Biology of Lipids Quiz

Lipids (Introduction and Classification)

• Lipids are biomolecules soluble in organic solvents but insoluble in water.
• Primarily esters of fatty acids and mostly hydrocarbons.
• Lipids are a major class of nutrients, providing energy.

Objectives

• Define lipids
• Differentiate two major classes of lipids (saponifiable and non-saponifiable)
• Understand biological significance of lipids
• Describe properties of lipids
• Classify lipids/fixed oils

What are Lipids?

• Lipids are biomolecules that are soluble in organic solvents and insoluble in water.
• They are esters of fatty acids.
• Mostly hydrocarbons.
• Carboxylic acids and esters are types.

What are Lipids (cont.)

• Utilized by living organisms:
• Energy storage: high energy yield upon oxidation.
• Membrane components: form insoluble compartments containing aqueous solutions.
• Messengers: act as chemical messengers.
  • Primary messengers: steroid hormones, eicosanoids
  • Secondary messengers: prostaglandins, thromboxanes

Biological Significance

• Secondary energy source
• Insulating material: prevents heat loss.
• Important cellular constituents: found in cell membranes and mitochondria, aids in transporting fatty acids.
• Storage form of energy
• Metabolic regulators (steroid hormones/prostaglandins)
• Surfactants/Detergents/Emulsifying agents (amphipathic lipids)
• Shape and contour of the body
• Cushoning effect on internal organs
• Aid in absorption of fat-soluble vitamins (A, D, E, K).
• Improve taste and palatability of food.
• Dietary constituents (essential fatty acids, fat-soluble vitamins, and micronutrients)
• Beneficial effects on chronic diseases (e.g., cardiovascular disease, rheumatoid arthritis, and dementia)

Properties of Lipids

• Greasy to touch, leaves oily stains on paper.
• Lighter than water.
• Soluble in organic solvents, insoluble in water.
• When pure, colorless with bland odor/taste.
• Yellow color in fat due to carotene (provitamin A).
• Heated strongly, undergoes decomposition and forms acrid flammable vapors, burning with a sooty flame.
• Acridity due to acrolein ("propenal").

Physical Constants

• Refractive index: increases with chain length and unsaturation.
• Melting/solidification point: range due to fatty acid mixture. The melting point is the temperature at which a lipid changes from a solid to a liquid state, and the solidification point is the temperature at which all lipids in a mixture are solid. The more saturated and longer the chain, the higher the melting point.
• Viscosity: resistance to flow.
• Specific gravity: less than 1, therefore floats on water.
• Optical rotation: ability to rotate plane polarized light to right or left.

Two Major Classes of Lipids

• Non-Saponifiable: cannot be broken down into smaller molecules by hydrolysis (e.g., steroids, fat-soluble vitamins, terpenes).
• Saponifiable: contain one or more ester groups, allowing hydrolysis (e.g., simple lipids, complex lipids).
  • Simple lipids: contain fatty acids and alcohol (e.g., triglycerides, waxes).
  • Complex lipids: contain more than two components (fatty acids, alcohol, and other components) (e.g., phospholipids, glycolipids).

Two Major Classes of Lipids (cont.)

• Non-Saponifiable:
  • Steroids (e.g. cholesterol, hormones testosterone, estrogen)
  • Fat-soluble Vitamins (A,D,E,K)
  • Terpenes (e.g., essential oils)
• Saponifiable: (can be broken into smaller molecules via hydrolysis)
  • Triglycerides: fats and oils
  • Phospholipids: cell membranes (e.g., lecithin)
  • Waxes: fatty acids + alcohols
  • Glycolipids: lipids + sugar group, cell signaling

What are Fatty Acids?

• Carboxyl group (-COOH) at one end (polar head)
• Hydrocarbon chain at the other end (nonpolar tail)
• Amphipathic: both polar and nonpolar groups.
• Mostly even numbers of carbon atoms (usually 14-24).

Two Main Types of Fatty Acids

• Saturated: only single carbon-carbon bonds (solid at room temperature, e.g., animal fats)
• Unsaturated: one or more double carbon-carbon bonds (liquid at room temperature, e.g., vegetable oils). Cis double bonds cause kinks in the hydrocarbon chain.

Saturated Fats

• Bonds between all carbons are single bonds.
• Solid at room temperature.
• Primarily from animal sources (e.g., bacon grease, lard).

Unsaturated Fats

• One or more double or triple bonds between carbon atoms
• Liquid at room temperature
• Primarily from plant sources (e.g., olive oil, peanut oil) and some fish.

Fatty Acids and Their Derivatives

• Short chain fatty acids (2-4 carbons)
• Medium chain fatty acids (6-12 carbons)
• Long chain fatty acids (14-26 carbons)

How to Name Fatty Acids

• Common name: (e.g. lauric acid)
• Systematic name: (e.g. dodecanoic acid)
• Notation (ex 12:0) = number of carbons : number of double bonds (saturated fatty acids)

Triacylglycerols (Triglycerides)

• Glycerol + 3 fatty acids = ester linkages
• Older name: triglycerides.
• Important for energy storage.

