Difference Between Photosynthesis and Cellular Respiration

Main Difference – Photosynthesis vs Cellular Respiration

Photosynthesis and cellular respiration are two basic metabolic processes occurring in the energy conversion of ecosystems. During photosynthesis, carbon dioxide and water are used in the synthesis of organic compounds with the aid of sunlight. This organic compounds can be used by cells as food. During cellular respiration, energy in the form of ATP is produced by breaking down food. The main difference between photosynthesis and cellular respiration is that photosynthesis is an anabolic process, where the synthesis of organic compounds occurs, storing energy whereas cellular respiration is a catabolic process, where the stored organic compounds are utilized, producing energy.

This article explores, 

1. What is Photosynthesis
      – Definition, Characteristics, Types, Process
2. What is Cellular Respiration
      – Definition, Characteristics, Types, Process
3. What is the difference between Photosynthesis and Cellular Respiration

What is Photosynthesis

Photosynthesis is the production of glucose from carbon dioxide and water by obtaining energy from sunlight. Oxygen gas is a by-product of photosynthesis. Pigments like chlorophyll, carotenoids and phycobilins are used to trap the light energy. Therefore, during photosynthesis, light energy is converted into potential chemical energy. Subsequently, glucose provides the metabolic energy to all cellular processes in the cell.

Types of Photosynthesis

Oxygenic photosynthesis and anoxygenic photosynthesis are the two types of photosynthesis found on earth. Plants, algae, and cyanobacteria carry out oxygenic photosynthesis while purple sulfur bacteria and green sulfur bacteria carry out anoxygenic photosynthesis. The electron donor in oxygenic photosynthesis is water whereas the electron donor in anoxygenic photosynthesis is a variant like hydrogen sulfide rather than water. Thereby, in anoxygenic photosynthesis, oxygen gas is not liberated as a by-product. The chemical reactions of both oxygenic and anoxygenic photosynthesis are shown below.

Figure 1: Photosynthesis in a plant

In plants, photosynthesis occurs in specialized plastids called chloroplasts that are found in the cytoplasm of photosynthetic cells. Photosynthesis occurs in thylakoid membrane and stroma regions of chloroplasts. The first stage of photosynthesis is the light reaction. In thylakoid membrane of grana, photocenters are found, organizing the photosynthetic pigments in them. The light is absorbed by photosystem I and II, which are the two protein complexes found in thylakoid membrane, and the absorbed light is transferred to photocenters. The generating high energy electrons are transferred into the third protein complex, cytochrome bf complex. The high energy electrons in PSI is transferred into a series of ferrodoxin carriers and ultimately, these electrons are transferred into NADP+ by NADPH reductase enzyme, forming NADP. During the light reaction, oxygen gas is produced by splitting water while producing NADP and ATP.

The second stage of photosynthesis is the dark reaction, where the NADPH and ATP produced in light reaction are used as the energy sources to synthesize glucose. Dark reaction occurs in the stroma. Dark reaction is also called the Calvin cycle. Other than glucose, 18 ATP and 12 NADPH are produced during the Calvin cycle. The 18 ATPs are used by the Calvin cycle itself. The 12 NADPH contain 24 electrons that are transported into the electron transport chain, which is the third stage of photosynthesis. ATP synthase enzyme on the thylakoid membrane transfers the 24 electrons into 12 water molecules, producing 6 oxygen molecules. This process of electron transportation is called photophosphorylation. The process of photosynthesis is shown in figure 2.

Figure 2: Photosynthesis Mechanism

What is Cellular Respiration

Cellular respiration is the process which converts the biochemical energy into energy in ATP, eliminating carbon dioxide and water as waste products. It occurs in all the organisms living on earth. Stored food like carbohydrates, fats and proteins in organisms are used in the form of glucose by cellular respiration.

Types of Cellular Respiration

Aerobic respiration and anaerobic respiration are the two types of respiration found on earth. In aerobic respiration, the oxidizing agent or the final electron acceptor is molecular oxygen. One glucose molecule contains energy sufficient to produce 30 ATPs by oxidative phosphorylation. During anaerobic respiration, the final electron acceptor is either inorganic sulfates or nitrates. Anaerobic respiration occurs in hydrothermal vents in the deep sea. Fermentation is also a kind of anaerobic respiration, which occurs when pyruvate is metabolized in the cytoplasm without oxygen. Lactic acid fermentation in muscle cells and ethanol fermentation in yeast are the two types of fermentation found among organisms. Only two ATPs are produced per glucose molecule in fermentation. The chemical reaction of cellular respiration is shown below.

Figure 3: Cellular Respiration in Humans

In eukaryotes, cellular respiration occurs in specialized organelles called mitochondria. In prokaryotes, it occurs in the cytoplasm itself. Cellular respiration occurs in the matrix, inner membrane of mitochondria and cytoplasm as well. The first stage of cellular respiration is the glycolysis. During glycolysis, glucose (C6) is broken down into two pyruvate (C3) molecules in the cytoplasm. Two pyruvate molecules are then imported into mitochondria. In the presence of oxygen, pyruvate combines with oxaloacetate (C4) in order to form citrate (C6), eliminating acetyl-CoA during the citric acid cycle. The citric acid cycle is the second stage of cellular respiration, which is also called Krebs cycle. During Krebs cycle, carbon dioxide is eliminated as a waste, while reducing NAD into NADH. 6NADH, 2FADH₂ and 2ATPs per one glucose molecule are produced by the Krebs cycle. Oxidative phosphorylation, which is the third stage of cellular respiration occurs in the mitochondrial cristae by the enzyme ATP synthase, producing 30ATPs. The process of cellular respiration is shown in figure 4.

