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Metabolism All parts of the body (muscles, brain, heart, and liver) need energy to work. This energy comes from the food we eat. It happens due to some sequential biochemical phenomenon known as metabolism.
What is metabolism ?

How does the body produce energy? It is through the phenomenal process known as - metabolism. The word "metabolism" comes from the Greek noun metabole, meaning "change". Metabolism provides energy required to sustain life through a sequence of biochemical reactions that take place in trillions of cells of our body. When we eat food, the digestive system produces the nutrients which are absorbed by the cells. The metabolism of these nutrients yield energy and also contribute to the enzyme activity that supports metabolic reactions in the cell. Metabolic reactions that take place within your body can be categorized as anabolic and catabolic reactions. Anabolic reactions consume energy to combine different molecules where as catabolic reactions release energy while splitting molecules apart. The energy stored in ATP created by catabolic reactions is the fuel for anabolic reactions which synthesize hormones, enzymes, sugars and other substances for cell growth, reproduction, and tissue repair

The endocrine system stimulates reactions of metabolism by releasing hormones like cortisol, glucagon and adrenaline; digestive system provides nutrients; nutrients through blood; respiratory system provides oxygen and excretory system eliminates waste. So metabolism which is the most important function in maintaining life can happen only with perfect coordination of all other systems in the body.

metabolism work Metabolism refers to all chemical reactions occurring in living organisms, including digestion and the transport of substances into and between different cells to sustain life. These processes allow the living organisms to grow and reproduce, maintain their structures, and respond to their environments.
How does metabolism work ?

Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. The word metabolism can also refer to all chemical reactions that occur in living organisms, including digestion and the transport of substances into and between different cells.

Metabolism is usually divided into two categories - catabolism and anabolism. Catabolism breaks down organic matter, for example to harvest energy in cellular respiration. Anabolism uses energy to construct components of cells such as proteins and nucleic acids. The chemical reactions of metabolism are organized into metabolic pathways, in which one chemical is transformed through a series of steps into another chemical, by a sequence of enzymes. Enzymes are crucial to metabolism because they allow organisms to drive desirable reactions that require energy and will not occur by themselves, by coupling them to spontaneous reactions that release energy. As enzymes act as catalysts they allow these reactions to proceed quickly and efficiently. Enzymes also allow the regulation of metabolic pathways in response to changes in the cell's environment or signals from other cells.

Metabolic pathways Lysis of glucose (a six carbon compound) is called Glycolysis, the metabolic pathway that converts glucose into pyruvate (a three carbon compound) that next enters the Krebs cycle, which is also known as the citric acid cycle. The high-energy electrons left in pyruvate complete cellular respiration by oxidizing pyruvate to form carbon dioxide. The free energy released during this process is used to form the high–energy ATP. This processs is an efficient way of releasing energy.
Metabolic pathways

Cell respiration is the means by which cells extract energy stored in food and transfer that energy to molecules of ATP. Energy that is temporarily stored in molecules of ATP is instantly available for every cellular activity such as passing an electrical impulse, contracting a muscle, moving cilia, or manufacturing a protein, etc. The reactions involved in respiration are catabolic reactions that involve the redox reaction (oxidation of one molecule and the reduction of another). Respiration is one of the key ways a cell gains useful energy to fuel cellular activity.

There are two major categories of respiration: aerobic and anaerobic. Aerobic respiration occurs in the presence of oxygen, while anaerobic respiration occurs in situations where oxygen is not available. Aerobic respiration involves three stages: glycolysis, the kreb's cycle, and oxidative phosphorylation

Glycolysis (lysis of glucose) is the metabolic pathway that converts glucose into pyruvate. The free energy released in this process is used to form the high–energy ATP. The pyruvate formed during glycolysis next enters the Krebs cycle, which is also known as the citric acid cycle. Glycolysis releases less than a quarter of the chemical energy stored in glucose; most of the energy remains stockpiled in the two molecules of pyruvate. There are many high-energy electrons left in pyruvate. Now, cells complete cellular respiration by oxidizing pyruvate to form carbon dioxide.

