Metabolic stores Flashcards
Where are most carbohydrates stored and what as?
- Stored as glycogen
* Mainly in the liver and muscle
How long would a typical glycogen store last for?
3-5 hours of moderate exercise
Describe the two ways glycogen is broken down
1) 90% of the time: Pi is added forming Glucose-6-phosphate (no ATP required)
2) H20 is added forming glucose, transported out of the cell then phosphorylated in the destination using ATP
Why are the two routes of breaking down glycogen needed?
- Glucose 6-phosphate cannot easily pass through the cell membrane
- Glucose can pass through the cell membrane
What is the major form in which fuel is stored in the body and why?
Lipids
• Weight for weight they release about six times the amount of energy as glycogen
What are triglycerides?
Esters of glycerol and fatty acids
Describe lipolysis
- Lipases digest triglycerides into glycerol and 3 fatty acids
- Glycerol (from the triglyceride) becomes a glycolysis substrate producing pyruvate
- Beta-oxidation chops 2 carbon acyl units off the fatty acids
- Acyl units become acetyl CoA and can be used in the citric acid cycle
Where are the majority of fatty acids and glycerol released by adipose tissue metabolised?
The liver
What happens due to the process of fatty acids being linked to coenzyme A?
- Hydrogen and electrons are removed during fatty acid oxidation and are passed along the respiratory chain
- Indirectly produces ATP
What does the breakdown of fatty acids lead to?
The formation of ketone bodies
What are the main ketone bodies produced in the breakdown of fatty acids?
- Acetoacetate
- 3-hydroxybutyrate
- Acetone
Why are ketone bodies made?
- Oxaloacetate becomes depleted because the liver converts it to pyruvate to produce glucose
- The acetyl group therefore cannot enter the cycle (can’t bind to anything)
Why may someone’s breath smell fruity?
- If someone has been fasting for some time
- Lipids are being used as a fuel
- Ketone bodies smell fruity
How can amino acids be used in metabolism?
- Amino acids can’t be stored so used as metabolic fuel
- Some amino acids can be deaminated, yielding NH4 and a keto acid which are intermediates of the glycolytic pathway or citric acid cycle
- Others that can’t undergo transamination, passing their amino acid on
Describe transamination
- Pass amino group to a ketone acid (usually alpha-ketoglutarate)
- Resulting keto acid can be fed into glycolysis or the citric acid cycle
- Resulting amino acid can be deaminated and used similarly
What is NH4 converted into and why?
- Urea
* Urea is less toxic
What is gluconeogenesis?
Formation of glucose from non-carbohydrate sources
• Glycerol and the carbon backbone of amino acids can be fed into the pathway
Where does the bulk of gluconeogenesis occur?
In the liver
Describe lipid synthesis
- Glycerol is made from glucose through glycolysis
- Two carbon acyl units from acetyl CoA are linked together by fatty acid synthetase to form fatty acids
- Glycerol plus 3 fatty acids make a triglyceride
Where are the majority of triglycerides stored?
In adipose tissue
Why would endurance athletes carbo load before an event?
- Lipids are not as effectively used as carbohydrate
* Energy production would therefore not be as high when we are using lipids as fuel rather than carbohydrate
How can we get amino acids?
- Some provided in the diet
* Some made by transferring amino groups to keto acids
What is the largest protein source?
Muscle
What fuel sources can the brain use and why?
- Normally glucose (so needs a constant supply in the blood)
- In times of starvation: ketone bodies (mainly acetoacetate and 3-hydroxybutyrate) formed in other tissues
- Lipids cannot cross the blood brain barrier
Which fuel sources can skeletal muscle use?
- Glucose
- Fatty acids
- Ketone bodies
Which fuel does resting muscle use?
• Fatty acids
Why can’t the brain or muscle transport glucose?
They can’t convert glucose-6-Phosphate or glycogen to glucose
What happens when muscle is active?
- Rate of glycolysis outpaces the citric acid cycle
- Pyruvate is converted to lactate
- Lactate passes into the blood and to the liver where it is converted back to glucose
How does the liver take some of the metabolic burden away from muscle?
- In transamination, many amino acids pass their amino group to pyruvate forming alanine
- Alanine is transported in the blood to the liver where it can be converted back to pyruvate which is used to synthesise glucose (gluconeogenesis)
what makes up adipose tissue?
• Adipocytes, each of which has a fat droplet that occupies most of the cell
What is the main function of adipose tissue?
- Synthesize and store triglycerides
* Release fatty acids and glycerol in times of need
What determines the rate at which fatty acids are esterified?
The availability of glucose
What hydrolyses triglycerides?
• Lipase (activity is hormone sensitive)
What ensures the liver has ready access to nutrients absorbed by the gut?
Hepatic portal system
What can the liver make new glucose from?
- lactate
- Alanine
- Glycerol
When fuel supplies are available, what does the liver do?
- Synthesizes fatty acids
- Esterifies them
- Secretes them into the blood as very low density lipoprotein (VLDL)
What does the liver do in fasting?
Generates ketone bodies for use in other tissues
