Lipids. Classifying Lipids

• Triacylglycerols (triglycerides): a storage form of energy not required for immediate use.

• Phospholipids, sphingolipids, and cholesterol (together with

Lipids

proteins) are the primary structural components of the membranes that surround all cells and organelles.

• Steroid hormones: including the sex hormones act as chemical messengers, initiating or altering activity in specific cells.

• Fat-soluble vitamins A, D, E, and K are required for a variety of physiological functions.

• Bile salts are needed for the digestion of lipids in the intestinal tract.

• The classification of a compound as a lipid is based on its solubility behavior rather than the presence of a common functional group.

• Compounds that dissolve in a nonpolar solvent such as toluene or carbon

Classifying Lipids

tetrachloride are classified as lipids. • Lipids:

• Hydrolyzable (saponifiable): Undergo hydrolytic cleavage in the presence of an acid, a base, or a digestive enzyme.

• Nonhydrolyzable (non-saponifiable): Do not undergo hydrolytic cleavage due to lack of an ester, amine, phosphate, or acetal groups.

Classifying Lipids

Fatty Acids

• Saturated fatty acids: contain no C=C bonds.

• Palmitic (C₁₆) and Stearic (C₁₈) acids are the most common.

Fatty Acids

• Unsaturated fatty acids: contain one or more C=C bonds (monounsaturated and polyunsaturated fatty acids).

• Oleic (C₁₈, one C=C) is the most common.

• Almost all natural unsaturated fatty acids contain cis double bonds. • Linoleic and Linolenic acids are essential fatty acids and are

synthesized only by plants. All other fatty acids are non-essential. Omega number ( ) The 1 carbon is the methyl group farthest • Omega number (ω-). The ω-1 carbon is the methyl group farthest

from the carbonyl carbon. • Linoleic acid ω-6 • Linolenic acid ω-3

• The length of the hydrocarbon chain and the number of double bonds affect the physical properties of fatty acids.

Fatty Acids

• Water solubility decreases as the number of carbon atoms increases: • Lauric acid (C12): 0.063 g/L at 30o C

• Stearic acid (C₁₈): 0.0034 g/L at 30o _C

• Glucose: 1100 g/L at 300_{C (MW ≈ Lauric acid)}

• Benzene solubility increases as the number of carbon atoms increases: • Benzene solubility increases as the number of carbon atoms increases:

• Lauric acid (C₁₂): 124 g/L at 30o _C • Stearic acid (C₁₈): 2600 g/L at 30o_C

• Increase as the number of carbon atoms increases. • Decrease as the number of

double bonds increases.

Melting Points

Some Common Fatty Acids

(Counting from the carboxyl group)

• Lauric acid (C12:0): Palm kernel oil Î CH₃(CH₂)₁₀COOH • Myristic acid (C14:0): Oil of nutmeg

Î CH (CH ) COOH Î CH3(CH2)12COOH • Palmitic acid (C16:0): Palm oil

Î CH3(CH2)14COOH • Stearic acid (C18:0): Beef tallow

Î CH₃(CH₂)₁₆COOH • Oleic acid (C18:1 cis Δ9_{): Olive oil}

Î CH (CH ) CH CH(CH ) COOH Î CH3(CH2)7CH=CH(CH2)7COOH • Linoleic acid (C18:2 cis Δ9, 12): Soybean oil (ω-6)

Î CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH • Linolenic acid (C18:3 cis Δ9, 12, 15): Fish oil (ω-3)

Î CH₃CH₂CH=CHCH₂CH=CHCH₂CH=CH(CH₂)₇COOH

• Triglycerides are triesters of glycerol:

Simple triglycerides : R1_=R2_=R3

Triacylglycerols (Triglycerides)

R R R

Complex or mixed triglycerides: R1_{, R}2_{, and R}3_{are different.}

• The percentage of unsaturated fatty acids varies with the source: • Animal sources other than fish: 40-60% • Fish (high % of polyunsaturation, ω-3): 75-80%

• Coconut oils are unusually high in saturated fatty acids (92%). • The high content of double bonds in plant triglycerides results in low

Triglycerides

The high content of double bonds in plant triglycerides results in low melting points.

• Plant triglycerides are liquid at room temperature • Animal triglycerides are solid.

• The terms fats & oils are used to indicate solid & liquid triglycerides. • All triglycerides are water insoluble.

Acid- or based-catalyzed hydrolysis at the ester groups. Saponification: Basic hydrolysis produces soaps.

Triglycerides: Hydrolysis

• Triglycerides are too large to diffuse through intestinal membranes. They are first digested in the small intestine (basic pH) with enzymes called lipases with the help of bile salts.

Digestion and Storage of Triglycerides

• The digestion in the intestines produces a mixture of primarily monoglycerides and fatty acids with some diglycerides and glycerol. • The intestinal cells then rebuild the triglycerides and combine them with

proteins into particles called chylomicrons.

• Chylomicrons are transported via the lymphatic system to the bloodstream where they are carried to various tissues.

