Digestion & Absorption Flashcards

1
Q

Define Digestion

A

Sequential ordered process with physiological links between each stage. Each specific stage allows the interaction of fluid and emulsifying agent, acid-base exchange, enzyme and substrate contact. Each stage is regulated and integrated.

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2
Q

Why is digestion important?

A

Food contains a mix of nutrients, often within a complex chemical / compound that cannot be directly absorbed and digestion allows for access to the desired nutrients

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3
Q

What are the two types of digestion

A

Chemical & Mechanical

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4
Q

What is the first part of digestion called

A

Mastication

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5
Q

Name the functions of the mouth in digestion?

A

Mechanical Digestion -> chewing
Chemical Digestion -> salivary lipases & amylase
Propulsion of food into oesophagus

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6
Q

Give 3 functions of saliva in digestion

A

Moistens food
Buffers pH of food
Antibacterial (immunoglobulins and lysozymes)

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7
Q

How does the oesophagus move food from mouth to stomach

A

Peristalsis (oesophagus tissue can secrete mucous for aid)

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8
Q

Give 3 cell types in the stomach

A

Epithelial, Parietal Epithelial, Chief Cells

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9
Q

What is the function of epithelial stomach cells?

A

Produce mucous and bicarbonate which makes an alkaline barrier to protect the epithelium and stomach from the acid produced -> i.e. cytoprotective

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10
Q

What is the function of stomach parietal epithelial cells

A

Secrete hydrogen and chloride ions to form HCl acid and also secretes intrinsic factor

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11
Q

What are the functions of stomach chief cells

A

Secretion of zymogens and enzymes (pepsinogen, chymosin (carbohydrate enzymes) and gastric lipase).

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12
Q

Give 4 function of the stomach

A
Stores chyme
Absorbs ethanol and lipid soluble drugs
Mechanical digestion -> churning
Chemical digestion -> gastric enzymes and acid
Sterilisation of food stuff
Inactivation of salivary amylase
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13
Q

What is absorbed in the stomach?

A

Ethanol

Lipid soluble drugs

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14
Q

Explain the structure of the small intestine in regards to surface area

A

Mucosal folds forms villi

Apical villi contain further folds called microvilli

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15
Q

Give 3 functions of the duodenum

A

Receives chyme from the stomach
Receives digestive juices from sphincter of Oddi (pancreatic, gall bladder and liver)
Duodenal gland cells secrete mucous and bicarb for lubrication and neutralisation
Fatty acid absorption

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16
Q

Give the main function of the jejunum

A

Bulk of absorption takes place here - particularly carbs and amino acids

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17
Q

Give 3 main functions of the ileum

A

Reabsorption of bile salts
Absorption of vitamins (+ particularly Vit B12 bound to IF)
Transport undigestible matter into colon via ileocaecal valve

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18
Q

Give 2 function of gall bladder/liver secretions in the lumen

A

pH neutralisation & fat emulsification

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19
Q

Give 2 main functions of pancreatic secretions in the lumen

A

pH neutralisation, chemical digestion (lipase, amylase and proteases)

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20
Q

Give the 3 main parts of the large colon along with one function

A

Caecum - receives undigested food
Colon - home of intestinal flora, absorbs water & mineral salts
Rectum - storage of faeces until defecation

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21
Q

What is starch made up of?

A

Two polysaccharide chains called Amylose & Amylopectin

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22
Q

What is the bonding involved with amylose?

A

unbranched alpha 1-4, glycosidic bonds

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23
Q

What is the bonding involved with amylopectin?

A

alpha 1-4 glycosidic bonds with alpha 1-6 branching glycosidic bonds every 12 subunits of glucose

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24
Q

Explain the digestion of starch

A

Amylases in saliva and pancreatic juice breaks alpha-1,4 bonds in starch down into oligosaccharides, disaccharides (maltose), dextrins (smaller units containing the alpha-1,6 bonds).

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25
Q

What type of enzymes are found in the brush-border that digests carbohydrates / disaccharides?

A

Hydrolase enzymes

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26
Q

Name the three most common brush border enzymes involved in disaccharide digestion and their targets with breakdown products

A

Maltase -> Maltose -> 2 x glucose
Sucrase -> Sucrose -> 1 glucose & 1 fructose
Lactase -> Lactose -> 1 glucose & 1 galactose

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27
Q

What brush border enzyme can break down dextrin and why?

A

Sucrase-Isomaltase -> can hydrolyse both the 1,6 and the 1,4 glycosidic bonds

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28
Q

What transporters are important in the absorption of monosaccharides

A
SGLT-1 = Glucose & Galactose into enterocyte
GLUT5 = Fructose into enterocyte
GLUT2 = All 3 out of cell basolaterally
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29
Q

Does GLUT2 require ATP?

