GIT and nutrition

Question 1

How is glucose transported into the cells epithelial cells of the GIT?

Answer 1

by the mechanism of active sodium-glucose cotransport, in which active transport of sodium provides energy for absorbing glucose against a concentration dif- ference.

Question 2

How is glucose transported into cells other than the GIT epithelial cells ?

Answer 2

facilitated diffusion, made possible by the special binding prop- erties of membrane glucose carrier protein.

Question 3

What hormone increases the facilitated diffusion of glucose into the cells?

Answer 3

insulin - when larger amounts are secreted the transport of glucose increases 10 fold

Question 4

Can the cells absorb enough glucose to allow for all essential metabolic functions in the abscence of insulin?

Answer 4

with the exception of liver and brain cells, without insulin too little glucose can enter the cells to supply the required energy for metabolism.

Question 5

What is the cells carbohydrate utilisation rate determined by

Answer 5

insulin

Question 6

What is the first step of glucose metabolism once the sugar enters the cell?

Answer 6

Phsophorilation by glukokinase in the liver and hexokinase in other cells -> glucose combines with a phosphate radical

Question 7

In what tissues is glocose phosphorilation by glukokinase or hexokinase reversible?

Answer 7

iver cells, renal tubular epithelial cells, and intestinal epithelial cells. In thse cells glucose phosphatase can dephosphorylate the glucose

Question 8

Once in the cell, what two pathways can glucose take metabolically?

Answer 8

1) it can be utulised for release of energy
2) can be stored as glycogen

Question 9

Which cells are best at storing glycogen?

Answer 9

All cells can store some glycogen, however, liver (5-8%) and muscle cells (1-3%) can sote lare amounts

Question 10

What is the molecular weight of glycogen and why ?

Answer 10

The glycogen molecules can be polymerized to almost any molecular weight, with the average molecular weight being 5 million or greater. This conversion of monosaccharides into a high- molecular-weight precipitated compound (glycogen) makes it possible to store large quantities of carbohydrates without significantly altering the osmotic pressure of the intracellular fluids. High concentrations of low-molecular- weight soluble monosaccharides would play havoc with the osmotic relations between intracellular and extracel- lular fluids.

Question 11

What is glycogenesis and glycogenolysis?

Answer 11

Glycogenesis - formation of glycogen from excess glucose
Glycogenolysis - breakdown of stored glycogen to reform glucose

Question 12

What is the role of phosphorylase in glycogen metabolism?
How is the function of this enzyme controlled?

Answer 12

The breakdown of glycogen into glucose occurs through severeal phosphorulation steps caused by phosphorylase.

Phosphorylase is usually innactive and has to be activated by cyclic-AMP which is formed by hormones such as glucagon or epinepherine

Question 13

What is the role of epinepherine in energy metabolism?

Answer 13

with increased sympathetic tone, epinepherine is released from the adrenal medulla, where it forms intracellular cyclic-AMP which activated phosphorylase which kicks off glycogenolysis. This is essentiial in the muscle and liver to promote glucose formation which is needed in the flight and fight response

Question 14

What is the role of glucagon in the production of energy ?

Answer 14

When glucose levels drop too low glucagon is released by the alpha-islets of the pancreas. This stimulates cyclic-AMP production intracellularly which activated phosphorylase inducing glycogenolysis

Question 15

How many moles ATP are produced by one mole of glucose

Answer 15

38

Question 16

What is glycolysis ?

Answer 16

The most important mean of releasing energy from glucose. This consists of 10 steps which result in the production of 2 pyruvic acid molecules for each molecule of glucose. Each pyruvic acid molecule will then enter its own Krebs cycle.

Question 17

What is the transition step between glycolysis and Krebs?

Answer 17

The two pyruvate molecuse bind to Coenzyme A to form acetyl-coa which can then enter the citirc acid (aka Krebs) cycle.

Question 18

what is the tricarbxylic acid cycle?

Answer 18

the same things as citric acid or krebs cycle

Question 19

What is the krebs cycle and where does this occur?

Answer 19

The citric acid cycle is a sequence of chemical reactions in which the acetyl portion of acetyl-CoA is degraded to carbon dioxide and hydrogen atoms. These reactions all occur in the matrix of mitochon- dria. The released hydrogen atoms add to the number of these atoms that will subsequently be oxidized through the electron transport chain

Question 20

How much hydrogen is produced in the krebs and glycolysis pathways and what happens to this ?

Answer 20

4 hydrogen atoms during glycolysis, 4 during formation of acetyl-CoA from pyruvic acid, and 16 in the citric acid cycle; thus a total of 24 hydrogen atoms are released for each original molecule of glucose. they are released in packets of two, and in each instance, the release is catalyzed by a specific protein enzyme called a dehydrogenase. Twenty of the 24 hydrogen atoms immediately combine with nicotinamide adenine dinucleotide (NAD+),

Question 21

Why is niacin an essential vitamin ?

Answer 21

because NAD+ is a derivative of this

Question 22

What is the electron transport chain and where is it ?

Answer 22

IT is in the mitocondria and does it do;
(1) split each hydrogen atom into a hydrogen ion and an electron and (2) use the electrons eventually to combine dis- solved oxygen of the fluids with water molecules to form hydroxyl ions. Then the hydrogen and hydroxyl ions combine with each other to form water. During this sequence of oxidative reactions, tremendous quantities of energy are released to form ATP.

Question 23

How is ADP converted to ATP in the ETC?

