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Anabolism and Catabolism: Definitions & Examples - Video & Lesson Transcript | tokyo-sinderera.info
Plant Metabolism National Coordinator: Bhatla, Department of Botany, University of Delhi. Concept of metabolism Lesson Developer: Shivaji College, University of Delhi. Kirori Mal College Language Editor: Cells are open systems which intake food in the form of carbohydrates, lipids and proteins, water, gases such as O 2.
Occurrence of metabolic pathways inside a cell is vital to its existence and sustenance. Metabolism allows the cells to synthesize and catabolize molecules as per their requirements, thus making it truly living. In every cell various biomolecules are broken down and others are synthesized in different pathways, forming a complex but highly regulated network of reactions.
We must therefore study the basic concepts governing the metabolic pathways. The concept of catabolism and anabolism Source: A series of consecutive chemical reactions occurring in an organized and ordered manner considered together constitute a metabolic pathway.
The tremendous diversity of metabolic processes allows the organism to perform all the normal life processes such as growth, reproduction, maintenance of cell structures, repair damage and respond to environment. Metabolic pathways do not occur at random inside the cells.
These are involved in performing specific functions such as breakdown of glucose or its synthesis, synthesis of ATP etc. These pathways are series of highly regulated, branched, interconnected enzymatic reactions, which have specific reactants and products. Schematic representation of major metabolic pathways Source: Usually, inside a cell more than metabolic reactions are known to occur, each reaction having a specific enzyme for catalysis.
Two or more pathways occurring in a cell can have certain reactions common to them. This links the various pathways inside the cell. Some well known common examples are glycolysis, citric acid cyclic, gluconeogenesis, photosynthesis. Many pathways are common to all cells whereas other may be specific to certain cells only.
The type of metabolic pathways occurring in an organism depends on the type of organism, its nutritional status and the developmental stage attained by it. It may also vary depending upon the type of cell within an organism. Characteristics of metabolic pathways: Metabolism is the sum total of the entire set of biochemical processes and all the chemical reactions occurring inside a cell or an organism for sustenance of life. The chemical reactions start with a primary compound, which acts as substrate or reactant.
After the completion of reaction the compound formed is known as product. Energy exchange facilitated mostly by ATP occurs during the reaction as well as oxidation-reduction mayalso occur during the metabolic reactions. All the biological reactions are catalyzed by enzymes, which are proteinaceous in nature. The aspects, which govern metabolic reactions, are discussed further in the following sections.
Thousands of biochemical reaction occurring inside cell can be divided into basic types namely: Bond cleavage with addition of H2O. Covalent bond formation requires ATP. Isomer formation by rearrangement of atoms. Involves transfer of some group e. Metabolism can be categorized into two types: These are convergent in nature, meaning by which all the complex molecules are degraded to simpler molecules which can be degraded further, e.
In similar way fats are hydrolyzed by beta oxidation and the products acetyl Co-A can enter TCA cycle showing convergent nature of the catabolic pathway. Unlike catabolism, energy is used up in such processes so these reactions are known to be endergonic.
Unlike catabolic pathway, these are divergent in nature. Catabolic and Anabolic reactions Source: A biochemical pathway involving both catabolism and anabolism, such as citric acid cycle, is known as amphibolic pathway.
Such pathways play dual metabolic role. Though we discussed anabolism and catabolism separately in previous section, inside cell, these two processes are interlinked to each other in terms of intermediates and energy. Tricarboxylic acid TCA cycle also known as citric acid cycle or Krebs cycle is an amphibolic pathway whichcomprises of both anabolic and catabolic reactions.
TCA cycle is the pathway that unifies carbohydrate, protein and fat metabolism, whose catabolism generates acetate. The first reaction is an anabolic reaction, involving condensation of oxaloacetate 4C with acetate 2C to form citrate 6C. Subsequently, citrate is isomerized to isocitrate 6C. Succinate is then converted again to oxaloacetate which enters another cycle of this pathway. Also, TCA cycle provides precursors for the biosynthesis of various biomolecules such as certain amino acid and fatty acids so it has a role in anabolic pathways.
