WEEK 3: OVERVIEW OF ABSORPTIVE STATE AND METABOLISM

Question 1

What is fed state?

Where is energy derived from during fed state?

Answer 1

The fed state is the absorptive state - the period after eating, when there is food in the gut which is being absorbed.

*It lasts for a few hours after a meal – when meals are regular it will last through the day for many humans
In this state energy required is being derived directly from food

*Food material taken into the blood from the gut is generally in excess of immediate requirements, and excess material is converted to appropriate form for storage then stored in appropriate places.

Question 2

Describe the route taken by glucose from the ileum to the heart.

Answer 2

1.Glucose is absorbed across the intestinal epithelium and enters the mesenteric capillaries.

2.These drain into the hepatic portal vein which takes mesenteric blood directly to the liver

-(note that a portal vein is one which carries blood from one capillary bed to another rather than draining blood back to the heart)

3.The hepatic portal vein divides in the liver into a capillary bed

4.Capillaries run between the liver lobes

5.Blood, containing food material, leaves the capillaries and trickles through the ‘cords’ of liver cells (hepatocytes) forming the lobes.

6.As the plasma passes over the surface of the hepatocyte’s material can be picked up by the cells and metabolized

7.The blood is collected into capillary branches of the hepatic vein, which drains blood out of the liver and back into the venous circulation, and back to the heart.

8.Thus blood containing material picked up from the intestine passes through the liver, and comes into intimate contact with liver cells, before it enters the general circulation

9.This is an important principle, affecting metabolism of foreign molecules, including drugs, as well as food materials.

Question 3

What is the glucose concentration in the hepatic portal vein after food absorption?

What is the glucose concentration of blood collected in the hepatic vein ?

Answer 3

1.During food absorption the glucose concentration in the hepatic portal vein can be as high as 10 – 15 mM, but it is never this high in the general circulation.

2.Glucose is removed from the blood by hepatocytes, and the blood collected in the hepatic vein has a glucose concentration not higher than about 7 – 8 mM.

Question 4

Describe blood glucose regulation, when high.

Answer 4

1.Blood glucose concentrations higher than about 5mM cause release of the hormone insulin from the b- cells of the Islets of Langerhans of the pancreas.

2.Insulin stimulates many cells, including muscle and adipose tissue cells, to take up glucose.

3.It does this by causing a glucose transport protein (Glut 4) to be moved to the plasma membrane, allowing the cells to take up glucose

4.The cells of these tissues are unable to take up glucose in the absence of insulin because they have no means of transporting it across the membrane.

-This is not true of liver, brain and CNS and a few other tissues which use different transport proteins which

Question 5

How is glucose taken into the cells?

Answer 5

Glucose is taken into cells by facilitated diffusion.

It requires a transporter to get it across the membrane but is transported along a concentration gradient without involvement of energy.

Question 6

State the 3 stages of cellular respiration.

Answer 6

1.Glycolysis
2.Citric cycle/ Krebs cycle
3.Oxidative phosphorylation

Question 7

What is glycolysis?
Where does glycolysis take place?
What are the end products of glycolysis?

Answer 7

1.The breakdown of glucose by enzymes, releasing energy and pyruvic acid.

2.Cytoplasm

3.The end products of the reaction include 2 pyruvate, 2 ATP and 2 NADH molecules.

Question 8

What is the citric cycle?
What is the other name for Krebs cycle?
Where does it take place?

Answer 8

1.It is a metabolic pathway that connects carbohydrate, fat, and protein metabolism.

2.The citric acid cycle is also known as the Krebs cycle or the TCA cycle (tricarboxylic acid cycle).

3.In the mitochondria.

Question 9

Outline the main steps of glycolysis (11 steps)

Answer 9

Glucose
Glucose-6-Phosphate
Fructose-6-Phosphate
Frustose-6,1-Diphosphate
Dihydroxyacetone-phosphate
Glyceraldehyde-phosphate
1,3-Biphosphoglycerate
3-phosphoglycerate
2-phosphoglycerate
Phosphoenolpyruvate
Pyruvate

Question 10

How is glycolysis linked to Krebs cycle?

Answer 10

During glycolysis, a single glucose molecule is split into two smaller, three-carbon molecules called pyruvate.

Pyruvate is then converted to acetyl CoA.

