56. Catabolism and absorption of carbohydrates

Question 1

Starch

Answer 1

• A giant polymer consisting of glucose molecules bound together by α-1-4 glycosidic bonds (straight chains) and α-1-6 glycosidic bonds (branched chains).
• Oral cavity:
• Saliva ptyalin will cleave the straight chains of the starch.
• Smaller and branched molecules are produced maltose, maltotrase, dextran
• Stomach:
• The effect of salivary amylase last until the gastric content is mixed.
• Later the low chemical reaction denaturates the enzyme.
• Small intestines: bi-phasic process
• α-amylase of the pancreatic juice hydrolyses starch to di- and oligosaccharides
• Brush border digestion:
• Small molecules bind to specific receptors having enzymatic effect, and are located on the surface of the enterocytes.
• Monosaccharides are generated this way, and can enter the blood circulation by absorption

Question 2

Absorption

Answer 2

• Fructose facilitated by GLUT-5 transporter, NO energy is needed. Transport last until concentration is equal on both sides of the membrane.

Question 3

• Glucose/galactose absorption:

Answer 3

• Secondary transport mechanism, with the need of a special receptor.
• Luminal receptor take up glucose/galactose only if Na+ binds before.
• If the molecules bind, the receptor goes through a conformational change, and “turns” into the IC space.
• Glucose/galactose and Na+ are diffused into the cytoplasm.
• Here glucose/galactose is taken up by GLUT-2 transporter, and the molecules are transported to the basal membrane. It enters the interstitium by a facilitated transport based on its chemical gradient.

Question 4

Cellulose

Answer 4

• Fibrous carbohydrate
• Can’t be digested my mammalian enzymes, and must be cleaved by microbial enzymes.
• Horse
• End-product is volatile fatty acids
• Occurs in the colon
• About 75% of the energy is provided by volatile fatty acids in the horse
• Carnivores:
• Doesn’t digest large amount of cellulose, but the microbial fermentation in the colon is still important:
• The non-digested carbohydrates would be lost if the microbial fermentation were absent.
• Carbohydrate molecules which are not absorbed represent a significant osmotic pressure in the colon, which would cause loss of water.

Question 5

Digestion of carbohydrates in ruminants

Answer 5

• Microflora of rumen: competition between protozoa and bacteria = good
• Fermentation: end products is mostly volatile fatty acids
• A part of the microbes, effectively deaminate amino acids, producing branched fatty acids.
• Part of them produces urease, by hydrolyzing carbamine, and release NH3.

Question 6

Absorption from ruminant stomach

Answer 6

• Volatile fatty acids:
• Are the end products of the carbohydrate fermentation, and they can diffuse through the mucosa into the blood.
• If the pH in the rumen decrease, the absorption of VFAs increases to maintain the optimal pH in rumen.
• Lactic acid:
• Products of easily digestible carbohydrates as grains.
• Usually low concentration in rumen, but sudden increase can cause damage to mucosa, and leads to acidosis.
• Ammonia:
• Significant amount of ammonia is released from the fermentative digestion of urea and protein, absorbed in the rumen.
• Ammonia goes to the liver via the portal vein, where urea is synthesized from it.
• Low pH decreases the ammonia absorption, and high pH leads to a significant absorption (toxic)