What is Digestion?
Digestion is breakdown of food by both chemical and mechanical processes into small and simple chemical compounds which absorbed into the blood stream.
Breaking down of larger food molecules into small molecules for use by body cell is called digestion.
1. Mechanical Breakdown (chewing)
2. Chemical breakdown by Enzyme
What is Absorption?
Absorption is the process by which digested food passes from the gastrointestinal tract into the cardiovascular (circulatory system) and lymphatic system for distribution.
1. Digestion in the Mouth
After ingestion food - taking food into the alimentary track - process of digestion start in the mouth.
Mechanical Digestion
Mechanical digestion occurs-
When food taken into mouth,
Mastification or chewed by teeth,
Moved round the mouth by the tongue and muscles of the cheeks,
Mixed with saliva and form bolus,
Ready for deglutition or swallowing
Chemical Digestion
Chemical digestion involve -
Salivary amylase or ptyalin enzyme secreted by salivary gland - Initiated breakdown of starch.
Carbohydrates are either Monosaccharides, Disaccharides, Polysaccharides
Amylase breakdown of chemical bound.
Saliva contains lingual lipase secreted by glands in tongue.
Start digestion of dietary triglycerides into fatty acids and monoglycerides.
Digestion in the Stomach
Mechanical Digestion
Mechanical digestion occurs when-
Through esophagus food reaches the stomach. Stomach causes mixing and liquification of bolus of food into rhyme. Breakdown bolus and mix it with gastric juice. Mixing waves pass over stomach every 15-25 sec.
Propel the stomach content toward pyloric sphincter,
Food remain in fundus1/2 an hours, digestion of starch.
Chemical Digestion
Chemical digestion involves digestion of proteins.
Pepsin secreted by stomach - Pepsin breakdown certain peptide bonds between amino acids making up proteins.
Principal chemical activity of stomach begin digestion of protein. Digestion of fat, gastric lipase enzyme of stomach splits short chain triglycerides molecule.
Digestion of Carbohydrate
Infant stomach secrete Rennin - Important in digestion of milk.
Absorption in the Stomach
Absorption in stomach occurs only small quantity of water, alcohol, glucose, electrolytes, asprin absorbed through gastric mucosa.
Digestion in the Small Intestine
When acidic chyme enters into small intestine. It mixed with pancreatic juice, bile and intestinal juice.
In small intestine digestion of all nutrient completed.
Carbohydrates to monosaccharides
Proteins to amino acids
Fats to fatty acids and glycerol
Mechanical Digestion
Movement of small intestine -
1. Segmentation
2. Peristalsis
1. Segmentation
Segmentation is major movement of small intestine. It mixes chyme with the digestive juice and bring the particles of food into contact with mucosa for absorption.
Segmentation depends mainly on intestinal distension which initiates nerve impulses to the central nervous system.
Returning parasympathetic impulses to increase motility.
Sympathetic impulses decrease intestinal motility.
2. Peristalsis
Peristalsis is a progressive type of movement.
Propelling material from point to point in GastroIntestinal track (GI).
Peristalysis propels chyme through the intestinal track
Chymes move through small intestine at rate 1cm/ minute
Chyme remains in small intestine for 3 to 10 hours.
Peristalsis initiated by mucosal stretching of longitudinal and circular muscle layers when chyme enters a segment of intestine.
Peristalsis is controlled by autonomic nervous system.
Chemical Digestion
In mouth - salivary amylase converts starch to maltose.
In stomach - Pepsin convert protein to peptides.
Chyme entering small intestine.
Chemical Digestion in the small intestine involve -
1. Digestion of carbohydrate
2. Digestion of proteins
3. Digestion of lipids
4. Digestion of Nucleic acid
1. Digestion of Carbohydrate
Pancreatic amylase enzyme in pancreatic juice act in small intestine convert all digestible polysaccharide (starch).
Brush border enzymes α-dextrin act on the a-dextrins (breaks into glucose).
There are 3 brush border enzyme digest the disaccharides into monosaccharides inside the cell of villi -
1. Maltase
Maltase splits- maltose and maltotriose into two or three molecules.
2. Sucrase
Sucrase breaks - sucrose into a molecules of glucose and a molecules of fructose.
3. Lactase
Lactase digest lactose into a molecules of glucose and molecules of galactose.
2. Digestion of Protein
Protein digestion starts in the in the stomach.
10-20% of total ingested protein - digested - by gastric pepsin into peptides.
Enzyme found in pancreatic juice -
Trypsin
Chemotrypsin
Elastase
Carboxypeptidase
1. Trypsin, Chymotrypsin and Elastase
Trypsin, chymotrypsin - convert some polypeptides to amino acids.
Trypsin, Pepsin, chymotrypsin , elastase - all convert whole proteins into peptides.
2. Carboxypeptidase
Carboxypeptidas act on peptides - breaks the peptide bond that attaches the terminal amino acid to the carboxy (acid) end - split off amino acid from carboxyl end.
3. Peptidases
Protein digested completed by peptidase produced by small intestinal cell.
1. Aminopeptidase
Act on peptides - break peptide bonds that attach amino acid.
2. Dipeptidas
Cleaved dipeptides into amino acids that can be absorbed.
Protein food contains -
Large molecule polypeptides
Smaller polypeptides
Smaller peptides
Amino acids
3. Digestion of Lipids
Almost all lipid digestion occurs in the small intestine.
1. Emulsification
Emulsification is first step in digestion of lipid. Process of transformation of large molecules into smaller droplets.
Emulsification process occurs in small intestine, bile is secreted by liver and stored in gall bladder, passes into the duodenum along with the pancreatic juice.
1. Bile salts, sodium taurocholate and sodium glycocholate break the fat into droplets.
2. Pancreatic lipase enzyme found in pancreatic juice, it hydrolysis or convert fat molecules into fatty acids and monoglycerides.
3. Intestinal lipase enzyme secreted by gland of small intestine, it splits natural fat to fatty acids and glycerol. This enzyme is present in intestinal mucosal cells.
5. Absorption of Electrolytes
Absorbed by small intestine.
Constituents of gastrointestinal secretion.
Component of ingested foods and liquids.
Sodium
It is able to move into the absorptive epithelial cells by diffusion and secondary active transport.
Negatively charged bicarbonate, chloride, iodine, and nitrate ions- actively transported.
Calcium ions actively transported.
Iron, potssium, magnesium and phosphate-move by active transport.
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