Introduction
Protein metabolism is a description of the physical and chemical processes that cause both the building, or synthesis, of amino acids into proteins and the breakdown, or catabolism, of proteins into amino acids. Amino acids are circulated through the blood and enter the body tissues, where they are synthesized back into protein. Balance between protein synthesis and catabolism is essential to maintain normal cell functioning.
Soft tissues require amino acids to manufacture the types of proteins needed for maintenance of life processes. Amino acid synthesis is necessary to form other essential compounds in the body, like histamine, neurotransmitters, and components of nucleotides. Any amino acids that are left over after synthesis are either stored as fat or converted into energy.
Amino acids can be classified as essential and non-essential. Essential amino acids cannot be made by the body but are essential to protein metabolism. These amino acids must be acquired from food. Non-essential amino acids are required for normal cell functioning and can be synthesized from other amino acids in the body. Once the proper amino acids are acquired, they combine to provide proteins the body tissues can use.
The liver is the center for breaking down needed proteins and sending the required amino acids into the blood. It constantly monitors and responds to the body’s protein needs. The liver is also responsible for processing and excreting the waste products that are produced as a by-product of protein metabolism.
Soft tissues require amino acids to manufacture the types of proteins needed for maintenance of life processes. Amino acid synthesis is necessary to form other essential compounds in the body, like histamine, neurotransmitters, and components of nucleotides. Any amino acids that are left over after synthesis are either stored as fat or converted into energy.
Amino acids can be classified as essential and non-essential. Essential amino acids cannot be made by the body but are essential to protein metabolism. These amino acids must be acquired from food. Non-essential amino acids are required for normal cell functioning and can be synthesized from other amino acids in the body. Once the proper amino acids are acquired, they combine to provide proteins the body tissues can use.
The liver is the center for breaking down needed proteins and sending the required amino acids into the blood. It constantly monitors and responds to the body’s protein needs. The liver is also responsible for processing and excreting the waste products that are produced as a by-product of protein metabolism.
Metabolism and energy
Metabolism: chemical reactions in cells
Countless chemical reactions take place in cells and are responsible for all the actions of organisms. Together, these reactions make up an organism's metabolism. The chemicals taking part in these reactions are called metabolites.
In all reactions:
- chemical bonds in the reacting molecules are broken; this takes in energy
- new chemical bonds form to make the products; this gives out energy
In an exergonic reaction, energy is released to the surroundings. The bonds being formed are stronger than the bonds being broken.
In an endergonic reaction, energy is absorbed from the surroundings. The bonds being formed are weaker than the bonds being broken
Anabolism and catabolism
Two types of metabolic reactions take place in the cell: 'building up' (anabolism) and 'breaking down' (catabolism).
Anabolic reactions use up energy. They are endergonic. In an anabolic reaction small molecules join to make larger ones. For example, the following condensation reactions that occur in cells are anabolic:
- amino acids join together to make dipeptides:
e.g. NH2CHRCOOH + NH2CHRCOOH NH2CHRCONHCHRCOOH + H2O
and the process continues as large protein molecules are built up - small sugar molecules join together to make dissacharides:
e.g. C6H12O6 + C6H12O6 C12H22O11 + H2O
and the process continues as large polysaccharide molecules are built up - glycerol reacts with fatty acids to make lipids:
e.g. CH2OHCH(OH)CH2OH + C17H35COOH CH2OHCH(OH)CH2OOCC17H35
and the process continues as the trigyleride is produced via similar reactions with the other two hydroxyl groups of the glycerol molecule - during photosynthesis carbon dioxide and water are used to produce glucose and oxygen:
e.g. 6CO2 + 6H2O C6H12O6 + 6O2
- 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
Molecules move and collideMolecules are constantly moving. Their bonds vibrate and rotate. In gases, liquids and solutions molecules move around, bumping into one another. When molecules collide there is the possibility of a reaction taking place, but only if the colliding molecules:
- have enough energy
- are aligned correctly
- The more molecules present, the faster the reaction. Therefore reactions take place faster in concentrated solutions than in solutions that are more dilute.
- At high temperatures molecules have more energy than at lower temperatures. Therefore collisions are more frequent and the likelihood of the molecules having enough energy is greater. Consequently the rate of chemical reactions increases with increasing temperature.
Reaction profiles: following the course of a one-step exergonic chemical reaction
It is more common for reactions between molecules to take place in a series of consecutive steps. After each step a reaction intermediate forms. Unlike an activated complex this has a real existence. For each step an activated complex is formed and there is an associated activation energy. The step with the highest activation energy is the rate-determining step in the reaction and controls how fast the overall reaction is.
Test your knowledge
Take quiz on Metabolism and energy : http://www.rsc.org/Education/Teachers/Resources/cfb/quiz.cfm?qzid=17
Take quiz on Metabolism and energy : http://www.rsc.org/Education/Teachers/Resources/cfb/quiz.cfm?qzid=17