The energy of a system can change increase or decrease when work is performed or when heat is exchanged. Free energy G of a system is the energy isothermally available that can be converted into useful work. Only those processes are spontaneous or feasible where free energy decreases i. When rearranged the equation can be written as.
It allows us to predict the direction of a reaction. A process must proceed to a state of lower free energy. A reaction will proceed in written direction only if energy is released during the process i. The presence of few irreversible reactions in a multistep pathway makes the entire processes unidirectional, as observed for most of the pathways.
This serves as a mechanism to make the metabolic pathway irreversible. The glycolytic pathway has three such points where the reactions are thermodynamically irreversible. Usually early in the pathway there is at least one committed step which makes the pathway proceed in downhill direction. These are the reactions where product of one reaction acts as substrate for the other reaction. The intermediate acts as bridging molecule between the two chemical reactions. These can be of two types: Coupled reaction — an example.
In cells it is the ATP molecules which are required for the energy coupled reactions, hydrolysis of ATP, which is exergonic reaction, is coupled with some other endergonic chemical reaction to drive the overall reaction in forward direction. The phosphate group of ATP is transferred to a biomolecule carbohydrates, amino acid, protein, lipids in the first step and thereafter removed during the second step.
Biosynthetic activities occurring in cell require energy and majority of it is supplied by ATP. It is the ATP molecule which is synthesized during the exergonic reactions.
So the reactions involving energy exchange are mediated by the energy currency of the cell ATP. Each ATP molecule is composed of a nitrogenous base adenine, one five-carbon sugar ribose and the three phosphate groups. The three phosphate groups are linked with each other by phosphoanhydride bonds and to the ribose by a phosphoester bond.
ATP — structure and hydrolysis. ATP hydrolysis and regeneration during exergonic and endergonic reactions. The energy released is used to perform various metabolic processes. This discussion can be summarized in the following figure.
The energy released during such hydrolysis is used by the cell to perform reactions which need input of energy. In many reactions, the phosphate ion instead of being released is transferred to some other molecule leading to phosphorylation of latter. The transfer of inorganic phosphate, formed during ATP hydrolysis, may activate or inactivate the enzyme. Proteins are phosphorylated by kinases and dephosphorylated by phosphatases.
This reversible phosphorylation and dephosporylation affects enzymatic activity. This enables the cell to have control over activity of enzymes, which govern the rate of metabolic pathways. This aspect will be dealt in detail when we discuss modulation of enzymes at the end of this chapter.
Second type of coupled reactions isoxidation-reduction reactions. During the chemical reactions of any metabolic pathway energy stored in chemical bonds of the starting molecule is transferred to other newly formed chemical bonds yielding one or more molecules. Such reactions are known as oxidation-reduction reactions or Redox reactions.
Loss of electron or hydrogen atom is known as oxidation. The lost electron or hydrogen is accepted by another atom, this gain of electron or hydrogen atom is known as reduction. For example, during photosynthesis electrons and hydrogen atoms are transferred from water molecules to carbon dioxide.
In this reaction, water is oxidized to oxygen and carbon dioxide is reduced to form sugar. Since electrons are transferred to higher energy level, energy input is required, which makes the process endergonic. This energy is provided by sunlight. On the contrary, during catabolic reaction of respiration. Glucose is oxidized to carbon dioxide, oxygen is reduced to water and the reaction is exergonic. If this entire energy were to be released in one step, most of it would be dissipated as heat, leading to increase in temperature, which could lead to cell death.
To avoid such condition, energy is released in a series of steps and it is stored in form of chemical bonds, which can be used later to release energy whenever and whereverit is required in the cell. The most important molecule facilitating such energy exchange is ATP adenosine triphosphate.
Such mechanisms enable the cell to maintain its metabolism in a regulated manner.