The oxidation of pyruvic acid into CO2 and water is called Krebs cycle. This cycle is also called citric acid cycle because the cycle begins with the formation of citric acid. After the Krebs cycle, comes the largest energy-producing step of them all: oxidative phosphorylation. Oxidative phosphorylation is a metabolic pathway that uses energy released by the oxidation of nutrients to produce adenosine triphosphate (ATP).

Although the many forms of life on earth use a range of different nutrients, almost all aerobic organisms carry out oxidative phosphorylation to produce ATP, the molecule that supplies energy to metabolism. This pathway is probably so pervasive because it is a highly efficient way of releasing energy.

Anaerobic respiration The respiration occurring without oxygen, is termed as anaerobic respiration. In this case of anaerobic respiration, glucose is broken down and the products generated from this are energy and either lactic acid or ethanol and CO2. This process is termed as Fermentation. On the other hand, aerobic respiration is a complicated process which involves chemical reactions utilizing oxygen to transform glucose into carbon dioxide and H2O. This process generates energy in the form of ATP.
Anaerobic respiration

Anaerobic respiration or fermentation occurs when oxygen is unavailable or cannot be used by the organism.

Two common types of fermentation are alcohol fermentation and lactic acid fermentation. In alcohol fermentation, pyruvate gives off carbon dioxide and is converted to ethyl alcohol (ethanol) in a two-step process. In lactic acid fermentation, pyruvate is converted to lactate (lactic acid).

Humans capitalize on both of these fermentation processes. Yeast undergo alcohol fermentation in the production of beer and wine. Certain bacteria and fungi undergo lactic acid fermentation, and are used to make cheese and yogurt. Thus, metabolism and respiration balance is augmented by the control of other enzymes at different key locations in cellular respiration.

Cells are thrifty, expedient, and responsive in their metabolism. Cells need energy to accomplish the tasks of life. Beginning with energy sources obtained from their environment in the form of sunlight and organic food molecules, cells make energy–rich molecules like ATP and NADH via energy pathways including photosynthesis, glycolysis, the citric acid cycle, and oxidative phosphorylation (All the reactions are balanced chemical equations occurs when the number of the atoms involved in the reactants side is equal to the number of atoms in the products side).

Any excess energy is then stored in larger, energy-rich molecules such as polysaccharides (starch and glycogen) and lipids. Thus, from glycolysis (lysis of glucose) to aerobic respiration (or through the anaerobic process of fermentation), ATP is produced as the prized power molecule.

cell metabolism The most common set of catabolic reactions in animals can be separated into three main stages. In the first, large organic molecules such as proteins, polysaccharides or lipids are digested into their smaller components outside cells. Next, these smaller molecules are taken up by cells and converted to yet smaller molecules, usually acetyl coenzyme A (acetyl-CoA), which releases some energy. Finally, the acetyl group on the CoA is oxidized to water and carbon dioxide in the citric acid cycle and electron transport chain, releasing the energy that is stored by reducing the coenzyme nicotinamide adenine dinucleotide (NAD+) into NADH.
Chemical reactions in cell metabolism

Metabolism involves a vast array of chemical reactions that involve the transfer of functional groups. Cells use a small set of metabolic intermediates to carry chemical groups between different reactions.

Cell metabolism explains series of chemical reactions e.g., the utilization of nutrients by cells. The nutrients penetrate the cell, which transforms them and uses them in the form of heat, energy, proteins and reserves. The waste products created by these transformations such as urea and carbon dioxide are evacuated. These group-transfer intermediates are called coenzymes.

Each class of group-transfer reaction is carried out by a particular coenzyme, which is the substrate for a set of enzymes that produce it, and a set of enzymes that consume it. These coenzymes are therefore continuously being made, consumed and then recycled.

