Digestion and Absorption Flashcards
1
Q
What is the definition of digestion?
A
- The mechanical, chemical and microbial breakdown of large, insoluble food molecules into simple absorbable compounds
2
Q
What is the definition of absorption?
A
- The process by which these simple compounds are taken across the intestinal membranes into the blood
3
Q
Where does most digestion and absorption occurs in an omnivore/carnivore?
A
- 90% occurs in the small intestine
4
Q
Where does most digestion and absorption occur in herbivores? (in both hindgut fermenters and ruminants)
A
- hind gut fermenters = more in the large intestine
- ruminants = significant uptake in the rumen too
5
Q
Where do digestive secretions come from and where do they secrete into?
A
- salivary glands - secrete into the mouth and tongue
- stomach
- exocrine pancreas - secretions into the small intestine
- liver - secretes bile into small intestine
- small intestinal glands and brush border
6
Q
What does alpha amylase do?
A
- catalyses the hydrolysis of starch into sugars
7
Q
What is the optimum pH of salivary alpha amylase and where is it inactivated?
A
- optimum pH = 6.6-6.8
- inactivated at stomach pH
8
Q
What is the function of salivary alpha amylase?
A
- to start carbohydrate digestion and act on starch
9
Q
Where is pancreatic alpha amylase secreted from?
A
- secreted by the exocrine pancreas
10
Q
What is the function of pancreatic alpha amylase?
A
- to end carbohydrate digestion and act on complex carbohydrates
11
Q
Where does most carbohydrate digestion occur?
A
- occurs in the small intestine by the action of pancreatic alpha amylase
12
Q
How do alpha amylases work?
A
- attack the alpha 1,4 glycosidic links only
- attack only in the middle of a CHO chain = endoglycosidases
13
Q
What are the two phases of carbohydrate digestion?
A
- luminal phase
- membranous phase
14
Q
Describe the luminal phase of carbohydrate digestion:
A
- starch and glycogen are degraded into compounds containing 2-9 glucose units
- products of luminal degradation cannot be absorbed by the epithelial cells
15
Q
Describe the membranous phase of carbohydrate digestion?
A
- di, tri and oligosaccharides are broken down to monosaccharides by enzymes bound to the apical membrane of the epithelial cells
16
Q
Where does the luminal phase of carbohydrate digestion occur?
A
- Luminal phase happens mostly in the duodenum as this is where the ducts of the pancreas empty
17
Q
What are enterocytes?
A
- columnar epithelial cells found at the surface of villi which act as absorptive functional units of the digestive tract
18
Q
What 3 types of vessel can be found within each of the individual villi?
A
- artery
- vein
- lymphatic vessel
19
Q
What is the lifespan for an enterocyte and what happens when they die?
A
- 2-3 days
- when they die they will either be passed out with faeces/degraded and reabsorbed
20
Q
What are the 4 different mechanisms for absorption?
A
- passive diffusion
- facilitated diffusion
- active transport
- endocytosis
21
Q
What happens once nutrients are within cells?
A
- they cross over the baso-lateral membrane and enter circulation, directed to the hepatic portal vein to liver or indirectly via lymphatic system
22
Q
What are the products of carbohydrate digestion?
A
- mainly glucose
- smaller amounts of fructose and galactose
23
Q
How is fructose absorbed?
A
- absorbed by carrier-mediated diffusion down its concentration gradients into the cell and the out of the cell into the blood
24
Q
What the transport of glucose and galactose into intestinal cells require?
A
- energy
25
Glucose transport has a number of characteristics which provide information about the mechanisms involved - what are these characteristics?
- saturable
- competitively inhibited by galactose
- inhibited by glucose analogues
- sodium dependent
- inhibited by the sodium/potassium ATPase inhibitor
- requires energy
26
How is glucose transported from the lumen of the GI tract into the blood?
1. glucose and sodium are co-transported together from the lumen into the intestinal cell
2. the concentration of sodium and glucose is higher inside the cell than in the blood
3. glucose is moved into the blood via facilitated diffusion
4. sodium pumped into the blood and potassium pumped out of the blood - needs energy
27
What is the general function of proteases?
