Intracellular and Extracellular Enzyme-Catalysed Reactions
- Enzymes are essential for metabolism, the sum of all chemical reactions in living organisms.
- These reactions can occur inside cells (intracellular) or outside cells (extracellular).
Definition
Intracellular enzymes
Intracellular enzymes operate within cells, driving critical metabolic pathways such as glycolysis and the Krebs cycle.
Definition
Extracellular enzymes
Extracellular enzymes work outside cells to break down large molecules into smaller, absorbable units.
Glycolysis: Breaking Down Glucose in the Cytoplasm
Definition
Glycolysis
Glycolysis is the first step in cellular respiration, where glucose is broken down into pyruvate, releasing energy in the form of ATP.
This process occurs in the cytoplasm and involves a series of enzyme-catalysed reactions.
1. Phosphorylation of Glucose:
- The enzyme hexokinase adds a phosphate group to glucose, forming glucose-6-phosphate.
- This step requires ATP and traps glucose in the cell, making it more reactive.
2. Conversion to Pyruvate:
- Through a series of steps, glucose is split into two three-carbon molecules called pyruvate.
- During this process, ATP and NADH(an electron carrier) are produced.
Hint
- Glycolysis is an anaerobic process, meaning it does not require oxygen.
- It is the first step in both aerobic and anaerobic respiration.
The Krebs Cycle: Energy Extraction in the Mitochondria
Definition
The Krebs cycle
The Krebs cycle (also known as the citric acid cycle) occurs in the mitochondrial matrix and is a key part of aerobic respiration.
- The Krebs cycle completes the breakdown of glucose by oxidizing acetyl-CoA (derived from pyruvate) into carbon dioxide.
- Formation of Citrate
- Acetyl-CoA combines with oxaloacetate to form citrate, catalysed by the enzyme citrate synthase.
- Energy Extraction
- The cycle involves a series of reactions that release energy.
- These are captured in the form of NADH, FADH2, and ATP (or GTP).
- Ultimately, releasing carbon dioxide as a waste product.
- Formation of Citrate
Example
- In the Krebs cycle, the enzyme fumarase catalyses the conversion of fumarate to malate.
- This step is crucial for regenerating oxaloacetate, allowing the cycle to continue.
Chemical Digestion in the Gut
In the digestive system, enzymes are secreted into the gut to break down food macromolecules into their monomers.
- Proteins: Proteases like pepsin (in the stomach) and trypsin (in the small intestine) break down proteins into amino acids.
- Carbohydrates: Amylase (in saliva and pancreatic juice) breaks down starch into maltose, which is further broken down into glucose by maltase.
- Lipids: Lipase (secreted by the pancreas) breaks down triglycerides into fatty acids and glycerol.
Note
- Extracellular enzymes are often secreted in an inactive form (zymogens) to prevent them from digesting the tissues that produce them.
- They are activated in the digestive tract.
Why These Reactions Matter
- Energy Production: Intracellular reactions like glycolysis and the Krebs cycle are essential for producing ATP, the energy currency of the cell.
- Nutrient Absorption: Extracellular digestion breaks down food into absorbable units, providing the raw materials for cellular processes.
Analogy
- Think of intracellular enzymes as workers inside a factory, breaking down raw materials to produce energy.
- Extracellular enzymes are like delivery trucks, breaking down large shipments into smaller packages that can be brought into the factory.
Challenges and Solutions
- Enzyme Specificity: Each enzyme is highly specific to its substrate. This ensures precise control over metabolic pathways.
- Environmental Conditions: Enzymes require optimal conditions (e.g., temperature, pH) to function.
Example
Pepsin works best in the acidic environment of the stomach, while trypsin operates in the alkaline conditions of the small intestine.
Common Mistake
- A common mistake is to assume that all enzymes work in the same conditions.
- Always consider the specific environment where the enzyme is active.
Self review
- Can you explain the difference between intracellular and extracellular enzyme-catalysed reactions?
- How do these processes contribute to metabolism?
