Carbohydrate Metabolism Flashcards
Body’s energy sources
Food intake
- carbs, fats, proteins
Metabolism
Why do cells need energy?
- survival/homeostasis
- growth
- reproduction
- repair
- movement
Adenosine triphosphate
Main energy source
- present in cytoplasm and nucleoplasm of all cells
- combo of adenine, ribose, and 3 phosphate radicals
- obtained by CHO, protein, and fat
ATP break down
Breaking of phosphate molecule from ATP causes a large energy release, which powers the muscle
- result = adenosine di-phosphate and an unattached phosphate molecule
Why does ATP provide so much energy?
Removal of each of the last 2 phosphate radicals liberates 12,000 calories of energy
= high energy bonds
ATP is used for _____, and ADP + Pi is used for _____
Energy utilization; energy production
Kinases
Adds a phosphate
- ex: glucokinase and hexokinase
Phosphatases
Removes a phosphate
- ex: glucose phosphatase
Phosphorylase
Spills a compound by adding phosphate
- similar to hydrolysis, but uses phosphate instead of water
_____ of carbs used by the body are used for ATP formation
90% or more
- final products of CHO digestion in the gut = fructose, galactose, and glucose (80%)
What is the final common pathway for transport/supply of CHO to all tissues?
Glucose
- after absorption from GIT, most of fructose and galactose are rapidly converted to glucose in liver via glucose phosphataes
Fate of glucose in the liver: option 1
CHO processed in liver –> glucose formation –> transport out of liver to needy tissues for energy use
- glucose has to go thru cell membrane
- then goes thru reactions inside the cell to produce ATP (glycolysis, citric acid cycle)
- H atoms that are released concurrently yield even more ATP (glycolysis, citric acid cycle)
Option 2
CHO processed in liver –> glucose formation –> glycogenesis (energy storage)
- glycogen is simply a large polymer of glucose: liver and muscle glycogen
Option 3
Glycogenolysis in the liver
- breakdown of stored glycogen in times of energy needs
Can glucose freely move into the cell?
No, its too large
- requires facilitated diffusion via carrier proteins in the cell membrane
- glucose transporter (GLUT) on outside and releases it intracellularly
- glucose moves from area of high concentration to low concentration
Insulin
Anabolic hormone secreted by pancreatic beta cells
- speeds up process of facilitated diffusion = increased efficiency and efficacy
- rate of CHO utilization by most cells is controlled by rate of insulin secreation
What cells do not require insulin for glucose transport?
RBC, brain, liver
GLUT transporters
GLUT 1 - RBCs GLUT 2 (most common) - liver - pancreas - GIT - kidney GLUT 3 - brain GLUT 4 - muscle - fat
First step after glucose is in the cell
It has to be “locked” in the cell
- phosphorylation via glucokinase (liver) or hexokinase (other tissues)
- glucose —-> glucokinase + ATP —> glucose 6 phosphate
Glycolysis
Splitting of glucose to form 2 pyruvic acid molecules
- occurs via 10 successive chemical reactions inside the cell
- does not require oxygen, but mostly occurs in the presence of oxygen
- process starts with glucose to create 2 pyruvic acid molecules = 2 ATP used, 4 ATP made and 4 H atoms are released*
Phophofructokinase (PFK)
Converts fructose-6-phosphate to fructose-1,6-diphosphate
- ATP inhibits PFK, while ADP activates PFK
Step 2
Citric acid cycle
- acetyl CoA is degraded into CO2 and H atoms in the mitochondria
- requires oxygen
- for each molecule of glucose, 2 acetyl CoA + H2O molecules enter TCA cycle
Citric acid cycle products
- 4 CO2
- 16 H atoms
- 2 coenzyme A
- 2 ATP (1 ATP per acetyl CoA molecule that enters cycle)
How many H atoms have been formed after step 2?
24
- most combine with nicotinamide adenine dinucleotide (NAD+) to form NADH+
- NADH+ will enter into multiple oxidative chemical reactions that form lots of ATP
Oxidative phophorylation
Oxidation of H atoms released from all previous stages of glucose metabolism
- forms 90% of ATP that comes from glucose metabolism
- *this process yields the most energy from glucose metabolism**
- requires oxygen!
Step 3 products
Net ATP produced per 2 H atoms = 3 ATP
- 20 H atoms enter this step
- 30 ATP are formed from oxidative phosphorylation
Recap of ATP formed so far:
- glycolysis = 2 ATP
- TCA cycle = 2 ATP
- oxidative phosphorylation = 30 ATP
- so: 34 ATP *
- 4 remaining H atoms produce 4 more ATP
- 38 ATP possible per glucose molecule!!!!*
What happens if oxygen is low?
Glycolysis can occur without oxygen (anaerobic glycolysis)
- most of pyruvic acid is converted to lactic acid, to provide energy for several minutes
Anaerobic glycolysis
Produces net gain of 2 ATP
- when oxygen becomes available again, lactic acid converts back to pyruvic acid and NADH+
= formation of ATP and re-formation of glucose
Pentose phosphate pathway
Alternative pathway for glucose
- responsible for 30% of glucose breakdown in liver and more in fat cells
- does not require same enzymes as TCA cycle
- cyclical process in which one molecule of glucose is metabolized per revolution of cycle
Pentose phosphate products
Yields:
- CO2
- H atoms
- 5 carbon sugar, D-ribulose
Glycogenesis
Glucose is stored in liver and muscle as glycogen
- any excess glucose after a meal is converted to glycogen
- any monosaccharide, including glucose, can enter into reactions that lead to glycogen formation
= glycogen is a polymer of glucose
Glycogenolysis
Breakdown of stored glycogen to re-form glucose in the cells
- occurs via phosphorylation, catalyzed by enzyme phosphorylase
- splits glucose molecules away from glycogen polymer
- phosphorylase enzyme inactive at rest, so glycogen remains stored
What activates phosphorylase?
Epinephrine
- sympathetic nervous system response
Glucagon
- triggered by hypoglycemia