- break down proteins and peptides
28
How do proteases work?
- They cause cleavage of the peptide bonds by hydrolysis
29
What are zymogens?
- zymogens are inactive precursors of proteases
30
What are the two groups of gastrointestinal proteases?
- endopeptidases
- exopeptidases
30
What are endopeptidases?
- attack and cleave in the middle of the amino acid chain
- secreted by the stomach and pancreas
30
What are exopeptidases?
- split off amino acids from the end of the chain
- secreted by pancreas and small intestine glands
31
What are pepsins?
- gastric proteases
32
where does protein digestion start?
- starts in the stomach via endopeptidase - pepsin
33
What pH does pepsin require, what does it do and what is it secreted by
- active at low pH <3
- attacks all proteins (except keratin and mucin)
- secreted by pepsinogen by chief cells
34
What is secreted simultaneously with pepsinogen? what is it secreted by?
* H+
* secreted by parietal cells
35
Where are both pepsin and H+ produced?
- gastric pits
36
How is HCl secreted by parietal cells?
- by carrier mediated active transport from the parietal cell into the lumen
37
Why is HCL needed?
- HCL activates pepsinogen to make activated pepsinogen
- activated pepsinogen loses 44 amino acids from its chain to make pepsin
38
How does pepsin make a positive feedback loop?
- when pepsin is made it stimulates more activated pepsinogen to become pepsin
39
Where do peptides from pepsin digestion in the stomach pass?
- into the small intestine
40
What are the 3 endopeptidases produced by the pancreas
- trypsinogen (trypsin)
- chymotrypsinogen (chymotrypsin)
- pro-elastase (elastase)
41
Why do pancreatic endopeptidases have an alkaline optimum pH?
- because carbonate HCO3- ions are also secreted from the pancreas
42
What happens to peptides after pancreatic endopeptidases digestion?
- peptides are attacked by exopeptidases
43
What are the two types of pancreatic exopeptidase?
- Carboxypeptidases
- Aminopeptidases
44
What do carboxypeptidases do and where are they secreted from?
- split amino acids from the carboxy terminal
- secreted by the pancreas as zymogens
45
What are aminopeptidases and where are they made and found?
- split amino acids from the N- terminal
- made in the small intestine glands
- found on intestinal cell membrane
46
Complete the sentence:
Proteases are both stored and secreted as ...
- zymogens
47
What do lipases do?
- digest fats, removing fatty acids from triacylglycerol's
48
Where is lingual lipase active?
- active in the mouth and stomach
49
Where is pancreatic lipase active?
- active in the small intestine
- requires phospholipids and bile acids for activation
50
When is bile released?
in response to CCK
51
What is bile, where is it stored, where is it synthesised?
- bile is a greeny-brown fluid which is synthesized in the liver and stored in the gall bladder
52
What does bile excrete?
- excess cholesterol and toxic breakdown products of haemoglobin (bile pigment)
53
How is bile returned to the liver?
- enterohepatic circulation
54
How is fat emulsified?
- Bile salt is sprayed through the pyloric sphincter
- lipid droplets are coated with bile salts, preventing them from coalescing
55
How is fat digested?
1. gastric lipids act o the fat droplets and starting to break them down
2. once in the duodenum pancreatic lipase binds to the droplets and with the aid of colipase start to breakdown the fats into fatty acids and monoglycerides
3. FA and monoglycerides diffuse into micelles which merge with the brush border and are absorbed into enterocytes
56
How are chylomicrons formed?
- TAG is conjugated with proteins and phospholipids
57
How are fatty acids released into the tissues?
- some of the TAG is broken down
58
Where vitamins and minerals absorbed?
- mainly in the small intestine
59
What mechanisms are used to absorb vitamins and minerals?
- passive diffusion
- carrier mediated transport
- active transport
60
How is water absorbed?
- the absorption of water follows electrolytes and is mainly linked to sodium movement, occurs in the small and large intestines
