Lipids Flashcards
What are lipids?
Lipids are group of hydrophobic molecules which are extracted in organic solvents.
Lipids have many different structures and functions
What are the types of nonpolar lipids?
Not water soluble
- Triacylglycerol(TAG or TG)
- Cholesteryl ester (CE)
What are the types of Amphipathic lipids ?
Poorly water soluble
- Fatty acids (FA have a negative charge)
- Cholesterol (polar hydroxyl group)
- Polar lipids of membranes
- Bile salts (negative charge)
Dietary lipids can be…
Beneficial or harmful for the human health
How can dietary fats be beneficial or harmful?
Dietary lipids provide essential fatty acids of the omega-3 and the omega-6 family
Dietary lipids rich in saturated fatty acids or trans-fatty acids can lead to atherosclerosis
How much ATP is from fatty acid degradation?
The degradation of fatty acids (B-oxidation) generates more ATP than degradation of carbohydrates or proteins. Fatty acids can be stored TAGs
Under what circumstances are stored TAGs degraded?
The stored TAGs in fat cells are only degraded under hormonal control to provide fatty acids for energy metabolism needed by the whole body
Describe the structure of fatty acids
Fatty acids contain a long hydrophobic hydrocarbon chain and a carboxyl group which is negatively charged at physiological pH
Describe the structure of TAGs
TAGs are a major storage form of fatty acids for energy metabolism. Fat droplets consist mainly of TAGs
What forms do natural fatty acids come in?
Natural fatty acids are saturated(no double bonds) or unsaturated with cis double bonds
What types are fatty acids are most common?
Long chain fatty acids
What are long chain fatty acids?
Contain 16-20
Which fatty acids are essential for humans?
Two fatty acids with 18C are dietary essential for humans:
Linoleic acid and a-linolenic acid
What are medium chain fatty acids?
MCFAs contain mainly 4-12 carbons and are dietary found in TAGs of milk
What are very long chain fatty acids?
VLCFAs are longer than 22 carbons and are found e.g. in membranes it brain and nervous tissues
What are the membrane polar lipids ?
Phospholipids and glycolipids are essential components of membranes
The steroid cholesterol is found only in the plasma membrane
How many layers of the membrane, are there? What compromises each layer?
Membranes are formed by two layers of polar lipids
- The polar head groups interact with the water phases
- The esterified fatty acids of the polar lipid bilayer form a lipophilic separation in membranes
What are the main groups of polar lipids found in membranes?
Phospholipids -contain phosphate
Glycolipids- contains sugars
What are glycerophospholipids?
Most phospholipids contain a glycerol backbone and are grouped as glycerophospholipids
What are sphingolipids?
Most glycolipids contain a sphingosine backbone and are grouped as sphingolipids
What is the backbone of glycero-phospholipids?
Phosphatidic acid
What are the major components of the membranes?
Phosphatidylethanolamine. (PE)
Phosphatidylcholine (PC)
Both phospholipids are uncharged
Describe thee structure of cholesterol
Free cholesterol is amphipathic as it is mostly nonpolar with exception of one hydroxyl group. It has 27 carbons and a steroid ring system with a double bond in ring B
What are the functions of cholesterol?
- An essential component of plasma membranes and regulates fluidity
- Cholesterol is precursor for steroid hormones, bile acids and vitamin D
How is cholesterol synthesized?
If not provided by the diet, cholesterol is mainly synthesized in the liver and distributed to other cells as nonpolar cholesteryl ester inside of lipoproteins
Why are dietary essential fatty acid needed for?
Dietary essential fatty acids are needed for membranes and for synthesis of eicosanoids (physiologically active mediators)
Fatty acids in general are used for energy metabolism or are stored in fat cells
Describe lipid digestion up until the small intestines
- Mouth: lingual lipase is mainly swallowed
- Stomach: lingual and gastric lipase degrade milk TAGs and release medium-chain fatty acids
- Small intestine: bile is needed for lipid degradation and absorption
Explain digestion of lipids in the Brush border cells and Intestinal mucous always cells
Brush border cells: uptakeof free fatty acids, 2-monoacylglycerols (MAG), free cholesterol and lipid-soluble vitamins needs mixed micelles with bile salts
Intestinal mucosal cells: TAGs and cholesteryl esters are re-formed and dietary lipids are released into the lymph inside of chylomicrons
Where does lipid digestion start?
In the stomach
What is the function of gastrin?
