Topic 4: Lipids

Introduction to Lipids

• Definition: Lipids are a group of organic substances that play a vital role in organisms, serving as an integral part of all cell membranes and as energy stores.

• Elemental Composition: They are composed of carbon, hydrogen, and oxygen.

• Comparison to Carbohydrates: Lipids have a much lower proportion of water and oxygen compared to molecules such as carbohydrates.

• Solubility Characteristics:     * They are nonpolar molecules.     * They are soluble only in nonpolar solvents.     * They are insoluble in water because water is a polar molecule.

• Human Synthesis and Sources:     * Lipids can be synthesized in the liver within the human body.     * Common dietary sources include oil, butter, whole milk, cheese, fried foods, and some red meats.

Fundamental Properties of Lipids

• Physical State: Lipids are oily or greasy nonpolar molecules.

• Storage: They are stored in the adipose tissue of the body.

• Chemical Identity: They are a heterogeneous group of compounds mainly composed of hydrocarbon chains.

• Energy Density: They are energy-rich organic molecules that provide energy for various life processes.

• Biological Significance: They form a mechanical barrier that divides a cell from its external environment, known as the cell membrane.

Biological Functions of Lipids

• Energy Storage: Act as a long-term reservoir for energy.

• Membrane Formation: Essential for making biological membranes.

• Insulation: Provide thermal insulation to maintain body temperature.

• Protection: Serve protective roles, such as preventing plant leaves from drying up.

• Buoyancy: Assist organisms in floating.

• Hormonal Activity: Act as hormones to regulate bodily functions.

Fats and Oils

• Classification: Fats and oils are similar chemically but differ in their physical state at room temperature:     * Fats: Solid at room temperature (e.g., butter); sourced mainly from animals.     * Oils: Liquid at room temperature (e.g., olive oil); sourced mainly from plants.

• Chemical Composition: All lipid molecules contain carbon, hydrogen, and oxygen, though the proportion of oxygen is much lower than in carbohydrates.

• Building Blocks: Fats and oils contain two types of organic chemical substances: fatty acids and glycerol. These components are combined using ester bonds.

Fatty Acids

• General Formula: $CH_3(CH_2)_nCOOH$     * $CH_3$: Represents an alkyl group.     * $(CH_2)_n$: Represents a variable hydrocarbon chain, where $n$ varies from 4 up to 14 to 24.     * $COOH$: Represents a carboxylic group of atoms, which provides the acidic characteristic of the molecule.

• Diversity: Living tissues contain more than 70 different types of fatty acids.

Fatty Acid Configuration

• Structure: They consist of a long hydrocarbon chain (folded backbone of carbon atoms with attached hydrogen atoms) and a carboxyl group ($-COOH$) at one end.

• Carbon Count: Chains usually contain an even number of carbon atoms, with 16 or 18 being the most common numbers.

• Variation Factors: Differences in fatty acids arise from:     1. The length of the carbon chain (often 15 to 17 carbon atoms long).     2. Whether the fatty acid is saturated or unsaturated.

• Lipid Nature: Fatty acids are classified as lipids because the nonpolar character of their hydrocarbon "tails" dominates their properties.

Saturated Fatty Acids

• Definition: These involve a hydrocarbon chain holding the maximum number of hydrogen atoms around the carbon atoms, as seen in stearic acid.

• Bonding: Every carbon atom in the hydrocarbon part has a single bond.

• Energy and Oxidation: The molecule is highly reduced or less oxidized, implying relatively higher energy reserves.

• Physical State: They have higher melting points due to their ability to pack molecules together into a straight rod-like shape. Thus, they are solids at ordinary temperature and pressure (room temperature).

• Biological Sources: Animal fats are the primary source.

• Formula for Stearic Acid: $CH_3(CH_2)_{16}COOH$

Unsaturated Fatty Acids

• Definition: The hydrocarbon part has one or more double bonds between carbon atoms, meaning there are fewer hydrogen atoms than possible.

• Naming Conventions:     * Monounsaturated: One double bond.     * Polyunsaturated: Two or more double bonds.

