Carbohydrate Classification, Structure, Digestion, Absorption, and Metabolism Flashcards
What are the monosaccharides?
Glucose, galactose, fructose
Name 3 common disaccharides
Sucrose (1 glucose and 1 fructose)
Maltose (2 glucose moieties)
Lactose (1 glucose and 1 galactose)
Are complex carbs water soluble or insoluble?
Insoluble with high molecular weights.
What are oligosaccharides?
Polysaccharides containing fewer than 10 glucose units
What are the straight chains in starch called?
Amylose
What are the branched chains in starch called?
Amylopectin
How do glucose and galactose enter enterocytes?
Through sodium-glucose transporter 1 (SGLT1), an adenosine triphosphate (ATP)-dependent active transporter
Which protein transports fructose?
GLUT5 (glucose transporter 5)
What are the 2 classes of glucose transport proteins?
The sodium dependent glucose transporters and the facilitative transporters
Describe the function of facilitative transporters
Function as passive diffusions channels (channels dependent on the gradient of glucose concentration between extra- and intracellular compartments) and do not require sodium or ATP. Present in many different tissues
Describe the function of sodium dependent glucose transporters
Involved in the active, ATP-dependent absorption of glucose from the intestinal lumen into cells. Also function in the reabsorption of filtered glucose in the proximal tubule of the kidney
Where are the highest concentrations of GLUT1?
Endothelial (major part of the blood-brain barrier) and glial cells of the brain
Which monosaccharides are transported by GLUT1?
Glucose and galactose
Where is GLUT2 mainly located?
Liver, small intestine, and kidneys
Describe the action of GLUT2 in the liver (1), kidneys (2), small intestine (3)
- In hepatocytes, GLUT2 has a low affinity but high capacity for glucose, allowing quick equilibration of intracellular and extracellular glucose concentrations across the membrane during fasting and fed states.
- GLUT2 transports the reabsorbed glucose back into the bloodstream from proximal tubular epithelia.
- GLUT2 is important in the transepithelial transport of the absorbed glucose
Where is the highest concentration of GLUT3?
The neuronal membranes of the brain
Describe the action of GLUT3 in neuronal cells
Controls the rate of glucose entry into neuronal cells
Where is GLUT4 mainly located?
Adipose (brown and white) and muscle (skeletal and cardiac) tissues. Mainly within the intracellular vesicles of these tissues but can be translocated to the cell surface in response to a rise in plasma insulin concentrations or muscle contraction
Where is GLUT5 mainly located?
The apical membrane of the jejunum
What is the formula that expresses the net chemical reaction of glucose catabolism?
C6H12O2 + 6O2 –> 6CO2 + 6H2O + Energy
Where in the cell does glycolysis take place?
The cytoplasm
Where in the cell does the TCA cycle take place?
Inside mitochondria
Glucose absorption into which organs is dependent on insulin? Independent?
Skeletal muscle and adipose tissue glucose absorption is insulin dependent.
Liver, kidney, brain, other tissue glucose absorption is insulin independent.
Name the substrates and major sites of expression for GLUT1
Substrates are glucose, galactose, mannose, and glucosamine
Major sites of expression are erythrocytes, CNS, blood-brain barrier, placenta, fetal tissues in general
Name the substrates and major sites of expression for GLUT2
Substrates are glucose, galactose, fructose, mannose, glucosamine
Major sites of expression are liver, beta-cells of pancreas, kidney, small intestine
Name the substrates and major sites of expression for GLUT3
Substrates are glucose, galactose, mannose, xylose, dehydroascorbic acid
Major sites of expression are brain (neurons), spermatozoa, placenta, preimplantation embryos
Name the substrates and major sites of expression for insulin dependent GLUT4
Substrates are glucose, glucosamine, dehydroascorbic acid
Major sites of expression are muscle, heart, brown and white adipocytes
Name the substrates and major sites of expression for GLUT5
Substrates are fructose, but not glucose
Major sites of expression are intestine, kidney, brain, skeletal muscle, adipose tissue
The net chemical reaction of glucose catabolism results in:
An equal number of moles of CO2 and oxygen, creating a respiratory quotient of 1
Glycosis converts glucose into:
Pyruvate
Where does the glycolytic pathway in the cell take place?
Cytoplasm
Approximately __ kcals are released per mole of ATP
7
What are the 3 major enzymes in glycolysis?
Glucokinase, phosphofructokinase, and pyruvate kinase
Where in the cell does the TCA cycle occur?
Mitochondria, only under aerobic conditions
What is the metabolic reaction of cell respiration?
The TCA cycle
Using the pyruvate-lactate pathway, how many ATPs can be generated per molecule of pyruvate?
2
The oxidation of 1 glucose molecule produces __ molecules of ATP total from which 2 types of reactions?
36 molecules of ATP. 4 molecules via substrate phosphorylation (2 from glycolysis and 2 from pyruvate decarboxylation) and 32 molecules via oxidative phosphorylation
Which pathway allows some organisms to survive under anaerobic conditions?
Pyruvate-lactate pathway
What are the 4 distinctive, irreversible reactions of gluconeogensis?
- Pyruvate is carboxylated in mitochondria to oxaloacetate
- The formed oxaloacetate is phosphorylated in the cytosol to phosphoenolpyruvate
- Special phosphatases hydrolyze fructose 1,6-biphosphate
- Special phosphatases hydrolyze glucose 6-phosphate
When can amino acids and fat provide carbon for gluconeogenesis?
During fasting or carbohydrate deprivation
What is the purpose of gluconeogenesis?
It is the endogenous formation of glucose and is important to maintain plasma glucose concentration in the normal range during fasting and allows constant provision of energy for metabolic needs
The body stores carbohydrate as:
Glycogen
In a 70 kg healthy person, the liver contains approximately __ gm of glycogen
100 gm (has the potential to provide 390 kcal)
Skeletal muscle contains about ___ gm of glycogen which suggests that an adult has enough glycogen to last for how long while performing normal activities?
Contains 300-400 gm glycogen, yielding less than 1560 kcal, suggesting that an adult stores enough glycogen for about a day of normal activities
The formation of glycogen is called ___, and the breakdown of glycogen is called ___
Glycogenesis
Glycogenolysis
Glycogen synthesis and glycogenolysis are regulated by the rate-limiting enzymes:
Glycogen synthase and glycogen phosphorylase, respectively
What are the primary substrates to gluconeogensis?
Lactate, glutamine, alanine, and glycerol
After 14 days of fasting, adipose tissue can provide more than what percent of daily energy requirements?
90%
After how many weeks of starvation is the brain able to adapt to using ketones as a source of energy?
1-2 weeks
Describe how stress hyperglycemia occurs?
During periods of trauma and illness, there is increased production of stress hormones such as epinephrine and cortisol, accompanied by an elevation in growth hormone and glucagon. These counterregulatory hormones oppose insulin action, causing increased glucose production by the liver (may exceed 500 gm glucose per day) and decreased utilization of glucose in peripheral tissues. Acute and chronic disease and injury can also increase production of cytokines which further contribute to hyperglycemia through stimulation of counterregulatory hormone release and suppression of insulin action
How can gastric emptying be delayed in regard to fiber intake?
Increased viscosity of gastric contents, which reduces pyloric flow
A minimum of __ gm of carbohydrates per day is required to avoid ketone production
50 gm
During moderate exercise, approximately __% of energy is provided by glucose
60%
What should be done if a pregnant patient with gestational diabetes loses weight?
If patient’s response to diagnosis of gestational diabetes is to self-starvation to avoid hyperglycemia, test urine for ketones and increase carb intake
