AP Syllabus focus:
‘Energy-related pathways are organized sequentially, so products of one reaction become reactants for the next, controlling energy transfer.’
Sequential metabolic pathways are the cell’s way of organising many chemical reactions into controlled “routes” for matter and energy. Their order, proximity, and regulation ensure energy is captured efficiently and released only when and where it is needed.
Core idea: reactions arranged in a sequence
A metabolic pathway is a series of connected, enzyme-catalysed reactions in which each step transforms a molecule, passing it to the next step.
Practice Questions
FAQ
Cells reduce exposure time of intermediates by organising enzymes into complexes and keeping intermediates at low steady-state concentrations.
Other strategies include:
rapid consumption by the next enzyme
localisation to specific regions to limit unintended interactions
It depends on what happens to the intermediates.
Linear: one main route from start to finish
Branched: one intermediate has multiple possible fates
Cyclic: intermediates are regenerated each turn, enabling repeated processing and tight control of throughput
Shared intermediates can be partitioned by controlling the enzymes at branch points. Cells can also regulate transport or localisation so that shared molecules are more available to one sequence than another under specific conditions.
Because every downstream reaction depends on upstream supply. If an early “commitment” step slows, all later steps receive less substrate, limiting both intermediate formation and the amount of energy that can be transferred through later reactions.
Coordination can involve moving intermediates between regions using transport proteins, and aligning enzyme activity with substrate delivery rates. Spatial separation can sharpen control by ensuring that only correctly delivered intermediates enter the next step in the sequence.
