2020A Question 11

An infusion of 50 mL of 50% dextrose is given to a healthy 70kg adult. Describe the possible metabolic pathways for the dextrose.

Examiner Report

In order to pass, an answer had to address the domains of:

  • Glucose utilisation pathways
  • Glucose storage pathways, namely glycogen and fat stores
  • Hormonal regulation of the pathways, in particular the effects of insulin on target tissues.

While it was not necessary to provide detailed reactions of the metabolic pathways, credit was awarded for naming of major intermediaries, enzymes, receptors, relative amounts of energy released by different pathways and other relevant information. Many good answers used flow diagrams effectively to demonstrate the relevant pathways. Common problems and errors included:

  • Entirely omitting one of the domains
  • Limiting discussion of storage pathways to either glycogen or fat
  • Incorrect utilisation of the terms “glycogenesis” and “glycogenolysis”
  • Nomenclature of GLUT transporters, their locations and insulin responsiveness
  • Calculation of the dose (ranging from 2.5g to 25,000g)
  • Identifying dextrose as a carbohydrate polymer or molecule that needs to be broken down or converted to glucose
  • Conversion of glucose to amino acids and/or proteins
  • Discussion of an osmotic effect and ADH response

Model Answer

Factor Detail

Occurs if ↓ insulin, ↑ glucagon
Aerobic: → 36 ATP (in all nucleated cells)

Anaerobic: → 2 ATP (in all cells)

Krebs cycle

Pyruvate → Acetyl CoA and enters CAC, to produce:

 - 4 x NADH+

 - 2 x FADH2

 - 2 x GTP

 - 4 x CO2

Oxidative phosphorylation

 - 1x ATP

 - 1 x H2O


Occurs if ↑ insulin, ↓ glucagon:

 -GLUT2: Constitutive

GLUT4: Inducible


Uptake into liver (GLUT2), skeletal muscle (GLUT4)

Dose is 25g. Limit is ~50g liver, 400g skeletal muscle

Can be later catabolised to yield glucose

De novo lipogenesis

Uptake into liver (GLUT2), adipose (GLUT4)

Lactate (Cori Cycle) Hepatic gluconeogenesis from lactate
Amino acids
(Cahill Cycle)
Hepatic gluconeogenesis from alanine (produced from muscle protein degradation)
HMP shunt For synthesis of nucleotides (ribose)
Rapoport-Luebering shunt Production of 2,3-DPG in RBCs

Renal threshold ~10mM may be exceeded if large dose

Glycosuria continuous if taking SGLT2 inhibitor

Glycosylation Chronic hyperglycaemia → Damage to proteins → Microvascular complications (nephropathy, neuropathy)

Last updated 2021-08-23

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