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.
