Stomach



What enzymes are in the Stomach?

Stomach:

Stomach is a J_ shaped sac having 4 regions: fundus, cardiac part with cardiac sphincter that checks regurgitation of food, body and pyloric part with pyloric sphincter that regulates entry of food from the stomach into the small intestine Stomach stores food, churns it by muscular contractions until it is liquefied ( chyme), and propels it into the small intestine in spurts. Stomach receives gastric juice from numerous microscopic gastric glands present in it wall.This juice is acidic( pH 2) and contains HCI, two proenzymes pepsinogen and prorennin, and an enzyme gastic lipase.

(1)Pepsinogen: 

It is first changed by hydrochloric acid into an active enzyme pepsin.Then the pepsin itself continues to activate additional pepsinogen. Such an activation is called autocatalytic reaction. Pepsin hydrolyses the proteins partially to proteoses¹ and peptones² in an acid medium ( pH 2). It can digest collagen of the white fibres of connective tissues but not keratin of hair, horns and nails.

        Pepsinogen + HC➡️ pepsin 

    
                 pepsin                                              Proteins ➡️➡️ Proteoses and peptones (Peptides) 

 ☆): Gastric Glands:

The gastric glands are innumerable, microscopic simple( straight ot branched), tubular glands in the wall of the stomach. They have 3 more common types of gland cells.

(i) Peptic (chief, zymogen) cells that are usually  basal in Location and produce enzymes,

(ii) Large, deep_ seated oxyntic( acid, parietal) cells that secrete HCI,

(iii) muscous cells which add mucus.

The three secretions together form an acidic gastric juice with pH 2. It is discharged into the stomach.it contains water, salts, mucinex, hydrochloric acid, two proenzymes¹ or zymogens¹: pepsinogen or propepsin and proprennin, and an enzyme gastric lipase. About 2 or 3 litrrs of gastric juice is secreted per day. Secretion of gastric juice is controlled by nerves and hormones. Gastric secretion is stimulated by throught of food, smell of food, chewing of food, and contract of food with stomach wall.

 The parietal cells also secrete the Castle's  intrinsic gastric factor that helps in the absorption of vitamins B12.

 The gastric glands also have two less common types of cells.

(i) argenataffin cells that are generally located at the base of the glands and secrete serotonin, a potent vasoconstriction.

(ii) endocrine cells which occur in the pyloric antrum and produce gastrin. The latter stimulates the secretion of enzymes and HCI.

The bolus enters the stomach through cardiac orifice guarded by cardiac sphincter. The latter regulates the entry of food into the stomach. Contraction and expansion of muscles of the wall of stomach bring about churning movement which help in thorough mixing of  the food with the  gastric juice secreted  by gastric glands.  Gastric juice is a mixture of hydrochloric acid ( HCI), pepsinogen, prorennin ( in infants and not in adults), lipase and mucus. The secretion of gastric juice takes place in two phases: 


(i) The cephalic phase: The sight, small or taste of food induces reflex stimulation of the vague nerve. As a result, the secretion of gastric juice from the gastric glands occurs much before the food reaches the stomach. 



 (ii) The gastric phase: The presence of food in the stomach stimulates the gastric mucosa to produce gastrin hormone. The latter is absorbed in the blood stream and stimulates gastric mucosa to produce juice. The goblet cells present in lining of stomach secrete mucus. Also, the pyloric tubular glands secrete mucin which lubricates and protects the epithelial surface from excoriation by HCI and digestion by enzymes. The chief cells, also called peptic cells or zymogen cells, secrete pepsinogen ( proenzyme) which is an inactive precursor of pepsin enzyme . In the presence of HCI , pepsinogen gets converted to an active form, i.e., pepsin. The activated pepsin by autocatlysis activites more pepsinogen to pepsin. This pepsin enzyme is
                          HCI
     Pepsinogen ➡️ Pepsin 
 ( Inactive form)   (Active form)


the principle protease or proteolytic enzyme of the stomach which hydrolyses proteins into peptides ( proteoses and peptones). Probably chief cells also secrete another enzyme in the infants in inactive form, i.e., prorennin. The latter gets activated to rennin in the presence of HCI and hydrolyses the soluble milk protein caesein into paracaesin and whey protein. Paracaesin  is spontaneously precipitated in the presence of calcium as insoluble calcium paracaesinate, forming soluble curd, or coagulated milk. The pepsin then converts calcium paracaesinate also into proteoses, peptones and polypeptides. 


