Human Digestion in stomach:

Human Digestion  in  stomach:

Mechanical digestion:  Food  from  pharynx enters the  stomach  through  the oesophagus.  Stomach  is  a bug like  organ.  Mechanical digestion  takes  place  by the churning  action  of the  stomach.  This  is  made more efficient by the fact  that  unlike  other regions  of the gut it  posseses  three  layers  of smooth  rnuscles instead  of two.  namely  the outer  longitudinal,  middle circular  and iner  oblique  layers.  Stomach  wall contains  gastric glands  which produces a iuice  collectively  known  as gastric juice.  As a result  of

mechanical  digestion  ttre  gastric  tuice  mixes  with the food  rnass.

Chemical digestion:

Approximately 2 liters of gastric  juice  are secreted  each  day. The components  of gastric iuice 

perform  the  chemical  digestion.Role  of gastric  juice  in  digestion: The  gastric juice  contains hydrochloric acid  (HCl). mucus and pepsinogen.

i. Hydrochloric  acid (HCl):  A. Kills bacteria. B. Converts  inactive  pepsinogen  into active  pepsin. C.  Makes the  stomach  contents  pH I-2.5,  ideal  for  the optimum activities  of the  stomach enzymes.  D.  Denatures  many proteins; their tertiary structure  is  altered, making them  unfold and  so easier to  digest.

ii. Mucus:  mucous  cells  protects  the stomach  lining  from  the  digestive action of  its  own secretion  and  HCl.

iii. Pepsin: Pepsin (protein breaking  enzymes  are  called  proteasei  so,  pepsin  is  a protease)  is  a powerful  endopeptidase (the  protease  that breaks bonds  between  amino acids within  proteins  thus producing  smaller  peptides  is  called endopeptidase and  that splits  off  amino  acids from proteins  i.e. polypeptides  is  called exopeptidase) enzyme:  it  breaks  specific  peptide bonds in  the  middle of  the protein chain,  turning protein molecules  into  polypeptides. The soluble protein  of  milk is caseinogen. Human  infants  digest this milk protein  with acidic  pepsin,  just  as  adults  do  (the  stomach  of ruminant mammal  secretes  a milk curdling  errzyme  called  rennin  which  clots  and  precipitates  the  soluble milk protein casein/caseinogen  into  insoluble  calcium  salt of casein  and  slows the movement of milk through the  stomach  which  is  then  digested  by pepsin;  Rennin should not  be confused  with  renin  which  also is an enzyme  produced  by  kidney).