Difference Between Photosynthesis and Chemosynthesis

The key difference between photosynthesis and chemosynthesis is that the photosynthesis is the process of converting the energy of sunlight into carbohydrates by photoautotrophs while chemosynthesis is the process of converting the chemical energy of inorganic compounds or methane into organic compounds by chemoautotrophs.

Photosynthesis and chemosynthesis are two important processes that allow living organisms to produce foods for them. Both photosynthesis and chemosynthesis help to sustain the living organisms. Though both processes use CO₂ and produce organic compounds, they differ from several characteristics as discussed in the article. As the names suggest, photo means sunlight and chemo means chemical. Hence, sunlight provides energy to photosynthesis, while the chemical energy of inorganic compounds provides energy to chemosynthesis.

CONTENTS

1. Overview and Key Difference
2. What is Photosynthesis
3. What is Chemosynthesis
4. Similarities Between Photosynthesis and Chemosynthesis
5. Side by Side Comparison – Photosynthesis vs Chemosynthesis in Tabular Form
6. Summary

What is Photosynthesis?

Photosynthesis is a metabolic process by which photoautotrophs convert solar energy into chemical energy in organic compounds such as carbohydrates using carbon dioxide and water as raw materials in the presence of chlorophyll. There are two main processes in photosynthesis; light reaction and dark reaction.

Light Reaction of Photosynthesis

The light reaction takes place in the thylakoid membrane. In the light reaction, pigment molecules absorb light energy and transfer to P680 chlorophyll molecules in the reaction centre of photosystem II. Once P680 absorbs energy, its electrons obtain high energy and become boosted. Primary electron acceptors pick up these high energy electrons and pass via a series of carrier molecules like cytochrome and finally pass to photosystem I. When electrons go through the carrier molecules, at each step, energy is released, and the released energy is stored in the form of ATP. It is the process called photophosphorylation.

At the same time, water molecules split by the light energy into O_2, and it is the process called photolysis of water. When four water molecules split, it produces 2 oxygen molecules, 4 protons and 4 electrons. The produced electrons from photolysis, replace the lost electrons of PS II. Eventually, the produced oxygen releases into the atmosphere.

Afterwards, when PS I obtain energy, its electrons also excite into high energy levels. Electron acceptors accept these electrons and pass into NADP molecules. Then the NADP molecules reduce into NADPH₂ molecules.

Dark Reaction of Photosynthesis

The dark reaction (Calvin cycle) takes place in the stroma of the chloroplast. It starts by the C 5 compound called ribulose bisphosphate. Ribulose bisphosphate accepts carbon dioxide and converts into two molecules of Phosphoglycerate (PGA). PGA is the first stable product of this photosynthesis process, and also it is the first carbohydrate. PGA then reduces into PGAL and this conversion utilizes all NADPH₂ and part of ATP produced during the light reaction. In this point, complex carbohydrates such as glucose and sucrose are produced from one part of PGA while the remaining PGA is used to generate RuBP. Likewise, the dark reaction takes place in a cyclic manner.

What is Chemosynthesis?

Chemosynthesis is the process by which chemoautotrophs produce foods (carbohydrates) for them. Unlike photosynthesis, chemosynthesis does not need sunlight. Hence, it occurs under dark conditions, mostly in the deep sea near to hydrothermal vents.

Hence, during the chemosynthesis, the chemical energy of inorganic compounds such as hydrogen gas, hydrogen sulfide or methane converts into carbohydrates. This type of food production employs mostly by prokaryotes such as sulfur-oxidizing gamma and epsilon proteobacteria, the Aquificae, the methanogenic archaea and the neutrophilic iron-oxidizing bacteria. Furthermore, chemosynthesis results in sulfur compounds as byproducts.

What are the Similarities Between Photosynthesis and Chemosynthesis?

• Both photosynthesis and chemosynthesis produce foods or carbohydrates.
• They convert energy into organic matter.
• In these processes, a series of reactions take place.
• Also, both processes use CO₂.
• Besides, both these processes help promote and sustain life on the Earth.

What is the Difference Between Photosynthesis and Chemosynthesis?

Photosynthesis is a process that utilizes sunlight to produce carbohydrates by plants, algae and cyanobacteria. On the other hand, chemosynthesis is a process that utilizes the energy of inorganic compounds to produce carbohydrates by bacteria. Therefore, this is the key difference between photosynthesis and chemosynthesis. Photoautotrophs carry out photosynthesis while chemoautotrophs carry out chemosynthesis. Furthermore, photosynthesis occurs when the sunlight is present while the chemosynthesis occurs under dark conditions mostly on the sea floor near the hydrothermal vents. Thus, it is another difference between photosynthesis and chemosynthesis.

Besides, one more difference between photosynthesis and chemosynthesis is that the presence of chlorophyll pigments is necessary to carry out photosynthesis while the chemosynthesis does not need chlorophylls. Moreover, photosynthesis produces oxygen as a byproduct while the chemosynthesis produces sulfur compounds as byproducts.

Below infographic on the difference between photosynthesis and chemosynthesis provides more difference between both processes.

Summary – Photosynthesis vs Chemosynthesis

Photosynthesis and chemosynthesis are two processes used by organisms to produce glucose. These two processes are extremely important since they provide foods for all living organisms including animals. The key difference between photosynthesis and chemosynthesis is the energy source. Photosynthesis utilizes energy from sunlight while the chemosynthesis utilizes the energy of inorganic compounds such as H₂, H₂S, methane, etc. Photoautotrophs produce glucose by photosynthesis while chemoautotrophs produce glucose by chemosynthesis. Furthermore, photosynthesis results in forming oxygen as a byproduct while the chemosynthesis results in forming sulfur compounds as by-products. Thus, this is the summary of photosynthesis and chemosynthesis.

Reference:

1. Nature News, Nature Publishing Group. Available here
2. “Chemosynthesis.” Wikipedia, Wikimedia Foundation, 10 Nov. 2018. Available here 

Image Courtesy:

1.”Simple photosynthesis overview”By Daniel Mayer – Own work, (CC BY-SA 4.0) via Commons Wikimedia 
2.”Venenivibrio”By No machine-readable author provided. (CC BY-SA 3.0) via Commons Wikimedia