Chapter 17: Lipids – Structure, Function, and Biological Importance

Lipids

Definition and General Properties

• Lipids are biomolecules that contain fatty acids or a steroid backbone.

• They are soluble in organic solvents but insoluble in water.

• Lipids are essential components of cell membranes, fat-soluble vitamins, and steroid hormones.

Lipids and Esters

• Many lipids contain esterified fatty acids, including:

  • Waxes

  • Triacylglycerols

  • Glycerophospholipids

  • Sphingolipids

Fatty Acids

Structure and Classification

• Fatty acids are long, unbranched carbon chains with a carboxylic acid group at one end.

• They typically contain 12–20 carbon atoms.

• Fatty acids are insoluble in water due to their long nonpolar hydrocarbon chains.

• Saturated fatty acids have no C=C double bonds.

• Unsaturated fatty acids contain one or more C=C double bonds.

Unsaturated Fatty Acids

• Most naturally occurring unsaturated fatty acids have cis double bonds, causing kinks in the chain.

Essential Fatty Acids

• Humans can synthesize some fatty acids but not enough polyunsaturated fatty acids.

• Linoleic acid, linolenic acid, and arachidonic acid are essential fatty acids that must be obtained from the diet.

Properties of Saturated Fats

• Saturated fatty acids pack closely together, resulting in stronger dispersion forces.

• This close packing leads to higher melting points and makes them less fluid.

Prostaglandins

Structure and Function

• Prostaglandins are signaling molecules derived from fatty acids, especially arachidonic acid.

• They are produced in response to injury and cause inflammation.

NSAIDs & Prostaglandins

• Nonsteroidal anti-inflammatory drugs (NSAIDs) block prostaglandin formation, reducing pain and inflammation.

Health Aspects of Fats

• Some polyunsaturated fats, especially omega-3 fatty acids, are important for preventing atherosclerosis and maintaining cardiovascular health.

Waxes

Structure and Function

• Waxes are esters formed from a long-chain alcohol and a long-chain fatty acid.

• They provide protective, waterproof coatings for plants and some animals.

Triacylglycerols

Structure and Biological Role

• Triacylglycerols are the main storage form of fat in the body and represent the most dense form of energy storage.

• They are esters of glycerol and three fatty acids.

Fats vs. Oils

• Fats: Solid at room temperature, more saturated, typically from animal sources.

• Oils: Liquid at room temperature, more unsaturated, typically from plant sources.

Hydrogenation of Fatty Acids

• Hydrogenation is the process of adding hydrogen to C=C double bonds, converting unsaturated fatty acids to saturated ones.

• Equation: $\mathrm{RCH=CHR' + H_2 \xrightarrow{catalyst} RCH_2CH_2R'}$

• This process also converts oils (liquids) into fats (solids).

Incomplete Saturation and Trans Fats

• Partial hydrogenation can produce trans fats, which are associated with negative health effects similar to saturated fats.

Hydrolysis and Saponification

• Hydrolysis: Breaking down fats with water (identical to ester acid hydrolysis).

• Saponification: Breaking down fats with base (identical to ester base hydrolysis), producing soap.

Soap Action

• The fatty acid chain is insoluble in water and binds to oils and dirt.

• The carboxylate ion is soluble in water, allowing soap to emulsify nonpolar substances.

Glycerophospholipids

Structure

• Composed of glycerol, two fatty acids, phosphoric acid, and an amino alcohol.

Phosphoric Acid

• Phosphate group attached to four oxygen atoms, with a negative charge.

Amino Alcohols

• Contain both an alcohol (-OH) and an amine (N) group.

• Charged at physiological pH.

• Common types: choline, serine, ethanolamine.

Lecithin vs. Cephalin

• Lecithin: Contains choline as the amino alcohol.

• Cephalin: Contains ethanolamine (or sometimes serine).

Snake Venom & Phospholipase

• Certain snake venoms contain phospholipases that hydrolyze the fatty acid on the center carbon of glycerophospholipids, producing a lysophospholipid.

• This degrades cell membranes, causing red blood cells to break down.

Sphingolipids

Structure and Function

• Contain sphingosine instead of glycerol, plus a fatty acid, phosphate, and amino alcohol.

Sphingomyelin

• Forms the protective white coat (myelin sheath) around nerve cells.

• Loss of sphingomyelin is associated with multiple sclerosis, leading to nerve damage and impaired signal transmission.

Sterols and Cholesterol

Structure

• Sterols have a characteristic structure of three 6-membered rings and one 5-membered ring.

Cholesterol

• The most important steroid in the body.

• Essential for cell membranes, brain and nerve tissue, steroid hormones, and vitamin D synthesis.

• High levels can lead to plaque formation and clogged arteries.

Bile Salts

• Synthesized in the liver from cholesterol and stored in the gallbladder.

• Have both polar and nonpolar regions, acting like soaps to emulsify fats and aid in cholesterol absorption.

• Excess cholesterol can form gallstones in the gallbladder.

Lipid Transport

Lipoproteins

• Lipids are transported in the body as lipoproteins, which are complexes of nonpolar lipids surrounded by polar lipids and proteins.

• This structure makes them soluble in water.

Types of Lipoproteins

Type	Main Function	Health Impact
LDL (Low Density Lipoprotein)	Delivers cholesterol to tissues	Excess leads to plaque formation
HDL (High Density Lipoprotein)	Returns cholesterol to liver for elimination	Protective; excess is eliminated

Steroid Hormones

Types and Functions

• All steroid hormones are derived from cholesterol and share the sterol backbone.

• They differ in their substituents and double bonds, and tend to have more polar groups.

• Main types:

  • Male sex hormones: Testosterone, androsterone

  • Female sex hormones: Estrogens, progesterone

  • Adrenal corticosteroids: Mineralocorticoids (electrolyte balance), glucocorticoids (glucose regulation)

Anabolic Steroids

• Derivatives of testosterone used medically for muscle loss recovery and illegally for muscle mass increase.

• Side effects include fluid retention, hair growth, sleep disturbance, and liver damage.

Adrenal Corticosteroids

• Produced by the adrenal glands (on top of each kidney).

• Aldosterone: Regulates electrolytes and water balance.

• Cortisone: Increases blood glucose level.

Cell Membranes and Lipids

Fluid Mosaic Model

• Cell membranes are flexible and dynamic, not rigid.

• Proteins move within the membrane, and the membrane can flex easily.

• Cis-double bonds in membrane lipids increase fluidity, while saturated fatty acids and cholesterol reduce fluidity.

Membrane Transport Mechanisms

• Simple Diffusion: Movement from higher to lower concentration without assistance.

• Facilitated Diffusion: Movement from higher to lower concentration via protein channels.

• Active Transport: Movement from lower to higher concentration using protein pumps and energy.