GIT Absorption (GIT) Flashcards
What are the two routes of absorption of molecules? Describe them.
Transcellular route: Through two cell membranes, apical and basolateral. Often uses active transport on the basolateral membrame creating a gradient, and open transporters on the apical membrane. Generally uses Na+ to create gradient.
Paracellular route: Through gaps in cells, passively. Permeability depends on structure of tight junctions: Tight TJs in colon, leaky TJs in the small intestine.
What are carbohydrates broken down into?
Glucose, galactose and fructose (monosaccharides)
Give two examples of glucose transported in the small intestine.
SGLT1 - sodium/glucose cotransporter 1
GLUT5 - facilitated glucose/fructose transporter 5
In the absence of Na+ transport, no glucose can be absorbed.
What digetsion products of proteins are absorbable?
How are amino acids taken up?
Amino acids and di-/tri-peptides are absorbable.
Multiple amino acid transporters are present on the apical membrane. They can also be taken up via Na+ co-transport.
This increases AA concentration inside cells, allowing them to diffuse out through transporters on the basolateral membrane.
What are the three classes of lipids and how do enzymes digest them?
- Triglycerides
- Phospholipids
- Cholesterol
Pancreatic lipase (MAJOR) breaks down lipids into fatty acids and monoglycerides after they have been emulsified by bile .
Describe the process of fat absorption and transport around the body.
Fats CAN diffuse through the lipid bilayer but it is very slow.
We use transporters: Fatty acid translocase (FAT/CD36). Once inside the cell they are packed into lipoprotein complexes to be distributed. LIpids are loaded into chylomicrons and taken to the liver.
Where is most water absorbed?
Give average values for consumed, secreted, reabsorbed and excreted.
Small intestine.
2000mL consumed
7000mL secreted
8800mL reabsorbed
200mL excreted.
Can we actively transport water?
No - must be via osmotic gradient.
Describe the absorption of Na+ in different regions: Small intestine and colon.
Absorption of Na+ is driven by basal Na+/K+ ATPase. Keeps cytoplasmic [Na+] low.
In regions where we are absorbing fats/proteins/carbs, we use glucose/amino acid co-transporters for Na+.
In the colon, we use Na+ channel diffusion.
Describe the absorption and secretion of chloride ions.
Chlored is absorbed via passive diffusion, or through Na+/H+ and Cl-/HCO3- exchange
Chloride is secreted through Cl- channels, such as CFTR
Explain how cystic fibrosis impacts chloride secretion.
Cystic fibrosis = mutations in CFTR gene that codes for the CFTR channels.
Inability to secrete chloride = can’t draw out enough water, makes mucus very thick and clogs up your lungs.
How is potassium absorbed and secreted?
Absorption is via paracellular diffusion or active K+/K+ exchange.
Secretion is via passive secretion or apical K+ channel powered basal Na+/K+ pump.
Describe the absorption of iron (hint: eaten in two forms)
Haem iron - iron in meat (blood, muscle) bound to haem. MOST READILY ABSORBED FORM.
Non-haem iron - iron in veggies. Ferrous (Fe2+) or ferric (Fe3+). Ferric iron is completely insoluble. Ferrous (Fe2+) is abosrbed through divalent metal transporter (DMT).
