20.1: What is Photosynthesis?
All living organisms on Earth are directly or indirectly dependent on photosynthesis. It is the only biological process that can capture energy from sunlight and convert it into chemical energy that every organism can use to power its metabolism. Photosynthesis is also the source of oxygen required by many living organisms.
Types of Organisms Based on their Modes of Nutrition
Broadly, there are two main categories of organisms based on their modes of nutrition — autotrophs and heterotrophs. An autotroph is an organism that can produce its own food. Plants, algae, and a group of bacteria called cyanobacteria are autotrophs. Because they use light to manufacture their own food in the form of carbohydrates, they are also called photoautotrophs. In contrast, heterotrophs, such as animals, fungi, and most bacteria, rely on the sugars produced by autotrophs for their energy needs. Even if an animal feeds on another animal, the origin of the food can be traced back to the autotrophs and the process of photosynthesis. Therefore, although the energy in the form of carbohydrates can only be produced in organisms that carry out photosynthesis, it is subsequently transferred to every living organism and drives the whole ecosystem.
Photosynthesis is a multi-step process that requires specific wavelengths of visible sunlight, carbon dioxide, and water as substrates. After the process is complete, it releases oxygen and produces glyceraldehyde-3-phosphate, as well as simple carbohydrate molecules that can then be converted into sugar molecules, such as glucose and sucrose. These sugar molecules contain energy and the energized carbon that all living things need to survive.
This text is adapted from Openstax, Biology 2e, Chapter 8, Section 8.1: Overview of Photosynthesis..