Why are algae and multicellular plants classified as autotrophs?

Algae and multicellular plants are classified as autotrophs because they have the ability to capture sunlight and use it to produce their own food in the form of sugars through a process called photosynthesis. In this process, chlorophyll, the green pigment found in these organisms, absorbs sunlight and converts carbon dioxide and water into glucose (a type of sugar) and oxygen. This self-sustaining ability to create organic compounds from inorganic sources distinguishes them from heterotrophs, which must consume other organisms for energy and nutrients.

The ability to perform photosynthesis not only enables algae and plants to thrive in various environments but also plays a crucial role in the Earth's ecosystems by contributing to the oxygen supply and serving as the base of food chains.