Nutrient digestion and absorption

Question 1

Major sources of carbohydrates

Answer 1

• Sucrose (disacch)
• Lactose (disacch)
• Starches (polysacch)

Question 2

Starch initial breakdown

Answer 2

Hydrolyzed initially by ptyalin (a-amylase) secreted in saliva
Hydrolysed to maltose and other small glucose polymers
PANCREATIC a-amylase is several times as powerful

Question 3

Enterocytes lining villi of small intestine

Answer 3

Lactase
Sucrase
Maltase
a-dextrinase

Question 4

What does lactose split into

Answer 4

Galactose and glucose

Question 5

What does sucrose split into

Answer 5

Fructose and glucose

Question 6

What about maltose and other small glucose molecules

Answer 6

Split into multiple small glucose molecules

Are water soluble and absorbed immediately into blood

Question 7

Pepsin role in protein digestion

Answer 7

Initiates the process
Converts protein to proteoses, peptones and a few polypeptides
Able to digest collagen
Its action terminated in the duodenum

Question 8

In duodenum and jejenum;

trypsin and chymotrypsin

Answer 8

split proteins into small polypeptides

Question 9

In duodenum and jejenum;

Carboxypolypeptidase

Answer 9

splits polypeptides into individual amino acids

Question 10

In duodenum and jejenum;

Proelastase

Answer 10

Converted to elastase

Digests elastin fibres that holds meats together

Question 11

Enterocytes role in protein digestion

Answer 11

• activates pancreatic proenzymes
• Peptidases aminopolypeptidase and several dipeptides split remaining large polypeptides into tr/dipeptides and aa. These are then transported through membrane into the enterocytes
• Inside there are more peptidases that split all into aa. These pass onto other side of enterocyte and into the blood

Question 12

What are enterocytes

Answer 12

Line the villi of small intestine (duodenum and jejenum).
Have a cell brush border that contain hundreds of microvilli containing peptidases that protrude through membrane to exterior and in contact w fluids

Question 13

Types of fat in the diet

Answer 13

Most abundant are Triglycerides; Glycerol nucleus and three fatty acid chains
Also have small quantities phopholipids, cholesterol, and cl esters

Question 14

Initial digestion of TG

Answer 14

By lingual lipase in the saliva. This is secreted by lingual glands in the mouth (less than 10% of fat digestion)

Question 15

Digestion of fat

1st step

Answer 15

Emulsification;
Breakdown of fat into very small sizes so that water soluble digestive enzymes can act on globule surface
Most happens in duodenum under influence of bile.

Question 16

Why is bile needed for emulsification

Answer 16

Doesn’t contain any digestive enzymes but does contain large quantity of bile salts and the phospholipid lechitin.
Polar parts of these are highly soluble in water, and the rest is highly soluble in fat.
SO soluble in fat part dissolves in the fat molecule. The polar parts are projecting out, making the fat particle now soluble and reduces the interfacial tension

Question 17

What does the reduced interfacial tension mean

Answer 17

That the particle can be broken up into many very small particles, increasing the fat’s total surface area.
This happens by agitation with the water in the small intestine.
Lipase enzymes are water soluble so can only attack on fats cell surface —> good increased SA

Question 18

Digestion of triglycerides by pancreatic lipase

Answer 18

Most important enzyme for digestion of the TG
Present in enormous quantities in pancreatic juice
The TG is split into fatty acids and 2 monoglycerides.

Question 19

Bile salts role to form micelles

Answer 19

Hydrolysis of TG is highly reversible so bile salts form micelles (when high enough conc in water). the fat-soluble sterol nucleus encompasses the MG and fatty acids, leaving the polar groups covering. As polar groups negative charge the whole micelle can dissolve in water.

Question 20

Where does the micelles take the MG and free fatty acids?

Answer 20

To the brush borders of interstinal epithelial cells. There the MG and fatty acids are absorbed into the blood for ABSORPTION.
Bile goes back to chyme to be used again

Question 21

Digestion of cholesterol esters

Answer 21

Most cholesterol in diet is in form of esters, which are combinations of cholesterol and one molecule fatty acid.
These are hydrolysed by enzyme cholesterol ester hydrolase in pancreatic secretion.
Transported via micelle

Question 22

Digestion of phospholipids

Answer 22

As these also contain fatty acids, they are hydrolysed by enzyme phospholipidase A2.
Transported via micelle

Question 23

Why does most absorption take place in the small intestine?