• Fatty acids not needed for energy are reconverted to triglycerides and stored in adipose cells (fat cells) as fat droplets

Absorption of Lipids

• Glycerol and Short-Medium Chain (6-10) Fatty Acids diffuse and are absorbed directly into the bloodstream.

• Monoglycerides and Long Chain (12-24) Fatty Acids form micelles, are absorbed, and are reformed into new triglycerides. With protein they are transported by Chylomicrons.

Bonds break

Bonds break

Triglyceride Monoglyceride + 2 fatty acids The triglyceride and two molecules of

water are split to give two fatty acids and a monoglyceride.

Fatty acids, monoglycerides, and glycerol are absorbed into intestinal cells.

• Triglycerides are the primary energy storage form in animals. • Triglycerides: 9.2 kcal/g

• Glycogen: 4 0 kcal/g

Triglycerides

• Glycogen: 4.0 kcal/g

• Triglycerides are more efficient for storing energy because they are more highly reduced than glycogen (more O₂can be added, therefore more energy can be generated).

• Normal liver and muscle glycogen levels can fill your energy needs for approximately 12 hours.

• Adipose tissue triglycerides can fill your energy needs for several weeks t l f th

to a couple of months.

• Hibernating animals live off of their accumulated body fat during their entire period of hibernation.

• Catalytic (Ni or Pt) hydrogenation:addition of H₂to alkene double bonds. • Conversion of plant oil into margarine and other products.

Catalytic Hydrogenation

• Double bonds in vegetable oils are hydrogenated in order to convert them into a solid, more palatable form.

• Vegetable oils contain a much higher percentage of unsaturated fatty acids and almost no cholesterol.

• Catalysts: some trans vs. cis double bonds.

• Evidence suggests that trans double bonds raise blood cholesterol levels more than cis double bonds and perhaps more than saturated fatty acids.

Catalytic Hydrogenation

• Decreasing double bonds of vegetable oils also increases their shelf life (oxidation of the double bonds causes rancidity).

• Air oxidation of alkene double bonds:

• Air oxidation of double bonds generates two fragments, each carbon of the double bond being converted into a –COOH group.

This generates low molecular weight volatile and offensive smelling • This generates low molecular weight, volatile and offensive-smelling

carboxylic and dicarboxylic acids.

• Rancidity reactions are slowed by refrigeration.

• Antioxidants such as BHA (butylated hydroxyanisole) or BHT (butylated hydroxytoluene) are often added to vegetable oils.

• Waxes have a variety of protective functions in plants and animals: • Coating on fruits, vegetables, and plant leaves to protect against

parasites, prevent mechanical damage, and prevent water loss.

Waxes

• Coating on hair, furs, feathers, and skin keeping them lubricated, pliable, and waterproof.

• Plankton and some other marine organism use waxes instead of triacylglycerols for energy storage.

• Cell-membrane:

• Glycerophospholipids (phospholipids): based on glycerol. • Sphingolipids: based on sphingosine.

Phospholipids & Sphingolipids

• Unlike the triglycerides, the glycerophospholipids and sphingolipids have one highly hydrophilic group.

• The hydrophilic group is responsible for the amphipathic nature of these lipids, which allows their assembly into cell membranes.

• Membrane lipids are organized into lipid bilayers as shown in the fluid mosaic model:

Membranes

Equilibrium

Passive Transport of Two Types of Molecule

Nonpolar vs. Polar

O

/CO

vs. Na

Osmosis

How Animal and Plant Cells Behave

H2O H2O

H2O

H2O

Isotonic solution Hypotonic solution Hypertonic solution

Animal cell Plant H2O H₂O H2O H2O Plasma membrane (1) Normal (2) Lyse (3) Shrivel (crenate) cell

(4) Flaccid (5) Turgid (6) Shrivel

Transport of a Solute Across a Membrane

Active Transport of a Solute Across a Membrane

Transport P Protein P P changes shape Phosphate detaches ATP ADP Solute protein

Solute binding Phosphorylation Transport Protein reversion

Catabolism of Various Food Molecules

(The reverse occurs when excess energy is available to the body)

Starch & Glycogen Hydrolyzed to

Glucose

Triglycerides Hydrolyzed to Glycerol & Fatty Acids

Proteins Hydrolyzed to Amino Acids (No Storage) Fatty Acids Hydrolyzed to Acetyl CoA (No Storage) Carbohydrates: Quick Energy Fats: Energy Storage

Compounds in pink are Glucogenic, as they can eventually be converted to glucose.

Compounds in blue are Ketogenic, as they can be converted to acetyl-CoA (ketone bodies).

Fatty Acids CAN NOT be converted to Fatty Acids CAN NOT be converted to glucose

Amino acids are a good source of glucose when carbohydrate is not available (Protein Sparing).

Urea Formation &

Excretion

Ammonia → Urea: a much less toxic

compound, formed in the liver.

Urea is excreted through the kidneys

Fat vs. Carbohydrate Energy

•

Excess Energy

9 Excess Protein is converted to fat but this is inefficient and

indirect. Its priority is other roles.

9 Excess Carbohydrate is converted to fat but this is

inefficient and indirect. Its priority is glycogen stores.