A

Yes

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30
Q

How efficient is protein digestion & absorption

A

Extremely efficient, 1g of nitrogen is excreted out of faeces every day regardless of intake (0-250g)

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31
Q

What part of protein digestion cycle is recyclable

A

Most enzymes used in the digestion of protein are all reabsorbed and recycled

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32
Q

Where is protein mostly absorbed?

A

upper two thirds small intestine

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33
Q

What is the ‘classification’ of enzymes involved with protein digestion? (Edwards question)

A

Protease & Peptidase
Protease digests proteins in peptides
Peptidases digest peptides but are a subclass of protease
The two terms are used interchangeably

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34
Q

What is the first sight and stage of protein digestion?

A

Stomach

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35
Q

How does the stomach digest proteins

A

Acid denatures proteins
Pepsinogen is secreted by chief cells in stomach and becomes activated to form pepsin by the acid which then acts as a protease to break down the protein into smaller peptides

36
Q

What is the key feature of chyme moving to the duodenum in reference to protein digestion

A

Neutralisation turns off pepsin and turns on and allows for the pancreatic peptidases to work

37
Q

What are the two main types of peptidases that work in the small intestine and how do they differ?

A

Endopeptidases - these attack internal peptide bonds giving smaller peptides / di/tri peptides etc
Exopeptidases - these cleave off AAs at the end of peptide chains

38
Q

What 3 main digestive enzymes are found in pancreatic juices that work on protein & classify them.

A

Trypsin, Chymotrypsin -> endopeptidases

Carboxypeptidases -> exopeptidases

39
Q

Name 2 examples each of endo and exo peptidases

A
Endo = trypsin & chymotrypsin 
Exo = carboxypeptidase & dipeptidase
40
Q

What is the final enzyme on the brush border on SI that works on dipeptides for final digestion before absorption?

A

Dipeptidases (a type of exopeptidase)

41
Q

Explain the breakdown of proteins in terms of location

A

In the stomach whole proteins are broken down into large and small peptides. In the SI these peptides are further broken down into smaller peptides, tri, di peptides and AAs via the juices from pancreas. Brush border enzymes results in AAs, di and sometimes tri peptides.

42
Q

How are amino acids and peptides absorbed?

A

A family of transporters that can absorb AAs, di and tri peptides into the enterocyte.

43
Q

What features do protein transporters exhibit?

A

Stereospecificity -> prefer specific AAs only
Broad Substrate Specificity -> likes multiple AAs
Overlapping Specificty -> Allows AAs to have a choice

44
Q

How can you classify amino acid and peptide transporters?

A

Based on amino acid it transports i.e. acid, basic etc

Based on mechanism of use i.e. facilitated, active etc

45
Q

How does absorption of amino acids work to get them in the blood?

A

Only AAs can be absorbed and cross the basolateral aspect of the enterocyte. So extra enzymes are required to digest di and tri peptides into AAs within the cytosol

46
Q

Explain the process of enzyme cascade in pancreatic juice/enzymes

A

Pancreas secretes zymogens - trypsinogen, chymotrypsinogen and procarboxypeptidase. Trypsinogen is converted to active trypsin via enterokinase / enteropeptidase (found in the lumen wall of jejunal and possibly duodenal cells). Activated trypsin converts chymotrypsinogen to chymotrypsin and then chymotrypsin converts procarboxypeptidases in carboxypeptidase

47
Q

What is the main type of fat in the diet?

A

Triacylglyceride (<90%)

48
Q

What are TAGs made up of?

A

Glycerol & Fatty acids esterfied

49
Q

Name three potential places where enzymes are secreted to aid lipid digestion and absorption

A

Lingual, Gastric, Pancreatic Lipase

50
Q

Name an enzyme other than lipase that can help digest lipids

A

Cholesterol Esterase or Phospholipase

51
Q

Why do we need to emulsify lipids?

A

Lipids are non-water soluble, all the enzymes that digest lipids are water soluble. Therefore they can only act on the lipid surface of the droplets and emulsification helps to form smaller droplets to increases the SA for enzymes to act upon

52
Q

What are some of the breakdown products from lipid digestion?

A

Glycerol, mono and di glyceride, fatty acids.

53
Q

How does emulsified droplets form?

A

Mechanical digestion physically breaks up droplets into smaller ones, then bile salts and phospholipids (amphiphatic molecules) bind to the droplet surface. THe polar regions of the amphiphatic molecules allows for repelling between droplets and therefore keeping them apart and small.

54
Q

how are micelles absorbed in the SI

A

The micelles are presented to the microvilli and the acidic micro-environment results in micelle breakdown and the contents of the micelle diffuse across apical membrane

55
Q

What is the differences between fatty acid chain lengths in relation to absorption and digestion

A

Small chain FAs - not really present in diet but if they were they wouldn’t be absorbed

Medium Chain FA - these are amphiphatic so cross the mucous layer and can diffuse across the cell membrane. These then diffuse across basolateral side without processing, attach to albumin in blood and enter blood system

Long Chain FAs - requires micelle transport to pass mucou layer, microenviroment being acidic breaks micelle down and releases contents to be diffused across membrane. Contents then enter cell and reformed into TAGs in sER and golgi apparatus packaged into chylomicrons and excreted into lacteals / lymphatic system

56
Q

What are found inside chylomicrons

A

Phospholipids, Cholesterol, Fat Soluble Vitamins and newly formed TAGs

57
Q

What is the difference in FA absorption rates vs cholesterol?

