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Lecture 5: Lipids and Carbohydrates

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  1. Lecture 5: Lipids and Carbohydrates Craig Kasper Aquaculture FAS 1012

  2. Part 1: Lipid Characteristics • Lipid = a compound that is insoluble in water, but soluble in an organic solvent (e.g., ether, benzene, acetone, chloroform) • “lipid” is synonymous with “fat”, but also includes phospholipids, sterols, etc. • chemical structure: glycerol + fatty acids

  3. Lipid Molecule

  4. Nutritional Uses of Lipids • We already know that lipids are concentrated sources of energy (9.45 kcal/g) • other functions include: • 1) provide means whereby fat-soluble nutrients (e.g., sterols, vitamins) can be absorbed by the body • 2) structural element of cell, subcellular components • 3) components of hormones and precursors for prostaglandin synthesis

  5. Lipid Classes • simple: FA’s esterified with glycerol • compound: same as simple, but with other compounds also attached • phospholipids: fats containing phosphoric acid and nitrogen (lecithin) • glycolipids: FA’s compounded with CHO, but no N • derivedlipids: substances from the above derived by hydrolysis • sterols: large molecular wt. alcohols found in nature and combined w/FA’s (e.g., cholesterol)

  6. Saturated vs. Unsaturated Fatty Acids • saturated: the SFA’s of a lipid have no double bonds between carbons in chain • polyunsaturated: more than one double bond in the chain • most common polyunsaturated fats contain the polyunsaturated fatty acids (PUFAs) oleic, linoleic and linolenic acid • unsaturated fats have lower melting points • stearic (SFA) melts at 70oC, oleic (PUFA) at 26oC

  7. Fatty Acids Commonly Found in Lipids

  8. Saturated vs. Unsaturated Fats • saturated fats tightly packed, clog arteries as atherosclerosis • because of double bonds, polyunsaturated fats do not pack well -- like building a wall with bricks vs. irregular-shaped objects • plant fats are much higher in PUFA’s than animal fats

  9. Saturated vs. Unsaturated FA’s Plant vs. Animal Fat

  10. Lipid Digestion/Absorption • Fats serve a structural function in cells, as sources of energy, and insulation • the poor water solubility of lipids presents a problem for digestion: substrates are not easily accessible to digestive enzymes • even if hydrolyzed, the products tend to aggregate to larger complexes that make poor contact with the cell surface and aren’t easily absorbed • to overcome these problems, changes in the physical state of lipids are connected to chemical changes during digestion and absorption

  11. Lipid Digestion/Absorption Five different phases: • hydrolysis of triglycerides (TG) to free fatty acids (FFA) and monoacylglycerols • solubilization of FFA and monoacylglycerols by detergents (bile acids) and transportation from the intestinal lumen toward the cell surface • uptake of FFA and monoacylglycerols into the cell and resynthesis to triglyceride • packaging of TG’s into chylomicrons • exocytosis of chylomicrons into lymph

  12. Enzymes Involved in Digestion of Lipids • lingual lipase: provides a stable interface with aqueous environment of stomach • pancreatic lipase: major enzyme affecting triglyceride hydrolysis • colipase: protein anchoring lipase to the lipid • lipid esterase: secreted by pancreas, acts on cholestrol esters, activated by bile • phospholipases: cleave phospholipids, activated by trypsin

  13. What about Bile??? • These are biological detergents synthesized by the liver and secreted into the intestine • they form the spherical structures (micelles) assisting in absorption • hydrophobic portion (tails of FA) are located to the inside of the micelle, with heads (hydrophillic portion) to the outside • they move lipids from the intestinal lumen to the cell surface • absorption is by diffusion (complete for FA and monoglycerides, less for others)

  14. Factors Affecting Absorption of Lipids • amount of fat consumed ( fat =  digestion =  absorption) • age of subject ( age =  digestion) • emulsifying agents • chain length of FA’s (> 18C =  digestibility) • degree of saturation of FA ( sat =  digestibility) • overheating and autooxidation (rancidification at double bond) • optimal dietary calcium = optimal FA absorption (high Ca =  absorption)

