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Small intestine and pancreas Flashcards

1
Q

Where is secretin released?

A

Secreted by S cells in duodenum

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2
Q

What is the action of secretin?

A
  • Causes secretion of bicarbonate to buffer pH of chyme entering SI
  • Triggers insulin release
  • Regulates secretion in stoamch, pancreas and liver
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3
Q

Under what circumstances is secretin released?

A

Stimulated by HCl

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4
Q

Where is cholecystokinin released from?

A

I cells in mucosal epithelium of small intestine (secreted into duodenum)

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5
Q

Under what circumstances is cholecystokinin released?

A

Stimulated by HCl, amino acids and fatty acids

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6
Q

What is the action of cholecystokinin?

A

Stimulates release of digestive enzymes from pancreas and bile from gall bladder

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7
Q

Where is gastric inhibitory polypeptide released from?

A

Synthesised by K cells in small intestine

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8
Q

Under what circumstances is gastric inhibitory polypeptide released?

A

Stimulated by hyperosmolarity of glucose in duodenum

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9
Q

What is the action of gastric inhibitory polypeptide?

A
  • Inhibits HCl secretion in stomach
  • Inihibits gastric motility by inhibiting secretion of acid and pepsin
  • Induces insulin secretion from pancreas
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10
Q

What are the 3 theories of appetite regulation in common domesticated species?

A
  • Glucostat
  • CCK
  • Lipostat
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11
Q

Briefly outline the glucostat theory of appetite regulation

A
  • Involves glucose, VFAs, AAss
  • high levels of these cause satiety centre to be activated
  • Inhibit appetite centre
  • Reduce feeding behaviour
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12
Q

Briefly outline lipostat theory of appetite regulation

A
  • Fat deposits and leptin

- Fat deposits trigger leptin release and activate satiety centre

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13
Q

What duodenal factors inhibit stomach emptying?

A
  • Secretin
  • Gastric inhibitory polypeptide (GIP)
  • Cholecystokinin (CCK)
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14
Q

Describe the intestinal phase of digestion in the duodenum

A
  • pH of stomach low, duodenum fragile
  • When pH in duodenum too low, triggers release of secretin
  • Triggers release of bicarbonate from pancreas
  • Contraction of gall bladder controlled by CCK
  • Sensitive to fat, bile reeased by gall bladder
  • Reduces size of fat blobs coming out of stomach
  • Enzymes used to digest protein
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15
Q

What is the effect of bicarbonate releas from the pancreas to the duodenum?

A
  • Increases pH to stop acid damaging duodenum
  • Creates environment stomach enzymes and proenzymes cannot survive in
  • Blocks overall digestion process
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16
Q

Describe the intestinal phase of digestion in the large intestine

A
  • Digestion carries on
  • Movement occurs slowly - 4 types of contraction
  • Segmentatino, peristalsis, antiperisalsis and mass movement
  • Mass movement driven by stretch of wall
  • Stomach and duodenum promote large intestine contraction via long reflex mesenteric nervous syste (Gastro-colic reflex)
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17
Q

What are the polysaccharides in starch?

A
  • Amylose

- Amylopectin

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18
Q

Describe amylose

A
  • Linear structure
  • alpha-1-4-linkage
  • Cabrons 1-4 are linked
  • Ppolar
  • H bonds between monomers hold it in spiral shape
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19
Q

Describe amylopectin

A
  • Branched chains
  • Alpha 1-6 linkage
  • Starch molecules
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20
Q

What does the structure of amylopectinmean for starch?

A

Starch molecules clustered in granules and are insoluble in water

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21
Q

Describe glycogen

A
  • Glucose store in animals
  • Made up of single highly branched polysaccharide
  • Has both alpha 1-4 and alpha 1-6 linkages
  • Usually more branched than amylopectin
  • More water soluble, increased branching means water can get trapped in spaces
  • Stored as granules in liver and skeletal muscle
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22
Q

What is the action of alpha-amylase on polysaccharides?