A
FA = 95% (diet up to 100g)
Cholesterol = 40%
58
Q

Name the components of bile (6)

A

Bile Acids & Bicarbonate - aids digestion

Cholesterol, Toxic Metals, Bile Pigments, Lecithin - waste products from the body

59
Q

What are the main bile acids?

A
Cholic &amp; Chenodeoxycholic Acid (synthesised by liver)
Deoxycholic Acid (synthesised by gut bacteria)
60
Q

Name 4 functions of bile acid

A

Acts as detergents for FA emulsification
Promotes cholesterol, phospholipid & lecithin excretion
Promotes colonic mobility
Prevents undue dehydration of faeces

61
Q

What two mechanisms are present to help newborns digest fat?

A

Lingual serous glands secrete lipase and maternal breast milk contains lipase

62
Q

Give two causes of maldigestion and then two causes of malabsorption of fats

A

M/D = pancreatitis, liver disease, pancreatic head tumours, gall bladder disease

M/A = coeliac, gastroenteritis, inflammatory conditions (due to the sensitised epithelium)

63
Q

Does resection of the ileum lead to maldigestion or malabsorption of fats & describe the mechanism as to why

A

Ileum resections means bile salts are not reabsorbed so therefore there will be a lack of them and a lack of emulsification so therefore cannot properly digest it = M/D.

64
Q

What regulates water absorption in the intestines?

A

Osmotic pressure gradients & aquaporins

65
Q

How are electrolytes and minerals absorbed?

A

Carrier Mediated Diffusion - i.e. requires a transporter but diffuses through. Then they bind to intracellular proteins and are secreted into the blood via actie transport

66
Q

What two minerals is the absorption tightly regulated for?

A

Calcium & Iron

67
Q

How are vitamins absorbed?

A

Fat Soluble -> with micelles and chylomicrons
Water Soluble -> diffusion
Vit B12 -> bound to intrinsic factor & absorbed in ileum

68
Q

Give 3 examples of anti-vitamins

A

Chelators -> effect Mg and Ca
Phytates -> bind to Ca, Fe, Zn complexes
Tanins -> Fe complexes

High FA conc. can prevent Mg and Ca absorption
Oxidising compounds can reduce Fe2+ to Fe3+ which is no longer absorbable

69
Q

What cells secrete gastrin hormone

A

G cells in the gastric pit

70
Q

Give 3 stimulants for gastrin hormone release

A

Amino acids / peptides in stomach
Distention of stomach
Vagal stimulation (cephalic phase)
GRP (gastrin releasing peptide) from short neural reflex in the ENS

71
Q

What inhibits gastrin secretion

A

Somatostatin

Low pH in stomach

72
Q

What is the function of gastrin

A

Stimulates parietal cells to release ions

Stimulates ECL cells to release histamine

73
Q

What cell secretes CCK?

A

Duodenal Mucosal Cells

74
Q

What is the main function for CCK?

A

Aid digestion of nutrients

75
Q

What stimulates CCK release?

A

Fat and amino acids in duodenum

76
Q

Give 3 effects of CCK

A

Reduces gastric emptying
Increases pancreatic enzyme secretion
Gall bladder contraction
Relaxation / Opening of Sphincter of Oddi

77
Q

What cells release secretin?

A

Duodenal Mucosal Cells

78
Q

What is the main function of secretin?

A

Acid Neutralisation

79
Q

What stimulates secretin release?

A

Acid present in the duodenum

80
Q

What 3 results are there following secretin release?

A

Inhibits HCl secretion by parietal cells
Reduces gastric emptying
Increases bicarb secretion from the pancreas, duodenal cells, and bile duct

81
Q

What does GIP and GLP-1 hormones stand for?

A
GIP = gastric inhibitory polypeptide 
GLP-1 = Glucagon like peptide -1
82
Q

What term encompasses the GIP and GLP-1 hormones?

A

Incretin Hormones

83
Q

What are the incretin homrones

A

GIP & GLP-1

Gastric inhibitory polypeptide, Glucagon like peptide1

84
Q

What stimulates incretin hormone release

A

Anything that stimulates insulin release from pancreatic b-cells -> amino acids and glucose presence in the duodenum

85
Q

What is the overall function of the incretin hormones

A

Lower blood glucose levels

86
Q

What 3 mechanisms does the incretin hormones directly do

A

Increases insulin secretion
Decreases glucagon secretion
Reduces gastric emptying