Lecture 20- Digestion 3

Question 1

What are the segmentation contractions?

Answer 1

-segmentation contractions occur in the large intestine and small intestine. While peristalsis involves one-way motion in the caudal direction, segmentation contractions move chyme in both directions, which allows greater mixing with the secretions of the intestines. Segmentation involves contractions of the circular muscles in the digestive tract, while peristalsis involves rhythmic contractions of the longitudinal muscles in the GI tract. Unlike peristalsis, segmentation actually can slow progression of chyme through the system.

• vigorous post meal
• Ileum: increase = by gastrin (gastro-ileal reflex)

Question 2

What is the gastro-ileal reflex?

Answer 2

-Gastroileal reflex is one of the ways in which gastric motility influences intestinal motility
When there is vigorous gastric peristalsis of increased secretion, it stimulates peristalsis in the ileum
This causes the ileal contents to be pushed into the colon, in turn stimulating colonic peristalsis and an urge to defecate

Question 3

What is the small intestine for?

Answer 3

• it is really long, small diameter most 10-12meters long
• main ingerdients mixed already so get more breakdown and absorption
• as it moves so it has finite time in which to absorb it- wasteful if gets to large intestine (in case of hindgut fermenters= horses, if too much carbohydrates then can’t break it all down and get interference with fermentation= bad)

Question 4

What is motilin?

Answer 4

• hormone
• The main function of motilin is to increase the migrating motility complex component of gastrointestinal motility and stimulate the production of pepsin. Motilin is also called “housekeeper of the gut” because it improves peristalsis in the small intestine and clears out the gut to prepare for the next meal

Question 5

What is the migrating motility complex?

Answer 5

• waves of activity that sweep through the intestines in a regular cycle during fasting state
• help trigger peristaltic waves, which facilitate transportation of indigestible substances such as bone, fiber, and foreign bodies from the stomach, through the small intestine, past the ileocecal sphincter, and into the colon
• Carries undigested material, mucosal debris, bacteria
• Weak peristaltic waves progressively move from stomach to colon
• this activity is increased by the secretion of motilin

Question 6

What is the ileocecal juncture?

Answer 6

• a sphincter or valve connecting the end of the small intesting (the ileum) with the beginning of the large intestine (the cecum)
• much larger and more pronounced in herbivores.
• regulates passage of material from ileum to the caecum(large intestine)

Question 7

What is the structure of the small intestine epithelium?

Answer 7

villi-microvilli= increase surface are 600x

• encased in “jelly-like” glycoprotein & mucous secreted by epithelial goblet cells
• also circular folds of the intestine make it possible to fit more into the body

Question 8

What is the lumenal phase of digestion?

Answer 8

• the stage of the digestion of fats that goes on in the lumen of the intestine
• pancreatic digestive enzyme
• pepsin
• salivary amylase

Question 9

What is the membraneous phase of digestion?

Answer 9

• Boundary layer of water; Unstirred by movement of luminal contents
• happens at the membrane by membrane bound proteins

Question 10

When are enzymes released in the small intestine and why?

Answer 10

• enzymes just before absorptions so the things are broken down to their contituents just before it
• there are tight junctions but it is more leaky than the stomach

Question 11

How do the tight junction in the small intestine differ to those in the stomach?

Answer 11

-the ones in the small intestine are much more leaky

Question 12

How are carbohydrates digested and absorbed?

Answer 12

• absorbed in the small intestine must be hydrolyzed to monosaccharides prior to absorption
• begins (starch) in mouth with amylase eventually broken down to maltose
• maltose= broken down by maltase into two glucose molecules
• absorption of most digested food occurs in the small intestine through the brush border of the epithelium covering the villi(small hair-like structure). It is not a simple diffusion of substances, but is active and requires energy use by the epithelial cells.

Question 13

How is lactose, maltose and sucrose digested and absorbed?

