Question

Describe the flow of foodstuff through the digestive tract starting with ingestion at the buccal cavity and end with defecation at anus. For each region state (1) which enzymes are secreted if any, (2) what the substrates of that enzyme would be in that region, and (3) what the products of chemical digestion would be in that region.

Answer 1

The food stuff is digested in the digestive system by different enzymes from different organ in digestive system. The food stuff is ingested in buccal cavity at first then it entery into stomach. The food stuff then is digested by pancreatic juice from pancreas. Then digested food materials enters into intestine and intestinal juice or succus entericus helps in the final digestion of the food stuff. So each region of the digestive tract secrets different enzymes for the digestion procedure of the food stuff.

1) Enzymes secretion by digestive tract: a) in buccal cavity, salivary juice is secreted from secretory acini. This salivary juice is contained ptyaline enzyme which helps in the digestion of starch. The starch is a carbohydrate molecule which is digested in oral cavity by this ptyaline enzyme.

b) in stomach, lipase is one of the enzyme which helps in the digestion of fat in the stomach. The pepsin, gilatinase are the main enzymes in stomach for the digestion of protein and gelatin respectively. HCl is one of the enzyme in stomach which digests sucrose carbohydrate.

c) in pancreas, trypsin, pancreatic amylase, pancreatic lipase, carboxypeptidase, aminopeptidase, ribonuclease and deoxyribonuclease are the main enzymes present in the pancreatic juice for the digestion of carbohydrates, protein and fat of food stuffs.

d) in intestine, moltase, lactase and sucrase are present in the intestinal juice for the digestion of carbohydrates. Intestinal lipases helps in the digestion of fat molecules present in the intestine. Erepsin, nuclease, nucleotidase and nucleosidase are the digestive enzymes for polypeptide, nucleic acid, nucleotide and nucleoside respectively.

2) substrate for the enzyme secreted by digestive tract: For CARBOHYDRATE digestion: In oral cavity, starch is the substrate for ptyaline enzyme. In stomach, sucrose is the substrate for HCl. The starch and amylose are the substrate for the pancreatic amylase of pancreatic juice. In intestine, moltose, lactose and sucrose is the main substrates for moltase, lactase and sucrase enzymes respectively.

For LIPID digestion: when the food stuff enters into stomach the lipid molecules are digested by gastric lipase. The lipid molecules of milk, butter, egg yolk is the substrate for lipase enzyme present in stomach. The pancreatic lipase also helps in digestion of lipid or fat molecules. Intestinal lipases uses lipid molecules as a substrate for digestion.

For protein digestion: The protein molecules are digested in stomach by pepsin enzyme. So the protein is the substrate for pepsin. Gilatine is the substrate for gilatinase enzyme in stomach. Rennin is enzyme present in the stomach of a new born baby for the digestion of caseinogen. So caseinogen is the substrate for rennin. Peptones are the substrate for trypsin enzyme in pancreatic juice. Aminopeptidase present in pancreatic juice acts on polypeptide (substrate). RNA is the substrate for ribonuclease enzyme and DNA is the substrate for deoxyribonuclease enzyme of pancreatic juice. In intestine, polypeptide is the substrate for erepsin. Nucleic acid is the substrate for nuclease. Nucleotide and nucleoside are the substrate for nucleotidase and nucleosidase enzyme respectively in the intestinal juice.

3) Products of chemical reaction of enzymes on their respective substrate: The enzymes of different organ of digestive tract acts on the substrate and produce products as a digestable molecules.

For carbohydrate digestion: starch is not digested in our body, so it transform into moltose and dextrin by ptyaline in salivary juice. So ptyaline acts on starch and produce moltose as a product. HCl acts on sucrose and produce glucose and fructose as a product. HCl cleaves the bond between glucose and fructose in sucrose molecule and splits into free glucose and fructose. Pancreatic amylase acts on starch and produce moltose as a product. In intestinal juice, moltase enzyme acts on moltose and produce glucose. Lactase acts on lactose and produce glucose and galactose. Sucrase acts on sucrose and produce gluvose and fructose as a product.

For LIPID digestion: In stomach, the gastric lipase acts on lipid molecules and transform into fatty acid and glycerol as the product because lipid molecules are constructed by the ester of glycerol and fatty acids. Pancreatic lipase and intestinal lipases also acts on lipd and produce fatty acids and glycerol. This lipase enzyme actually cleaves the ester bond between fatty acids and glycerol of lipid.

For protein digestion: The Pepsin enzyme of gastric juice acts on protein and produce peptone because pepsin cleaves the peptide bond between amino acids. Long chain protein then cleaved by pepsin into small peptones. Gilatinase acts on gilatine and produce peptone. Rennin acts on caseinogen and transform into casein as a product. The aminopeptidase enzyme of pancreatic juice cleaves the polypeptide chain into free amino acids molecules as the product. Ribonuclease acts on RNA and cleaves all the hydrogen bond between nucleotide and produce single nucleotide molecules as products. Deoxyribonuclease also produce nucleotide as products . In intestinal juice, nuclease enzyme acts on nucleic acid and produce purine and pyrimidine (Nitrogenous base). Nucleotidase acts on nucleotide and cleaves the ribose sugar part from the nitrogenous base and produce free ribose sugar and phosphate molecules as the product.

• So, these digestive enzymes acts on carbohydrate, protein and fat for the production of small digestable molecules (product) in the body. The digestable part of the food stuff then reabsorbed from the intestine for the body metabolism. The indigestible food stuffs are excreted from the body.