Adenosine triphosphate (ATP), the universal energy currency of cells is an important coenzyme used to transfer chemical energy between different chemical reactions. ATP acts as a bridge between catabolism and anabolism, with catabolic reactions generating ATP and anabolic reactions consuming it.

Thus, the metabolism has got its complexity, its efficiency, its integration, and its responsiveness to subtle changes. The concepts of metabolism that we learn in this chapter will help us to understand how matter and energy flow during life's processes and how that flow is regulated.

Coupled Reactions Two types of metabolic reactions take place in the cell: 'building up' (anabolism) and 'breaking down' (catabolism).
Coupled Reactions

Anabolism and catabolism : Metabolism is usually divided into two categories. Two types of metabolic reactions take place in the cell: 'building up' (anabolism) and 'breaking down' (catabolism).

Catabolism which break down large molecules to harvest energy in cellular respiration and anabolism uses energy released in catabolism to construct components of cells such as proteins and nucleic acids

Anabolism Cholesterol is a type of fatty acid and plays a crucial role in the synthesis of hormones and bile salts. It also helps to transport fats in the bloodstream to tissues throughout the body.
Anabolism

It is the set of constructive metabolic processes where the energy released by catabolism is used to synthesize complex molecules. Anabolism is powered by catabolism, where large molecules are broken down into smaller parts and then used up in respiration (metabolism disrupts with different types of lung diseases).

Many anabolic processes are powered by adenosine triphosphate (ATP). Anabolism involves production of precursors such as amino acids ( Amino acids are much critical to life since they have a role in functions of metabolism. One particularly important function of aminoacids is to serve as the building blocks of proteins), monosaccharides and nucleotides, their activation into reactive forms using energy from ATP, and finally the assembly of these precursors into complex molecules such as proteins, polysaccharides, lipids and nucleic acids.

Anabolic processes tend toward "building up" organs and tissues through growth and differentiation of cells. Examples of anabolic processes include the growth and mineralization of bone and increases the muscle mass. Anabolic reactions use up energy. They are endergonic. For example, the following condensation reactions that occur in cells are anabolic:

1. During photosynthesis, carbon dioxide and water are used to produce glucose and oxygen:
6CO2 + 6H2O  →   C6H12O6 + 6O2

2. Amino acids join together to make dipeptides:
NH2CHRCOOH + NH2CHRCOOH   →  NH2CHRCONHCHRCOOH + H2O

3. Small sugar molecules join together to make dissacharides:
C6H12O6 + C6H12O6  →  C12H22O11 + H2O
and the process continues as large polysaccharide molecules are built up.

Catabolism In catabolism, large molecules such as polysaccharides, fat, nucleic acids and proteins are broken down into smaller units such as monosaccharides, fatty acids. Cholesterol is a type of fatty acid and an important constituent of cells.
Catabolism

It is the set of metabolic pathways, which break down molecules into smaller units and release energy.

In catabolism, large molecules such as polysaccharides, fat, nucleic acids and proteins are broken down into smaller units such as monosaccharides, fatty acids(Cholesterol is a type of fatty acid and an important constituent of cells. It plays a crucial role in the synthesis of hormones and bile salts. It also helps in transporting fats in the bloodstream to tissues throughout the body), nucleotides and amino acids, respectively.

These include breaking down and oxidizing food molecules. The purpose of the catabolic reactions is to provide the energy. Catabolic reactions give out energy. They are exergonic. A simple example of a catabolic reaction that occurs in cells is the decomposition of hydrogen peroxide into water and oxygen:

2H2O2 →   2H2O + O2
The conversion of glucose during respiration to produce carbon dioxide and water is another common example:

C6H12O6 + 6O2  →   6CO2 + 6H2O
The most common set of catabolic reactions in animals can be separated into different stages. In the first, large organic molecules such as proteins, polysaccharides or lipids (Lipoproteins are the major carriers of cholesterol in the blood and are mainly composed of lipids) are digested into their smaller components outside the cells. Next, these smaller molecules are taken up by cells and converted to yet smaller molecules, releasing the energy that is stored.