Answer 13

-start with quite simple sugars but has to be broken down further
also has Na co transport channel= actively transported into cells and creates gradient so these sugars will enter circulation

• lactose= broken into galactose and glucose by lactase then with help of Na+ co-transport channel absorbed
• maltose= broken into two glucose molecules and also Na+ co-transport channel for absorption
• sucrose= the same only it’s broken down to glucose and fructose

Question 14

What is the Na+ glucose co-transport system?

Answer 14

• transport the glucose from the lumen into epithelial cells from which it can reach the blood
• it is an active process driven by the concentration gradient of Na+
• glucose and Na+ are actively transported down the Na+ gradient into the epithelial cells= no energy is required
• via glucose carrier the glucose enters blood

Question 15

How is glucose absorbed into the blood?

Answer 15

-most of it is done by facilitated diffusion
-only when very low level of glucose does the co-transport system come into play
=these three things are at play

1. Active transport of sodium into blood (Na+ K+ pump)
2. Cotransport of glucose and sodium from lumen into ileum epithelial cells(passive= no energy)
3. Facilitated diffusion of glucose into blood

-the sodium is high in the lumen and low in epithelial cells, glucose is low in lumen and high in epithelial cells

=cotransport of sodium and glucose = passive

cotrasport= involves more than one type of particle being transported in the same direction at the same time by the same mechanism
-Occurs from the lumen into the epithelial cell, absorbing a sodium ion and glucose molecule together : COTRANSPORT down a concentration gradient for SODIUM… but AGAINST for glucose

Question 16

What does the Na+ K+ pump do?

Answer 16

• example of counterexport=in which two kinds of particles are transported at the same time in opposite directions by the same mechanism.
• pumps the sodium into the blood from the epithelial cells so the Na+ conc is low in the epithelium= that allows the Na+ glucose cotransport to work
• -have to maintain the sodium gradient= done by the Na+ K+ pump=Sodium ions are actively transported from the epithelial cell (low Na+ conc.) into the blood (high Na+ conc.) in exchange for potassium ions.=This maintains the diffusion gradient for sodium ions from the lumen (high Na+ conc.) into the epithelial cell (low Na+ conc.)
• important for the glucose absorption in the small intestine

Question 17

How is protein digested? (plus how is single amino acid absorbed)

Answer 17

• begins in stomach (pepsin= about 15% of digestion)
• break down of protein into peptides and eventually into amino acids
• mostly happens in the small intestine
• Before the absorption in the small intestine, most proteins are already reduced to single amino acid or peptides of several amino acids
• have to have Na+ gradient as the single amino acids are co-transported with Na+
• this gradient is once again maintained by the potassium sodium pump= It pumps sodium out of the epithelial cells into the blood in exchange for the potassium(uses energy)
• there are specific cotransporters at the epithelial membrane(Na+ cotransport)
• then at the epithelial cell to blood (basolateral membrane) =another transporter for the amino acid (specific) and it is transported with the help of Na+ coming from the blood (so Na+ going into the epithelial cells and the amino acid going into blood)

Question 18

How are dipeptides or tripeptides absorbed into the blood?

Answer 18

• pump, have Na+ gradient (lower in epithelial so wants to come in) when the Na+ comes in, H+ goes to the lumen so eventually will have quite a lot H+ in the lumen and fewer in the epithelial= again concentration gradient
• another pump, this time H+ co transport so the H+ goes into the epithelial cells and takes with it a dipeptide or tripeptide
• at the basolateral membrane (epithelial into blood) another pump, pumps the H+ into the epithelial cells and the dipeptides and tripeptieds out into the blood

Question 19

How is fat digested and absorbed?

Answer 19

-different because fats= hydrophobic
-lipids= long chains of triglycerides (glycerol with three fatty chains)
-first bile emulsifies the fat
=breaks them down to smaller balls
-the lipase=breaks down triglyceride into two fatty acids and one fatty acid with a glycerol (monoglycerol (all hydrophobic)
-they can just diffuse through the cell membrane as hydrophobic
-the free fatty acids just diffuse
-micelles= the free fatty acids and glycerol coated by lipoprotein

Question 20

What is a cholymicron?

