Define digestion. What are the three types of digestion
Digestion is the breaking down of macromolecules into smaller components so they can be absorbed in the lumen of the GI tract
1. Luminal- enzymes in the lumen of the GI tract
2. Membrane or contact- Digestive enzymes synthesize by entercytes in the brush border of the GI lumen.
3. Cytosol- intracellular cytosolic peptidases break down absorbed di/tri peptides into single AA acids
Define absorption and how does it occur
Permeation of molecules from the lumen of the GI tract, through the epithelial cell (enterocyte) layer, into the blood and lymph vascular compartment.
• Occurs via diffusion, facilitated or active transport mechanisms across apical membrane.
• Enterocytes release absorbed nutrients into blood (e.g. amino acids, sugar) or lymphatic circulation (e.g. lipids) for delivery to sites of metabolism/utilization.
Two common secondary transport in the GI tract
Active transport through secondary
Antiport- Na/Ca exchange
Sites of absorption what are unique features of the SI
Features that increase the SA (absorptive surface) such as circular folds, villa, and microvilli. These can increase the SA by a factor of 600 compared to the simple tube of the small intestine.
Villa icnrease by 30x so imagine the damage of Celiacs and the inability to absorb
Where are nutrients absorbed? Mostly in what specific part
ONLY in the SI
Majority of the carbds, proteins and lipids are in the Duodenum then the jejunum and least amount inte ileum
Where is bile acids absorbed? mainly where?
Throughout the SI and LI
Mostly actively in the ileum and the uporcessed parts in the proximal part of the colon
Breakdown of total amount of water intake
2 liters through diet
7 liters in GI sections
Lose 100mL (5%) through feces
SO the average absoprtion is about 9L
How is water absorbed?
Generally passive and determiend by the differenes in osmolarity. (low to high osmolarity)
In the colon water can be absorbed in the absense or even against a gradient b/c the LI establishes a local gradient by extruding Na into thebasolateral space (blood) (Cl- follows for electronegative balance) so a gradient is established and water follows into the intercellular space .
How does water move between the GI and circularoty system?
Either through transcellular(through epithelial cells) or paracellular(between epithelial cells/tight junctions)
Tight junctions in the GI tract are leaky and allow water and ions through (paracellular)
Solvent drag (paracellular)- large columes of water absorbed from lumen to blood with small solutes
Acitvely in the jejunum ileium and colon
K+ absorption and Cl-
K+- Mainly passively
Cl- absorbed througtout
HCO3--Absorbed in jejunum and secreted in ileum and colon
Net absorption of SI and Colon
SI- net absorbtion of water, Na, CL, K ad net secretor of HCO3
LI- net absorb og water, Na+, Cl-, but is a net secretor of K+ and HCO3
Dicuss calcium absorption
Calcium is Actively in the Duodenum and passively in the jejunum and ileum (ileum has the least absorption of calcium)
Active is transcellular- calcium enters cytosol of enterocytes and binds Ca to protein (calbindin) and they it is released through the Ca-ATPase out of the cell into circulation-- this process is facilitated by Vitamin D. When low plasma Ca2+ Parathyoid hormone is converted and 25 Dhydrocy Vitamin D converted into 1-25 hydroxy vitamin D which stimulates the calium bindign proteins and Ca2+ ATPase
Passive is through paracellular (no cytosol) not under Vitamin D just dependent on the gradient
Discuss absorption of magnesium
Active process is in the ileum but poorly described
Minor passive (paracellular)
Discuss iron absroption
Too low can lead to anemia
Excessive absoprtion of iron through idiopathetic hemochromatosis
2 forms of iron 1/ Heme compound and 2. Non-heme
1. Heme compound comes from meats and is absorbed by faciliated transport
2.Non-heme iron- either Fe3+ (insoluble) or Fe2+-- the apical membrance has na iron reductase that convert the 3+ intot he 2+ making it absorbable. THe iron doesn't just diffuse it has Divalent Metal Transporter I from the lumen to the cytosol
organic substances needed in small quantities for normal metabolic function, growth and maintenance of the body
Which ones are fat and water soluble? And discuss the difference in absorption
Fat (have to come from diet)- A, D, E, and K-- absorbed by diffusion passively in the SI and make micelles
Water- C, B1, B2, B6, B12, biotin, choline folic acid and inositol-- absorbed in the SI by Na-dependent or faciliated mechanisms
Absorption of Vitamin B12
Gastric phase- B12 in binds to R proteins protected from the acidic stomach and an intrinsic factor.
Once it enters the SI the R protein is degraded but the intrinsic factor transfer it to the ileum
Intertinal Phase- the Vitamin B12 complex is absorbed at the terminal ileum via IF-B12 receptor