MLT GlobeChemistry: Carbohydrates & diabetes – page 2
121 Chemistry MCQs on Carbohydrates & diabetes with answers and explanations.
During an overnight fast, which organ is the main site of gluconeogenesis?
The liver (with a smaller kidney contribution) has glucose-6-phosphatase and releases new glucose into blood. Muscle lacks this enzyme and cannot export glucose.
D-glucose and L-glucose are non-superimposable mirror images. They are best described as:
Mirror-image stereoisomers are enantiomers. Diastereomers, including epimers, are stereoisomers that are not mirror images; anomers differ only at the anomeric carbon.
Which glycolytic enzyme is the main control point, allosterically inhibited by high ATP and citrate?
PFK-1 catalyses the committed step and is inhibited when ATP and citrate signal energy abundance; AMP and fructose-2,6-bisphosphate activate it. Hexokinase is inhibited by its product glucose-6-phosphate.
Which tissue converts glucose into galactose in large amounts to synthesise lactose?
UDP-glucose 4-epimerase forms UDP-galactose, which lactose synthase joins to glucose in the lactating breast. The intestine absorbs galactose but does not make lactose.
The rate-limiting enzyme of the hexose monophosphate (pentose phosphate) pathway is:
G6PD commits glucose-6-phosphate to the pathway and generates NADPH. α-Ketoglutarate and malate dehydrogenases are Krebs cycle enzymes.
After glycogen phosphorylase releases glucose-1-phosphate, which enzyme converts it to glucose-6-phosphate?
Phosphoglucomutase moves the phosphate from C-1 to C-6. Phosphoglucose isomerase converts glucose-6-phosphate to fructose-6-phosphate.
Which polysaccharide makes up much of plant dietary fibre, because human enzymes cannot digest its beta-1,4 bonds?
Humans lack cellulase, so cellulose passes undigested and adds bulk. Starch, glycogen and dextrin have alpha bonds that amylase breaks.
According to ADA criteria, which result is diagnostic of diabetes mellitus (confirmed on repeat testing)?
ADA criteria are HbA1c 6.5% or more, fasting glucose 126 mg/dL (7.0 mmol/L) or more, 2-hour OGTT glucose 200 mg/dL or more, or random glucose 200 mg/dL or more with symptoms.
In the hexokinase method for glucose, which change is measured?
Hexokinase converts glucose to glucose-6-phosphate; G6PD then oxidises it and reduces NADP+ to NADPH, raising absorbance at 340 nm. Hydrogen peroxide and dye colour belong to the glucose oxidase (Trinder) method.
Compared with whole-blood glucose from the same sample, plasma glucose is usually:
Plasma has more water than red cells, so plasma glucose is about 10–15% (around 11%) higher than whole-blood glucose. The values are not identical.
A red-top tube for glucose was left unspun at room temperature for 4 hours before testing. The result will most likely be:
Cells continue to use glucose; the loss is about 5–7% per hour at room temperature. Serum must be separated quickly or a glycolytic inhibitor used.
For a standard adult oral glucose tolerance test, the glucose load given after an overnight fast is:
The WHO/ADA adult OGTT uses 75 g of anhydrous glucose dissolved in water, with a 2-hour sample. 50 g is used for the non-fasting screening step in pregnancy.
HbA1c mainly reflects average blood glucose over the previous:
Glycation is slow and lasts for the life of the red cell (about 120 days), so HbA1c reflects the last 2–3 months. Fructosamine reflects 2–3 weeks.
Whipple triad, used to confirm true hypoglycemia, consists of:
Whipple triad links symptoms to a documented low glucose and their relief after glucose is raised. Insulin and C-peptide are used later to find the cause.
In a healthy adult, CSF glucose is normally about what fraction of plasma glucose?
CSF glucose is about 60–70% of plasma glucose, so a paired plasma sample is needed for interpretation. Low values suggest bacterial or fungal meningitis.
A plasma glucose of 126 mg/dL is equal to approximately:
To convert glucose from mg/dL to mmol/L divide by 18 (126 / 18 = 7.0). 5.6 mmol/L equals 100 mg/dL.
Sucrose is a non-reducing disaccharide. Which two monosaccharides form it?
Sucrose is glucose linked to fructose through both anomeric carbons, so it has no free reducing group. Glucose + galactose is lactose; two glucose units form maltose.
In which form is glucose mainly stored in the liver and skeletal muscle?
Animals store glucose as glycogen, a branched glucose polymer, mainly in liver and muscle. Starch is the storage polysaccharide of plants.
Which hormone is the principal hormone that lowers blood glucose concentration?
Insulin increases glucose uptake by muscle and fat and promotes glycogen synthesis. Glucagon, cortisol and growth hormone are counter-regulatory hormones that raise glucose.
What is the main action of glucagon on glucose metabolism?
Glucagon, released from pancreatic alpha cells during fasting, raises blood glucose by stimulating liver glycogen breakdown and glucose formation. The other actions are effects of insulin.