Week 2: Lipids and Metabolism Flashcards
What is the energy content per gram of carbohydrates, lipids and proteins?
Proteins: 4kcal/g
Lipids: 9kcal/g
Carbohydrates: 4kcal/g
How is energy released from organic molecules?
Oxidation reactions (loss of electrons)
(For organic molecules, the more carbon/hydrogen and less oxygen they contain, the more scope there is for oxidising them)
Why do lipids contain more energy?
They are oxygen poor and hydrogen and carbon rich
What constitutes an ‘unsaturated’ fatty acid?
What is the significance of this?
Contains at least 1 double bond, meaning they contain less hydrogens. There is no rotation around the double bond, meaning they have fixed configuration at each double bond.
Double bond provides rigidity to the fatty acid- e.g. provides more rigidity to a cell membrane.
What is meant by ‘trans’ and ‘cis’ configurations?
Trans configurations: carbon chain crosses the double bond
Cis configurations: carbon chain kinks back on itself
What does the ‘omega’ nomenclature describe?
The position of the final double bond, counting from the hydrocarbon end.
How are short chain fatty acids absorbed?
Simple diffusion across brush border of cell membrane and diffusion across basolateral membrane into blood.
How are long chain fatty acids absorbed?
Fats (triglycerides) emulsified by bile salts and phospholipids into emulsion droplets.
Digested by pancreatic lipase to monoglycerides and fatty acids.
Held in micelles, combined with bile salts and phospholipids.
Micelles diffuse into an ‘unstirred layer’ next to the surface of epithelial cells.
Monoglycerides and fatty acids diffuse into cell membrane.
Inside the cell monoglycerides and fatty acids are reassembled into triglycerides and packaged into chylomicrons
Exported across the basolateral membrane and leave the intestinal villus via the lacteals of its lymph system and into the systemic circulation.
Describe the structure of a lipoprotein
Lipids with a hydrophobic core containing triglycerides and cholesterol esters and a hydrophilic surface containing phopsholipids, free cholesterol and proteins.
What releases fatty acids from chylomicrons and VLDLs into the tissues?
Lipoprotein lipase
What are the main functions of chylomicrons?
Describe their composition
Deliver dietary (exogenous) triacylglycerols (TAG) to peripheral tissues.
High triacylglycerol, low cholesterol
What are the main functions of VLDLs?
Describe their composition
Deliver endogenous triacylglycerols to peripheral tissues.
High triacylglycerols, low cholesterol (less TAG and more cholesterol however than chylomicrons)
What is the main function of LDLs?
Describe their composition
Deliver cholesterol to peripheral tissues and liver
Low triacylglycerols, highest cholesterol (compared to chylomicrons, LDLs and HDLs)
What is the function of HDLs?
Describe their composition
Deliver cholesterol from peripheral tissues to the liver for elimination.
Lowest tryacylglycerol content (compared with chylomicrons, LDLs and VLDLs) and high cholesterol
Describe the exogenous lipid cycle
Dietary lipids absorbed in the GI tract and packaged into chylomicrons which enter the systemic circulation.
Broken down by lipoprotein lipase in the systemic circulation to remove lipid content.
Remnants of chylomicrons reach the liver via the systemic circulation.
Liver breaks down remnants of chylomicrons into cholesterol which is re-introduced to the GI tract via the biliary system.