Difference Between Photosynthesis and Chemosynthesis

Photosynthesis vs Chemosynthesis


Photoautotrophs use light as the source of energy and inorganic carbon as the source of carbon. Photosynthesis is a metabolic process by which solar energy is converted into chemical energy in organic compounds like carbohydrates using carbon dioxide and water  as raw materials in the presence of chlorophyll. Light reaction takes place in the thylakoid membrane.

In the light reaction, light energy absorbed by the pigment molecules is transferred to P 680 chlorophyll a molecules in the reaction centre of photosystem II. When energy is transferred to P 680, its electrons are boosted to high energy levels. These electrons are picked up by primary electron acceptor molecules and finally to photosystem I through a series of carrier molecules like cytochrome. When electrons are transferred through electron carriers of low energy levels, some of the energy released is used in the synthesis of ATP from ADP. This process is called photophosphorylation.

At the same time, water molecules are split by the light energy and this process is called photolysis of water.  As a result of photolysis of 4 water molecules, 2 oxygen molecules, 4 protons and 4 electrons are produced. Electrons produced replace electrons lost from chlorophyll a molecule of PS II. Oxygen is evolved as a biproduct. At PS I also light energy is absorbed, when P 700 chlorophyll a molecules of photosystem I are excited. Then its electrons are boosted to higher energy levels and are accepted by primary electron acceptors. Through acceptor molecules finally transferred to NADP molecules, which is reduced to NADPH2 using protons produced in photolysis.

In PS I, the electron that is excited may be an electron from chlorophyll a or the electron that comes from PS II. The dark reaction takes place in the stroma of the chloroplast. Carbon dioxide is accepted by ribulose bisphosphate, which is a C5 compound. This reaction is catalyzed by an enzyme called RuBP carboxylase and takes place in the stroma. First an unstable C6 compound is produced. Finally, 2 PGA molecules, which are C3 compounds, are produced.

PGA is the first stable product of this photosynthesis process and also it is the first carbohydrate. PGA is reduced to PGAL. All NADPH2 and part of ATP produced during the light reaction is used up in this reaction. Part of the PGA formed is used to synthesize more complex carbohydrates like glucose, sucrose, starch etc. Remaining part is used for regeneration of RuBP through RuMP, using remaining ATP.  Dark reaction takes place in a cyclic manner, and this is called the Calvin cycle.


Chemoautotrophs use inorganic carbon as the source of energy. Their source of carbon is also inorganic carbon. Chemosynthesis is carried out by bacteria such as Nitosomonas and Nitrobacter.


What is the difference between Photosynthesis and Chemosynthesis?

• Photoautotrophs use light as the source of energy whereas chemoautotrophs use inorganic carbon as the source of energy, but they both use inorganic carbon as the source of carbon.