Answer 23

• Contains many lumen-protruding folds called valvulae conniventes, that extend circularly around
• These folds are lined with villi projections
• These villi have microvilli on their surfaces (brush border). These have actinfilaments which cause continuous movement of the microvilli
  Therefore the total surface area is very great

Question 24

1. Absorption of Na via active transport

Answer 24

Active transport of Na out of the cell through basolateral walls via Na+/K+?ATPase pump. Into paracellular spaces
This passively drags Cl- out
This reduces Na- conc of brush cell, causing Na+ from the chyme to move in via apical membrane

Question 25

Transporters which facilitate Na+ movement into cell

Answer 25

• Na+/glucose co-transporter
• Na+/amino acid co-tranporter
• Na+/H+ cotransporter

Question 26

2. Osmosis of water

Answer 26

Next step is movement of water into paracellular spaces as the increased concentration of Na+ ions in paracellular space has increased the osmotic gradient.
Osmosis between the tight junctions and through the cells
Finally moves into the circulating blood of the villus

Question 27

Aldosterone effect on Na+ absorption

Answer 27

Causes increased activation of the enzyme and transport mechanisms for all aspects of Na+ absorption.
Conserves NaCl and water in the body when a person becomes dehydrated

Question 28

Absorption of Cl- ions

Answer 28

In duodenum and jejenum the Cl- absop is quick as it follows the Na+
In ileum and parts of large intestine it is absorbed by Cl-/HCO3- exchanger
Exits on basolateral side via chloride channels

Question 29

Absorption of HCO3- in duodenum and jejenum

Answer 29

• When Na+ absorbed in brush cells from chyme, exchanged for H+
• These combine with the HCO3- to form carbonic acid, which then dissociates into water and CO2.
• Water remains in chyme, CO2 is readily absorbed into blood and expired through lungs

Question 30

What are carbohydrates absorbed as?

Answer 30

80% of all carbohydrates are absorbed as glucose

Remaining 20% are galactose and fructose

Question 31

Glucose and galactose absorption

Answer 31

The movement of Na+ out of basolateral membrane makes Na+ conc in cell very low.
Therefore Na+ from chyme wants to move into brush cell, however only carrier SGLT1 does so by facilitated diffusion
Therefore binds to glucose or galactose and pull them into cell

Question 32

Fructose absorption

Answer 32

transported by facilitated diffusion via GLUT5 all the way through epithelium
On entering the cell, most will be converted into glucose and transported with rest of glucose into the blood

Question 33

How are proteins absorbed

Answer 33

Most require co-transport mechanism (transporters on apical membrane)
Few transported by facilitated diffusion

Question 34

How are anionic amino acids absorbed

Answer 34

via SLC1A1 transporter
Requires cotransporter of Na+
H+ out and K+ out

Question 35

How are neutral and cationic aminoacids absorbed

Answer 35

via SLC5A14 transporeer

Also requires Na+ contransporter

Question 36

Absorption of fats after micelles

Answer 36

The micelles that hold MG and fatty acids penetrate into recesses of microvilli and let MG and fatty acids diffuseinto the interior of the brush cells (as lipids are soluble in the membrane).
IF local acid pH then this can happen

Question 37

Absorption of the fats if it is not a local acid pH

Answer 37

Then can be transported using transport proteins FAT(P1) plus CD36

Question 38

What happens to fats in the cell

Answer 38

• move to endoplasmic reticulum where they bind with lipoproteins
• then packaged into chylomicrons (vesicles)
• These are exocitosed and they travel to liver via the lymphatics system

Question 39

Iron absorption

Answer 39

Most of diet iron is Fe3+, but it is the Fe2+ form that is absorbed.
Fe3+ reductase activity occur near brush borders. Gastric secretions dissolve the iron
Iron absorption occurs in the duodenum via DMT1.

Question 40

What happens after iron is in the cell

Answer 40

Some is stored as ferritin
Remainder is transported out into blood through basolateral transporter ferroportin 1. Hp facilitates
In the blood it is converted back to Fe3+ and bound to transferrin

Question 41

Composition of foeces

Answer 41

three quarters water
one quarter solid matter
the solid matter is;
- 30% dead bacteria
- 10-20% fat
- 10-20% inorganic matter
- 2-3% protein
- 30% undigested roughages

Question 42

What makes the feces brown

Answer 42

Stercobilin
Urobilin
(derivatives from bilirubin)