Metabolism comprises of two major parts: anabolism and catabolism. Catabolism is the set of metabolic processes that break down large molecules. These more complex molecules are broken down to produce energy necessary for various functions of the body. The energy is utilized for building or anabolic processes.
The exact nature of these catabolic reactions differs from organism to organism and organisms can be classified based on their sources of energy and carbon:
In humans and animals, redox reactions involve complex organic molecules being broken down to simpler molecules, such as carbon dioxide and water.
In photosynthetic organisms such as plants and cyanobacteria, these electron-transfer reactions do not release energy. These reactions just help store energy absorbed from sunlight.
The exact nature of these catabolic reactions differs from organism to organism and organisms can be classified based on their sources of energy and carbon:
- in organotrophs, organic sources are used as a source of energy
- in lithotrophs, inorganic substrates are used
- in phototrophs, sunlight is used as chemical energy
In humans and animals, redox reactions involve complex organic molecules being broken down to simpler molecules, such as carbon dioxide and water.
In photosynthetic organisms such as plants and cyanobacteria, these electron-transfer reactions do not release energy. These reactions just help store energy absorbed from sunlight.
Catabolism can be broken down into 3 main stages.
Stage 1 – Stage of Digestion
The large organic molecules like proteins, lipids and polysaccharides are digested into their smaller components outside cells. This stage acts on starch, cellulose or proteins that cannot be directly absorbed by the cells and need to be broken into their smaller units before they can be used in cell metabolism.
Digestive enzymes include glycoside hydrolases that digest polysaccharides into monosaccharides or simple sugars.
The primary enzyme involved in protein digestion is pepsin which catalyzes the nonspecific hydrolysis of peptide bonds at an optimal pH of 2. In the lumen of the small intestine, the pancreas secretes zymogens of trypsin, chymotrypsin, elastase etc. These proteolytic enzymes break the proteins down into free amino acids as well as dipeptides and tripeptides. The free amino acids as well as the di and tripeptides are absorbed by the intestinal mucosa cells which subsequently are released into the blood stream where they are absorbed by other tissues.
The amino acids and sugars are then pumped into cells by specific active transport proteins.
Stage 1 – Stage of Digestion
The large organic molecules like proteins, lipids and polysaccharides are digested into their smaller components outside cells. This stage acts on starch, cellulose or proteins that cannot be directly absorbed by the cells and need to be broken into their smaller units before they can be used in cell metabolism.
Digestive enzymes include glycoside hydrolases that digest polysaccharides into monosaccharides or simple sugars.
The primary enzyme involved in protein digestion is pepsin which catalyzes the nonspecific hydrolysis of peptide bonds at an optimal pH of 2. In the lumen of the small intestine, the pancreas secretes zymogens of trypsin, chymotrypsin, elastase etc. These proteolytic enzymes break the proteins down into free amino acids as well as dipeptides and tripeptides. The free amino acids as well as the di and tripeptides are absorbed by the intestinal mucosa cells which subsequently are released into the blood stream where they are absorbed by other tissues.
The amino acids and sugars are then pumped into cells by specific active transport proteins.
Stage 2 – Release of energy
Converrted to yet smaller molecules, usually acetyl coenzyme A (acetyl-CoA), which releases some energy.
Stage 3 - The acetyl group on the CoA is oxidised 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.