A metabolic pathway is a connected sequence or network of biochemical reactions in which products from one reaction become substrates for others.

Instead of viewing each enzyme-catalyzed reaction in isolation, metabolism tracks how matter flows through shared molecular intermediates.

A simple linear pathway can be represented as

A --E1--> B --E2--> C --E3--> D

where each arrow is a reaction and each E represents an enzyme catalyzing that step.

Real metabolic systems can also branch, merge and form cycles. The same metabolite can therefore feed several possible routes or be produced by several routes.

Flux

The amount of material passing through a pathway per unit time is its flux. Flux depends on several factors at once, including

• substrate availability;
• enzyme abundance and catalytic activity;
• product removal;
• competition with branching pathways;
• transport between cellular compartments.

A slow or strongly regulated step can influence pathway behavior, but biological control is often distributed across several reactions rather than governed by one universal 'rate-limiting enzyme.'

Catabolism and anabolism

Catabolic pathways break larger or more complex molecules into products that can supply reusable building blocks or feed other energy-releasing processes.

Anabolic pathways construct larger molecules from smaller precursors and require appropriate sources of material and thermodynamic driving force.

The two are connected: intermediates produced by catabolism can become precursors for anabolism, while reaction-coupling chemistry can link favorable and unfavorable transformations.

Feedback regulation of pathway flux

Cells regulate metabolic flux by changing enzyme activity, enzyme abundance, substrate access or compartmentalization.

A common application of negative feedback is feedback inhibition: a downstream product reduces the activity of an earlier enzyme, so product accumulation slows further product formation.

A → B → C → D
        ↑     |
        └─────┘ inhibition

If D becomes scarce, that inhibitory influence can weaken and flux can rise again. Other regulatory signals can activate pathways when demand increases.

Metabolism is a network

Named pathways are useful learning modules, but inside a cell their intermediates connect into a larger reaction network. Understanding metabolism therefore requires more than memorizing sequences of arrows. The central questions are where matter enters and leaves, what determines flux, how reactions are coupled to favorable chemistry, and how regulation redistributes that flux when cellular conditions change.