Let us now take a look at the flow chart of the process of digestion:
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Digestion process
Digestion of different biomolecules present in food:
Digestion of polymers into monomers
We have already seen that food comprises complex insoluble macromolecules or polymers - carbohydrates, fats, proteins and nucleic acids. These polymers, in the presence of enzymes, are broken down into monomers. Now we shall see how these polymers are converted to monomers.
Digestion of carbohydratesbegins in the buccal cavity by saliva. Food is mixed with saliva in the mouth, which contains ptyalin. Ptyalin or salivary amylase converts starch into maltose, isomaltose and dextrins. Approximately \(30%\) of starch is converted here.
StarchpH6.8Salivaryamylase Maltose + Isomaltose + Dextrins
In the small intestine, mucus is secreted by goblet cells which protect the wall of the duodenum. Pancreatic juice contains α-amylase starch into maltose, isomaltose and dextrins.
Starch αamylasePancreatic Maltose + Isomaltose + Dextrins
Intestinal juice contains maltase, isomaltase, sucrase, lactase and dextrinase converting maltose, isomaltose, sucrose, lactose and dextrins to simpler monomers glucose, fructose and galactose.
Protein digestion starts in the stomach. Gastric glands of the stomach secrete gastric juice. Gastric juice contains \(HCl\), mucus and proenzymespepsinogen and prorenin.
The \(HCl\) acid converts the enzymespepsinogen and prorenin into pepsin and rennin. Pepsin convertsproteins into peptones and peptides. Rennin digests the milk protein.
ProteinPepsin Peptones + Proteases
In the small intestine, pancreatic juice and intestinal juice digests the protein completely.
The action of pancreatic juice:
Pancreatic juice contains proenzymes trypsinogen, chymotrypsinogen and procarboxypeptidase. In addition, bile provides an alkaline medium. Trypsinogen is converted to trypsin. Trypsin converts chymotrypsinogen and procarboxypeptidase into chymotrypsin and carboxypeptidase.
ProteinsTrypsin Dipeptides
ProteinsChymotrypsin Dipeptides
ProteasesCarboxypeptidase Dipeptides
The action of intestinal juice:
Intestinal juice contains aminopeptidase and dipeptidases. Aminopeptidase and dipeptidase convert peptides and dipeptides into amino acids.
Peptides AminopeptidaseAmino acids
Dipeptides DipeptidaseAmino acids 
The digestion of fats starts in the stomach. Gastric juice contains a small amount of gastric lipase, which converts fats into monoglycerides and fatty acids. Fat is primarily digested in the small intestine. Bile salts break down fat into smaller fat globules through fat emulsification.
Lipase present in the pancreatic juice and intestinal juice digests the emulsified fat. Pancreatic lipase is the primary enzyme that digestsfat into glycerol and fatty acids.
Emulsified fatlipasePancreatic Fatty acid + Glycerol
Nucleic acids:
Nucleic acids digestion starts in the small intestine. Pancreatic juice and intestinal juice digests nucleic acids.
The pancreatic juice contains Deoxyribonuclease (DNase) and Ribonuclease (RNase). DNase and RNase convert DNA and RNA to deoxyribonucleotides and ribonucleotides.
Intestinal juice contains nucleotidases and nucleosidases that converts nucleotides to nucleosides. Nucleosidases converts nucleosides into nitrogenous bases and pentose sugar.