L9: absorption of carbohydrates and proteins Flashcards
Importance of carbohydrates/saccharides
- meet the body’s energy demands - glucose
- proteins and fats are vital building components of cells and tissue so should not be wasted for energy
- feed brain and nervous system
- keep digestive system fit
- add bulk to foods
- indigestible carbohydrates also beneficial
Examples of polysaccharides
Starch - storage form of glucose in plants. Moderately branched.
Glycogen - storage form of glucose in animals. Highly branched.
Fibres/cellulose - forms support structures of leaves, stems and plants. Cannot be digested - not branched.
What is a monosaccharide with example?
Smallest unit or sugar
E.g., glucose (blood sugar), fructose (fruit sugar), galactose (milk sugar)
What are disaccharides with examples?
Two sugar units joined together
Sucrose (glucose and fructose), lactose (glucose and galactose), maltose (glucose and glucose).
How are polysaccharides broken down?
Polysaccharides into disaccharides:
- salivary amylase
- pancreatic amylase
- intestinal (brush border) enzymes
How are disaccharides broken down into monosaccharides?
Intestinal (brush border) enzymes:
- intestinal maltase
- intestinal sucrase
- intestinal lactase
How do monosaccharides cross the enterocytes to the blood from the intestinal lumen?
After a meal, the glucose is higher in the intestinal lumen than in the blood.
After a while diffusion will not pick up all of the glucose as it will equilibrate if passive.
- Basolateral side (closest to the blood) are sodium potassium ATPases. 3 sodium out 2 potassium into enterocyte using ATP. Primary active transport process.
- Low concentration of sodium in enterocytes
- Sodium glucose/galactose cotransporters on the luminal side - SGLT-1 channel, will transport sodium into the cell only if glucose or galactose is coupled to it. Secondary ATP transport since this channel does not require ATP.
Fructose can enter the enterocytes by facilitated diffusion, via GLUT5 channel.
Then all move by facilitated diffusion from enterocyte to blood, via GLUT2.
What does SGLT stand for?
Sodium Glucose Linked Transporter protein
Transports 2 sodium’s with a glucose or galactose
What happens when the monosaccharides are absorbed?
Fructose and galactose converted to glucose
- Glucose is released to blood and sent to all cells that need it for energy
- Conversion to glycogen stored in liver and muscles
- Conversion into adipose tissue
Why is protein important?
- absorption is highly efficient
- used to build and repair x provides essential amino acids for protein synthesis and N atoms for nitrogen containing groups
- can get energy from proteins when carbohydrate and lipid resources are not available - inefficient
- lose a lot of protein daily (mucins, cell debris etc)
Various forms of protein entering the gut
- diet (only half!)
- digestive enzymes
- microorganisms
- mucus
- plasma proteins
- desquamated epithelial cells
Structure of protein
Protein Polypeptides Peptides Di- and tri-peptides Amino acids
Amino acids and di and tri peptides can be absorbed by the SI
Location of protein digestion
Stomach (10-15%):
- mechanical digestion
- HCl converts pepsinogen into active pepsin
- HCl denatures protein to increase SA for pepsin
- pepsin digests large proteins into smaller peptides
Intestines (85-90%):
- receives denatured polypeptides from stomach
- pancreatic enzymes
- extensive and rapid digestion
- breaks small peptides into AAs
Enzymes involved in protein digestions
Endopeptidases: attacks bonds in centre of peptide chains, e.g. gastric pepsin, pancreatic trypsin
Exopeptidases: split AAs one at a time, e.g. pancreatic enzymes, intestinal (brush border) enzymes, enterocyte cytoplasm enzymes
So luminal and cellular digestion
Small peptide and AA absorption
Channels on the brush border that will pick up single amino acids and they will leave via the basolateral surface to enter the bloodstream.
There are also channels that can pick up di and tri peptides. There are enterocyte peptidases which will degrade them into amino acids - cellular digestion.
