Photosynthesis, the process by which plants, algae, and some bacteria convert light energy into chemical energy, often raises questions about its relationship with oxygen. Understanding the role of oxygen in photosynthesis requires a clear distinction between processes that require oxygen and those that do not.
Defining Aerobic and Anaerobic
The terms “aerobic” and “anaerobic” describe whether a metabolic process requires oxygen. An aerobic process depends on the presence of oxygen. Cellular respiration in humans, where oxygen is consumed to break down glucose for energy, is a common example.
Conversely, an anaerobic process takes place in the absence of oxygen. Fermentation, which produces less ATP than aerobic respiration, is a well-known anaerobic example.
Photosynthesis and Its Oxygen Byproduct
The most common form of photosynthesis, known as oxygenic photosynthesis, is carried out by plants, algae, and cyanobacteria. This process converts light energy into chemical energy, primarily in the form of glucose. It produces oxygen as a byproduct.
During oxygenic photosynthesis, water molecules are split to provide electrons for the reaction, releasing oxygen into the atmosphere. Therefore, photosynthesis does not consume oxygen; rather, it generates it. The overall equation for this process illustrates that carbon dioxide and water, along with light energy, yield glucose and oxygen.
Photosynthesis Without Oxygen Production
While the familiar form of photosynthesis releases oxygen, other types of photosynthetic processes do not. This is known as anoxygenic photosynthesis, performed by certain bacteria. These organisms, such as purple sulfur bacteria and green sulfur bacteria, use different electron donors instead of water.
Anoxygenic phototrophs use hydrogen sulfide (H₂S), elemental sulfur (S⁰), or thiosulfate (S₂O₃²⁻) as electron donors. Since water is not split in these reactions, oxygen is not produced as a byproduct. While oxygenic photosynthesis is a major source of atmospheric oxygen, not all photosynthetic pathways produce it.
Photosynthesis Versus Cellular Respiration
Photosynthesis and cellular respiration are complementary processes, often confused regarding their relationship with oxygen. Cellular respiration, particularly aerobic respiration, is a process that consumes oxygen to break down glucose and release energy in the form of ATP. This occurs in most living organisms, including plants.
In contrast, photosynthesis uses carbon dioxide and water to synthesize glucose and release oxygen. These two processes essentially reverse each other in terms of their inputs and outputs concerning glucose, carbon dioxide, and oxygen. Photosynthesis stores energy in glucose, while cellular respiration releases that stored energy for cellular functions.