Linear pathways move from substrate to final product, while cyclical pathways regenerate intermediates that re-enter the pathway for reuse in metabolism.
Coverage 2023–2025 · Updated 15 Jul 2026
A linear metabolic pathway converts an initial substrate through a sequence to a terminal product; a cyclical pathway regenerates an acceptor needed for the next turn.
Glycolysis is linear: glucose passes through intermediates and ends as pyruvate. The Krebs cycle regenerates oxaloacetate, and the Calvin cycle regenerates ribulose bisphosphate while processing new carbon input.
Identify pathway form by tracing carbon or the acceptor: if a starting acceptor is re-formed, the pathway is cyclical; if intermediates lead to a different terminal product without regeneration, it is linear.
Each Krebs-cycle turn combines an acetyl group with oxaloacetate and later regenerates oxaloacetate, allowing another acetyl group to enter when substrates and cofactors are available.
A cycle is not self-sustaining perpetual motion: it requires new inputs and energy transfers. A linear pathway can also contain reversible steps without becoming a cycle.
This objective is assessed through structured response, commonly using Distinguish.
Distinguish
Build the answer around this relationship: Linear pathways proceed from substrate to final product.
Representative question
Distinguish between processes involved in cyclical and linear metabolic pathways.
Enzymes can act inside or outside cells, metabolic reactions release heat, pathways may be linear or cyclical, and pathway output is regulated by inhibition. Competitive inhibitors bind active sites and can be overcome by more substrate; non-competitive inhibitors bind allosteric sites; feedback inhibition uses end products to inhibit earlier enzymes; mechanism-based inhibitors trap enzymes after reaction begins.
