Q.Explain why photosynthesis is classified as a means of autotrophic nutrition, and briefly state its ecological significance for heterotrophic organisms, including humans.

[!TLDR]

Photosynthesis is autotrophic because plants build organic food from inorganic CO2 and water using light energy; this food, and the O2 released, sustains almost all heterotrophic life.

[!ANSWER]

Photosynthesis is autotrophic nutrition because the plant manufactures its own organic food from inorganic CO2 and water, using light energy, without consuming any other organism. Ecologically, this makes photosynthesising organisms the primary producers at the base of almost every food chain, and the source of the atmospheric O2 that supports aerobic respiration in heterotrophic organisms, including humans.

Autotrophic nutrition is defined by the ability to synthesise organic food from purely inorganic raw materials, which is exactly what photosynthesis achieves using CO2, water and light energy. Because no heterotroph -- from a herbivorous insect to a human -- can build organic molecules from scratch in this way, all heterotrophic life ultimately depends, directly or indirectly, on the organic carbon and stored chemical energy that photosynthesis first creates. Photosynthesis is also, separately, the source of essentially all atmospheric O2, meaning it sustains heterotrophic life in two distinct ways at once: as food, and as breathable oxygen.

[!ANSWER]

Photosynthesis synthesises organic food from inorganic CO2 and water using light energy (the definition of autotrophic nutrition), and this food and the O2 it releases sustain almost all heterotrophic life as the base of the food chain and the source of atmospheric oxygen.

Define autotrophic nutrition first, then connect the definition outward to its two distinct ecological consequences: food supply and oxygen supply.

Discussing only the food-chain role and forgetting the separate, equally important O2-supply role.