Lipid Digestion

Question 1

What are the different lipids?

Answer 1

Fats/oils
Phospholipids
Cholesterol
Fatty acids

Question 2

What is saturated?

Answer 2

Single bonds only

Question 3

What is monounsaturated?

Answer 3

One double bond

Question 4

What is polyunsaturated?

Answer 4

2 or more double bonds

Question 5

Describe general process of lipid digestion

Answer 5

Emulsification
Enzymatically digested to yield monoglycerides + FAs

Question 6

What does lingual lipases do (mouth)?

Answer 6

Hydrolysis of triglycerides at sn3
Cleaves short chain FAs

Question 7

What do gastric lipases do?

Answer 7

Highest activity against short + medium chain triglycerides
Hydrolysis gets faster as goes on = FAs produce act as surfactants

Question 8

What does pancreatic lipase do?

Answer 8

Hydrolysis of dietary triglycerides
Hydrolyses FA at 1 + 3 position
Produces 2-monoacylglycerol + 2 FAs

Question 9

What do chyme do?

Answer 9

Stimulate secretin, cholecystokinin (CCK) + pancreatic juice secretion

Question 10

What is secretin?

Answer 10

Increase secretion of electrolyte + fluids of pancreatic juice

Question 11

What does CCK do?

Answer 11

Cause contraction of gall bladder + discharge of bile into intestines + stimulates secretion of enzymes

Question 12

Describe pancreatic lipase

Answer 12

Main digestive enzyme
*Aided by bile salts from gall bladder
HCO3- in pancreatic juice neutralises gastric acid: pH for optimal enzyme action

Question 13

Briefly describe what micelles do

Answer 13

Carry monoglycerides and FFA to brush border and diffuse into enterocytes

Question 14

Why can triglycerides not diffuse across brush border?

Answer 14

Too big

Question 15

What are the role of bile salts in lipid digestion?

Answer 15

Surround fat and break it into smaller droplets, preventing fat from reclumping
Increases the surface area for pancreatic lipase to act upon fat droplets

Question 16

What does failure to secrete bile salts result in?

Answer 16

Lipid malabsorption
Secondary vitamin deficiency

Question 17

What do bile salts do to the lipid droplet?

Answer 17

Expose hydrophobic core
= change surface charge

Question 18

What are the primary products of dietary lipid digestion?

Answer 18

Free FAs
Free cholesterol
Monoglycerides
Lysophospholipids

Question 19

What happens to fat soluble vitamins?

Answer 19

Fat soluble vitamins packaged + absorbed from mixed micelles

Question 20

Describe lipid digestion

Answer 20

Emulsification of fat droplets by bile salts
Hydrolysis of triglycerides in emulsified fat droplets into FAs + monoglycerides
Dissolving of FAs + monoglycerides into micelles to produce “mixed micelles”

Question 21

What happens to mixed micelles?

Answer 21

Trapped in brush border of intestinal mucosa
Lipid components absorbed by passive diffusion

Question 22

What happens to short + medium triglycerides?

Answer 22

Triglycerides absorbed + hydrolysed by intestinal lipases

Question 23

What do short + medium FAs NOT require?

Answer 23

Bile salts

Question 24

Describe absorption + transport in the blood

Answer 24

FA + monoglyceride enter enterocyte by simple diffusion
Transported to endoplasmic reticulum + converted to triglycerides
Golgi = packaged with cholesterol
Chylomicrons into vesicles
Vesicles transported to basolateral surface of enterocyte
Vesicles fuse with plasma membrane + undergo exocytosis
= transfer chylomicrons outside
Transported to lymphatic vessel
Lymph drains into lymphatic system then into blood
Excreted into lacteals + circulate into blood

Question 25

What does lipoprotein lipase (LPL) do?

Answer 25

Hydrolyses triglycerides held within chylomicron

Question 26

What do free FAs bind to?

Answer 26

Albumin + circulate in plasma

Question 27

What does lipoprotein core contain?

Answer 27

Cholesterol ester + TG

Question 28

What does outer layer of lipoprotein particle contain?

Answer 28

Free cholesterol, phospholipid + specific apolipoproteins

Question 29

What do apolipoproteins do?

Answer 29

Coat lipoprotein particles
= important in recognition

Question 30

Describe VLDL

Answer 30

Bind to capillary cells
Become IDLs

Question 31

How are IDL converted to LDL?

Answer 31

By hepatic lipase

Question 32

Describe LDL

Answer 32

Product of VLDL losing triglycerides
Carry majority of cholesterol in blood
In peripheral cells

Question 33

How do peripheral cells utilise LDL cholesterol?

Answer 33

For cell membrane structure + production of hormones

Question 34

Describe HDL

Answer 34

Produce in peripheral tissue
Bind to receptors in vessel walls

Question 35

What does lecithin-cholesterol acyltransferase (LCAT) do?

Answer 35

Convert free cholesterol in cholesterol ester, which moves into centre of HDL particle

Question 36

How does the absorption of cholesterol work?

Answer 36

Transport by endocytosis by NPC1L1 protein

Question 37

What are higher levels of LDL associated with?

Answer 37

Increased cardiovascular disease risk

Question 38

What are increased HDL levels associated with?

Answer 38

Decreased risk for coronary heart disease
Low levels = RISK

Question 39

What is the cardioprotective role of HDL?

Answer 39

Facilitate transfer of cholesterol from lipoproteins to liver

Question 40

What happens to fat soluble vitamins?
A, D, E + K

Answer 40

Incorporated into mixed micelles
Passively transported
Incorporated into chylomicrons
Distributed by intestinal lymphatics

Question 41

What happens to H2O-soluble vitamins?

Answer 41

Energy dependent process
Require Na+ in/dependent transporters

Question 42

How is vitamin B12 (cobalamin) absorbed?

Answer 42

Ingested in food
Salivary glands secrete haptocorin
Stomach acid released B12
Haptocorin binds B12
Stomach parietal cells release intrinsic factor
Pancreatic protease digest haptocorin in small intestine, B12 released
B12 binds to intrinsic factor
Absorbed in terminal ileum by endocytosis