Key Difference – Oxygenic vs Anoxygenic Photosynthesis
Photosynthesis is a process which synthesizes carbohydrates (glucose) from water and carbon dioxide, utilizing the energy from sunlight by green plants, algae, and cyanobacteria. As a result of photosynthesis, gaseous oxygen is released to the environment. It is an extremely important process for the existence of life on earth. Photosynthesis can be divided into two categories such as oxygenic and anoxygenic photosynthesis based on the generation of oxygen. The key difference between oxygenic and anoxygenic photosynthesis is that oxygenic photosynthesis generates molecular oxygen during the synthesis of sugar from carbon dioxide and water while anoxygenic photosynthesis does not generate oxygen.
CONTENTS
1. Overview and Key Difference
2. What is Oxygenic Photosynthesis
3. What is Anoxygenic Photosynthesis
4. Side by Side Comparison – Oxygenic vs Anoxygenic Photosynthesis
5. Summary
What is Oxygenic Photosynthesis?
The energy of sunlight is converted into chemical energy by photosynthesis. The light is captured by the green pigments called chlorophylls possessed by photosynthetic organisms. Using this absorbed energy, chlorophyll reaction centers of the photosystems are excited and release electrons which contain high energy. These high energy electrons flow via several electron carriers and convert water and carbon dioxide into glucose and molecular oxygen. The excited electrons travel in a noncyclic chain and terminate at the NADPH. Due to the generation of molecular oxygen, this process is known as oxygenic photosynthesis and also called noncyclic photophosphorylation.
Oxygenic photosynthesis has two photosystems named PS I and PS II. These two photosynthetic apparatus contain two reaction centers P700 and P680. Upon absorption of light, the reaction center P680 become excited and releases high energy electrons. These electrons travel via several electron carriers and release some energy and are handed over to P700. P700 become excited due to this energy and release high energy electrons. These electrons flow through several carriers again and finally reach the terminal electron acceptor NADP+ and become reducing power NADPH. The water molecule hydrolyzes near PS II and donates electrons and liberates molecular oxygen. During the electron transport chain, proton motive force is created and is used to synthesize ATP from ADP.
Oxygenic photosynthesis is extremely important since it is the process which is responsible for the transformation of the Earth’s primitive anoxygenic atmosphere to oxygen rich atmosphere.
Figure 01: Oxygenic Photosynthesis
What is Anoxygenic Photosynthesis?
Anoxygenic photosynthesis is the process where light energy is converted to chemical energy without generating molecular oxygen as a byproduct. This process is seen in several bacterial groups such as purple bacteria, green sulfur and nonsulfur bacteria, heliobacteria and acidobacteria. Without generating oxygen, ATP is produced by these bacterial groups. Water is not used as the initial electron donor in anoxygenic photosynthesis. This is why oxygen is not generated during this process. Only one photosystem is involved with anoxygenic photosynthesis. Hence electrons are transported in a cyclic chain and returned to the same photosystem. Therefore, anoxygenic photosynthesis is also known as cyclic photophosphorylation.
Anoxygenic photosynthesis depends on bacteriochlorophylls as opposed to chlorophylls used in oxygenic photosynthesis. Purple bacteria possess photosystem I with P870 reaction center. Different electron acceptors such as bacteriopheophytin are involved with this process.
Figure 02: Anoxygenic Photosynthesis
What is the difference between Oxygenic and Anoxygenic Photosynthesis?
Oxygenic vs Anoxygenic Photosynthesis |
|
| Oxygenic photosynthesis is the process which converts light energy to chemical energy by certain photoautotrophs by generating molecular oxygen. | Anoxygenic photosynthesis is the process which converts light energy to chemical energy by certain bacteria without generating molecular oxygen. |
| Generation of Oxygen | |
| Oxygen is released as a by-product. | Oxygen is not released or generated. |
| Organisms | |
| Oxygenic photosynthesis is shown by cyanobacteria, algae, and green plants. | Anoxygenic photosynthesis is mainly shown by purple bacteria, green sulfur and nonsulfur bacteria, heliobacteria, and acidobacteria. |
| Electron Transport Chain | |
| Electrons travel via several electron carriers. | It occurs via cyclic photosynthetic electron chain. |
| Water as an Electron Donor | |
| Water is used as the initial electron donor. | Water is not used as an electron donor. |
| Photosystem | |
| Photosystem I and II are involved in oxygenic photosynthesis | Photosystem II is not present in anoxygenic photosynthesis |
| Generation of NADPH (reducing power) | |
| NADPH is generated during the oxygenic photosynthesis. | NADPH is not generated because electrons cycle back to the system. Hence reducing power is obtained from other reactions. |
Summary – Oxygenic vs Anoxygenic Photosynthesis
Photosynthesis is the process where light energy is converted into chemical energy by photosynthetic organisms. It can happen in two ways: oxygenic photosynthesis and anoxygenic photosynthesis. Oxygenic photosynthesis is the photosynthetic process which liberates molecular oxygen to the atmosphere and it is seen in green plants, aglae, and cyanobacteria which possess chlorophylls. Anoxygenic photosynthesis is a photosynthetic process which does not generate molecular oxygen and is used by certain bacterial groups which possess bacteriochlorophylls. Thus, the difference between oxygenic and anoxygenic photosynthesis mainly depends on the generation of oxygen.
Reference:
1. “Oxygenic and Anoxygenic Photosynthesis in Bacteria.” Biology Discussion. N.p., 16 Sept. 2016. Web. 13 May 2017. <http://www.biologydiscussion.com/microbiology-2/microbial-photosynthesis/oxygenic-and-anoxygenic-photosynthesis-in-bacteria/55365>.
2. “Generating Oxygen (or Not): Oxygenic and Anoxygenic Photosynthesis.”Dummies. N.p., n.d. Web. 13 May 2017. <http://www.dummies.com/education/science/biology/generating-oxygen-or-not-oxygenic-and-anoxygenic-photosynthesis/>.
Image Courtesy:
1. “Thylakoid membrane” By Tameeria at English Wikipedia – Transferred from en.wikipedia to Commons. (Public Domain) via Commons Wikimedia
2. “Anoxygene Photosynthese P870 final” By Yikrazuul – Own work (CC BY-SA 3.0) via Commons Wikimedia