Question

What are organisms that cannot make their own food and must obtain energy from external sources called?

Answer 1

Hint: To grow and stay healthy, all living things require nourishment. The biochemical and physiological mechanism by which an organism uses food to sustain its life is known as nutrition. The study of the chemical substances required for plant development and physiology is known as plant nutrition.

Complete answer:
Organisms that cannot make their own food and must obtain energy from external sources are called Heterotrophs. Heterotroph is a Greek word where hetero, means "other", and trophe, means "nutrition".
Heterotrophs are incapable of converting inorganic to organic molecules. They must subsist on an organic carbon source that originated in another living thing. Heterotrophs rely on autotrophs for nutrients and food energy, either directly or indirectly.
Heterotrophs are classified based on their energy supplier. A chemoheterotroph is a heterotroph that utilizes chemical energy. It's a photoheterotroph if it gets its energy from light. Heterotrophs can be also classified as Organotrophs and Lithotrophs. Organotrophs use reduced carbon molecules like carbohydrates, lipids, and proteins from plants and animals as electron sources. Lithotrophs, on the contrary, get electrons from inorganic substances such as ammonium, nitrite or sulphur.
Heterotrophs are consumers in the food chain or food web, which means they eat other living things. Unlike the producers, they are unable to create their own food. Heterotrophs can be encountered at all stages of the food chain in a given habitat, and each stage plays an important role in that environment's ecosystem.

Note:
A heterotroph is an organism that lacks the ability to chemically create its own nutrition from inorganic molecules. Heterotrophs rely on other types of life to meet their organic food requisites due to this limitation. Heterotrophs include bacteria, fungi, yeast, cows, dogs, bats, birds, insects, humans etc.