Metabolism refers to all of the chemical reactions that take place inside living 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:
When a chemical reaction takes place energy is either taken in or released. This depends on the relative strengths of bonds being broken and bonds being formed.
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.
Two types of metabolic reactions take place in the cell:
Anabolic reactions use up energy. They are endergonic. In an anabolic reaction small molecules join to make larger ones. For example,
Catabolic reactions give out energy. They are exergonic. In a catabolic reaction large molecules are broken down into smaller ones. For example
In respiration, glucose is combined with oxygen and releases usable energy, carbon dioxide and water. This usable energy is stored in a compound called ATP (adenosine triphosphate). ATP is the power molecule used by all the cells of an organism to power the secondary reactions that keep us alive. ATP is a chemical energy nucleotide that links catabolism and anabolism.
Amphibolic pathway – A biochemical pathway that serves both anabolic and catabolic processes is called an amphibolic pathway. An important example of an amphibolic pathway is the Krebs cycle, which involves both the catabolism of carbohydrates and fatty acids and the synthesis of anabolic precursors for amino-acid synthesis.
All metabolic pathways have to be regulated and controlled to stop the build-up of an end product that isn’t needed. The cell can control a metabolic pathway by the presence or absence of a particular enzyme. Enzymes are special protein molecules whose function is to facilitate or otherwise accelerate most chemical reactions in cells. They are simply biological catalysts.
Different chemicals can influence enzyme activity. Inhibitors can be used to stop an enzyme from binding to its substrate. As a result, inhibitors can directly control the progress of a metabolic pathway.
There are three types of inhibition:
a. Competitive inhibition – This occurs when an inhibitor molecule binds to the active site of the enzyme and stops the substrate from binding. They can compete with the substrate because they have a similar molecular shape. Example: sarin
b. Non-competitive inhibition – This occurs when an inhibitor does not bind to the active site but does bind to a different part of the enzyme and changes the active site shape. This stops the substrate binding to the enzyme and decreases the reaction time. Non-competitive inhibition cannot be reversed by increasing the substrate concentration. Example: cyanide, mercury, and silver.
c. Feedback inhibition – Another way a metabolic pathway can be controlled is by feedback inhibition. This is when the end product in a metabolic pathway binds to an enzyme at the start of the pathway. This process stops the metabolic pathway and so prevents the further synthesis of the end product until the end product concentration decreases. The higher the concentration of the end product, the quicker the metabolic pathway stops.
