3.3.3 Digestion and absorption Flashcards
Describe the process of digestion in general
- Digestion is the process by which complex food materials are broken down into
smaller water soluble molecules. - These water soluble substances can be absorbed by the cells
- The food to be broken may include large molecules or macromolecules like
carbohydrates, lipids, proteins and small molecules or micromolecules like
minerals and vitamins and water - The macromolecules or large molecules are broken down or hydrolysed by the
enzyme called hydrolases - Hydrolases are enzymes which help in splitting up of a molecule by adding water
and the process is hydrolysis - Examples of Hydrolases: digestive enzymes like amylase, lipase, protease etc.
Describe digestion of carbohydrates
● Digestion of carbohydrates in mammals takes place in mouth as well as small
intestine.
● Digestion of carbohydrates especially starch takes place in mouth under the
action of ptyalin or salivary amylase. Salivary amylase functions at an alkaline pH
of 7.5 to 8.
● Its action continues up to oesophagus but stops at stomach due to acidic pH. So,
carbohydrate digestion resumes at small intestine again, where pH is again
alkaline.
● Digestion of carbohydrates in jejunum and ileum of small intestine takes place
with the help of pancreatic juice and intestinal juice.
How are starch and glycogen digested ?
Amalyse hydrolyses them into maltose disaccharides and then maltase hydrolyses them into glucose
How are sucrose disaccharides broken down ?
The enzyme sucrase hydrolyses them into glucose and fructose
How is the dissacharide lactose broken down ?
Lactase hydrolyses lactose into glucose and galactose
How are fats and lipids digested ?
▸ Digestion of fats and lipids takes place in duodenum of small intestine.
▸ Fats do not dissolve in water but lipases are present in aqueous medium.
▸ Fats are broken down into small droplets to increase the surface area.
▸ Lipases are present in pancreatic juice and intestinal juice. But apart from that bile
also plays important role in digestion of fats and lipids.
▸ Bile juice also makes the medium alkaline which is essential for lipase activity. Bile
juice also kills bacteria.
▸ Emulsification of fats: breaking down of fats into small droplets in presence of bile
salts.
How are triglycerides digested in stages ?
1)Lipids are emulsified by the bile
2)Lipases breakdown triglycerides into fatty acids and monoglycerides
3) Fatty acids and monoglycerides are packaged into micelles that are absorbed by microvilli
4)Fatty acids and monoglycerides converted back into triglycerides that aggregate with cholerstrol , proteins and phospholipids to form chylomicrons ( absorptive epithelial cell )
5) Chylomicrons move into a lymph capillary which transports them to the rest of the body
How are proteins digested ?
▸ Digestion of proteins takes place in stomach and duodenum of small intestine.
▸ When food enters into stomach it has alkaline pH which becomes acidic as it mixes
with gastric juice (HCL, enzymes and mucus).
▸ HCl maintains acidic pH of 1-2, which is optimum for pepsin action.
▸ It kills bacteris and other germs too due to acidic nature.
How are protein digested in the small intestine ?
▸ Endopeptidases break the peptide bond in the middle of the peptide chain.
▸ Exopeptidases acts at the end of the peptide chain and helps in releasing the last
amino acid.
▸ Pancreatic juice contains trypsinogen, chymotrypsinogen and carboxypeptidases.
What are the actions of exopeptidases ?
▸ Peptides + Carboxypeptidases → smaller peptide chain + amino acid
What are the mechanism for absorption of the products of digestion ?
▸ Active transport.
▸ Passive diffusion.
▸ Facilitated diffusion.
▸ Co-transport (secondary active transport).
▸ Endocytosis.
How are carbohydrates absorbed ?
▸ Absorption as monosaccharides.
▸ Transport of monosaccharides glucose and galactose is facilitated by symporters,
which can transfer two substances in the same direction. An example of a
symporter is the sodium-glucose symporter. This process is called Co-transport
(secondary active transport). This is followed by facilitated diffusion
▸ Monosaccharide fructose is absorbed as well as transported through facilitated
diffusion.
How are proteins absorbed ?
▸ Active transport in small intestine mostly takes place at duodenum and jejunum
absorbing about 98% of amino acids.
▸ Co-transport of di and tripeptides is also possible with H+
ions via the PepT1
transporter.
How are fats and lipids absorbed ?
▸ Fatty acids are absorbed by lacteals and not by blood capillaries.
▸ Fatty acids are not directly absorbed.
▸ They are first made into small spherical water soluble droplets called micells.
▸ This is done with the help of bile salts.
▸ Fatty acids, fat soluble vitamins and sterols are absorbed by intestinal cells from the
micells.