  15. Lipid Metabolism/Absorption • short chain FA’s are absorbed and enter the portal vein to the liver • those FA’s with more than 10 carbons are resynthesized by the liver to triglycerides • they are then converted into chylomicrons and pass to the lymphatic system • some FA’s entering the liver are oxidized for energy, others stored • blood lipids: 45% P-lipids, 35% triglycerides, 15% cholestrol esters, 5% free FA’s

  16. Lipid Digestion/Absorption

  17. Lipid Digestion/Absorption

  18. Characteristics of Fat Storage • Most of the body’s energy stores are triglycerides • storage is in adipose, source is dietary or anabolism (synthesis) from COH or AA carbon skeletons • remember obesity? • adipose can remove FA’s from the blood and enzymes can put them back

  19. Fatty Acid Nomenclature • Nomenclature reflects location of double bonds • also used are common names (e.g., oleic, stearic, palmitic) • linoleic is also known as 18:2 n-6 • this means the FA is 18 carbons in length, has 2 double bonds, the first of which is on the 6th carbon • arachidonic = 20:4 n-6

  20. Essential Fatty Acids • Only recently determined as essential (1930) • body can synthesize cholesterol, phospholipids • research: same as AA’s but via addition (EFA’s added improved growth, NEFA’s didn’t) • requirement determined by depleting fat reserves of subject animal: difficult

  21. Essential Fatty Acids (fish) • Most NEAA found in marine food webs • Essential fatty acids (to date): • linoleic (18:2 n-6; terrestrials; fish - not really) • linolenic (18:3 n-3; terrestrials; fish) • arachidonic (20:4 n-6; marine maybe) • eicosopentaenoic acid (20:5 n-3, marine) • docosohexaenoic (22:6 n-3, marine) • Why? Because elongation beyond 18 carbons is very difficult in marine fish (lack pathways) • actual EFA requirement is a matter of whether the fish is FW/SW or WW/CW

  22. Essential Fatty Acids (most animals) • salmonids need n-3 FA’s for membrane flexibility in cold water • trout can elongate and desaturate n-3 FA’s • Linoleic acid (18:2 n-6) is the most essential • addition of arachidonic is also helpful in deficient diets, but can be synthesized from linoleic (maybe sparing effect) • EFA’s, like EAA’s, must be dietary


  24. Lipids as Crustacean Energy Sources • Largely, n-6 FA’s (linoleic) used for energy • as temperature drops, requirement for monounsaturated and PUFA’s increases • change in temperature = change in diet • cold water species = increased dietary HUFA’s • maturation animals: increased requirement for 20:4 n-6, 20:5 n-3 and 22:6 n-3 for proper spawning

  25. Part 2: Carbohydrate Characteristics From: Lovell; D’Abramo et al.

  26. General Comments • Carbohydrates often written as “COH” • much of what we need to know about them, besides their structure, was covered in “Bioenergetics, Parts 1&2” • here, we cover structure

  27. Carbohydrate Structure • Basic chemical structure consists of sugar units • found as aldehydes or ketones derived from polyhydric alcohols • contain: C, H, O • often shown as aliphatic or linear structures, but exist in nature as ringed structures

  28. Glucose Structure O C-H H- C-OH HO-C-H H-C-OH H-C-OH CH2OH CH2OH O H H OH H OH HO H OH Haworth perspective

  29. Carbohydrate Classification • Usually by the number of sugar units in the molecule: • monosaccharides (glucose) • disaccharides (2 units) • maltose (2 glucose units) • sucrose (glucose + fructose) • polysaccharides (long chain polymers of monosaccharides • most important polysaccharides to animals are starch and cellulose

  30. Starch and Cellulose CH2OH CH2OH O O H H H H starch OH H OH H O O O H OH H OH CH2OH CH2OH H O O H O O O OH H OH H H H cellulose H OH H OH

  31. Starch and Cellulose • Starch contains -D-glucose linkage • Cellulose has a -D-glucose linkage • we store starch in muscle tissues as glycogen, peeled off by enzymes when needed • cellulose is primary component of plant tissue, largely indigestible to monogastrics • must have enzyme, “cellulase”