Figure 4: Cellular respiration mechanism

Difference Between Photosynthesis and Cellular Respiration

Presence

Photosynthesis: Photosynthesis is found only in chlorophyllous cells.

Cellular Respiration: Cellular respiration is found in all the cells on earth.

Definition

Photosynthesis: Photosynthesis is the production of glucose from carbon dioxide and water by obtaining energy from sunlight.

Cellular Respiration: Cellular respiration is the process which converts the biochemical energy into energy in ATP, eliminating carbon dioxide and water as waste products.

Organelles

Photosynthesis: Photosynthesis occurs in thylakoid membrane and stroma of chloroplasts in plants.

Cellular Respiration: Cellular respiration occurs in the matrix and the inner membrane of mitochondria and cytoplasm in eukaryotes.

Darkness/Light

Photosynthesis: Photosynthesis only occurs in light.

Cellular Respiration: Cellular respiration occurs in both light and dark.

Steps

Photosynthesis: Light reaction, dark reaction, and photolysis are the three steps in photosynthesis.

Cellular Respiration: Glycolysis, Citric acid cycle and electron transport chain are the three steps in cellular respiration.

Oxygen/Carbon Dioxide/Water

Photosynthesis: Carbon dioxide and water are utilized and oxygen is released during photosynthesis.

Cellular Respiration: Oxygen is utilized and carbon dioxide and water are released during cellular respiration.

Metabolism

Photosynthesis: Photosynthesis is an anabolic process, which synthesizes complex organic compounds.

Cellular Respiration: Cellular respiration is a catabolic process, which degrades organic compounds.

Carbohydrates

Photosynthesis: Carbohydrates are synthesized during photosynthesis.

Cellular Respiration: Carbohydrates are used during cellular respiration.

Energy

Photosynthesis: Energy is stored during photosynthesis. Hence photosynthesis is an endothermic process.

Cellular Respiration: Energy is liberated during cellular respiration. Hence, cellular respiration is an exothermic process.

Form of Energy

Photosynthesis: Chemical energy is stored in the bonds of forming organic compounds.

Cellular Respiration: Energy is liberated in the form of ATP, which can be utilized by other cellular processes.

Dry Weight

Photosynthesis: Dry weight of the plant is increased during photosynthesis.

Cellular Respiration: Dry weight of the organism is decreased during cellular respiration.

Type of Phosphorylation

Photosynthesis: Photophosphorylation occurs during photosynthesis.

Cellular Respiration: Oxidative phosphorylation occurs during cellular respiration.

Energy Conversion

Photosynthesis: During photosynthesis, light energy is converted into potential energy.

Cellular Respiration: During cellular respiration, potential energy is converted into kinetic energy.

Final Electron Acceptor

Photosynthesis: Final electron acceptor is water.

Cellular Respiration: Final electron acceptor is molecular oxygen.

Pigments

Photosynthesis: Chlorophyll is the main type of pigment involving in photosynthesis.

Cellular Respiration: Pigments are not involved in cellular respiration.

Coenzymes

Photosynthesis: NADP is the coenzyme used in photosynthesis.

Cellular Respiration: NAD and FAD are the coenzymes used in cellular respiration.

Conclusion

Photosynthesis and cellular respiration are the main two metabolic processes that occur in organisms, driving all the cellular processes in the body. Photosynthesis only occurs in chlorophyllous organisms. It has the highest contribution in producing food for all the living forms on earth. Hence, photosynthetic organisms are found as primary producers in food chains. During photosynthesis, glucose is produced from carbon dioxide and water using the energy from sunlight. Photosynthetic organisms contain special pigments like chlorophyll and carotenoids in order to trap the light. In contrast, cellular respiration occurs in all living forms on earth. During respiration, food is oxidized in order to obtain the potential energy stored in the form of ATP. ATP powers almost all the cellular processes in the cell. Carbon dioxide and water is produced as waste during cellular respiration. Oxygen gas is liberated during photosynthesis, which can be used in cellular respiration. Therefore, the main difference between photosynthesis and cellular respiration is their contribution in the metabolism of a cell.

Reference:
Cooper, Geoffrey M. “Photosynthesis.” The Cell: A Molecular Approach. 2nd edition. U.S. National Library of Medicine, 01 Jan. 1970. Web. 03 Apr. 2017.
Berg, Jeremy M. “The Citric Acid Cycle.” Biochemistry. 5th edition. U.S. National Library of Medicine, 01 Jan. 1970. Web. 04 Apr. 2017.
Cooper, Geoffrey M. “Metabolic Energy.” The Cell: A Molecular Approach. 2nd edition. U.S. National Library of Medicine, 01 Jan. 1970. Web. 04 Apr. 2017.

Image Courtesy:
1. “Photosynthesis Images” By Masroor.nida.ns – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Photosynthesis” By Cykl_Calvina.svg: PisumThylakoid_membrane.svg: YikrazuulThylakoide.png: Tameeria pour la version anglaise, Pascal Corpet pour la traductionderivative work: Marek M (talk) – Cykl_Calvina.svgThylakoid_membrane.svgThylakoide.png (CC BY-SA 3.0) via Commons Wikimedia
3. “Energy and life” By Mikael Häggström – All images are of Public Domain license.File:Pikilia.JPG (Public Domain) via Commons Wikimedia
4. “CellRespiration”By RegisFrey – Own work (CC BY-SA 3.0) via Commons Wikimedia