Absorption of Fat

Question 1

What are the types of lipids?

Answer 1

• Triglycerides - 95% of all fats
○ glycerol covalently bonded to 3 fatty acids ( 3 ester bonds)
○ Source of energy
○ Triglycerides are too large to be absorbed
○ Chemical form of which fat exists
○ Phospholipid bilayer is made out of lipids
• Cholesterol, phospholipids and essential lipids - 5% of all fats
○ Essential lipids carry fat soluble vitamin A,D, E and K
○ Essential fatty acids - linolenic acid, alpha-linolenic acid

• Essential lipids- carry fat soluble vitamins
	○ Vitamin A: helps with vision (night vision) vitamins A deficiency reduces ability to see in the dark
	○ Vitamin D:needed for Calcium absorption ( vitamin D deficiency means less calcium for bones and teeth)-Ca2= metabolism
	○ Vitamin E:  protecting the integrity of membranes in the nervous system
	○ Vitamin K: used to increase proper blood clotting
• Essential fatty acids
	○ Regulate the nervous system
	○ Maintain cholesterol levels at a stable level 
	○ For the structural components of the cell membrane

Question 2

Why is insolubility of lipids a problem?

Answer 2

• Lipids are hydrophobic so don’t dissolve in water
• If shaken droplets will form which will eventually coalesce ( clump back together)
○ Therefore won’t move in and area with lots of water e.g.; bile, small intestine or blood
○ Needs an emulsifier to break up large lipid into many smaller droplets
• Bile salts needed to emulsify the lipid into many smaller droplets (forms micelles)
○ increase surface area for attachment of lipase- so increase rate of digestion
○ Makes lipids soluble in water as micelles have formed so lipid can move in small intestine and not coalesce
○ With bile absorption of fats=97% without bile absorption=50-60%

Question 3

How is fat digested?

Answer 3

1. Fatty chyme enters the duodenum and stimulates the relace of CCK
   
   2. CCK travels to the: stomach to stop acid production, pancreas to stimulate pancreatic juice secretion (contains lipase and HCO3-) and goes to gall bladder to cause contraction and the release of bile
   3. Bile goes down the bile duct and joins the pancreatic juice then entry the duodenum though the Ampulla of Varat
      ○ With bile absorption of fats=97% without bile absorption=50-60%
      ○ Bile solubilises lipids in chyme
      ○ Contains HCO3- so neutralises acid
   4. Bile salts have a hydrophilic head and hydrophobic tail. So they cluster around the lipid with fails facing towards lipid and head facing towards the water. After more attach to lipid clumps of the lipid will fall off into droplets. The bile salts will surround these clumps forming micelles
      ○ This will keep the fat suspended in the aqueous solution, increase surface area for enzyme attachment and prevent large clump reforming
      ○ Amphipathic structure
      ○ This is mechanical breakdown not chemical digestion
   5. lipase will attach to the triglycerides and hydrolyse 2 ester bonds. This creates 1 nonglyceride and 2 fatty acid chains
   6. Fatty acids and monoglycerides are carried to the cell membrane of ilium cells where the micelle breaks up and relaces the Fatty acids and monoglycerides
   7. Fatty acids and monoglycerides diffuse by simple diffusion into the ilium cell
   8. Fatty acids and monoglycerides are reformed into triglycerides at the smooth endoplasmic reticulum
      ○ Recreation of triglycerides is called re-esterification
   9. Golgi apparatus triglycerides packages triglycerides into a chylomicron and coats them in a proteins (to make them water soluble)
   10. chylomicron is too large to exit be by diffusion so must exit by exocytosis
       ○ Not good to put lipid into blood as they will stick together and block capillary so they are put into the lacteal/lymphatic capillary.
   11. The chylomicron is too large to enter the blood capillary so must enter the lymphatic
       capillary called lacteal (as the gaps are bigger)
   12. The chylomicron will join the blood at the subclavian vein
       ○ Lymphatic capillaries are white in colour
       ○ Lacteal -> thoracic duct -> subclavian vein

Question 4

Explain what emulsification of fats is and how micelles are formed.

Answer 4

• Bile salts have a hydrophilic head and hydrophobic tail. So they cluster around the lipid with fails facing towards lipid and head facing towards the water. After more attach to lipid clumps of the lipid will fall off into droplets. The bile salts will surround these clumps forming micelles
○ This will keep the fat suspended in the aqueous solution, increase surface area for enzyme attachment and prevent large clump reforming
○ Amphipathic structure
○ This is mechanical breakdown not chemical digestion
• Emulsification of fats is breaking up a large fat glob into many smaller droplets and sensing the droplets cant coalesce

Question 5

What is fat malabsorption? What are the symptoms and why does this occur?

Answer 5

• Unable to digest or absorb lipids
• e.g. could be due to:
○ Liver or/and gall bladder disease- gall stones, cholecystectomy
○ Pancreatic disease-cancer, pancreatitis, cystic fibrosis
○ Intestinal disease- Crohn’s disease, coeliac disease, short bowel
• Symptoms:
○ Fat in faeces (steatorrhea)
○ Greasy, foule smell
○ Essential fatty acid deficiency
○ Sat soluble Vitamin deficiency