• Kinking Factor: The molecule bends at the site of the double bonds and cannot be straight. This prevents tight packing because the attraction between molecules is weak.

• Physical State: They are liquids at ordinary temperature and pressure (oils).

• Melting Point Factors:     * The more double bonds, the lower the melting point.     * As the tail length increases, the melting point increases.

• Biological Sources: Plants are the primary source.

• Formula for Oleic Acid: $CH_3(CH_2)_7CH=CH(CH_2)_7COOH$

Glycerol

• Molecular Data:     * Formula: $C_3H_8O_3$     * Molecular Weight: $92.09\,g/mol$     * IUPAC Name: 1, 2, 3-Propanetriol or 1, 2, 3-Trihydroxypropane.

• Biological Function: Acts as an intermediate in carbohydrate and lipid metabolism. Surplus carbohydrate can be converted into long-chain fatty acids and esterified with the three hydroxyl groups of glycerol.

Uses of Glycerol

• Industrial: Production of dynamite, which propelled industrial development and mining.

• Cosmetics: Moisture-control reagent and emollient to enhance the texture of lotions and creams.

• Food Industry: Forms inter-molecular hydrogen bonds with water to increase moisture in preserved food without compromising shelf life; enhances viscosity and texture.

• Pharmaceuticals: Used as a low-toxicity emulsifier, improves smoothness and taste of medications, used in tablet coatings for easier swallowing, and found in cough lozenges.

• Medical: Suppositories act as laxatives by irritating the anal mucosa.

Glycerides

• Definition: Fatty acids linked by an ester bond to glycerol (a trihydric alcohol) or other alcohols like cholesterol.

• Synthesis: Formed via a condensation reaction between glycerol and fatty acids.

• Classification of Glycerides:     * Monoglyceride: Glycerol esterified to one fatty acid.     * Diglyceride: Glycerol esterified to two fatty acids.     * Triglyceride: Glycerol esterified to three fatty acids (a triester).

• Plant vs. Animal Glycerides: Plant glycerides are oily due to higher double bond content and low melting points; animal fats (rich in stearic or palmitic acid) melt at higher temperatures.

• Triglycerides in Mammals: Primarily stored in adipose tissue, replacing large portions of cytoplasm in specialized connective tissue cells with lipid droplets.

Energy Characteristics of Triglycerides

• Triglycerides possess a higher proportion of hydrogen than carbohydrates or proteins.

• They are more reduced than carbohydrates and act as strong reducing agents.

• Energy Yield: They yield $38\,kJ/g$, which is twice the yield of carbohydrates.

• Table 1: Metabolic Substrate Comparison     * Carbohydrate: Energy output = $17.2\,kJ\,g^{-1}$; Water produced = $0.56\,g\,g^{-1}$; Oxygen consumed = $0.83\,Dm^3\,g^{-1}$.     * Lipid: Energy output = $38.9\,kJ\,g^{-1}$; Water produced = $1.07\,g\,g^{-1}$; Oxygen consumed = $2.02\,Dm^3\,g^{-1}$.

Synthesis and Hydrolysis of Triglycerides

• Bond Formation: A hydroxyl ($-OH$) group from glycerol reacts with the hydroxyl group of the fatty acid's carboxyl group.

• Condensation: A water molecule ($H_2O$) is removed to form a covalent ester bond.

• Hydrolysis: Fats can be broken down into glycerol and fatty acids via the addition of water and the enzyme lipase. This is essential for digestion.

Phospholipids

• Structure: Similar to lipids, but a phosphate group ($PO_4^{3-}$) replaces one of the fatty acid chains. This makes the molecule polar/negatively charged at the phosphate end.

• Amphipathic Nature: Phospholipids have both polar (hydrophilic) and nonpolar (hydrophobic) regions.     * In water, they form spheres with hydrophilic "heads" facing outward and hydrophobic "tails" buried inside.

• Transport Role: They function as transporters of hydrophobic substances in hydrophilic environments. For example, lipids bond to amphipathic molecules to circulate in the blood without causing obstructions.

