Every living organism is an open thermodynamic system, constantly exchanging matter and energy with its environment. Indeed, the capacity to harness and expend energy is one of the defining characteristics of life.

Organisms acquire energy through diverse pathways. Autotrophs synthesize organic matter from inorganic compounds: plants, algae, and cyanobacteria utilize sunlight, while certain bacteria and archaea exploit the energy of chemical reactions. Heterotrophs subsist on preformed organic compounds, and mixotrophs combine both strategies.

The universal ‘energy currency’ of the cell is ATP (adenosine triphosphate). Energy is temporarily stored within its chemical bonds and subsequently expended to power cellular machinery. The primary processes that convert external energy into a form usable by living systems are photosynthesis and chemosynthesis.

Life begins where the energy of the external world transforms into the internal power of the cell!

METABOLIC MODES

Heterotrophs

Consume preformed organic matter synthesized by other organisms.

Saprotrophs: Obtain nutrients by decomposing dead biomass, organic detritus, or waste products.

Biotrophs (comprising zoophages/carnivores and phytophages/herbivores): Feed directly on other living organisms.

Autotrophs

Synthesize organic matter directly from inorganic compounds.

Phototrophs: Utilize solar energy.

Chemotrophs: Derive energy from chemical reactions by oxidizing various substances.

Mixotrophs

Combine both nutritional strategies.