Macro and Micronutrient Transport Flashcards
3 macronutrients
Sugars
Proteins
Lipids
What are micronutrients
Vitamins and minerals
Water soluble vitamines needed as co-factors for enzymes
Fat soluble vitamins have varied functions
3 ways the intestine can increase its surface area
Microvilli
Villi
Folds of Kerckring (wrinkles in the tube)
5 cells found in the villus and crypts
Absorptive cells
Goblet cells
Enteric endocrine cells (contain hormones)
Stem/progenitor cells (replace other cell types)
Paneth cells (protection for stem cells)
5 Barries to free movement of nutrients from lumen into blood or lymph
Glycocalyx (coat on microvilli) Apical PM Apical tight junctions Basolateral PM Basement membrane
Paracellular transport
Movement of molecules between cells
Tight junctions set up transepithelial resistance (can be regulated)
When the resistance is low, small charged ions can move down concentration gradients through tight junctions between cells
Where is transepithelial resistance lowest?
In the small intestine (because a lot of things need to be absorbed)
Transcellular transport
Material moved through cells by diffusion, facilitated transport, active transport, and other methods
Passive diffusion
Free movement of lipid soluble molecules or water (through pores) across the lipid bilayer of the PM
No energy is required
Facilitated diffusion
Transport of molecules by a membrane carrier or an ion channel
No energy is required due to movement with/down a gradient
Active transport
Molecularly specific uptake/release system that is often coupled to the Na/K ATPase
Energy is required due to movement against/up a gradient
Micropinocytosis/endocytosis
Uptake of large molecules by the formation of small vesicles that form at the base of microvilli of intestinal enterocytes
Requires energy
Generally, where do carbs/proteins/lips get absorbed in the small intestine?
Most carb and proteins are absorbed in the first 50% of the small intestine (duodenum and jejunum)
Lipid absorption occurs slightly later due to longer digestion time
5 general patterns of digestion and absorption
None (glucose passes directly through cells into interstitial space)
Luminal hydrolysis of polymers to monomers (proteins to aas)
Brushborder hydrolysis of oligomer to monomer (sucrose to fructose and glucose)
Intracellular hydrolysis (peptides)
Luminal hydrolysis followed by intracellular resynthesis (TAGs)
Where is fat breakdown initiated? Where does it continue?
Initiated in the stomach
Dispersion/emulsification of fats begins in the stomach
Further hydrolysis and micelle formation in the small intestine
Some complex fats are re-synthesized in enterocytes
4 major types of dietary fats
Triglycerides
Cholesterol (and esters)
Phospholipids
Fat soluble vitamins
6 steps in fat digestion and absorption
Initial hydrolysis in the stomach
Emulsification in the SI
Hydrolysis to FFAs and 2-monoglycerides
Micelle formation
Absorption of FFAs and 2-monoglycerides into epithelial cells
Active absorption of conjugated bile acids
Gastric lipase
Present in early neonatal development
Contributes to initial dispersion of large fat globules
Efficient at low pH
Preference for fatty acids in 3-position of TAGs
Emulsification
The release of FFAs, mixing, and the interaction with protein breakdown products begins the process of dispersion of large fat globules into small lipid droplets in the chyme
Lipid hydrolysis by pancreatic lipases in the small intestine
Generates lipids with polar head groups that facilitate further emulsification
Colipase
Helps ancho lipase to emulsification droplets once they are coated with bile salts
Micelles
Shell is made of bile salts that have interacted with FAs and 2-monoglycerides
Inside are poorly soluble lipid breakdown products
Micelles are able to move through the aqueous unstirred water layer to reach the apical surface of enterocytes
How do short/medium chain FAs pass into the enterocyte?
Pass directly by partitioning into the microvillus membrane (major pathway)
Or by binding carrier proteins in the membrane
Does not require the solubilization of the micelle (most of these are on the micelle surface)
How are long chain FAs, monoglycerides, and cholesterol absorbed?
Usually requires the dissociation/breakdown of the micelle (often inside it)
Most require binding proteins (long chain FAs) in the microvillar membranes of the enterocytes for absorption
Cholesterol and monoglycerides can diffuse in
How are
1. Small/medium chain FAs
2. Long chain FAs
transported to blood
- Released basolaterally and diffuse into the blood
- FAs and monoglycerides are re-esterified with glycerol to form TAGs. They are packaged into apolipoproteins in the Golgi and exocytosed as chylomicrons
4 major types of dietary carbs, and if they are digestible or not
Starch (yes) Dietary fiber (no) Dietary disaccharides (yes) Dietary monosaccharides (yes)
Enzyme responsible for the initial breakdown of long chain carbs
Salivary amylase
Salivary amylase
Hydrolyses 1:4 and 1:6 linkages (initiates breakdown of liner and branched starch)
Gastric acid breaks it down
Activity is reduced as chyme is released into the duodenum through sphincter
Pancreatic amylase
Completes initial digestion in the lumen of the duodenum
Breaks down to di and trisaccharides
How is carb digestion to monosaccharides completed?
By brush border enzymes on the microvilli of intestinal enterocytes
Transporters then bring them into the cell
How does 1. Glucose 2. Galactose 3. Fructose get into the cell?
- SGLT1
- SGLT1
- GLUT5
How does 1. Glucose 2. Galactose 3. Fructose get out of the cell?
All via GLUT2
Diffusion
SGLT1
Sodium-Glucose-Galactose transporter 1
Transports a Na into the cell along with a glucose or galactose
The concentration gradient of Na is driven by the Na/K ATPase
Protein digestion in stomach
Pepsiongen to pepsin
HCl and autocatalysis activate more pepsiongen
Protein digestion in the small intestine
Enterocytes release enterokinase, which cleaves trypsinogen into trypsin
Trypsin can autocatalyze, and cleave many other pancreatic proteases and lipases
2 ways peptides can be digested and absorbed by the brush border
Direct transport (di and tri peptides through transporter into cell) Hydrolysis and uptake (peptides hydrolyzed and aas are transported)
Newborn intestine
Macromolecules ingested by pinocytosis
Mucosa is leaky and immunologically deficient
Epithelial cells mature with age, passage of macromolecules decreases (closes at 3 months)
How are 1. Fat soluble vitamins 2. Water soluble vitamins 3. Minerals absorbed?
- Passively from micelles
- Many ways (facilitated diffusion, active transport, endocytosis, etc)
- Many ways (active transport, facilitation by vitamin, alternate transport)
