Chapter 67 Metabolism Of Carbohydrates And Formation Of ATP Flashcards
What are the final products of carbohydrate digestion in the alimentary tract?
Almost entirely glucose, fructose and galactose with glucose representing on average 80 percent of these.
How is glucose transported through the cell membrane into the cytoplasm so that it can be used?
Penetrating through the lipid matrix of the cell membrane are large numbers of protein carrier molecules that can bind with glucose. In this bound form, the glucose can be transported by the carrier from one side of the membrane to the other side and then released.
What is phosphorylation and what is the purpose?
Immediately on entry into cells, glucose combined with a phosphate radical with the purpose to capture the glucose in the cell.
When the liver releases monosaccharides back into the blood, the final product is almost always glucose. What is the reason for this?
The liver cells contain large amounts of glucose phosphatase. Therefore glucose-6-phosphate can be degraded to glucose and phosphate and the glucose can be transported through the liver cell membrane back into the blood.
What is the mechanism called when glucose is transported through the membrane of the body’s tissue cells?
Facilitated diffusion
Describe the phosphorylation of glucose?
Glucose turns into glucose-6-phosphate (with help of glucokinase or hexokinase) plus ATP
Phosphorylation is almost completely irreversible except in some cells - which are these?
Liver cells, renal tubular epithelial cells and the intestinal epithelial cells.
In these cells - another enzyme: glucose phosphatase, is also available and when this activated, it can reverse the reaction.
After absorption into a cell, glucose can be used immediately for release of energy to the cell or it can be stored in the form of glycogen. Which cells can store glycogen?
All cells of the body are capable of storing at least some glycogen but certain cells can store large amounts - especially liver cells which can store up to 5-8 % of their weight as glycogen, and muscle cells which can store up to 1-3% glycogen.
What is glycogenolysis and how does it occur?
It’s the breakdown of the cell’s stored glycogen to reform glucose in the cell. It does not occur by reversal of the same chemical reaction that form glycogen but instead each succeeding glucose molecule on each branch of the glycogen polymer is split way by phosphorylation catalysed by the enzyme phosphorylase.
Which 2 hormones can activate phosphorylase and thereby cause rapid glycogenolysis?
Epinephrine
Glucagon
The initial effect of each of these hormones is to promote the formation of cyclic AMP in the cells which then initiates a cascade of chemical reactions that activates the phosphorylase.
Where is glucagon secreted from?
By the alpha cells of the pancreas when the blood glucose concentration falls to low. It stimulates formation of cyclic AMP mainly in the liver cells and this in turn promotes conversion of liver glycogen into glucose and its release into the blood.
How many moles of ATP are formed for each mole of glucose metabolised by the cells?
A total of 38 moles of ATP
What does glycolysis mean?
Splitting of the glucose to form two molecules of pyruvic acid.
The end products of glycolysis are then oxidised to provide energy.
How many moles of ATP are formed for each mole of fructose-1,6-diphosphate that is split into pyruvic acid?
4 moles
Yet 2 moles of ATP are required to phosphorylate the original glucose to form fructose-1,6-diphosphate before glycolysis could begin.
Therefore the net gain in ATP is only 2 moles for each mole of glucose utilised.
What is the citric acid cycle (Krebs cycle)?
A sequence of chemical reactions in which the acetyl portion of Acetyl CoA is degraded to carbon dioxide and hydrogen atoms. These reactions all occur in the matrix of the mitochondrion.
