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GI Anatomy & Phyiology > Nutrient digestion & absorption > Flashcards

Nutrient digestion & absorption Flashcards

1
Q

What are the principle dietary constituents?

A 
Carbohydrates
Protein
Fat
Vitamins
Minerals
Water
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2
Q

Monosaccharides:

  1. Give examples of hexose sugars.
  2. Where are they absorbed?
A
  1. Glucose, galactose, fructose

2. Small intestine.

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3
Q

Disaccharides:

  1. What links 2 monosaccharides together?
  2. What breaks them down?
  3. Function of lactase?
  4. Function of sucrase?
  5. Function of maltase?
A
  1. Glycosidic bond
  2. brush border enzymes in small intestine to monomers.
  3. Lactose = glucose & galactose.
  4. Sucrose = glucose & fructose
  5. Maltose = glucose & glucose
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4
Q

Give examples of polysaccharides.

A

starch
cellulose
glycogen

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5
Q

What is starch and name 2 forms of it?

A

Plant storage form of glucose. a-amylose: glucose linked in straight chains. Amylopectin: glucose chains highly branched.

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6
Q
  1. What is cellulose?

2. How is it digested?

A
  1. Constituent of plant cell walls. Unbranched, linear chains of glucose linked by b-1,4 glycosidic bonds.
  2. Dietary fibre - not digested in vertebrates. Bacteria does it using cellulase.
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7
Q

What is glycogen?

A

Animal storage form of glucose. Glucose linked by a-1,4 glycosidic bonds.

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8
Q

Which enzymes hydrolyses glucose monomers linked by a-1,4 glycosidic bonds found in starch and glycogen? Does it digest cellulose as well?

A

Amylases (saliva, pancreas): breaks down polymer to disaccharides. a-amylase can break a-1,4 bonds but not b-1, 4 bonds found in cellulose.

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9
Q

List ways molecules can enter/leave a cell.

A
  • Transcellular
  • Paracellular (around the cell - through gap junctions?)
  • Vectorial (unidirectional) transport.
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10
Q

How does glucose enter the cell?

A

Movement of Na out into blood & K into cell by Na/K ATPase (primary active transporter: uses ATP) sets up an electrochemically gradient. Glucose & Na are transported down that gradient from lumen to cell by a symporter (has binding site for both Na & glucose) SGLT1 (secondary active transporter: doesn’t use ATP directly). GLUT-2 transports glucose from cell to blood (facilitated diffusion). Na transported to blood via ATPase. overall movement of Na from lumen to blood creates osmotic gradient which H2O follows to get from lumen to blood.

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11
Q
  1. T/F: fructose is transported into cell from lumen by GLUT-5.
  2. T/F: Water is also transported with fructose.
A
  1. True. SGLT1 transports glucose & galactose.

2. False. No Na is moved across so no water absorption.

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12
Q
  1. What are proteins?
  2. Give examples of post-transitional modification of proteins.
  3. What is a peptide
A
  1. Polymers of AAs linked by peptide bonds.
  2. Addition of CHO = glycoprotein; lipid = lipoprotein.
  3. small proteins, 3-10 AAs long.
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13
Q
  1. What are peptide bond hydrolysing enzymes called?

2. What are the 2 classes of enzymes?

A
  1. Proteases: digest proteins. Peptidases: digest peptides.
  2. Endopeptidases (breaks down interior of proteins to give 2 peptides).
    Exopeptidases (work only on terminal branches to make single AAs & a peptide): aminopeptidase work on NH2 end, carboxypeptidases work on COOH end.
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14
Q

How are AAs transported from lumen to blood?

A

Similar to glucose (Na dependent transport). SAAT1 (Na, AA transporter) transports both Na & AA from lumen to cell. ATPase transports Na to blood. Another transporter takes AA from cell to blood. Water also transported as Na moves from lumen to blood as well.

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15
Q

What other ways protein is transported from lumen to blood?

A

PepT1 (proton dependent transporter) moves H & dipeptide into cell from lumen. H transported back to lumen by NHE3 (Na H exchanger) which creates acidic microclimate (proton layer by apical membrane) & Na transported into cell. Na then uses Na/K ATPase to get from cell to blood. Unknown transporter takes dipeptides from cell to blood. 70% of protein transfer via this system.

