Q.In a way green plants and cyanobacteria have synthesized all the food on the earth. Comment.

It is indeed accurate to say that green plants and cyanobacteria, between them, have synthesised virtually all the food on earth.

• Green plants are autotrophs — they are unique in being able to manufacture, or synthesise, their own organic food, and every other organism (heterotrophs) depends on them for its nutrition.
• Photosynthesis, the process by which they do this, is described as the primary source of all food on earth, using light energy to build organic compounds from CO2 and water. …

All animals, including human beings, ultimately depend on plants for their food, because green plants are unique in being able to synthesise their own organic food — a capacity called autotrophic nutrition. Every other organism that must obtain its food from green plants is, by contrast, called a heterotroph. Photosynthesis, the physico-chemical process by which green plants use light energy to make organic compounds, is described as important for two major reasons: it is the primary source of all food on earth, and it is responsible for the oxygen released into the atmosphere on which most life depends for respiration. …

Method 1 — Reason from the definitions of autotroph/heterotroph and photosynthesis's role

1. Recall that green plants are autotrophs, uniquely able to synthesise their own organic food, while every other organism (a heterotroph) depends on them.
2. Recall that photosynthesis is described as the primary source of all food on earth.
3. Recall that cyanobacteria are also photosynthetic organisms, carrying out the same light-driven synthesis of organic food, particularly significant in aquatic ecosystems. …