Cells and Energy - Part 1


from the Greek metabole, meaning "change"


Metabolism is an emergent property of life.

It includes all the chemical processes needed by cells to maintain homeostasis. Respiration, synthesis of biochemicals, and breakdown of wastes are all carried out by various metabolic pathways which have been illustrated at ExPASy. For example click on the sample below to peruse the metabolic steps in glycolysis.

The thousands of different reactions that occur in a cell can be thought of as an intricately branched road map -- pathways needed to supply the cell with the molecules it uses for energy, structure and function.

Each metabolic pathway, a branch in the road map, is an orderly series of enzyme-mediated chemical reactions resulting in the desired product, each step catalyzed by its own specific enzyme. Click here for more about metabolic pathways.


All of metabolism can be categorized as either anabolic or catabolic. In cells:

  • anabolism
    • is the synthesis of molecules, a building process that requires energy input resulting in a more highly ordered chemical organization and the increase of free energy of the products. Entropy decreases (order increases).

      A major downhill avenue of catabolism is cellular respiration. Glucose is oxidized to water, CO2 and energy (see ATP).

  • catabolism
    • produces the breakdown of complex molecules to simpler ones, releasing energy which results in the decrease of chemical organization as well as the supply of free energy. Entropy increases (order decreases)

    These opposing metabolic pathways are frequently coupled so that energy released by "downhill" exergonic reactions can be used to fuel the "uphill" endergonic ones. Coupling often involves ionic gradients.

Enzymes direct traffic by selectively accelerating those steps which are most necessary to the cell at any given time during its life cycle. Homeostatic mechanisms that regulate enzymes balance metabolic supply and demand - resulting in the most energy efficient system possible.

It is evolution and the process of natural selection that has fine tuned the diverse metabolic pathways so that as little energy as possible is wasted.

Modified July 10, 2005