Macro and Micronutrient Transport

Question 1

3 macronutrients

Answer 1

Sugars
Proteins
Lipids

Question 2

What are micronutrients

Answer 2

Vitamins and minerals
Water soluble vitamines needed as co-factors for enzymes
Fat soluble vitamins have varied functions

Question 3

3 ways the intestine can increase its surface area

Answer 3

Microvilli
Villi
Folds of Kerckring (wrinkles in the tube)

Question 4

5 cells found in the villus and crypts

Answer 4

Absorptive cells
Goblet cells
Enteric endocrine cells (contain hormones)
Stem/progenitor cells (replace other cell types)
Paneth cells (protection for stem cells)

Question 5

5 Barries to free movement of nutrients from lumen into blood or lymph

Answer 5

Glycocalyx (coat on microvilli)
Apical PM
Apical tight junctions
Basolateral PM
Basement membrane

Question 6

Paracellular transport

Answer 6

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

Question 7

Where is transepithelial resistance lowest?

Answer 7

In the small intestine (because a lot of things need to be absorbed)

Question 8

Transcellular transport

Answer 8

Material moved through cells by diffusion, facilitated transport, active transport, and other methods

Question 9

Passive diffusion

Answer 9

Free movement of lipid soluble molecules or water (through pores) across the lipid bilayer of the PM
No energy is required

Question 10

Facilitated diffusion

Answer 10

Transport of molecules by a membrane carrier or an ion channel
No energy is required due to movement with/down a gradient

Question 11

Active transport

Answer 11

Molecularly specific uptake/release system that is often coupled to the Na/K ATPase
Energy is required due to movement against/up a gradient

Question 12

Micropinocytosis/endocytosis

Answer 12

Uptake of large molecules by the formation of small vesicles that form at the base of microvilli of intestinal enterocytes
Requires energy

Question 13

Generally, where do carbs/proteins/lips get absorbed in the small intestine?

Answer 13

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

Question 14

5 general patterns of digestion and absorption

Answer 14

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)

Question 15

Where is fat breakdown initiated? Where does it continue?

Answer 15

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

Question 16

4 major types of dietary fats

Answer 16

Triglycerides
Cholesterol (and esters)
Phospholipids
Fat soluble vitamins

Question 17

6 steps in fat digestion and absorption

Answer 17

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

Question 18

Gastric lipase

Answer 18

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

Question 19

Emulsification

Answer 19

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

Question 20

Lipid hydrolysis by pancreatic lipases in the small intestine

Answer 20

Generates lipids with polar head groups that facilitate further emulsification

Question 21

Colipase

Answer 21

Helps ancho lipase to emulsification droplets once they are coated with bile salts

Question 22

Micelles

Answer 22

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

Question 23

How do short/medium chain FAs pass into the enterocyte?

Answer 23

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)

Question 24

How are long chain FAs, monoglycerides, and cholesterol absorbed?

Answer 24

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

Question 25

How are
1. Small/medium chain FAs
2. Long chain FAs
transported to blood

Answer 25

1. Released basolaterally and diffuse into the blood
2. FAs and monoglycerides are re-esterified with glycerol to form TAGs. They are packaged into apolipoproteins in the Golgi and exocytosed as chylomicrons

Question 26

4 major types of dietary carbs, and if they are digestible or not

Answer 26

Starch (yes)
Dietary fiber (no)
Dietary disaccharides (yes)
Dietary monosaccharides (yes)

Question 27

Enzyme responsible for the initial breakdown of long chain carbs

Answer 27

Salivary amylase

Question 28

Salivary amylase

Answer 28

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

Question 29

Pancreatic amylase

Answer 29

Completes initial digestion in the lumen of the duodenum

Breaks down to di and trisaccharides

Question 30

How is carb digestion to monosaccharides completed?

Answer 30

By brush border enzymes on the microvilli of intestinal enterocytes
Transporters then bring them into the cell

Question 31

How does
1. Glucose
2. Galactose
3. Fructose
get into the cell?

Answer 31

1. SGLT1
2. SGLT1
3. GLUT5

Question 32

How does
1. Glucose
2. Galactose
3. Fructose
get out of the cell?

Answer 32

All via GLUT2

Diffusion

Question 33

SGLT1

Answer 33

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

Question 34

Protein digestion in stomach

Answer 34

Pepsiongen to pepsin

HCl and autocatalysis activate more pepsiongen

Question 35

Protein digestion in the small intestine

Answer 35

Enterocytes release enterokinase, which cleaves trypsinogen into trypsin
Trypsin can autocatalyze, and cleave many other pancreatic proteases and lipases

Question 36

2 ways peptides can be digested and absorbed by the brush border

Answer 36

Direct transport (di and tri peptides through transporter into cell)
Hydrolysis and uptake (peptides hydrolyzed and aas are transported)

Question 37

Newborn intestine

Answer 37

Macromolecules ingested by pinocytosis
Mucosa is leaky and immunologically deficient
Epithelial cells mature with age, passage of macromolecules decreases (closes at 3 months)

Question 38

How are
1. Fat soluble vitamins
2. Water soluble vitamins
3. Minerals
absorbed?

Answer 38

1. Passively from micelles
2. Many ways (facilitated diffusion, active transport, endocytosis, etc)
3. Many ways (active transport, facilitation by vitamin, alternate transport)