Metabolic pathways are an organism
's methods of synthesizing and deconstructing important molecule
s to the living process. They are extremely efficient reactions, with yields near or equal to 100%. To make sure this tremendous mechanical efficiency is properly maintained, metabolic pathways are not used randomly but in a highly regulated manner.
The greatest advantage of regulated reaction rates is that it allows an organism to dynamically respond to environmental changes. It's what allows something to be 'alive'. The metabolism of a cell must change dramatically depending on the surroundings it encounters, and without any control mechanism on the metabolic pathways that fuel metabolism, you'd be stuck. The modification of pathways to accelerate or decelerate their processes can hapen very quickly, or over a longer period of time. For example, the transfer of ions across the cell membrane and the metabolic pathways involved in that mechanism undergo dramatic changes in velocity over a matter of milliseconds. This quick response is vital to the transmission of nerve impulses and muscle contraction, which depend on ion movement.
One mechanism of control in metabolic pathways is irreversibility. Most pathways can only work in one direction, they cannot travel backwards. This does not mean that all steps of the reaction are irreversible, but many will be. A good metaphor is the show Who Wants to Be a Millionaire?. There are certain key points along the winnings ladder from which you can't turn back. You may mess up the $48,000 question, but you'll still get $32,000, that step was irreversible. Such to metabolic pathways.
Another internalized mechanism of control for metabolic pathways is feedback inhibition and feedforward activation. The former slows down a reaction, the latter speeds it up. These processes are dependant on the presence of an earlier or later product in the chain affecting another part of the pathway. They make sure that a reaction doesn't run away with itself or grind to a halt by engaging in an automatic system of checks and balances.
External factors can act as regulatory patterns for metabolic pathways as well. The removal of substrates, products, catalyzing enzymes, and inhibitors can all profoundly affect the activation of a pathway. Many times the availability of materials for one pathway will be dependant on another, intertwining the two and making sure they can't operate without each other. A regulatory mechanism kicking in for one pathway can slow down a whole collection of hundreds or even thousands of pathways dependant on that one pathway's products.
Though these regulatory mechanisms are listed invidually, it's important to note that several can be operating within a pathway simultaneously. It gets complicated.