Acetyl CoA is then utilized within the Krebs cycle to produce several major products.

Question 11

Outline the summary of the krebs cycle.

Answer 11

Here is a summary of the Krebs cycle:

Acetyl-CoA combines with oxaloacetate to form citrate.
Citrate is converted into its isomer, isocitrate.
Isocitrate is oxidized to alpha-ketoglutarate.
Alpha-ketoglutarate is oxidized to succinyl-CoA.
Succinyl-CoA is converted into succinate.
Succinate is oxidized to fumarate.
Fumarate is converted into malate.
Malate is oxidized to oxaloacetate

Question 12

How is the Krebs cycle linked to Oxidative phosphorylation?

Answer 12

It forms electron carriers (NADH and FADH2) cannot provide energy to cellular process directly.

Instead, the processes of the electron transport chain and oxidative phosphorylation will use the energy from these molecules to activate the enzyme complex ATP synthase, which produces ATP.

Question 13

What is oxidative phosphorylation?
Where does it occur?
Briefly describe oxidative phosphorylation process.

Answer 13

1.Oxidative phosphorylation is the final and most efficient step of cellular respiration, producing most of the ATP that cells need for energy.

2. It takes place in the mitochondria

3.Electrons from NADH and FADH2 are transferred to oxygen through the electron transport chain, which pump protons across the inner mitochondrial membrane, creating a gradient that drives the synthesis of ATP by a complex called ATP synthase.

Question 14

Describe the Storage of Excess Glucose.

Answer 14

*Glycogen stores in humans are quite limited, about 500g is stored in total in an adult.

1. About 350g, is stored in the muscles, where it can make up about 1% of the wet weight.

2.The rest – about 150g- is stored in the liver cells, which can store up to about 5% of their wet weight.

Question 15

Describe the fat storage in the body.

Answer 15

1.Fat is stored in the body in the form of triacylglycerol (triglyceride)

2.TAG is stored in a special tissue called adipose tissue.

Question 16

Describe the transport of Transport of Triacyl Glycerol.

Question 17

What is the Metabolic Fate of Amino Acids.

Answer 17

Amino acids in excess of requirements are first deaminated. This usually involves the amino acid exchanging its amino group with 2-oxo glutarate, forming glutamate and leaving the carbon skeleton of the amino acid in keto-acid form.

*The amino acid nitrogen is removed in the urea cycle and converted into urea, which is eliminated in the kidneys.

*The remaining carbon skeletons are fed into intermediary metabolism.
Most amino acids can be converted to pyruvate, or four or five carbon intermediates of the Krebs’ cycle.

*These amino acids will be catabolized to CO2 or converted to fatty acid in the fed animal, but during fasting can be converted to glucose, so are said to be glucogenic

*However in the fasted state it can be used as a precursor of ketone bodies, so these amino acids are said to be ketogenic or partially ketogenic

Question 18

What happens in Transamination (Aminotransferase) Reactions?

Answer 18

In these reactions the amino group of an amino acid is transferred to a keto- acid acceptor, converting the acceptor into an amino acid and leaving the carbon skeleton of the donor amino acid in keto-acid form.

The acceptor is usually pyruvate, which forms alanine, (alanine amino transferase AAT, ALT), 2-oxo glutamate, which forms glutamate (glutamate aminotransferase GAT, GGT) or oxaloacetate which forms aspartate.

Question 19

Name the water-soluble non-toxic molecule, which can be eliminated from the blood by the kidneys that amino acid nitrogen is converted in the liver.

Name the process by which this occurs.

Name the enzyme that produce ammonia in the liver from amino acid glutamate.

Name the compound that ammonia enters the Urea cycle as.

Answer 19

1.Amino acid nitrogen can be removed as ammonia, but it is important to avoid ammonia build up since this is very toxic.

2.Urea

3.Urea production occurs in a pathway called the urea cycle which takes place partly in the liver mitochondria and partly in the cytoplasm.

4.Ammonia is produced in the liver by the enzyme glutamate dehydrogenase from the amino acid glutamate.

5.Ammonia enters the urea cycle as carbamoyl phosphate, which is synthesized from ammonia and carbon dioxide in the liver mitochondria.

6.The amino acid, aspartate, also supplies a –NH2 group, which forms one of the nitrogen’s of the urea molecule.