Nutrient Digestion Flashcards
Four Functions of the Gut
Digestion
Absorption
Secretion
Motility
Monosaccharides Examples
Hexose Sugars (6C) Glucose, Fructose, Galactose
Disaccharides and their monomer constiuents)
2 hexose sugars linked together by a glycosidic bond
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Lactose
glucose and galactose
Sucrose
(glucose and fructose)
Maltose
(glucose and glucose)
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Polysaccarides
Complex Carbohydrates
Starch ( plant storage of glucose)
Cellulose (plant cell walls)
Glycogen (animal storage of glucose)
What breaks down Starch
Amylases
Where is amylase produced and what is it’s action
The pancreas and the salivary glands. They hydrolyse α-1,4 glycosidic bonds, to break starch into it’s monomer constituents
How is cellulose broken down?
Mammals do not produce the necessary enzymes but some of our gut bacteria does.
Indigestible cellulose = fibre.
Where are carbs absorbed ?
Carbohydrates are absorbed by the columnar epithelial of the small intestine.
They travel via transcellular transport
How do carbs pass the apical layer of the columnar epithelial
SGLT - Utilises the Na+ gradient to passively transport glucose (&galactose, GLUT-5 transports fructose by the same mechanism)
How do carbs pass the basal-lateral layer of the columnar epithelial
Two Components
A) Na/K ATpase - (active transport) maintains the osmotic gradient between the lumen and the ICF
B) GLUT-2 - facilitated diffusion across the basal lateral membrane (fructose, glucose and galactose)
Define ParaCellular Transport
Transfer of substances between cells. Water in carbohydrate absorption travels through the tight junction between cells,. The Na+ gradient means water moves across the intestinal lumen into circulation
Name the enzymes groups that break down proteins
Proteases (endopeptidases)
Peptidases
Action of Proteases
Proteases hydrolyses internal peptide bonds to break larger chains into smaller fragments.
Metabolism of Proteases
Proteases are secreted as proenzymes called Zymogens by epithelial cells in the head of the pancreas, stomach and intestine.
They are activated in the GI tract.
Common Proteases
Pepsin (stomach)
Trypsin
Chymotrypsin (pancreas)
Action of Peptidases
Peptide hydrolyses of terminal amino acids.
Absorption of Amino Acids through the apical surface and basal lateral surface
Apical: SAAT1 (sodium, amino acid transporter)
Basal/Lateral: Facilitated diffusion through a transport protein.
Na/K+ maintains Na+ gradient.
Absorption of Dipeptides through the apical and basal/lateral membrance
Apical: PepT1 (paired hydrogen/dipeptide transporter)
NHE3: The hydrogen ion is returned to the lumen via a Na/H transporter.
Unknown how dipeptide crosses basal/lateral membrance
Structure of Triglycerides
Condensation reaction between glycerol and three long-chain fatty acids.
Location of fat digestion
Fat digestion takes place in the small intestine via pancreatic lipase.
Describe Lipid Digestion
- Fatty Droplets suspended in water due to insolubility in water. Lipase can only act on the external surface.
- Emulsification breaks down the fatty droplets into smaller globules. Increasing surface area for the action of lipase
- Micelle Formation (smaller)
Describe Lipase
Water soluble enzyme. Secreted by the pancreas. Breakdowns fat into constituents.
Requirements of Emulsification
1) Mechanical Disruption - Smooth muscle contraction of the intestine mix the luminal contents
2) Emulsifying agent - bile salts and phospholipids amiphatic molecules, secreted by the gallbladder
Lipid Absorption
Micelles breakdown at the cell membrane and lipids diffuse across the cell membrane.
Journey of lipids inside the cell
- FFA & Monoglycerides, diffuse across the cell membrane.
- In the sER, the absorbed constituents are reformed to triglycerides and emulsified.
- The emulsified triglycerides travel trough the vesicles of the sER to the golgi apparatus.
- The golgi apparatus exocytoses into the ECF at the serosal membrane as chylomicrons.
- Chylomicrons pass into the lacteals, which drain into the thoracic duct before going into the IVC
Define Chylomicrons
Extracellular fat droplets, also containing phosolipids, cholesterol and fat soluble vitamins ~1um.
What are lacteals
Lymphatic capillary beds of the epithelia.
Absorption of Fat Soluble Vitamins
Absorbed in the same way as fats - A D E K
Absorption of Water Soluble vitamins ex
B12 - binds to intrinsic factor (secreted by the stomach) and is absorbed by a transport protein in the ileum
Iron Absorption
Transported across the ‘brush border membrane’ via DMT1.
Fate of Iron
- Stored with Ferritin as a intracellular iron store
2. Transferred to the blood, and binds to transferrin.
Hyperaemia
Increased Ferritin levels cause more iron to be bound to the entreocytes
Anaemia
Decreased ferritin levels cause more iron to be released into the blood
Pernicious Anaemia
B12 deficiency resulting in failure of red blood cell maturation
