Processes of photosynthesis and cell respiration are the central link in the plant metabolism. They are closely associated with other metabolic processes. It is important that despite the opposite direction of the two central processes of plant organism and on their localization in different cell organelles, there is a close relationship between them.
In plants, the respiration is the opposite process of photosynthesis. A molecule of glucose is oxidized by atmospheric oxygen to carbon dioxide and water with the release of the energy contained in carbohydrates. This energy is used to implement and support all life processes: absorption and evaporation of water and mineral salts, plant growth and development.
There are three main differences between photosynthesis and respiration.
1) Photosynthesis is a process of absorption of carbon dioxide and release of oxygen, which is used for respiration. Respiration, in turn, is a process of absorption of oxygen and release of carbon dioxide, which is used for photosynthesis.
2) The end product of photosynthesis is glucose, which is used further for synthesis of other important organic compounds. As a result of respiration, organic compounds are oxidized to inorganic.
3) As a result of photosynthesis, solar energy is stored in chemical bonds of organic substances and by respiration this energy is released. This energy is required for enzyme synthesis, including the enzymes of photosynthesis.
Photosynthesis is the formation of organic substances from water and carbon dioxide in the light with the involvement of photosynthetic pigments (chlorophyll in plants and bacteriochlorophyll and bacteriorhodopsin in bacteria). Photosynthetic tissue of higher plants has many large organells called chloroplasts, which contain chlorophylls, the specialized light-absorbing green pigments.
Modern plant physiology explains photosynthesis as a phototrophic function of absorption processes, energy conversion and use of light quanta in various reactions, including the conversion of carbon dioxide into organic substances.
There are anoxic and oxygenic photosynthesis types. Oxygenic type is much more widespread, it is carried out by plants, cyanobacteria and prochlorophyta.
Total photosynthesis equation is usually written like this: CO2+H2O=CH2nO+CO2 (Alberts et al, 2002).
There are four stages of photosynthesis: absorption of light (light harvesting), electron transport, generation of ATP and carbon fixation (Lodish et al, 2000). Each stage occurs in a defined area of chloroplast.
During the light harvesting stage chlorophylls in the thylakoid membranes absorb the light to remove electrons from water (as a result, oxygen is produced). The second step is the charge separation in the reaction center and electron transfer on photosynthetic electron transport chain. High-energy compounds ATP and NADPH are the end products of these reactions. They are called thylakoid reactions, because they all take place on special inner membranes of chloroplasts (thylakoids). Conversion of light energy into chemical energy is carried out by two photosystems with different functions.
The fourth stage do not requires light. It includes biochemical reactions of synthesis of organic compounds with the use of energy accumulated in the light-dependent stage. These reactions are called carbon fixation reactions. The most common of these reactions are Calvin cycle and gluconeogenesis, the formation of sugars and starch from carbon dioxide air.
Plant cell respiration
Respiration is the main form of dissimilation in humans, animals, plants and many microorganisms. During the respiration energy-rich substances are completely decomposed to inorganic poor energy end-products (carbon dioxide and water), with the use molecular oxygen. An external respiration is understood as a gas exchange between the organism and its environment, including the absorption of oxygen and carbon dioxide emission and transport of these gases within the body. An internal (cellular) respiration includes biochemical processes in cytoplasm and mitochondrion of cell that results in energy release (Millar et al, 2011).
Total respiration equation is usually written like this: C6H12O6+6O2=6H2O+6CO2
Glycolytic pathway. This pathway consists of two phases: an anaerobic (glycolysis) and aerobic (Krebs cycle).
Reactions of glycolysis take place in cytoplasm and chloroplasts. It results in formation of 2 molecules of pyruvic acid and 4 molecules of ATP from 1 molecule of glucose.
Since high-energy bond is formed directly on the substrate to be oxidized, a process of formation of ATP is called substrate phosphorylation. Two molecules of ATP cover the cost of the initial activation of the substrate due to phosphorylation. Consequently, 2 molecules of ATP are accumulated. Moreover, 2 molecules of NAD are reduced to NADH during the glycolysis. Their oxidation in electron transport chain provides synthesis of 6 molecules of ATP. Total 8 ATP molecules are formed. Two molecules of pyruvic acid come into the aerobic phase of respiration.
Aerobic stage of respiration takes place in mitochondrion. Acetyl coenzyme A that is the derivative of pyruvic acid is oxidized to water and carbon dioxide in citric acid cycle. Acetyl coenzyme A is formed by an oxidative decarboxylation of pyruvic acid.
This process consists of a series of reactions catalyzed by multienzyme complex system of three enzymes and five co-enzymes, and called pyruvatcarboxylase.
Oxidation of 1 molecule of pyruvic acid results in formation of 3 molecules of NADH, 1 molecule of NADPH and 1 molecule of FADH2. Their oxidation in respiration electron transport chain results in synthesis of 14 molecules of ATP. Moreover, 1 molecule of ATP is formed in a process of substrate phosphorylation (Taiz et al, 2015).
Glyoxylate pathway. Glyoxylate pathway is a modification of Crebs cycle. It takes place not in mitochondrion, but in glyoxysomes. These organelles provide formation of isocitric acid as the Krebs cycle. Then it breaks down into glyoxylic and succinic acid by isocitrate lyase.
