Cyclical and Linear Pathways in Metabolism
- Metabolic pathways are sequences of enzyme-catalyzed reactions that transform molecules in cells.
- Thousands of these reactions occur constantly to support vital processes: cellular respiration, muscle contraction, nerve impulses, biosynthesis.
- Pathways can be organized in two main ways: linear or cyclical.
Linear Pathways: One-Way Sequences
Definition
Linear pathway
A straight sequence of reactions where each product becomes the substrate for the next step, moving in one direction without looping back.
- Characteristics of Linear Pathways
- Products do not re-enter the pathway.
- The pathway has a clear starting substrate and end product.
- Each step depends on the previous one, like dominoes falling in sequence.
Example of a linear pathway: Glycolysis
- Glycolysis is a linear pathway that takes place in the cytoplasm, breaking down glucose (6C) into two pyruvate molecules (3C), producing ATP and NADH.
- It's linear because glucose enters one end, and pyruvate exits out the other.
- Pyruvate never re-enters, it moves on to the link reaction (in aerobic conditions) or fermentation (in anaerobic conditions).
- Glycolysis is like a one-way assembly line.
- Raw materials (glucose) enter, undergo transformations at each station, and finished products (pyruvate) exit without returning.
Cyclical Pathways: Closed Loops
Definition
Cyclical pathway
A closed loop of reactions where intermediates are regenerated, allowing the cycle to repeat continuously as long as substrates are available.
- Characteristics of Cyclical Pathways
- The starting molecule is regenerated at the end of the cycle.
- The pathway can run indefinitely as long as the input substrate (e.g., acetyl-CoA, CO₂) is supplied.
- Intermediates are constantly recycled.
Example 1 of a closed loop pathway: Krebs Cycle
- The Krebs cycle is a cyclical pathway that oxidizes acetyl-CoA to produce ATP, NADH, FADH₂, and CO₂, occurring in the mitochondrial matrix.
- It's cyclical because oxaloacetate is regenerated at the end, allowing the cycle to continue as long as acetyl-CoA is available.
- In other words, the intermediates are reused instead of consumed.
- The Krebs cycle is like a revolving door.
- Acetyl-CoA enters, gets processed as it goes around, and the door returns to its starting position, ready for the next molecule.
- Remember: The Krebs cycle does not directly produce large amounts of ATP.
- Its main role is to generate high-energy carriers (NADH and FADH₂) for the electron transport chain.
Example 2 of a closed loop pathway: The Calvin Cycle
- The Calvin cycle is a cyclical pathway in photosynthesis that occurs in the chloroplast stroma.
- It uses ATP and NADPH (produced in light-dependent reactions) to fix carbon dioxide into glucose.
- It's a closed loop pathway because RuBP is regenerated at the end, so the cycle can keep fixing CO₂ as long as ATP and NADPH are supplied by the light-dependent reactions.
- The Calvin cycle is like a factory with a conveyor belt.
- CO₂ enters, gets processed into products (triose phosphate), and the belt (RuBP) is reset to accept more CO₂.
Comparing Linear and Cyclical Pathways
| Feature | Linear Pathways | Cyclical Pathways |
|---|---|---|
| Direction | One-way, no looping | Closed loop, regenerates starting molecule |
| Example | Glycolysis | Krebs cycle, Calvin cycle |
| Starting molecule | Consumed (e.g., glucose) | Regenerated (e.g., oxaloacetate, RuBP) |
| Products | Exit the pathway | Some exit, intermediates recycled |
| Purpose | Often catabolic (breaking down molecules for energy) | Versatile, catabolic (Krebs) or anabolic (Calvin) |
| Continuation | Ends when substrate is consumed | Continues as long as input substrates are available |
Think of cyclical pathways like recycling and reusing.
Self review- What is a metabolic pathway?
- What is the difference between a linear and a cyclical pathway?
- Is glycolysis linear or cyclical? Why?
- What is the starting molecule and end product of glycolysis?
- Is the Krebs cycle linear or cyclical? Why?
- s the Calvin cycle linear or cyclical? Why?
- What molecule is regenerated in the Calvin cycle?
- Why do cyclical pathways not run out of intermediates?