A
  • Break bonds between monosaccharides
  • Hydrolysis
  • Substitutes bond for water molecule
  • Preferentially breaks alpha-1-6-linkages
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23
Q

Desscribe glucose digestion and absorption in the small intestine

A
  • In mouth, salivary apha-amylase begins starch digestion to maltose, some glucose and dextrins
  • Complete in small intestine by pancreatic enzymes
  • Luminal phase then membranous phase to complete digestion
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24
Q

Describe the luminal phase of carbohydrate digestion in the small intestine

A
  • Starch breakdown started by alpha-amylase completed by pancreatic amylase to maltose
  • Cannot yet be absorbed
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25
Q

Describe the membranous phase of carbohydate digestion in the small intestine

A
  • Disaccharides to monosaccharides by glucosidase enzymes (maltase, sucrase, lactase)
  • Known as brush border enzymes as are located here
  • Resulting monosaccharides transported across intestinal lumen
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26
Q

How are glucose and galactose absorbed in the small intestine?

A

Actively

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27
Q

How is fructose absorbed in the small intestine?

A

Facilitate transport

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28
Q

What is the absorption of glucose limited by?

A

The rate of epithelial transport only

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29
Q

What is the rate of lactose digestion limited by?

A

Rate of hydrolysis

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30
Q

Describe cellulose

A
  • Non-starch polysaccharide
  • beta 1-4 linkages
  • Structural role, major component of plant cell wall
  • Polysaccharide of glucose
  • No branching
  • Insoluble in water, indigestible in humans
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31
Q

Describe the locationof the pancreas

A
  • Adjacent to stomach and duodenum

- One lobe lies within duodenal flexure

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32
Q

List the products released by the endocrine function of the pancreas

A
  • Glucagon
  • Insulin
  • Somatostatin
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33
Q

What cell type secretes glucagon?

A

Alpha cells

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34
Q

What cell type secretes insulin?

A

Beta cells

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35
Q

What cell type secretes somatostatin?

A

Delta cells

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36
Q

What cell type secretes digestive enzymes?

A

Cells of the pancreatic acini

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37
Q

What is produced by the exocrine function of the pancreas?

A

Digestive enzymes

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38
Q

Where are the digestive enzymes produced in the pancreas?

A

Pancreatic acini

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39
Q

What cells are in the Islets of Langerhans?

A
  • Alpha, beta, delta
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40
Q

List the enzymes produced by the exocrine pancreas

A
  • Trypsin
  • Chymotrypsin
  • (Pro)elastase
  • Pro-carboxypeptidase
  • Lipase
  • Phospholipase
  • Amylase
  • Ribonuclease
  • Deoxyribonuclease
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41
Q

What are the functions of trypsin, chymotrypsin and (pro)elastase?

A
  • Act on proteins and peptides
  • Cleave interior peptide bonds
  • Are endonucleases
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42
Q

What is the functin of pro-carboxypeptidase?

A
  • Acts on peptides/proteins
  • Releases amino acid at carboxy terminal
  • Exopeptidase
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43
Q

What is the function of lipase?

A
  • Acts on triglycerides

- Cleaves ester bond at 1&3 positions yeilding free fatty acids and monoglycerides

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44
Q

What is the function of phospholipase?

A
  • Acts on phospholipids

- Cleaves ester bond at 2- position of phospholipids

45
Q

What is the function of amylase?

A
  • Acts on starch (polysaccharides)

- Cleaves to maltose

46
Q

What is the function of ribonuclease?

A
  • Acts on RNA

- Cleaves RNA to nucleotides

47
Q

What is the function of deoxyribonuclease?