  Prorennin + HCI ➡️Rennin 

                             Rennin                                                          Casesin ➡️ ➡️ Paracaesin + Whey Protein

Paracaesin + Calcium ➡️ Calcium Paracaesinate
                                          Pepsin 
  Calcium Paracaesinate➡️ ➡️ Peptones 

 Gastric juice also secretes gastric lipase enzymes. It converts neglibal amount of fats into monoglycerides and fatty acids because the enzyme is sensitive to free acid and is destroyed by HCI.Also, stomach lacks fatemulsifying agent.

2):Gastric lipase: 

Gadtric Lipase on the other hand ,is an enzyme produced by cells in the stomach called chief cells and the pyloric glands. It functions breakdown triglyceride ( fats) into fatty acids and monoglycerides. Gastric lipase active in the acidic environment of the stomach but its role in fat digestion is minimal compared to other enzymes, such as pancreatic lipase, which play a more significant role in breaking down fats in the small intestine. 

The semidigested food from the stomach mixed with gastric secretion( chyme) is passed into duodenum through pyloric sphincter from time to time . Once, in the small intestine, the chyme receives three alkaline secretions, namely, bile (pH 8.0) from the liver, pancreatic juice ( pH 8.8) from the pancreas and the intestinal juice ( pH 8.3) from the intestinal glands. Their alkalinity stops the active of pepsin.


  The pancreatic juice from the pancreas and the bile from the  liver ( stored in the gall bladder) are poured into the duodenum through hepato__ pancreatic duct.

Pancreatic juice: It is secreted by acinar cells of pancreas. It contains a carbohydrate digestive enzyme, i.e., pancreatic amylase, protein digesting enzymes in inactive forms  namely, trypsinogen, chymotrypsinogen and carboxypolypeptidases, a fat digesting  enzymes,  pancreatic lipase, nucleic acids digesting enzymes, bicarbonates and water. Bicarbonates neutralize hydrochloric acid of the chyme.


(i) Pancreatic amylase hydrolyses more starch and glycogen to dextrins and the latter to maltose, isomaltose and ' limit' dextrins.

                             Pancreatic 
Starch+Glycogen➡️➡️Maltose+ Isomaltose+ 'Limit' Dextrins
 

 (ii) Protein digesting enzymes hydrolyse the protein components of the food.


 (a) Trypsinogen gets activated to trypsin by a non_ digestive enzyme enterokinase present in the intestinal juice.
 
  It then hydrolyses proteins into peptides.

                       Enterokinase                                     Trypsinogen➡️➡️➡️➡️➡️ Trypsin
 
                       Trypsin                                                    Proteins ➡️➡️➡️ Peptides 

 (b) Chymotrypsinogen is activated to Chymotrypsin by trypsin. The latter than splits proteins into  peptides. 

                                   Trypsin 
 Chymotrypsinogen ➡️➡️➡️Chymotrypsin

                                Chymotrypsin                                        Proteins ➡️➡️➡️➡️➡️➡️Peptides
   

(c) Procarboxypeptidases These are activated to carboxpeptidases by trypsin. These enzymes separate individual amino acids adjacent to the free terminal carboxyl group of the peptides, reducing then to dipeptides.

                                            Trypsin                    Procarboxypeptidases➡️➡️➡️Carboxpeptidases 

             Carboxpeptidases                                      Peptides➡️➡️➡️➡️➡️➡️ Dipeptide+Amino acids

( iii) Pancreatic lipase hydrolyses fats in stages. Lipase separates three fatty acids one by one from the emulsified tryglyceride, changing it into diglyceride, then into monoglyceride, and finally into glycerol.Thus, complete hydrolysis of a fat molecule give three fatty acids molecules and a glycerol enough for the completion of fat hydrolysis. Thus, the end products of fat hydrolysis are fatty acids, glycerol,monoglycerides, diglycerides and even triglycerides.