Answer 20

-re lipoprotein particles that consist of triglycerides (85-92%), phospholipids (6-12%), cholesterol (1-3%), and proteins (1-2%).[1] They transport dietary lipids from the intestines to other locations in the body. Chylomicrons are one of the five major groups of lipoproteins (chylomicrons, VLDL, IDL, LDL, HDL) that enable fats and cholesterol to move within the water-based solution of the bloodstream.

Question 21

How is Na+ and Cl- transported?

Answer 21

Na+- can be co-transport
or simple diffusion at apical membrane

• Cl- paracellular diffusion (between cells)
• Cl will pass passively to balance it

Question 22

How is Na+ and Cl- transported? Part 2

Answer 22

-also can be transported via antiports that use HCO3(bicarb) and H+ ions to drag Na+ and Cl- into the cell (the HCO3 and H+ are made from a reaction of CO2 and H2O)

Question 23

How is K+ absorbed?

Answer 23

• Paracellular passive diffusion from lumen to extracellular fluid
• No active uptake of K+ on apical membrane of SI epithelium
• [K+] high in luminal contents ␣ Lots of K+ in animal diets

Question 24

How is water absorbed?

Answer 24

• Always follows net osmotic gradient

- there will be lot of water in the small intestine still

Question 25

What is the large intestine for?

Answer 25

- Drying & storage organ | - horses etc. can have fermentation there= bigger then

Question 26

How does the large intestine move?

Answer 26

- Segmentation contractions - Antiperistaltic contractions propagate toward the illeum - Mass movements

Question 27

How is water absorbed in the large intestine?

Answer 27

- mechanisms similar to small intestine Na+ ions transported from lumen across epithelium by very active sodium pumps= then water follows - colonic epithelium is more efficient at absorbing water than small intestine

Question 28

What is bicarb excretion for in large intestine?

Answer 28

-aids in neutralization of acids generated by microbial fermentation in small intestine

Question 29

What is special about ruminant digestion?

Answer 29

- use microbial organisms to digest carbohydrates(especially cellulose and starch) and proteins in the rumen but also in the hind gut - starch and cellulose are broken down into volatile fatty acid (VFA)

Question 30

What are the three most important volatile fatty acids in ruminants?

Answer 30

- Acetic CH3COOH - Propionic CH3CH2COOH - n-Butyric CH3CH2CH2COOH

Question 31

How does the absorption of volatile fatty acids in ruminants occur?

Answer 31

- through simple diffusion - Occurs directly across rumenal epithelia - also in small intestine, cecum, large intestine of ruminants and non-ruminants

Question 32

How is the concentration gradient for volatile fatty acids diffusion maintained in ruminants?

Answer 32

=concentration gradient is maintained via 1. rapid utilization/conversion by absorptive cells 2. rapid transport to portal blood - onto the liver and the rest of the system

Question 33

What is the rate of uptake of the volatile fatty acids in ruminants affected by?

Answer 33

-pH - dissociated vs. nondissociated form -production of bicarb also if lot of starch in= lactate is made = can harm and destroy some of the bacteria as pH changes

Question 34

What happens to the three main volatile fatty acids in ruminants when absorbed?

Answer 34

- Most butyrate converted to beta-hydroxybutyrate - Some propionate to lactate - Some acetate oxidized - most just absorbed

Question 35

What is the n Buteric acid for in ruminants?

Answer 35

-18-20% of VFAs produced is butyric acid. It provides energy to the rumen wall and is used in milk fat synthesis and for body fat, when excess energy is present in the diet. It doesn't vary in proportion to other volatile fatty acids, therefore has little influence in milk fat content.

Question 36

What is the Propionic acid for in ruminants?

Answer 36

12-18% of VFAs produced is propanoic acid. It predominates on a high concentrate diet and provides energy via the conversion of blood glucose in the liver. It is used in lactose (milk sugar) synthesis. The molecular formula is, CH3.CH2.COOH.

Question 37

What is the acetic acid for in ruminants?

Answer 37

-50-60% of VFAs produced is acetic acid. It predominates on a high roughage diet and is a precursor for mammalian milk fat. Some is also used for muscle metabolism and body fat. The molecular formula is, CH3.COOH.