A
  • Acts on DNA

- Cleaves DNA to nucleotides

48
Q

Define “zymogen” and give examples

A

Inactive nezyme precursor

- Trypsinogen, chymotrypsinogen, pepsinogen, proelastase, procarboxypeptidase

49
Q

Define exopeptidase

A

Detach terinal acids from polypeptides e.g. aminopeptidases (on membrane of small intestine only) or carboxypeptidases

50
Q

Define endopeptidase

A

Hydrolyse internal peptide bonds of a protein (e.g. typsin, chymotrypsin, pepsin, elastase)

51
Q

Define protease

A

An enzyme that conducts proteolysis i.e. begins protein catabolism by hydrolysis of the peptide bonds between adjacent amino acids in polypeptide chain

52
Q

Name the groups of proteases

A
  • Serine proteases (trypsin, chymotrypsiin)
  • Threonine proteases
  • Cysteine proteases
  • Aspartic acid proteases
  • Metallo proteases
  • Glutamic acid proteases
53
Q

Describe the mechanism of action of serine endopeptidases

A
  • Synthesised by pancreatic acinar cells
  • Secreted into SI
  • Share similar structure, differ in peptide bond they cleave
  • Highly specific to regions of polypeptide chain, based on side chains of amino acid residues surrounding site of cleavage
54
Q

What bonds are cleaved by chymotrypsin?

A
  • Bonds following large hydrophobic amino acid residues

- e.g. on C-terminal side of phenylalanine, tryptophan and tyrosine residue

55
Q

What bonds are cleaved by trypsin?

A
  • Bonds after positively-charged amino acid residue

- e.g. C-terminal side of arginine and lysine residue

56
Q

What bonds are cleaved by elastase?

A
  • Bonds after small neutral amino acid residue

- e.g. alanine, glycine and valine (connective tissues in meat)

57
Q

What is an aminopeptidase?

A

An exopeptidase that attacks amino terminal (N-terminal) of peptides secreted from small intestine

58
Q

What are the 2 main pathways of protein absorption?

A
  • Peptide transport

- Single amino acid transport

59
Q

Describe the peptide transporter

A
  • High afinity for di-tripeptides
  • Prefers peptides with L-amino acids and is driven by electrochemical gradient produced by Na+ pump
  • Represents majority of peptide transport i.e. few enter as single amino acids
60
Q

Describe the amino acid transporter

A
  • From intestinal lumen is an active process
  • Involves Na+ dependent, carrier mediated cotransport system similar to that for glucose
  • Selective carrier systems for certain groups of amino acids
  • Some sharing of transporters
61
Q

What are the 4 groups of amino acids within which transporters are shared?

A
  • Neutral amino acids (neutral brush border system)
  • Acidic (dicarboxylic) amino acids
  • Imino amino acids (glycine, proline and hydroxyproline)
  • Basic amino acids
62
Q

Define exocrine

A
  • “out of the body” into gut

- Ducts to get where needs to be

63
Q

Define endocrine

A
  • Into the body (tissues)

- No ducts

64
Q

Name the major components of bile

A
  • Polar derivatives of cholesterol
  • Lecithin
  • Bile pigments (bilirubin)
  • Bila salts and bile acids
65
Q

What are bile acids and where are they formed?

A
  • Polar derivatives of cholesterol
  • Amphipathic molecules with detergent like properties, emulsion
  • Formed in liver, secreted into gall bladder
66
Q

Describe the breakdown of triacylglycerols by lipases

A
  • CCK stimulates bile secretion into intestine, aid digestion of fats and fat-soluble vitamins
  • Bile acids start emulsion process
  • Phospholipase A2 (pancreas) transforms lecithin (bile) into lysophospholipid that acts as a strong detergent
  • Large drop of fat covered by components of bile to form emulsion
  • Lingual lipase first then pancreatic lipase
  • Pancreatic lipase secreted from pancreas as proenzyme activated by trypsin
  • Catalyse hydrolysis of TAGs at positions 1 an 3, forming 1,2-diacylglycerols and then 2 monoacylglycerols
  • Colipase aids binding of enzyme at lipid-water interface
67
Q

Why is the majority of lipid taken up as monoacylglycerol?