( iv) Nucleic acids  are digested by nucleic acid digesting enzymes. The action of these enzymes.


Bile juice: Bile contains no enzyme, and has no chemical properties action on food. However, it has salts, namely, sodium glycocholate and sodium taurocholate.These salts reduce the surface tension of large fat droplets and break them into many small ones. This process is called emulsification .They also form thin coating around tiny fat droplets to keep them from coalescing. This increases lipase action on fats.

                       Bile salts
Triglycerides ➡️➡️➡️➡️Emulsified triglycerides 


Intestinal Juice: Intestinal gland secrete intestinal juice in the small intestine. It contains six carbohydrates ( intestinal amylase,maltase, isomaltase, 'Limit' dextrinase, sucrase and lactase), two proteases (aminopeptidases and dipeptidases) and a non_ digestive enzyme enterokinase, intestinal lipase, nucleic acid digesting enzymes (nucleases,nucleotidases, nucleosidases), mucus, carbonates and water.

(i) Carbohydrases ultimately form monosaccharides: Intestinal amylase hydrolyses remaining starch and glycogen to ultimately maltose, isomaltose and ' limit'dextrins. Actions of other carbohydrases are summarized below:

                Maltase
  Maltose ➡️➡️➡️ Glucose 

                          Isomaltose                                            Isomaltose➡️➡️➡️➡️Glucose                             
                          ' Limit' 
'Limit' dextrin➡️➡️➡️➡️Glucose                                                  Dextrinase                                                                                                                                            Sucrase                                                   Sucrose➡️➡️➡️➡️ Glucose + Fructose                                                                                                                  Lactase                                                    Lactose ➡️➡️➡️Glucose + Galactose 
   
                           


( ii) Proteases act on peptides. 


(a) Aminopeptidases  separate individual amino acids adjacent to the free terminal amino group of the peptides, reducing them to dipeptides.                                                                   

                 Aminopeptidases                      Peptides➡️➡️➡️➡️➡️➡️➡️Dipeptide +Amino Acids


(b) Dipeptidases hydrolyses dipeptides into individual amino acids.   
         

                    Dipeptidases                                         Dipeptides ➡️➡️➡️➡️➡️Amino Acids

(iii) Intestinal Lipase hydrolyses some triglycerides, diglycerides and monoglycerides to fatty acids and glycerol like the pancreatic lipase.


(iv) Nucleic acids are digested in small intestine by the nucleases, nucleotidases and nucleosidases secreted by pancreatic and intestinal juices.

 (a) Pancreatic juice contains two nucleases, namely deoxyribonuclease and ribonuclease which hydrolyse DNA and RNA to yield deoxyribonucleotides and ribonucleotides respectively.                                   

          Deoxyribonuclease                      
DNA  ➡️➡️➡️➡️➡️➡️  Deoxyribonucletides                  
           Ribonuclease                     
    RNA ➡️➡️➡️➡️➡️Ribonucleotides         

 
(b) Intestinal juice  contains  nucleotidases and nucleosides.  Their chemical action is briefly presented below: 

                                                                                                                                                                                                               Nucleotidases 
Deoxyribonucleotides and 
Ribonucleotides ➡️➡️➡️➡️➡️➡️➡️➡️➡️ Deoxyribonucleosides + Ribonucleotides + Inorganic phosphates 

                                 Ribonucleotidases                 Deoxyribonucleoside and Ribonucleotides ➡️➡️➡️➡️➡️➡️➡️ Nitrogenous bases + Pentose sugars¹ 
 The end products of nucleic acid digestion are pentose sugars ( deoxyribose sugar and ribose sugar), nitrogenous bases ( purines and Pyrimidines) and inorganic phosphates.















































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