Organisms that synthesise their own organic food from simple inorganic substances (such as carbon dioxide and water) are known by which term?

Introduction / Context:
Living organisms can be broadly divided into those that produce their own food and those that must obtain food from other sources. This fundamental distinction lies at the heart of ecology and energy flow in ecosystems. The question asks for the correct term used to describe organisms that can synthesise their own organic food from simple inorganic raw materials using an external energy source such as sunlight or chemical reactions.

Given Data / Assumptions:

The organisms in question can make their own food from inorganic substances. Typical examples include green plants and certain bacteria. Options include autotrophs, heterotrophs, auxotrophs, saprotrophs and parasites. We assume standard ecological and nutritional definitions.

Concept / Approach:
Autotrophs are organisms that can synthesise organic compounds from inorganic substances. Photoautotrophs, such as green plants, use light energy to drive photosynthesis, while chemoautotrophs, such as some bacteria, use energy from chemical reactions. In contrast, heterotrophs cannot make their own food and must consume organic material produced by other organisms. Auxotrophs are mutant organisms that require a specific additional nutrient that the wild type does not need. Saprotrophs feed on dead and decaying organic matter, and parasites live on or in a host, deriving nutrients from it. Only autotrophs match the description of synthesising their own food from inorganic sources.

Step-by-Step Solution:
Step 1: Focus on the phrase “synthesise their own food from inorganic substances”. Step 2: Recall that auto means self and troph means feeding, so autotroph literally means self feeding. Step 3: Recognise that green plants, algae and some bacteria are classic autotrophs producing sugars from carbon dioxide and water. Step 4: Distinguish this from heterotrophs, which must obtain ready made organic food from external sources. Step 5: Note that auxotrophs, saprotrophs and parasites are all specific types of heterotrophs, not organisms that create food from purely inorganic materials.

Verification / Alternative check:
Check familiar examples: a green plant using sunlight to produce glucose from carbon dioxide and water is always described as an autotroph. A lion eating other animals is a heterotroph, not an autotroph. A mushroom decomposing dead wood is a saprotroph, feeding on pre existing organic matter. A tapeworm living in a host intestine is a parasite. Textbooks and ecological pyramids always place autotrophs at the base as producers, reinforcing that this is the correct term for organisms that synthesise their own food from inorganic raw materials.

Why Other Options Are Wrong:
Heterotrophs cannot synthesise all their food from inorganic substances and instead rely on consuming organic materials produced by other organisms. Auxotrophs are genetically altered organisms that require additional specific nutrients and are still heterotrophic. Saprotrophs obtain food by secreting enzymes on dead and decaying matter and absorbing the digested nutrients, not by making organic molecules from inorganic carbon dioxide. Parasites feed on living hosts and depend on host derived nutrients. None of these categories fit the description of independent food synthesis from inorganic sources as directly as autotrophs do.

Common Pitfalls:
A common mistake is to mix up autotrophs with heterotrophs because both ends in trophs and sound similar. Another pitfall is to think of saprotrophs as self sufficient simply because they can digest dead matter externally. To avoid confusion, remember that autotrophs build organic molecules from inorganic ones, while all other trophic types rely on pre formed organic substances. Associating autotrophs with producers at the base of the food chain can help you quickly recall the correct term in exam questions.

Final Answer:
Organisms that synthesise their own food from inorganic substances are called autotrophs.