Session 2: Energy Production from Carbohydrates

What is stage three of the process of catabolism of carbohydrates?

The TCA cycle

Where does the Krebs cycle take place?

matrix of mitochondria

What is stage four of catabolism of carbohydrates?

Oxidative phosphorylation

What is starch an example of?

A polysaccharide

What is glycogen?

A polysaccharide - major store of glucose in mammals

Where is glycogen stored?

In the liver

List common disaccharides and their monosaccharide components

Lactose = galactose & glucose

Sucrose = fructose & glucose

Maltose = glucose & glucose

List common monosaccharides

Fructose, glucose, galactose

Give an example of an oligosaccharide made up of glucose monomers (3-10 monosaccharides)

Dextrins

What is the name of the more thermodynamically stable structure of glucose?

Hawthorne projection (ring structures)

List examples of common polysaccharides

Glycogen, starch, cellulose (polymers of glucose)

In the polymerisation of monosaccharides - what is the name of the bond that forms between monosaccharides?

Glycosidic bonds

What is an example of a highly branched polysaccharide of glucose?

Glycogen is highly branched

What is one example of an enzyme secreted in the mouth in the digestion of carbohydrates?

Salivary alpha-amylase

Starch/glycogen —> dextrins/disaccharides

What are some enzymes in the small intestine which assist in the digestion of carbohydrates

Disaccharidases - attached to brush border of epithelial cells (enterocytes)

- Lactase

- Sucrase

- Maltase

- Isomaltase

What is isomaltase

An isomer of maltose

What is the name of the adaptation of epithelial cells in the small intestine which facilitates absorption of nutrients?

Microvilli (brush border) - apical membrane extensions of epithelial cells

How are glucose, galactose and fructose transported to enterocytes (epithelial cells)?

By facilitated or active transport

What is the role of GLUT2 (glucose transporter type 2)

Monosaccharides transported from enterocytes into the bloodstream by GLUT2

What is the role of GLUT1-14

Glucose transporters which transport monosaccharides to the target tissues

What is the cause of lactose intolerance?

Loss of reduction of lactase activity = lactose is not hydrolysed to glucose and galactose

What are the symptoms of lactose intolerance?

Diarrhoea, abdominal cramps, bloating, abdominal pain, nausea (appearing 30-120 minutes post-consumption)

How is lactose intolerance diagnosed?

Positive hydrogen breath test, positive stool acidity test

How do you manage lactose intolerance?

Decrease or eliminate amount of lactose (lactose free-diet), consumption of lactase-treated foods or lactase supplementation

What are the glucose requirements of tissues needed per day in grams

Around 180g of glucose is needed per day

Which tissues have an absolute requirement for glucose? How much glucose per day in grams?

RBC, WBC, kidney medulla, testes, lens and cornea of eye

- Around 40g/day

Approximately how much does glucose does CNS (prefer) everyday?

CNS prefers glucose as a fuel (~140g/day)

What is the name of stage two in catabolism of carbohydrates?

Glycolysis

What is the product of glycolysis?

Pyruvate

Where does glycolysis occur?

cytosol of the cell (of all tissues)

What is the only catabolic pathway that can operate anaerobically?

Glycolysis

Is glycolysis an endergonic or exergonic pathway?

Glycolysis is a catabolic process - it is exergonic (releases energy and ATP)

What is the name given to aldehyde-containing sugars? Give an example

Aldoses e.g. glucose, galactose

What is the name given to keto-containing sugars? Give an example

Ketones e.g. fructose

Are carbohydrates hydrophobic or hydrophilic?

Simple carbohydrates are small POLAR molecules - contain many (-OH) groups which makes them hydrophilic

Which is the naturally occurring form of stereoisomers?

D-isomers

The position of the ______ group on carbon-__ determines whether D-glucose has alpha or beta structure

The position of the OH group on carbon-1 determines whether D-glucose has alpha or beta structure

Roughly how many monosaccharides do oligosaccharides contain e.g., dextrins

3-10 monosaccharides

Roughly how many monosaccharides do polysaccharides contain?

10-1000 monosaccharides

What is Dextrin an example of?

A glucose oligosaccharide

How many (NET) ATP molecules are produced per glucose in glycolysis

2 ATP

Is glycolysis an exergonic or endergonic reaction

Glycolysis is an exergonic reaction

Glycolysis is the only catabolic pathway that can also operate in the absence of _______

Glycolysis if the only catabolic pathway that can also operate in the absence of oxygen

Is glycolysis an oxidative or reductive reaction?

Glycolysis is oxidative

What enzyme catalyses the phosphorylation of glucose in glycolysis to G-6-P?

Hexokinase enzyme

What are the major dietary monosaccharides?

Glucose, fructose, galactose

Name the dietary macronutrient which should provide you with the most energy

Lipids

Which polysaccharide cannot be digested in the human body?

Cellulose

β 1-4 linkages cannot be hydrolysed

Which are the three key fuel molecules that undergo oxidation during catabolism?

Fatty acids, glucose and amino acids

The end-product of glycolysis under anaerobic conditions

Lactate

Name the facilitated transporter that transports monosaccharides from enterocytes to the blood

GLUT2

Name the insulin-responsive glucose transporter that occurs in skeletal muscle and white adipose tissue

GLUT4

Which cells/tissues have an absolute requirements for glucose as a fuel?

Testes, lens/cornea, kidney medulla, red and white blood cells

What is the net synthesis of ATP from one mole of glucose?

2

Which polysaccharide is highly branched and allows for an efficient way of storing glucose in a small space?

Glycogen

Glycolysis is the only catabolic pathway that can occur in the absence of ______.

Glycolysis is the only catabolic pathway that can occur in the absence of oxygen.

Which of the following metabolic intermediates in glycolysis gives rise to glycerol 3 phosphate?

- Dihydroxyacetone phosphate

- Glucose 6 phosphate

- 3 phosphoglycerate

- 1,3-Bisphosphoglycerate

Dihydroxyacetone phosphate (DHAP)

Which of the following control enzymes in glycolysis is activated by insulin through dephosphorylation?

Pyruvate kinase

Name the key enzyme in anaerobic glycolysis?

Lactate dehydrogenase

Where is lactate metabolised?

In the liver, heart and kidney

Name the enzyme of which deficiency causes fructose intolerance

Aldolase

GLUTs

Stage one of carbohydrate metabolism occurs extracellularly (GI tract)

Describe some of the major enzymes involved in this process...

Difference between α- and β-glycosidic bonds

Starch is made up what?

Amylose α1‐4 glycosidic bonds

Amylopectin α1‐6 glycosidic bonds

Genetic cause of lactase deficiency

Lactase activity is high in infants but decreases in childhood in most populations (especially African and Asian)

Nongenetic cause of lactase deficiency

Injury to the small intestine (e.g. inflammatory bowel disease, surgery, infection)

Consequences of lactose intolerance

- Undigested lactose is passed to the large intestine

- Colonic bacteria ferment lactose and produce organic acids and gases

- Lactose and organic acids increase osmotic pressure and draw in water causing = diarrhoea

- Gases cause abdominal cramps and bloating

Blood glucose is regulated at what level in the blood?

~5 mM