A
  • Lipase removes 2 fatty acids per TAG

- Ratio of 2:1 monoacylglycerols to cleaved TAG, so 70% lipid taken up as MAG

68
Q

Outline the basics of lipid absorption by intestinal cells

A
  • micells needed for lipid to be able to enter cells
  • Concentration gradient aids passive absorption into enterocyte
  • Intracellular fatty-acid binding protein maintains this gradient
  • Fat components accumulate inside ER of epithelial cells
  • TAG molecules reformed
  • Chol transformed into cholesterase
69
Q

Describe chylomicrons

A
  • TAG and cholsterol esters carried in core
  • Outer skin of chylomicrons made up partly of amphipathic phospholipids, partly of free cholesterol, partly of protein
  • Cholesterol esterified to increase hydrophobicity
  • TAGs/lipids/FAs/chol/cholesterols gather in ER to form chylomicrons (lipoproteins)
70
Q

What are the different of lipoprotein?

A
  • Chylomicron
  • Very low density lipoprotein
  • Intermediate density lipoprotein
  • Low density lipoprotein
  • High density lipoprotein
71
Q

What is the major protein in chylomicrons?

A

Apolipoprotein B

72
Q

What is the function of apolipoprotein B in chylomicrons?

A
  • Forms hydrophilic shell around lipid layer and allows it to form a stable structure in the blood
  • Activates lipoprotein lipase
73
Q

How do lipoproteins get around the body?

A
  • use lymphatic circulation cia thoracic duct
  • Too big to enter systemic circulation via capillaries
  • Bypass hepatic metabolism
74
Q

What is the function of lipoprotein lipase?

A
  • Enzyme attached to lumenal surface of small blood vessels

- Catalyses hydrolytic cleavage of fatty acids from TAGs of chylomicron

75
Q

What happens the the fatty acids and monoacylglycerols released from chylomicrons?

A

Picked up by body cells for use as energy sources

76
Q

Describe chylomicrons in terms of density

A

Largest, lowest in density

  • High lipid/protein ration
  • Highest % weight of TAG
77
Q

Describe VLDLs in terms of density

A

Very low density lipoprotein

- 2nd highest in TAG as % weight

78
Q

What does IDL stand for?

A

Intermediate density lipoprotein

79
Q

Describe LDLs in terms of density

A
  • low density lipoprotein

- Highest in cholesteryl esters as % of weight

80
Q

Describe HDLs in terms of density

A
  • High density lipoproteins

- Highest in density due to high protein/lipid ratio

81
Q

What is meant by Michaelis-Menton kinetics?

A
  • At low substrate concentrations the substrate is rate limitigng - 1st order kinetics (kinetics reduces over time as substrate is used up)
  • At high substrate concentration the enzyme is at maximal capacity - zero order kinetics
82
Q

What is Vmax?

A

The macimum rate of the reaction at the maximum substrate concentration

83
Q

What is Km?

A

The rate constant

  • Measure of enzyme-substrate complex stability
  • Indicator of enzyme affinity for substrate
84
Q

What is the importance of Km?

A
  • Describes the enzyme affinity for substate
  • Concentrations fo susbtrates in cells should be approx. the Km
  • Used in lab assays to get results in correct range
  • Depends on precise relationship between substrate and enzyme
85
Q

Why should the concentration of substrates in cells be close to the Km?

A

Allows good physiological regulation

86
Q

What is shown by a low Km?

A

High affinity

87
Q

What is shown by a high Km?

A

Low affinity

88
Q

What are the main types of enzyme inhibitors?

A
  • Competitive vs non-competitive
  • Irreversible vs reversible
  • Competitive reversible
  • Non-competitive
  • Irreversible
89
Q

Descrieb the basics of a competitive reversible enzyme inhibitor

A
  • Mimics substrate or transition state analogue
  • Substance competes with substrate for active site
  • Negative feedback signal sent to turn off production or release of susbtrate
  • Not transformed into a product
  • Reversible
  • Amount of inhibition depends on Ki
  • Increases Vm, no change in Vmax
90
Q

What is a transition state analogue?

