Digestion and Absorption

Define digestion

the process of breaking down complex nutrients into simple molecules

Define absorption

the process of transporting simple molecules across the intestinal epithelium

where to enterocytes in the villi live?

within a multilayer niche

what allows for the flow of chyme?

unstirred water layer

how are the molecules near the apical membrane trapped?

the mucus blends into the glycocalyx so that the two layers form a viscous coating to trap molecules near the apical membrane

what processes are involved in digestion?

the physical breakdown of nutrients (mastication, peristalsis) and the chemical breakdown of nutrients (amylase in saliva, pepsin and HCl in stomach)

mastication

chewing

peristalsis

grinding within stomach

what kinds of molecules are involved in the luminal phase?

large polymeric molecules like starch and protein

what enzymes are active in the lumen of the gut?

digestive enzymes from salivary, gastric, and pancreatic glands

what kinds of molecules are involved in the membranous phase?

small polymer molecules like polysaccharides and peptides

what happens when enzymes are active at the surface of the gut during the membranous phase?

monomeric molecules become suitable for absorption

Digestion of carbohydrates in the luminal phase involve…?

complex sugars like maltose, isomaltose, and maltotriose that have 2 or 3 glucose units, which are formed in the gut as intermediate products of starch digestion

where can dietary proteins come from?

plant or animal sources

what happens to proteins during luminal digestion?

like carbohydrates, large molecular proteins are broken down into small peptide chains via luminal digestion

what kind of luminal digestive enzymes are there for proteins?

endopeptidase and exopeptidase

where and why does luminal phase protein digestion begin?

in the stomach as HCl has hydrolytic properties and activates zymogens—gastric pH=1.3, which is beneficial for optimal pepsin activity (especially digestion of collagen-rich connective tissue)

where and how is luminal phase digestion completed?

in the small intestine by the action of pancreatic enzymes

what happens in the membranous phase digestion of peptides?

peptidases on enterocyte surface membrane extend into the glycocalyx, then the enzymes hydrolyze the peptide products of luminal phase protein digestion, yielding free amino acids

Is the digestion for the membranous phase complete? What happens that can affect this process?

Even though digestion is sometimes incomplete, dipeptides and tripeptides can also be directly absorbed and then get hydrolyzed intracellularly

Define intestinal absorption

it is the movement of the products of digestion across the intestinal mucosa and into the vascular system for distribution

primary active transport

ATP used to move ions across gradients (Na+ K+ - ATPase): keeps interior of cells electrically negative, and decreases sodium concentration in the cells

secondary active transport

Co-transport of 2Na+ + glucose; also see a Na+/H+ antiport that also uses that Na+ electrochemical gradient

tertiary active transport

Na+/K+ -ATPase drives Na+/H+ antiport, which drives Cl-/HCO3- exchange

passive (non-mediated) transport

can see paracellular absorption of water and small organic ions through tight junctions of duodenum and jejunum, which works in a complementary way to transcellular absorption

facilitated diffusion

movement of glucose out of enterocyte from uniporter

how is absorption of water and electrolytes important to sustaining life?

conservation of body’s supply of water and electrolytes, primarily Na+, K+, Cl-, and HCO3-, is a high priority for sustaining life

how does the absorption of HCO3- maintain blood buffering?

NaHCO3 enters intestinal lumen to neutralize HCl from the stomach → H2O + CO2 + NaCl → absorption of HCO3- and H+ ions

what happens to the HCO3- after it is absorbed?

the HCO3- remains in the intestine after the neutralization of stomach acid; reabsorbed primarily in the ileum and colon via ion-exchange mechanism

What happens to the K+ in passive diffusion paracellularly?

High K+ levels in the intestinal lumen (abundant in diets and other ions) are being transported into cells

where are low K+ levels found?

in lateral spaces

what is the primary mechanism of K+ absorption?

paracellular passive diffusion (because of K+ gradient)

What is associated with K+ movement?

water

how are lipids related to water?

they are hydrophobic and thus do not dissolve in water

dietary lipids include…

• triglycerides from animals and plants

• cholesterol and cholesteryl from animals

• waxes from plants

• phospholipids from animals and plants

• lipid soluble vitamins A, D, E, K

The process of lipid assimilation involves…?

1. emulsification

2. hydrolysis

3. micelle formation

4. absorption

what are chylomicrons and how are they formed?

absorbed lipids picked up by carrier molecules and taken to ER, then lipids are re-esterified to form triglycerides and phospholipids, and then packed with cholesterol, other lipids and proteins from RER form chylomicron