Carbohydrates 2 Flashcards
What carbohydrates are found in our diet?
Starch Glycogen Cellulose and hemicellulose ogliosaccharides containing (a1-6) linked galactose Lactose, sucrose, maltose Glucose, fructose
Describe the digestion of carbohydrates in the mouth
Salivary amylase hydrolyses (a1-4) bonds of starch.
Describe the digestion of carbohydrates in the duodenum.
Pancreatic amylase works as in the mouth.
Describe the digestion of carbohydrates in the jejunum.
Final digestion by mucosal cell-surface enzymes
1. Ismaltase- hydrolyses (a1-6) bonds
2. Glucoamylase- removes Glc sequentially from non-reducing ends.
3. Sucrase- hydrolyses sucrose
Lactase- hydrolyses lactose
What are the main products of carbohydrate digestion?
Glc
Gal
Fru
What process allows for the absorption of glucose?
Glucose is absorbed through an indirect ATP-powered process.
ATP-driven Na pump maintains low cellular Na so glucose can continually be moved in to the epithelial cells.This system continues to work even if glucose has to be moved into the epithelial cells against it’s concentration gradient (i.e. When blood glucose is high)
Galactose has a similar mode of absorption as glucose, utilising gradients to facilitate it’s transport
How is fructose absorbed?
Fructose binds to the channel protein GLUT5. Simply moves down its concentration gradient (high in gut lumen, low in blood)
What are cellulose and hemicellulose used for?
Increase faecal bulk and decrease transit time.
What can a lack of ogliosaccharides in the diet lead to?
Poor health
Polymers are broken down by the gut to yield what?
CH4
H2
What can disaccharidase deficiencies result from?
It may be genetic Severe intestinal infection Other inflammation of the gut lining Drugs injuring the gut wall Surgical removal of the intestine.
What are disaccharidase deficiencies characterised by?
abdominal distension and cramps
How are disaccharidasse deficiencies diagnosed?
Requires enzyme tests of intestinal secretions. Usually checking for lactase , maltase or sucrose activity.
Why does the ingestion of milk give disaccharidase deficiency if lactase is lacking?
Undigested lactose is broken down by the gut bacteria causing gas build up an irritant acids.
Lactose is osmotically active, thus drawing water from the gut into the lumen causing diarrhoea.
How can symptoms of lactose intolerance be avoided?
Avoiding milk products
Using milk products treated with fungal lactase.
Supplementing diet with lactase.
What is the fate of absorbed glucose?
Glc diffuses through the intestinal epithelium cells into the portal blood and on to the liver
Glc is immediately phosphorylated into glucose 6-phosphate by the hepatocytes (or any other cell glucose enters)
How does glucose get trapped in the cell?
Glucose 6-phosphate cannot diffuse out of the cell because GLUT transporters wont recognise it
What are the enzyme catalysts involved in the fate of absorbed glucose?
Glucokinase (liver)
Hexokinase (other tissues)
glucokinase/ hexokinase
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Where is 90% of glycogen found?
The liver and skeletal muscle
What happens when blood glucose levels fall in the liver?
glycogen >G-6-P>(using glucose 6 phosphatase) Glc into the blood
What happens when blood glucose levels fall in the skeletal muscle?
In skeletal muscle: there is no glucose 6-phosphatase so G-6-P straight to lactate
What does the process of glycogenin involve?
Glycogen does not form directly from Glc monomers
Glycogenin begins the process by covalently binding Glc from uracil-diphosphate (UDP)-glucose to form chains of approx. 8 Glc residues
What process extends the glucose chains?
Glycogen synthase takes over and extends the Glc chains.
