Lipid Digestion And Absorption

Question 1

Major lipid components of the diet

How many grams per day?

Answer 1

90% triacylglycerol; rest is cholesterol, cholesterol esters (cholesterol with fatty acid associated), phospholipids, and FFAs

81 grams per day

Question 2

1. Name the two acid lipases
2. What do they target
3. Where do they function
4. What do they release/digest

Answer 2

1. Lingual lipase and gastric lipase
2. TAG containing short and medium chain fatty acids
3. In the stomach at pH 4-6
4. Milk fat; especially important in neonates when milk is primary food source (gastric lipase specifically)

Question 3

1. Lingual lipase is secreted from?

2. Gastric lipase is secreted from?

Answer 3

1. Glands at the back of the tongue (does not digest FAs in the mouth)
2. Gastric mucosa

Question 4

Emulsification requires two complementary actions:

Answer 4

Mechanical agitation and bile salts secretion

Question 5

1. Purpose of mechanical agitation?

2. Purpose of bile salt secretion?

Answer 5

1. Increases lipid droplet SA
2. stabilize particles as they become smaller, prevents them from coalescing (Made in liver, stored in gallbladder, secreted to small intestine); also helps keep things somewhat water soluble

~making components smaller increases SA

Question 6

Pancreatic enzymes digest what 3 things

Process of digestion?

Answer 6

Dietary TAG, cholesteryl esters and phospholipids

Removal of specific FAs by breaking ester bonds attaching the fatty acids

Question 7

Pancreatic lipase

1. Target substrates
2. Relative efficiency
3. Products produced

Answer 7

1. TAG digestion: Fatty acids at carbon 1 and 3 of glycerol backbone
2. High catalytic efficiency
3. 2 FFA, a 2-monoacylglycerol

Question 8

Function of colipase

Answer 8

Binds to pancreatic lipase to anchor it at the lipid-aqueous interface to promote its activity when inhibitory bile salts are present

~bile salts inhibit pancreatic lipase; colipase will help prevent that

Question 9

Cholesterol esterase

1. Target substrates
2. Products produced
3. Secreted to

Answer 9

1. Cholesteryl esters (mostly nonesterified cholesterol, but also esterified cholesterol)
2. Cholesterol and FFAs (nonesterified)
3. From pancreas to small intestine

~bile salts promote cholesterol esterase activity

Question 10

Phospholipase A2

1. Target substrates
2. Important for
3. Products produced

Answer 10

1. Phospholipids
2. Releasing arachnoidic acid from phospholipid bilayer
3. Lysophospholipid and a FFA

Question 11

Lysophospholipase

1. Target substrates
2. Products produced

Answer 11

1. Lysophospholipid formed from phospholipase A2 activity

2. One fatty acid and one glycerylphosphoryl base

Question 12

What do gut endocrine cells (in mucosa of small intestine) sense? (2 things)

What two hormones do these cells release?

Answer 12

Lipids and partially digested proteins, and the low pH of chyme entering the intestine

Cholecystokinin (CCK) and secretin

Question 13

1. When will CCK be secreted to the blood?
2. What two things does this hormone cause?
3. What does this hormone inhibit?

Answer 13

1. In response to the presence of lipids and partially digested proteins
2. Promotes pancreatic enzyme secretion and causes gallbladder to release bile
3. Decreases gastric motility, reducing release rate of gastric contents to small intestine

~CCK stimulates digestion of fat and protein

Question 14

1. When will secretin be secreted into the blood?

2. Secretin promotes the release of?

Answer 14

1. In response to low pH of chyme entering the intestine
2. Bicarbonate rich solution from pancreas to small intestine, providing the appropriate pH for optimal pancreatic enzyme function (neutralizes the acid)

Question 15

Lipid digestion in the jejunum generates 3 primary products:

What do these 3 products form?

Answer 15

FFAs, free cholesterol, and 2-monoacylglycerol

Micelles

Question 16

Micelles:

1. Structure
2. Formed by
3. How are they absorbed?
4. Function

Answer 16

1. Disk shaped clusters with hydrophobic interior and hydrophilic exterior
2. FFAs, free cholesterol, 2 monoacylglycerol, vitamins ADEK, and bile salts
3. Associate with the brush border membrane of enterocytes where they are absorbed
4. Necessary for cholesterol to be absorbed (since cholesterol is hydrophobic); short/medium chain FAs do not need micelles)

Question 17

What happens if you are not eating a lot of dietary fat?

Answer 17

You will have a problem making micelles, and will have a hard time absorbing fat soluble vitamins into your body

Question 18

Lipids are processed where in the cell?

Answer 18

Smooth ER (site of lipid biosynthesis)

Question 19

1. What enzyme converts long chain fatty acids to their activated form
2. Function of TAG synthase?
3. Function of acyl CoA:cholesterol acyltransferase

Answer 19

1. Thiokinase (fatty acyl-CoA synthetase)
2. Converts 2-monoacylglycerol to TAG (by adding activated FAs one at a time)
3. Esterifies cholesterol with a fatty acid

Question 20

What is the fate of short and medium chain fatty acids?

Answer 20

They do not become activated; they are released into portal circulation (likely carried by serum albumin to the liver)

Question 21

Since TAG and cholesteryl esters are hydrophobic, what has to happen in order for them to be secreted by intestinal enterocytes

Answer 21

They need to be packaged into stable lipid droplets called chylomicrons

Question 22

Chylomicrons:

1. Composition
2. Where are they produced?
3. Fate?

Answer 22

1. TAG and cholesteryl esters surrounded by a single layer of phospholipids, cholesterol and apolipoprotein B-48
2. Produced at ER of the enterocytes
3. Exocytosed into the lacteals (lymph vessels that originate at the villi of small intestine)

Question 23

What is chyle

Answer 23

Lymph which contains chylomicrons

Question 24

Lipoprotein lipase

1. Origin
2. Localization
3. Action

Answer 24

1. Associates with lumen endothelial cells of capillary beds
2. Synthesized and secreted primarily by muscle and adipose (but also heart, lung, kidney, and liver)
3. Enzymatically degrades TAG in the circulating chylomicrons to FFAs and glycerol

Question 25

Fate of glycerol

Answer 25

Taken up by the liver and used to produce glycerol-3-phosphate which can enter glycolysis or gluconeogenesis or be used to make TAG (storage)

Question 26

1. Lipid digestion and absorption problems can cause?

2. Causes of this (4)

Answer 26

1. Increased excretion of essential dietary lipids in the feces
2. Pancreatic insufficiency, cystic fibrosis, defective mucosal cells, or shortened bowel

Question 27

Cystic fibrosis

1. This disease affects
2. How is that relevant to lipid digestion?
3. How does it present in patients
4. Treatment?

Answer 27

1. Cl- ion channel called CFTR that is important for hydration (viscosity of mucus)
2. Reduced function of CFTR in secretory ducts of pancreas results in thick mucus that blocks secretion of pancreatic enzymes to small intestine (pancreatic insufficiency)
3. Delayed growth and energy deficiency (due to reduction in dietary calories and decreased fat soluble vitamin uptake)
4. Dietary needs assessment; enzyme replacement and fat soluble vitamin supplements