TOPIC 7 CARBOHYDRATES OBJs Flashcards
- Identify the major sites of digestion and absorption in humans
a. Mouth (limited)
b. Stomach (some)
c. Small intestine (bulk)
Wheres does the bulk of carbohydrate digestion and absorption occur
Small intestine
Ceullose source, subunit and bonds
plant
B-glucose
1-4
Starch source, subunit, bonds
Amylose
Plant
a-glucose
1-4
Amylopectin
a-glucose
1.4 , 1,6
Glycogen source, subunit, bonds
Animal
a-glucose
1,4 and 1,6
Carbohydrate digestion occurs by break of the ____ by enzymes called _________
glycosidic bonds
endoglycosidases
Digestion of glycogen, starch and cellulose in humans
The mouth
- α-amylase, secreted from the parotid salivary glands, acts on starch and glycogen
- Breaks α -1-4 glycosidic bonds
- Digestion is limited – amount of time spent there
The stomach
o Salivary amylase is inactivated due to low pH
o No digestion here!
o Oligosaccharides pass into small intestine
The small intestine
o Pancreas releases pancreatic α-amylase
o Digests the oligosaccharides into disaccharides
Cellulose?
o Cellulose (β1-4) cannot be broken down in most animal guts
o In humans, cellulose passed out via the faeces undigested, as we lack the enzyme capable of breaking the bond
What are sucrose, lactose, maltose and isomaltose
disaccharides
Carbohydrates from starch and glycogen breakdown digests into
disaccharides
what can be absobed by small intestine
monosaccharide
Mastose goes to ________ by what enzyme
2 glucose by maltase
isomaltose goes to _____ by what enzyme
2 glucose by isomaltase
Sucrose goes to _____ by what enzyme
glucose and fructose by sucrase
lactose goes to _____ by what enzyme
galactose and glucose by lactase
disacchardies are associated with
brush border epithelial cells of small intestine
- Explain how glucose, fructose and galactose are absorbed in the small intestine
- SGLT-1 – absorbs glucose and galactose
- GLUT-5 – facilitated transporter, uptake of fructose
- All three are transported into circulation through GLUT-2
- Distinguish between the roles of glycogen in animal skeletal muscle and liver cells
- In skeletal muscle cells, glycogen acts as an energy store for strenuous exercise or ‘fight or flight’ response
- In liver cells, glycogen is primarily a store of glucose to provide to body cells when blood glucose is low
- Identify the reducing and non-reducing ends of a glycogen molecule and relate the non-reducing end to the sites of glycogen synthesis and degradation
- Enzymes act on non-reducing ends for synthesis and degradation
- Having many means can be rapid
- Define the roles of the branching and debranching enzymes and glycogenin
- Branching enzymes: forms the α-1,6 bonds
- Debranching enzymes: breaks or makes the α-1,6 bonds
- Glycogenin: can initiate glycogenesis when no glycogen molecules are available
- Explain the importance of branching in glycogen
- Two main roles:
1. Increases solubility of glycogen
2. Creates more non-reducing ends, which are the reactive sites upon which phosphorylase and glycogen synthase act. - Therefore, increases the rate synthesis and degradation
What is glycogen
- Glycogen is a polymer of glucoses bonded by glycosidic bonds (have branches too which make it easier to add and take)
- Glycogenesis high in fed state
- Liver and skeletal muscles
Glycogenesis
glycogen synthesis
4 steps of glycogenesis
1) Attaching uridine diphosphate (UDP) to glucose
- Phosphoglucomutase
- UDP-glucose phosphorylase
2) Creating glycogen,
- Glycogen synthase catalyzes the bonding of the glucose of one UDP-glucose to another, forming an alpha 1,4 glycosidic bond
- Glycogenin
3) Adding branches to glycogen molecule or polymer
- Branching enzyme
- Glycogen synthase
Glycogenolysis
Glycogen degradation
Glycogenolysis steps
1) Starts with the branches
- Glycogen phosphorylase cleaves alpha 1,4 glycosidic bonds, and catalyzes transfer of a phosphate to the free glucose – releasing 1 glucose-1-phosphate molecule at a time, does this until 4 glucose made
- Debranching enzyme cleaves 1,6 bonds and they are added back to 1,4 linear one, extending it & can also cleave the 1,6 releasing a free glucose
-
2) Conversion to **glucose-6-phosphate **by **phosphoglucomutase **
Liver:
o Has glucose-6-phosphatase, which removes the phosphate and released into the blood
Skeletal muscle:
o DOES NOT have glucose-6-phosphatase, so it sends glucose-6-phosphate into glycolysis to release energy
- Explain the cAMP-dependent pathway by which adrenaline and glucagon trigger the breakdown of glycogen
- They bind to G-coupled protein receptor which activates adenylyl cyclase which converts ATP to cAMP
- cAMP activates protein kinase A, which adds a phosphate to glycogen phosphorylase kinase , activating phosphorylase, increasing glycogen breakdown and decreasing glycogen synthesis by glycogen synthase
Glycogen synthase
Makes glycogen
Insulin increases this because its increasing the stores
Glycogen phosphorylase
Removes phosphates
Glucagon increases this to release glucose
To access stored energy from macromolecules they need to be
digested and absorbed
plant carbohydrates
starch and cellulose
animal carbohydrates
glycogen, disac lactose and succrose
glycogen is a
major glucose storage in liver
Digestion of carbohydrate in mouth
a-amylase secreted from parotid salivary glands acts on starch and glycogen, breaking the a 1-4 glycosidic bonds, results in oligosaccharide
- amylose, amylopectin or glycogen
what is oligosacharide
small chain of glucose joined together
Carbohydrate digestion in stomach
none
Carbohydrate digestion in small intestine
Pancreas releases pancreatic a-amylase
oligosaccharides are digested to disaccharides
also by cleaving a-1-4 glycosidic bonds
- maltose and isomaltose result
What about cellulose
cellulose (b1-4) cannot be broken down by body enzymes
disaccharidases act where
small intestine
disaccharidases are associated with the
brush border epithelial cells
how do monosaccharides occur
disaccharidases cleave disaccharides into monosaccharides and now they can be absorbed
monosaccharides
glucose
fructose
galactose
how is glucose absorbed
Na+/glucose symporter
Na+/glucose symporter is an example of
secondary active transport
glucose transport in humans is by _____ systems
GLUT
what does the body do with excess glucose
stored as glycogen or fat
Glycogen storage occurs in what state and where
fed state
liver and skeletal muscle
what triggers glycogen breakdown
adrenaline (fight or flight) or low blood glucose (glucagon)
