Lecture 6- CHO Digestion and Function Flashcards
Mouth
○ Alpha-amylase breaks down alpha-1,4-glycosidic bonds
○ Cellulose, lactose, and alpha-1,6-bonds are resistant
Produces only a few monosaccharide
Stomach
○ Alpha amylase digestion continues until pH drops, and the enzyme is deactivated (No further CHO digestion within the stomach)
At this point, carbohydrates are small polysaccharides and maltose
Small Intestine
○ Pancreatic juice- mix of digestive enzymes that contains pancreatic alpha-amylase (not salivary alpha-amylase- was deactivated in the stomach)
○ Pancreas releases bicarbonate, neutralizes pH so that alpha amylase can work (needs neutral pH)
○ Pancreatic alpha-amylase continues break down of CHO, result in mono and disaccharide
○ Alpha-1,6 bonds leads to the production of isomaltose
§ Highly branched structure broken down from each end of the branches, but can’t break down the junction of the branches (alpha-1,6 bonds)
Alpha-amylase can’t break down alpha-1,6 glycosidic bonds
Brush Border Enzyme Activity:
• Breakdown of disaccharides to monosaccharides in the small intestine
• Enzymes that break down CHO located on the top of cell (facing lumen- apical side) within the intestinal lumen
CHO resistant to amylase digestion are broken down by brush border enzymes
Diagram
Alpha-dextrinase/ Isomaltase:
Breaks down alpha-1,6 glycosidic bonds into 2 glucose molecules
Maltase:
Breaks down maltose into 2 glucose molecules
Invertase/Sucrase:
Invertase (also known as sucrase) breaks down sucrose into glucose and fructose
Lactase:
Lactase breaks down lactate to glucose and galactose
Lactose Intolerance:
• Lactose enzyme no longer functioning effectively
• When lactose isn’t being broken down in the small intestine, it travels to the large intestine
• Bacteria can break it down, but generates lactic acid in the process
• Factors contributing to lactose intolerance
○ Digestive ability decreases with age
○ Ethnicity- Asian vs. Northern Europeans
Genetics- variants in the LCT (codes for lactase)
Diagram
Monosaccharide Absorption:
• Very efficient
• Enterocytes (intestinal cells) are polarized, have an “up” and “down”
• Nearly all monosaccharides are absorbed by the end of the jejenum
○ 15 % of glucose leaks back out of the lumen
○ 25 % of glucose diffuses into the circulation via the basolateral membrane
○ 60 % of glucose is transported into the circulation by GLUT2
• Glucose and galactose transported via active transport
○ Apical transporter, SGLT1 (sodium glucose transport 1) needs to bind both sodium and glucose (on apical site) to become functional (cotransport)
○ SGLT1 works on a concentration gradient (if there is a buildup of sodium and glucose within the cell, the gradient breaks down)
§ Glucose moves through cell from GLUT 2 into the blood (results in constant low level of glucose within the cell)
§ Na-K pump (ATPase) on basolateral side uses ATP to pump sodium out of the cell into the blood
§ Low concentration of sodium and glucose in cell allows SGLT1 to bring in sodium and glucose into cell
• Fructose transported via facilitated transport
○ GLUT5 on apical surface
GLUT2 on basolateral surface
Diagram
Functions of CHO in the Body:
• Glucose is the primary source of energy for cells
○ Red blood cells completely dependent on glucose (doesn’t have mitochondria, can’t undergo beta oxidation)
○ Excess glucose stored as glycogen
• CHO “spare” protein
○ Prevent breakdown of protein for energy
○ Allows protein to continue building, repairing, and maintaining body tissue
§ Proteins make up digestive enzymes, transporters…etc, don’t want them being broken down
• Prevents ketosis
○ When CHO are limited, fats are broken down for energy, leads to the production of ketone bodies, causes body pH to be acidic
○ During overnight fast (sleeping), body switches to using fat as a source of energy, switching permanently to using fat as energy produces ketone bodies, puts stress on body
• Primary source of energy for the brain
Fermentable CHO (not glucose) ensures growth of healthy bacteria in the gut