Answer 23

ATP synthase, which protrudes through the mitocondrial membrane oxidatively phosphorylates the the ADP to ATP

Question 24

What are the main factors that control the quantity of ATP that is produced by the ETC?

Answer 24

1) [ATP and ADP]
- high [ATP] causes phosphofrutokinase inhibition - glycolysis rate reduction. Vice versa for ADP
2) citrate ion in the krebs cycle; an eccess inhibits phosphofrutokinae preventing glycolysis
3) Excess ATP concentration will prevent further formatio until the store is depleted (except with strenous exercise)

Question 25

Why is anaerobic metabolism not preferentially used?

Answer 25

This process is extremely wasteful of glucose because only 24,000 calories of energy are used to form ATP for each molecule of glucose metabolized, which represents only a little over 3 percent of the total energy in the glucose molecule.

Question 26

Which of the following 3 can function in an anaerobic environement?
1) glycolysis
2) Krebs
3) ETC

Answer 26

Glycolysisas the formation of 2 pyruvate does not need O2

Question 27

How is the anaerobic metabolism pathway kicked in?

Answer 27

Krebs and ETC cannot work without O2. At the end of glycolysis you end up with 2x pyruvate + 2 H+. These build up in concentration and can rate limit glycolisis through phosphofructokinase. When their concentration accumlates excessively the end products react with each other forming lactic acid

Question 28

What is the benefit of lactic acid?

Answer 28

Lactic acid diffuses readily out of the cells into the extracellular fluids and even into the intracellular fluids of other less active cells. Therefore, lactic acid represents a type of “sinkhole” into which the glycolytic end products can disappear, thus allowing glycolysis to proceed far longer than would oth- erwise be possible.

Heart muscle is especially capable of converting lactic acid to pyruvic acid and then using the pyruvic acid for energy. This is beneficial in heavy exercise as large volumes of lactic acid are produced which can be used by th e heart for energy and is a salvage mechanisms during infactation

When a person begins to breathe oxygen again after a period of anaerobic metabo- lism, the lactic acid is rapidly reconverted to pyruvic acid and NADH plus H+. Large portions of these substances are immediately oxidized to form large quantities of ATP. This excess ATP then causes as much as three fourths of the remaining excess pyruvic acid to be converted back into glucose.

Question 29

Other than glycolysis which other important first step pathway exists for energy metabolism;

Answer 29

Pentose phosphate patheway (phosphocluconate pathwy) which is responsibe for 30% of the glucose breakdon in the live and even more in the fat cells

Question 30

why does the pentose pathway exist?

Answer 30

This pathway is especially important because it can provide energy independently of all the enzymes of the citric acid cycle and therefore is an alternative pathway for energy metabolism when certain enzymatic abnormalities occur in cells. It has a special capacity for providing energy to multiple cellular synthetic processes.

Question 31

What key differece exists in hydrogen untilisation in the pentose phosphate pathway compared to glycolysis?

Answer 31

The hydrogen released during the pentose phosphate cycle does not combine with NAD+ as in the glycolytic pathway but combines with nicotinamide adenine dinucleotide phosphate (NADP+), which is almost identical to NAD+ except for an extra phosphate radical, P. This difference is extremely significant because only hydrogen bound with NADP+ in the form of NADPH can be used for the synthe- sis of fats from carbohydrates - hence why this pathway is common in fat cells

Question 32

What happens to the pentose pathway when glycolysis is downregulated due to cellular innactivity ?

Answer 32

he pentose phosphate pathway remains operative (mainly in the liver) to break down any excess glucose that continues to be transported into the cells, and NADPH becomes abundant to help convert acetyl-CoA, also derived from glucose, into long fatty acid chains. This is another way in which energy in the glucose molecule is used other than for the formation of ATP—in this instance, for the formation and storage of fat in the body.

Question 33

What is excess glucose stored as?

Answer 33

Glucose is prefer- entially stored as glycogen until the cells have stored as much glycogen as they can—an amount sufficient to supply the energy needs of the body for only 12 to 24 hours.
When the glycogen-storing cells (primarily liver and muscle cells) approach saturation with glycogen, the addi- tional glucose is converted into fat in liver and fat cells and is stored as fat in the fat cells.

Question 34

what is gluconeogensis ?

Answer 34

When the body’s stores of carbohydrates decrease below normal, moderate quantities of glucose can be formed from amino acids and the glycerol portion of fat. This process is called gluconeogenesis.

Question 35

During prolonged fasting which processes mantain glucose levels?

Answer 35

Gluconeogenesis breaks down fats. kidneys also synthesize considerable amounts of glucose from amino acids and other precursors.

About 60 percent of the amino acids in the body proteins can be converted easily into carbohydrates; the remaining 40 percent have chemical configurations that make this conversion difficult or impossible.

Question 36

What is the role of corticotropin and glucocorticoids on gluconeogenesis?

Answer 36

When normal quantities of carbohydrates are not available to the cells, the adenohypophysis, for reasons not completely understood, secretes increased quantities of the hormone corticotropin. This secretion stimulates the adrenal cortex to produce large quantities of glucocorticoid hormones, especially cortisol. In turn, cortisol mobilizes proteins from essentially all cells of the body, making these proteins available in the form of amino acids in the body fluids. A high proportion of these amino acids immediately become deaminated in the liver and provide ideal sub- strates for conversion into glucose. Thus, one of the most important means by which gluconeogenesis is promoted is through the release of glucocorticoids from the adrenal cortex.

Question 37

What is the normal Bg of a person?

Answer 37

90mg/dl during fasting, 140mg/dl after a large meal.