Download pbl – night starvation - UQMBBS-2013

PBL 8 – NIGHT STARVATION
1999 Paper 3, Part F: Q6
Digestive Cycle Mouth-SI
CHO in diet: Starch, lactose & sucrose are the 3 main sources. Also small amounts of others,
including alcohol, glycogen, lactic acid, dextrins. Cellulose is starch we eat but is not digested.
Salivary amylase begins hydrolysis of CHO in the mouth.
In stomach, the low pH stops digestion.
Digestion continues in the small intestine from pancreatic amylase. The amylase breaks down
starch to maltose (diasaccharide), maltotriose (trisacch) and dextrins.
The disaccharides are further hydrolysed to monosacchardes by the brush border enzymes on
the intestinal epithelium (enzymes are Lactase, maltase and sucrase).
Lactose (by lactase)  glucose + galactose
Maltose (by maltase)  glucose + glucose
Sucrose (by sucrase)  glucose + fructose
Dextrins (by dextrinase)  small glucose polymers & monosaccharides.
Thus, all the final products are monosaccharides. They are taken up by Na-coupled transporter
(facilitated diffusion) on the luminal side of the enterocytes, then glucose diffuses into the portal
blood via a GLUT-2 transorter on the basolateral side of the enterocytes. Galactose is handled in
the same way. Fructose is usually converted to glucose within the enterocytes, then absorbed as
per glucose.
2000 Paper 1, Part A: Q1-2, 4-6
A hiker is lost in the Kosciusko ranges for 14 days without food, but with
access to water. When rescued, he is weak but alert.
Question 1. (2 marks)
What is the major metabolic process responsible for the maintenance of blood
glucose concentrations
(a) 12 hours after the last meal?
Glycogenolysis which has kicked in after 4-6 hours
(b) 3 days after the last meal?
Gluconeogenesis is in full flight from the liver. Other tissues are now using ketones as a fuel
Question 2. (9 marks)
(a) Explain in what form glucose is stored in the liver, adipose tissue and muscle in
the fed state (3 marks)
In the fed state, glucose is stored as glycogen within the liver and muscle. It
is stored in this state by conversion of glucose to G-6-P and is then joined together into the the
highly branched glycogen state. In adipose tissue it is converted into acetyl-CoA and then stored
as FA's and TAGS.
(b) State whether energy stores in these organs can be used to maintain blood
glucose concentrations during fasting, and if not, explain why (3 marks)
Liver glycogen can be degraded into glucose and released into the blood to maintain BGL.
Muscle glycogen is broken down the glucose but cannot exit the cells into circulation. TAG's
undergo Beta oxidation to produce acetyl CoA and ketone bodies; neither of which can be
converted into glucose.
(c) Explain the effects of insulin on pathways of glucose metabolism in the liver. (3
marks)
Insulin directly affects glucose metabolism in the liver. It achieves this by activating
phosphofructokinase which will cause phosphorylation of fructose 1 P to F-1,6-P, and it will also
upregulate other enzymes like pyruvate kinase which will convert pyruvate into acetyl CoA to be
used as an energy source for the TCA cycle.
Question 4. (5 marks)
Organs such as the liver use fatty acid oxidation to meet most of their energy
requirements during fasting. Outline the changes in metabolic regulation that occur
during fasting, to promote fatty acid mobilization in adipose tissue, and oxidation in
the liver.
During fasting, insulin levels will fall to minimal levels due to a decreasing glucose concentration.
This decrease in insulin will stop glucose uptake by insulin-mediated uptake into adipose tissues.
Subsequently, the decreasing levels of circulating glucose will cause an increase in the levels of
glucagon, which will upregulate lipolysis in the adipose tissues. In an insulin dominant state, the
levels of malonyl CoA are high and this inhibits the carnitine acyl transporter 1. With low insulin
this inhibition is negated and FAs can be uptaken by the hepatic cells. Here, beta-oxidation will
occur, creating large amounts of acetyl CoA to be oxidised and used as an energy source.
NB: Stops uptake of TAGs into the adipose and the glucose uptake is downreg.
Question 5. (6 marks)
With the aid of a diagram/s, illustrate how energy derived from the oxidation of fatty
acids is used to drive ATP synthesis.
Energy yield from Beta oxn. is high and each removal of a C-C yields 1 FADH2 & 1NADH & ATP
from TCA. Ketone bodies are usually only in a very low concentration in blood. During fasting, its
concentration increases to provide energy to peripheral tissues. KB -> A CoA -> TCA
Question 6. (4 marks)
Explain how an increase in ATP demand (for example to drive gluconeogenesis)
results in an increase in O2 consumption.
With an increase in ATP demand, there is an increase in O2 consumption because of an
increased need to retain an electric gradient across the inner mitochondrial membrane. When
ATP is made at a faster rate, H+ will be shuttled through the 5th ATPase bound membrane
protein at a faster rate, resulting in increased H+ concentration within the cell. If this was allowed
to continue, there would eventually be an equalising of the electrostatic charge across the
membrane, leading to no further H+'s travelling through the ATPase bound protein. Increased O2
will take up and bind these H+'s and remove them from the inner membrane, allowing the electric
gradient to be maintained.
2003 Paper 2, Part 2: Q2-4
Question 2. (10 marks)
In the fed state, excess carbohydrate is stored as fat. Draw a simple diagram to
illustrate how glucose in the liver is converted to fat that is transported to, and
ultimately stored in adipose tissue.
Question 3. (10 marks)
With the aid of a diagram, illustrate how energy derived from the oxidation of NADH
in the electron transport chain is used to drive ATP synthesis.
Question 4. (8 marks)
Organs such as the liver use fatty acid oxidation to meet most of their energy
requirements during fasting. Outline the changes in metabolic regulation that occur
to promote fatty acid mobilization and oxidation in fasting.
2005 Paper 1: Part A: Q3, 4
Question 3. (10 marks)
In the fed state, excess carbohydrate is stored as fat. Draw a simple diagram to
illustrate the process in which glucose is converted to fat in the liver, and then
transported to, and ultimately stored in, adipose tissue.
Question 4. (8 marks)
Organs such as the liver use fatty acid oxidation to meet most of their energy
requirements during fasting. Outline the changes in metabolic regulation that occur
to promote fatty acid mobilization and oxidation in fasting.
2005 Paper 2, Part C: Q1
A 25 year man (Mr Peter D) is found unconscious by his wife in the garage of
their house. He was slumped over the steering wheel. The engine of the car
was running. He has been brought in by the Ambulance Service to the
Emergency Department of the local hospital.
Question 1. (10 marks)
Below is a list of conditions that can lead to coma in any person. For each, state the
underlying mechanism responsible.
Condition
Diabetes
Carbon monoxide Poisoning
Renal Failure
Head Injury
Heart Failure
Mechanism
Decrease in glucose, brain runs out of energy
to stay active
Decreased oxygen in blood, brain is affected
Blood is full of unwanted substances, not
cleared properly, affects heart, and brain
Haemorrhage causing ischaemia in brain
Decreased blood to brain