Photosynthesis: How and Why?

Abstract

The total solar energy absorbed by Earth is approximately 3,850,000 exajoules per year. This was more energy in one hour than the world used in one year! Nature uses very wonderful and interesting strategies to capture the energy in an interesting process: Photosynthesis. To know more about photosynthesis, the first we should know about phototrophy. Phototrophy is the process by which organisms trap photons and store energy as chemical energy in the form of adenosine triphosphate (ATP). ATP transports chemical energy within cells for metabolism. There are three major types of phototrophy: Oxygenic and Anoxygenic photosynthesis, and Rhodopsin-based phototrophy. Photosynthesis is a chemical process that converts carbon dioxide into different organic compounds using solar energy. Oxygenic and anoxygenic photosynthesis undergo different reactions in the presence and absence of light (called light and dark reactions, respectively). In anoxygenic photosynthesis, light energy is captured and stored as ATP, without the production of oxygen. This means water is not used as primary electron donor. Phototrophic green bacteria, phototrophic purple bacteria, and heliobacteria are three groups of bacteria that use anoxygenic photosynthesis. Anoxygenic phototrophs have photosynthetic pigments called bacteriochlorophylls. Bacteriochlorophyll a and b have maxima wavelength absorption at 775 nm and 790 nm, respectively in ether. Unlike oxygenic phototrophs, anoxygenic photosynthesis only functions using a single photosystem. This restricts them to cyclic electron flow only, and they are therefore unable to produce O2 from the oxidization of H2O. In plants, algae and cyanobacteria, the photosynthetic processes results not only in the fixation of carbon dioxide (CO2) from the atmosphere but also release of molecular oxygen to the atmosphere. This process is known as oxygenic photosynthesis. Photosynthesis captures approximately 3,000 EJ per year in biomass and produces more than 100 billion tons of dry biomass annually (Barber, 2009). Photosynthesis is also necessary for maintaining the normal level of oxygen in the atmosphere. It is believed that the first photosynthetic organisms evolved about 3,500 million years ago. In that condition, the atmosphere had much more carbon dioxide and organisms used hydrogen or hydrogen sulfide as sources of electron (Olson, 2006). Around 3,000 million years ago, Cyanobacteria appeared later and changed the Earth when they began to oxygenate the atmosphere, beginning about 2,400 million years ago. This new atmosphere was a revolution for complex life. The chloroplasts in modern plants are the descendants of