Triglycerides (cont.)

• Simple triacylglyerols: all 3 fatty acids the same. (e.g. trisatearin)
• Mixed triacylglycerols: 2 or 3 different fatty acids.
• Found primarily in the adipose tissue (body fat), which serves as depots or storage site for lipids. Fat versus oil: the physical state at room temperature.

Properties of Triglycerides

• Physical state:
  • Fats: mixture of triglycerides, high proportion of long-chain saturated fatty acids (solid at room temperature, except cod liver oil).
  • Oils: mixture of triglycerides, high proportion of long-chain unsaturated fatty acids or short-chain saturated fatty acids (liquid at room temperature, except cocoa butter).

Triglycerides (Physical Properties)

• Melting points increase with the increasing number of carbons and decreasing double bonds in the hydrocarbon chains.
• Triglycerides rich in unsaturated fatty acids are generally liquid at room temperature; these are called oils. Triglycerides rich in saturated fatty acids are generally semi-solid or solid at room temperature; these are called fats.

Hydrogenation

• Addition of hydrogen to unsaturated fatty acids, converting double bonds to single bonds.
• Decreases the level of unsaturation in the fatty acid.
• Used to turn liquid oils into solid fats (e.g., margarine).

Saponification

• Base-promoted hydrolysis of fats/oils in aqueous NaOH.
• Produces glycerol and a mixture of fatty acid sodium salts: soaps.

Specific Functions of Lipids

• Energy-stored lipids: triacylglycerols
• Membrane lipids: phospholipids, sphingoglycolipids, cholesterol
• Emulsification lipids: bile acids (synthesized in liver, stored in gall bladder)
• Chemical messengers: steroid hormones, eicosanoids
• Protective-coating lipids: biological waxes (e.g., canola wax, paraffin wax)

Classification of Lipids

• Simple Lipids:
  • Fats: liquid or solid at room temp (saturated/unsaturated)
  • Oils: liquid at room temp (mostly unsaturated)
  • Waxes: esters of high molecular weight fatty acids and alcohol
• Complex/Compound Lipids:
  • Phospholipids (glycerophospholipids, sphingomyelin)
  • Glycolipids (cerebrosides, gangliosides, sulfolipids)
• Derived Lipids:
  • Fatty acids
  • Alcohols

Lipids as Messengers

• Prostaglandins, thromboxanes, leukotrienes

Prostaglandins

• Family of compounds with 20-carbon skeleton of prostanoic acid.
• Produced in response to specific physiological triggers (tissue damage/infection).
• Control processes like inflammation, blood flow, blood clot formation, and labor induction.

COX Enzymes

• Cyclooxygenase enzymes: catalyze the production of prostaglandins.
• COX-1: normal production
• COX-2: production during inflammation.
• Aspirin/NSAIDs inhibit COX enzymes/production of prostaglandins.

Thromboxanes

• Derived from arachidonic acid.
• Induce platelet aggregation and vasoconstriction.
• Aspirin/NSAIDs inhibit thromboxane synthesis by inhibiting COX enzymes.

Leukotrienes

• Synthesized from arachidonic acid.
• Occur mainly in leukocytes.
• Cause potent muscle contractions (especially in lungs), leading to asthma-like attacks.
• Several anti-asthma drugs inhibit leukotriene synthesis.

Cholesterol

• Most abundant sterol in the human body.
• Component of plasma membranes in animal cells.
• Precursor for steroid hormones and bile acids.

Steroid Hormones

• Produced from cholesterol.
• Examples:
  • Adrenocorticoids (e.g. cortisol, aldosterone) for carbohydrate and ion regulation, produced in the adrenal gland.
  • Sex hormones (e.g., testosterone, estrogen) for development of secondary sex characteristics

Lipoproteins

• Carriers of cholesterol. Four main classes
• HDL (high-density lipoprotein): carries cholesterol from tissues to liver ("good" cholesterol)
• LDL (low-density lipoprotein): carries cholesterol from liver to tissues, higher cholesterol content ("bad" cholesterol).
• VLDL (very low-density lipoprotein): carries triglycerides from liver to tissues.
• Chylomicrons: transport dietary lipids from intestines.

Cholesterol Transport

• VLDL transports cholesterol from the liver.
• Triglycerides are removed, leaving LDL (low density protein).
• LDL carries cholesterol to tissues.
• LDL binds to LDL receptors in the cell.
• Cholesterol is removed.

Reverse Cholesterol Transport

• HDL transports cholesterol from peripheral tissues to the liver.
• Free cholesterol in HDL is converted to cholesteryl esters.
• HDL delivers cholesteryl esters to liver cells for steroid hormone and bile acid synthesis.

Bile Salts

• Oxidation products of cholesterol.
• Synthesized in the liver and stored in the gallbladder.
• Released into intestines to emulsify dietary fats and aid in absorption and digestion. Glycine or taurine portions.

Test your understanding of lipids with this quiz covering their significance, properties, and functions in the body. From triglycerides to fatty acids, explore the various aspects that make lipids essential dietary constituents.