• Properties:     * Signal mediators.     * Anchor proteins within cell membranes.     * Major constituents of cell membranes (providing fluidity).     * Components of bile and lipoproteins.

• Functions:     * Regulate membrane permeability.     * Aid in the absorption of fat from the intestine.     * Help in the Electron Transport Chain in mitochondria.     * Act as emulsifying agents.     * Prevent fat accumulation in the liver.     * Transport and remove cholesterol from cells.     * Act as surfactants in the respiratory system.     * Involved in blood coagulation.     * Help synthesize lipoproteins, prostacyclins, and prostaglandins.

Glycolipids

• Definition: Lipids containing a sugar residue (monosaccharide, oligosaccharide, or polysaccharide).

• Distribution: Found in tissue, brain, and nerve cells.

• Structure: Composed of a sugar group attached to a sphingolipid or a glycerol group with one or two fatty acids (yielding glycosphingolipids and glycoglycerolipids).

• Attachment: The hydrophobic lipid tail anchors the molecule to the surface of the plasma membrane by interacting with the lipid bilayer.

Waxes

• Definition: Esters formed from long-chain alcohols and long-chain carboxylic acids.

• Properties: Insoluble in water, solid at ambient temperature, liquid when melted, and plastic (bends under pressure without heat).

• Natural Examples:     * Plants: Waxy coatings on fruits and leaves safeguard against dehydration and small predators.     * Animals: Fur and feathers use waxy coatings as water repellents.

Animal Wax: Beeswax

• Source: Produced by worker bees (Apis mellifera) via eight wax-producing glands.

• Approximate Formula: $C_{15}H_{31}COOC_{30}H_{61}$.

• Usage: Building honeycomb cells. Edible but has low nutritional value for humans as monoesters are poorly hydrolyzed in the gut (some birds like honeyguides can digest it).

• Applications: Cheese coatings (prevents mold), cosmetics (pomades, moustache wax, eye shadow), pharmaceuticals, and highly flammable candles.

Vegetable Wax: Carnauba Wax

• Source: From leaves of the palm plant Copernicia prunifera in Brazil; known as the "queen of waxes."

• Chemical Properties: Hardest wax, high melting point ($78-85^\circ C$), contains compounds in the $C_{26}-C_{30}$ range.

• Applications: High-durability polishes (cars, shoes, floors) when mixed with beeswax, food glazes (candies, gums), cosmetics (lipsticks), and paper coating.

Steroids

• Structure: Identifiable by a set of four ring structures.

• Nature: Hydrophilic hydroxyl groups make them slightly water-soluble, but they are generally hydrophobic and pass through cell membranes freely.

• Examples:     * Sex Hormones: Testosterone and Estrogen.     * Vitamin D: Formed from cholesterol via UV radiation.

• Cholesterol ($C_{27}H_{45}OH$):     * Composed of a hydrocarbon tail, four rings, and a hydroxyl group.     * Maintains structural integrity of cell membranes.     * Precursor for all other steroids.     * Travels in blood via lipoproteins: Low density lipoproteins (LDL) and high density lipoproteins (HDL).

Summary of the Roles of Lipids

• Energy and Water Source:     * Lipids are concentrated fuel. $C-C$ and $C-H$ bonds contain more energy than $C-O$ bonds.     * They provide concentrated energy reserves during plenty and are accessed during starvation.     * Oxidation of fat produces high amounts of water. This is vital for desert animals; Kangaroo rats satisfy their water needs through fat oxidation and often do not drink water even if available. Unhatched chicks also rely on this metabolic water.

• Membranes: Structural components of cell membranes.

• Insulation: Poor conductors of heat. Adipose tissue reduces heat loss in winter for mammals. Plants use lipid coatings to prevent moisture loss.

• Diet: Many essential vitamins are lipids.

• Shock Absorbers: Protect delicate organs. For instance, kidneys are cushioned by thick layers of fat. Adipose tissue surrounds critical organs to absorb mechanical damage.

• Buoyancy: Single-celled aquatic organisms produce oil droplets to float.

• Hormones: Certain lipids allow glands to control metabolic activity in remote tissues.