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Carbohydrate digestion and metabolism

This set of exercises introduces the main pathways of carbohydrate metabolism: glycolysis, gluconeogenesis and the pentose phosphate pathway, as well as the synthesis and utilisation of glycogen.

The following exercises are available in this section:

Sugars, starches and the glycaemic index (Classification of carbohydrates, digestion and absorption, glycaemic index, resistant starch and on-starch polysaccharides)

Bloating, flatulence and diarrhoea after drinking milk - but not yoghurt or cheese (Alactasia, anaerobic glycolysis)

Experiments on glucose metabolism (The pathway of glycolysis)

Life-threatening acidosis in an alcoholic - and in a hunger striker given intravenous glucose (Pyruvate dehydrogenase and the role of thiamin)

Breathless after sprinting (Cori cycle, anaerobic glycolysis in muscle and gluconeogenesis in liver)

Weight loss in a patient with advanced cancer (Cancer cachexia, energy yield of muscle, anaerobic glycolysis in tumour and gluconeogenesis in liver)

How is NADH from glycolysis normally re-oxidised? (Malate-aspartate and glycerol phosphate shuttles)

An adverse response to antimalarial medication - and a fatal reaction to fava beans (Pentose phosphate pathway)

Fasting hypoglycaemia in an infant - and poor exercise tolerance in two brothers (Glycogen synthesis and utilisation, glycogen storage disease type 0)

Alanine released from muscle in fasting (Glucose-alanine cycle)

A hypoglycaemic adolescent with an enlarged liver and gout (Glycogen storage disease type I - glucose 6-phosphatase deficiency)

Two diabetic patients in coma (Hypoglycaemic and hyperglycaemic coma)

Was she murdered by insulin injection? (Insulin synthesis, pro-insulin and C-peptide)

An unusual cause of diabetes - how the pancreas senses a rise in blood glucose (The role of glucokinase in sensing an increase in blood glucose)

 

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Objectives

After working through these exercises you should be able to:

Describe the main dietary sources of carbohydrate and explain what is meant by the term glycaemic index

Describe the digestion and absorption of carbohydrates

Describe the pathway of glycolysis and differentiate between aerobic and anaerobic glycolysis, explain what is meant by the term oxygen debt and explain why glycolysis is essentially irreversible

Explain how reducing equivalents from glycolysis are transferred from the cytosol into the mitochondria

Describe the pentose phosphate pathway and explain its importance as a source of pentose sugars and NADPH

Describein outline the entry of other sugars into the glycolytic pathway

Describe the reaction of pyruvate dehydrogenase and explain the importance of thiamin (vitamin B1)

Describe how the irreversible steps in glycolysis are circumvented for gluconeogenesis

Describe how glycogen is synthesised in the fed state and how it is utilised in the fasting state, explain how muscle glycogen can be an indirect source of blood glucose in the fasting state

Explain the metabolic consequences of excessive and inadequate insulin as may be seen in people with diabetes mellitus

Describe the steps involved in the synthesis and secretion of insulin

Explain how the pancreas senses an increase in the blood glucose concentration to stimulate insulin secretion