Carbohydrates 2 Flashcards
What are some carbohydrates in the diet?
Starch (cereals, potatoes)
Glycogen (meat)
Cellulose and hemicellulose (plant cell walls)
Oligosaccharides containing a1-6 linked galactose (peas, beans)
Lactose, succrose, maltose (milk, sugar, beer)
Glucose, fructose (fruit, honey)
Where are 3 places that carbohydrates are digested?
Mouth
Duodenum
Jejanum
How are carbohydrates digested in the mouth?
Salivary amylase hydrolyses a1-4 bonds of starch
How are carbohydrates digested in the duodenum?
Pancreatic amylase works as in the mouth (hydrolyses a1-4- bonds)
How are carbohydrates digested in the jejunum?
Final digestion by mucosal cell surface enzymes
What are enzymes that digest carbohydrates in the jejunum and what do they do?
Isomaltase (hydrolysis a1-6 bonds)
Glucoamylase (removes glucose from non reducing end)
Sucrase (hydrolysis sucrose)
Lactase (hydrolysis lactose)
What is the deudenum and jejunum part of?
Small intestine
Are any carbohydrates digested in the stomach?
No
Where are glucose transporters found?
In the microvilli of epithelial cells facing the lumen of the intestine
What does our high intake os salt cause?
High Na concentration outside of cells
What is the glucose transporter driven by?
Na, it takes in 2 Na and 1 glucose
What maintains the concentration gradient of Na that can drive glucose into the cells?
Na+/K+ pumps
How is glucose then pumped from within the cell into the blood?
Pump on the basal surface of the cell move glucose to blood, only transporting glucose due to the high concentration gradient
What the exact process for the absorption of glucose?
- High [Na+] out of the cell due to high salt intake
- Na+/K+ pump maintains the concentration gradient
- Transporters on microvilli bring in 2 Na+ and 1 glucose
- Transporter on basal surface lets 1 glucose leave into blood due to concentration gradient
Why can the absorption of glucose be described as an indirect ATP process?
Because of the use of the Na+ pump
Why will glucose be pumped in the cell even if its up its concentration gradient?
Because of the Na+ concentration which drives the pump
How is galactose absorbed?
Similarly to glucose utilising gradients
How is fructose absorbed?
- Binds to channel protein GLUT5
- Simply moves down its concentration gradient
What happens to cellulose and hemicellulose?
Cannot be digested by the gut, are broken down into methane and hydrogen by gut bacteria
What do cellulose and hemicellulose do?
Increase facial bulk and decrease transit time
Cause smelly farts (broken down into methane)
What be disaccaride deficiencies be caused by?
Genetic
Severe intestinal infection
Inflammation of the gut lining
Drugs injuring the gut wall
Surgical removal of part of the intestine
What does diagnosis of disaccaride defficiencies require?
Enzyme tests of intestinal secretions for lactose, maltose or sucrose activity
What is the most common disaccharide deficiency?
Lactose intolerence
What causes the symptoms of lactose intolerence?
Undigested lactose broken down by gut bacteria causing gas build up and acid
Lactose is osmotically active, drawing water into the lumen from the gut causing diarrhoea
What is the fate of absorbed glucose?
- Diffuses through intestinal epithelium cells into blood and then the liver
- Immedietely phosphorylated into glucose-6-phosphate by the hepatocytes
Why is glucose converted into glucose-6-phosphate?
Because glucose-6-phosphate cannot escape the cell as GLUT transporters don’t recognise it
What enzymes control the reaction of glucose to glucose-6-phophate?
Glucokinase (liver)
Hexokinase (skeletal muscle)
Where is glucokinase found?
In the liver
Where is hexokinase found?
In skeletal muscle
What are the kenetic properties of glucokinase and hexokinase?
Glucokinase has high KM and high vmax
Hexokinase has low KM and low vmax
What happens when the concetration of blood glucose is low, in terms of glucokinase and hexokinase?
Liver doesn’t grab all of it (due to high KM of glucokinase) so other tissues can use it
What happens when glucose concentration is high, in terms of glucokinase and hexokinase?
The liver grabs a lot more of it
Why would glucose-6-phosphate be converted back to glucose in the liver?
To enter the blood stream and travel to where it is needed
What is substrate level phosphorylation?
Formation of ATP by direct transfer of a phosphoryl group (PO3) to ADP from another phosphorylated compound
What is oxidative phosphorylation?
ATP is formed from the transfer of electrons from NADH or FADH2 to O2 by a series of electron carriers
What does the pathway of glycogen in the liver to glucose in the blood look like?
What does the fate of glycogen in skeletal muscle look like?
Where is the enzyme glucose-6-phosphotase found?
In the liver, not the skeletal muscle as the genes that code for it are not active
What does glucose-6-phosphatase do?
Breaks down glucose-6-phosphate into glucose
What is the exact process of the synthesis of glycogen?
- Glycogenin begins process by covalently binding glucose from uracil diphosphate (UDP) glucose to form a chain of about 8 monomers
- Glycogen synthase takes over and extends the glucose chain
- Chains formed are broken by glycogen branching enzymes and reattacthed via a1-6- bonds to give branching points
- Glycogen synthase extends the chain and repeat
How does glycogenin catalyse the synthesis of glycogen?
Producing a tiny amount of glucose to start the chain
What is uracil diphospahte (UDP)?
A carrier molecule that allows the cell to defferentiate what glucose is to be used for the synthesis of glycogen
What is the exact process of the degradation of glycogen?
- Glucose monomers are removed one at a time from the non reducing end by glycogen phophorylase to produce G-1-P
- Branch is removed by debranching enzyme
- Transferase removes 3 glucose monomers and attatches them to nearest non reducing end via a1-4 bonds)
- Glucosidase removes final glucose monomer by breaking a1-6 bond and releases a free glucose
- Process repeats
What is G-1-P readily converted to and what does this look like?
G-6-P
What are some common diseases associated with glycogen storage?
van Gierke’s disease
McArdle’s disease
What is van Gierke’s disease caused by?
A G-6-P defficiency in the liver, kidney and intestine
What are the symptoms of van Gierke’s disease?
High [liver glycogen]
Low [blood glucose]
High [blood lactate]
What is the treatment of van Gierke’s disease?
Regular carbohydrate feeding