Gastrin leads to increase of proton concentration in the stomach and to the release of gastric lipase
Where is lingual lipase obtained?
Lingual lipase is mostly swallowed and acts in the lumen of the stomach
What are lingual and gastric lipases?
Lingual and gastric lipases are acid stable and active at low pH in the stomach and prefer TAGs with medium-chain FAs (found in milk)
What happens to MCFA in digestion after digestion?
The formed MCFA are later on directly released in the hepatic vein and reach the liver. Important for infants with milk as main nutrient.
Note: LCFA do not reach directly the liver
What causes release of secretin and CCK
The acidic chyme reaches the duodenum resulting in the release of secretin and CCK from endocrine cells of the duodenum
What is the function of secretin ?
Secretin leads to release of bicarbonate from the pancreas which is needed for pH adjustments to about pH 6-7 for pancreatic enzyme activities
What is the function of Cholecystokinin(CKK)?
Cholecystokinin (CKK) leads to the contraction of the gallbladder and release of bile
CKK leads also to release of pancreatic enzymes which include pancreatic lipase, cholesterol esterase and Phospholipase A2
What is bile?
Bile is an alkaline fluid that is released by hepatocytes into the bile ducts.
Only the liver synthesizes bile acids from cholesterol and forms bile salts by conjugation of bile acids with glycine or taurine
What does bile contain?
Bile salts, phosphatidylcholine, small amount of free cholesterol and conjugated bilirubin. Bile can be stored and concentrated in the gallbladder
What is a bile acid?
It is an amphipathic molecule that interacts with lipids and water
Cholic acid
Has a hydrophobic side and hydrophilic side
Negatively charge from COO-
Why does the liver form bile salts?
A negative charge in bile is wanted as it improves lipid emulsification
-hepatic conjugation of bile acids with glycine or taurine lowers the pKa and leads to negatively charged molecules which form salts with sodium ions “Bile salts”
What is the function of bile salts as transport?
Transportation of free cholesterol in the bile from the liver to the duodenum for release of cholesterol in feces
What are the functions of bile salts in terms of solubility?
Bile salts increase the solubility of free cholesterol in bile. Imbalance can lead to cholesterol gallstones
What are the functions of bile salts in terms of digestion ?
In the small intestine bile salts emulsify dietary lipids for digestion by pancreatic lipase, pancreatic Phospholipase A2 and cholesteryl esterase
What are the functions of bile salts in bile?
- Transportation of free cholesterol in the bile from the liver to the duodenum for release of cholesterol in feces
Bile salts increase the solubility of free cholesterol in bile. Imbalance can lead to cholesterol gallstones
- In the small intestine bile salts emulsify dietary lipids for digestion by pancreatic lipase, pancreatic Phospholipase A2 and cholesteryl esterase
- Bile salts from mixed micelles for uptake of digested lipids into intestinal mucosal cells
What are the functions of micelles in terms of mixed micelles?
Bile salts form mixed micelles for uptake digested lipids into the intestinal mucosal cells
What are the functions of pancreatic lipase?
Dietary TAGs are covered by bile salts. Colipase pushes bile salts away and anchors pancreatic lipase at the lipid-aqueous interface
Are MAG degraded?
MAGs aren’t degraded any further but taken up into mucousal cell
What are the functions of hepatocytes?
Release bile salts into bile canaliculi and the bile reaches the duodenum
Summarize what bile salts are needed for
Bile salts are needed for cholesterol transport into duodenum and lipid emulsification for digestion and absorption of dietary lipids in the duodenum, jejunum and ileum
In the small intestine ….
BILE is needed for lipid digestion
Give an example of a bile acid
Chalice acid
Give the enterohepatic circulation of bile salt
- Secreted bile salts consist of 95% old, recycled bile salts and 5% newly synthesized bile salts
- 95% of bile salts are reabsorbed by the small intestine
- Reabsorbed bile salts are recycled by enterohepatic curculation into terminal ileum into the hepatic portal vein (colonic bacteria changes primary bile salts to secondary bile salts)
- 5% of bile salts are lost in feces
What is the function of Phospholipase A2?
Cleaves phospholipids including phosphotidylcholine provided in bile which helps with emulsification
What is the function of pancreatic lipase?
Is anchored by colipase to the fat droplet covered by bile salts and cleaves TAG only to MAG and 2 free fatty acids
When is TAG resynthesized?
After uptake into intestinal mucousal cells
