LIPIDS

1. Overview of Lipids

What are lipids?

Lipids are a diverse group of biological molecules that are generally hydrophobic (water-insoluble) or amphipathic.

The word lipid comes from the Greek word lipos, meaning fat.

General properties

• Most lipids are insoluble or poorly soluble in water.

• They are generally soluble in non`polar organic solvents.

• They are important for:

  • Energy storage

  • Cell membranes

  • Insulation and protection

  • Hormone/signaling molecules

  • Waterproofing

  • Absorption of fat-soluble vitamins

Important correction

Lipids are NOT simply "hydrocarbons."

Hydrocarbons contain only carbon and hydrogen. Many lipids contain oxygen, phosphorus, nitrogen, and/or sulfur in addition to carbon and hydrogen.

2. Major Functions of Lipids

Lipids have several important biological functions.

A. Energy storage

Triacylglycerols (triglycerides) are the major long-term energy-storage molecules in animals.

Fat stores a large amount of energy because its carbon atoms are highly reduced and therefore yield lots of energy when oxidized.

B. Cell membranes

Lipids are major components of the plasma membrane.

The most important membrane lipids include:

• Glycerophospholipids

• