy storage |
Primarily contributes to immediate energy needs. |
Supports energy storage indirectly through NADPH. |
|
30 |
Key precursor molecule |
Ī±-Ketoglutarate is a key precursor for various biosynthetic pathways. |
Ribose-5-phosphate is a key precursor for nucleotide synthesis. |
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Frequently Asked Questions (FAQs)
1. What outcomes does the citric acid cycle produce?
Each cycle of the citric acid cycle results in the production of three molecules of NADH, one of FADH2, one of ATP (or GTP), and two molecules of carbon dioxide (CO2). These substances transport energetic electrons that are utilized by the electron transport chain to increase ATP production.
2. What part does the citric acid cycle play in the creation of energy?
The fundamental function of the citric acid cycle is to produce high-energy electron carriers (NADH and FADH2), as well as ATP/GTP and high-energy electron carriers (NADH and FADH2), which are then utilized in the electron transport chain to produce ATP by oxidative phosphorylation.
3. What function does the pentose phosphate pathway serve in the production of nucleotides?
A precursor for the synthesis of nucleotides, ribose-5-phosphate, is created by the PPP. DNA and RNA synthesis require nucleotides, which are necessary for cell growth and replication.
4. What are the Pentose Phosphate Pathway's two stages?
The oxidative phase and the non-oxidative phase are the two stages that make up the PPP. In contrast to the non-oxidative phase, which involves the interconversion of different sugar phosphates, the oxidative phase involves the production of NADPH and ribulose-5-phosphate through a sequence of processes.
5. The Pentose Phosphate Pathway occurs where?
The Pentose Phosphate Pathway takes place in tissues where NADPH is required for numerous biosynthetic and redox processes, notably in the cytoplasm of cells.