Glyoxylic acid reacts with the second molecule of acetyl coenzyme A to form the malic acid, which is then oxidized to oxaloacetic acid. Succinic acid exits glyoxysome and converts into oxaloacetic acid.
Respiratory electron transport chain and oxidative phosphorylation. Respiration electron transport chain contains electron transporters, which transfer electrons from substrate to oxygen. Location of carriers is determined by their redox potential. The aim starts from NADH with potential of –0.32 V, and ends with oxygen with potential of +0.82 V. Carriers are arranged on both sides of the inner mitochondrial membrane and they cross it. On the inner side of the membrane, located in the mitochondrial matrix, two protons and two electrons from NADH move to flavin mononucleotide and iron-sulfur proteins. Flavin mononucleotide after receiving protons transfers them to the outside of the membrane, where it gives protons to the intermembrane space. Iron-sulfur proteins inside the membrane transfer electrons from NADH to oxidized ubiquinone Q. It adds two more protons and diffuses to the membrane cytochromes. Cytochrome b560 gives two electrons to ubiquinone, which is joined by two protons from the matrix, transfers two electrons to cytochrome b556 and two electrons to cytochrome c1. Protons enter the intermembrane space. On the outer side of the membrane cytochrome c receives two electrons from cytochrome c1, and transmits them to cytochrome a, which transports them through a membrane cytochrome a3. Cytochrome a3 binds oxygen and gives electrons to it. Oxygen attaches two protons to form water (Taiz, 2012).
Thus, the electron transport in the respiratory electron transport chain is accompanied by the transmembrane proton transfer. The resulting potential difference on both sides of the inner mitochondrial membrane is used to synthesize ATP (oxidative phosphorylation). As a result of this process, 3 molecules of ATP are formed.
The relationship of cell respiration and photosynthesis
Carbohydrates and other organic substances produced during the photosynthesis are used by plant cells as nutrients. Cells of non-green parts of plants and all cells in the absence of light heterotrophically eat carbohydrate substances as well as animals. The most important stage of nutrition of organic matter at the cellular level is a process of respiration.
The opposite process of photosynthesis is the oxidation of organic substances in the cells, which results in release of energy required for the life of the organism. This is a complex process of respiration, which begins with the gas exchange between the organism and the environment, and ends in the cells of the body.
Cell respiration requires as substrates carbohydrates, produced by photosynthesis. The end product of photosynthesis is glucose, which is used further for synthesis of other important organic compounds. As a result of respiration, organic compounds are oxidized to inorganic. As a result of photosynthesis, solar energy is stored in chemical bonds of organic substances and by respiration this energy is released. This energy is required for enzyme synthesis, including the enzymes of photosynthesis.
Thus, photosynthesis provides formation of organic compounds that plants depend on and respiration releases the energy stored in those compounds.
Significance of photosynthesis and cell respiration to the evolution and diversity of life
The energy victory of living organisms is associated with the improvement of the photosynthetic apparatus, which led to an increase in productivity and the rate of photosynthesis. As a result, systems of photoreceptors, physico-chemical reactions of electron transport chain, systems of carbon dioxide deposition and regulatory mechanisms of photosynthesis were improved.
Appearance of many specific plant biochemical pathways is associated with offshoot of main photosynthetic pathway. Its intensification is caused by the further morphological evolution of plants.
Gradual accumulation of oxygen in the atmosphere made the progress and morphological differentiation of nature an irreversible phenomenon, which led to the increase of energy transfer and geochemical transformation on Earth.
Both cell respiration and photosynthesis are provided by cell organelles with two membranes. Originally these organelles were symbionts of unicellular organisms. Appearance of chloroplasts and mitochondrion was a huge step in evolution that led to the appearance of first eukaryotic organisms 2 billion years ago. This led to the fact that eukaryotes have come to dominate the world.
Initially, the oxygen in the atmosphere of the ancient Earth was very small. After the phenomenon, called “The Great Oxidation Event” that occurred 2.4 billion years ago, there was an increase of oxygen level in the atmosphere. Today air contains 21% of oxygen. Mostly, this oxygen is a product of photosynthesis. Each year plants and other photosynthetic organisms produce about 120 billion tons of oxygen.
Photosynthesis and cell respiration are two directly opposite processes in plant physiology, which are important for the existence of life on Earth. Photosynthesis is a process of formation of organic compounds from inorganic (carbon dioxide and water), which takes place in green parts of plants in the presence of light. Respiration provides the intake of oxygen in plants, oxygen using by cells and tissues, as well as the allocation from the body of carbon dioxide.
Alberts, B., Johnson, A., Lewis, J. (2002). Chloroplasts and Photosynthesis. Molecular Biology of the Cell. 4th edition. New York: Garland Science
Lodish, H., Berk, A., Zipursky, S. L., (2000). Photosynthetic Stages and Light-Absorbing Pigments. Molecular Cell Biology. 4th edition. New York: W. H. Freeman
Millar, A. H., Whelan, J., Soole, K. L., Day, D. A. (2011). Organization and Regulation of Mitochondrial Respiration in Plants. Annual Review of Plant Biology, 62, 79-104
Taiz, L., Zeiger, E. (2012). Respiration and Lipid Metabolism. Plant Physiology. Fifth Edition. Sunderland, Massachusetts: Sinauer Associates Inc.
Taiz, L., Zeiger, E., Møller, I. M. Murphy, A. (2015). Plant Physiology and Development. California: Sinauer Associates
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