A
  • Enzyme and substrate combined together

- Enzyme-substrate complex

91
Q

What is meant by Ki?

A
  • Enzyme affinity for inhibitor

- Enzyme may bind preferentially to inhibitor over substrate

92
Q

Describe non-competitive inhibition

A
  • Substrate and inhibitor bind at different site
  • Little to no resembalnce to substrate
  • Able to bind to enzyme even if E already bound to substrate
  • Causes change in 3D structure
  • Decreases Vmac, no effect on Km (no change to substrate concentration)
  • End product design to bind to allosteric site when high concentrations present to stop production of product, prevent accumulation
93
Q

Describe irreversible inhibition

A
  • Forms covalent bond to amino acid near/at active site
  • Permanently inactivates enzyme
  • Susceptible AA residues e.g. -OH, -SH (ser, cys)
94
Q

Define the term enzyme

A

Proteins that function by accelerating chemical reactions in biological systems

95
Q

List the key features of enzyme action

A
  • Higher reaction rates
  • Milder conditions required
  • Greater reaction specificity
  • Capacity for regulation - allostery, covalent modification
96
Q

Explain how enzyme specificity is produced

A
  • Small cleft on surface
  • Often non-polar
  • Binds complementary substrate
  • Highly conserved (same sacross families and species)
  • Specific due to shape
  • 2 models of substrate binding - lock and key and induced fit
  • Depends on specific spatial arrangement of amino acids/atoms
  • Able to distinguish stero-isomer
97
Q

Describe the lock and key mechanism of enzyme-substrate binding

A

Substrate fits exactly into the active site

98
Q

Describe the induced fit mechanism of enzyme-substrate binding

A
  • Enzyme fits around the substrate
  • substrate binding induces conformation change in enzyme
  • optimal orietation for formation of transition state
  • Stabilisation of transition state
  • Exlusion of water
99
Q

Define activation energy

A

The energy required to make a reaction happen

- Is lowered by enzymes

100
Q

Define specific activity

A

The way in which an enzyme will only act on one or two substrate

101
Q

List the steps involved in enzyme catalysis

A
  • Stabolose transition state
  • Alter substrate orientation
  • Change substrate reactivity
  • Induce substrate strain
  • Exclude water
102
Q

Describe the enzymes function in controlling substrate orientation in enzyme catalysis

A
  • Hold substrate together correctly to allow reaction to take place
  • Lines things up to allow interaction
103
Q

Describe the enzyme’s role in changing substrate reactivity in enzymes catalysis

A
  • Take susbtrate with slightly wrong ionic charge, temporarily give correct ionic charge and allow substrate to become reactive
  • Can be done by acid base catalysis (removal/addition of protins - donor/acceptor in active site)
  • Also by covalent catalysis - nucelophilic/electrophilic substitution, reactive amino acids and cofactors
104
Q

Explain why substrate strain is induced in enzyme catalysis

A
  • Expose correct part of substrate

- Allows that part to tak epart in reaction

105
Q

Explain why water is excluded in enzyme catalysis

A

Allows reaction to tak eplace without interference

106
Q

Describe the histological appearance of the duodenum

A
  • Villi loonger

- Submucosa expanded due to presence of Brunner’s glands

107
Q

Describe the histological features of the ileum

A
  • Lymph tissue (Peyer’s patches) present

- Short villi

108
Q

Describe the histological appearance of the jejunum

A
  • Shorter villi

- Thinner submucosa than duodenum

109
Q

Describe the histological appearance of the pancreas

A
  • Glands present
  • Lobules and conective tissue septa = looks similar to salivary glands
  • Islets’s of Langerhans present
  • Secretory acini present
  • Islets are paler than acini (but look quite similar)

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