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16
Q
  1. Almost all ingested fat is in form of?
  2. Triacylglycerol is made of?
  3. All fat digestion happen where and by whom in adults?
  4. Triacylglycerol digested by which enzyme into what?
A
  1. Triacylglycerol.
  2. Glycerol & 3 stearic acids.
  3. Small intestine by pancreatic lipase.
  4. Lipase. into monoglyceride + 2 fatty acids. This is very slow as digestion can only take place at surface of droplet due to lipase being water soluble rather than fat soluble so can’t access the core of droplet.
17
Q
  1. Define emulsification.

2. What does emulsification requires?

A
  1. dividing large lipid droplets into smaller droplets (~1 mm diameter), this increases surface area and accessibility to lipase action.
  2. Mechanical disruption: of large lipid droplets into small droplets. Smooth muscle contraction grinds and mixes luminal contents.
    - Emulsifying agent: prevents small droplets reforming into large droplets. Bile salts & phospholipids secreted in bile. Amphipathic molecules. Non-polar portions associate with non-polar interior of lipid droplet leaving polar portions exposed at water surface. Polar portions repel other small lipid droplets (also coated with bile salts/phospholipids) thus preventing reformation into large ones.
18
Q

How is the absorption of lipase digestion products enhanced?

A

Via formation of micelles.

19
Q

What are micelles made of?

A

Micelle = bile salt + monoglycerides + fatty acids + phospholipids. They are 4-7 micrometre in diameter. Micelle surface = polar portions of molecules. Micelle core = non-polar portions of molecule.

20
Q
  1. What does micelle breakdown release?

2. T/F micelles are not absorbed themselves.

A
  1. Small amounts of free fatty acids (FFA) & monoglycerides into solution. These diffuse across plasma membrane of absorbing cells.
  2. True.
21
Q

What happens to fatty acids & monoglycerides when they enter the epithelial cell?

A

They enter sER where they are reformed into triacylglycerols by enzymes located within the sER.

22
Q

What happens to reformed triacylglycerols droplets in the epithelial cell?

A

They are coated with amphiphatic protein (emulsification) & then transported through cell in vesicles formed from sER membrane (processed through Golgi apparatus & exocytosed into ECF at serosal membrane.

23
Q

What are triacylglycerols called in ECF and what additional stuff do they contain?

A

Chylomicrons (1 micrometre diameter). Alongside fat digestion products, they also contain phospholipids, cholesterol & fat-soluble vitamins.

24
Q

Where do chylomicrons in ECF go?

A

Pass into lacteals (branch of lymphatic system which then drains into lymphatic duct which drains into IVC) between endothelial cells (cannot pass through capillary basement membrane due to being too big - good as don’t want fat in blood). Once in circulation it is always bound to some proteins.

25
Q

Name fat soluble vitamins.

A

Vitamin A, D, E, K (follow same absorptive path as fat).

26
Q
  1. Name water-soluble vitamins.
  2. How is Vitamin B12 absorbed?
  3. What does B12 deficiency cause?
A
  1. B group, C & folic acid (either absorbed by passive diffusion or carrier-mediated transport).
  2. B12 large charged molecule so binds to intrinsic factor in stomach to form complex which is absorbed via specific transport mechanism in distal ileum.
  3. Pernicious anaemia (failure of RBC maturation).
27
Q
  1. How much of daily ingested iron absorbed?
  2. How is it transported from intestinal lumen to blood?
  3. What does iron bind to in blood?
  4. What happens to ferritin levels during Hyperaemia?
  5. What happens to ferritin levels during Anaemia?
A
  1. 10%
  2. Transported across brush border membrane (via DMT1) into duodenal enterocyte. Inside cell iron incorporated into ferritin (protein-Fe complex) to store it. Unbound iron transported across serosal membrane to blood.
  3. Transferrin
  4. Increased so more iron bound in enterocytes (less released in blood).
  5. Decreased so more iron released to blood.

GI Anatomy & Phyiology (9 decks)

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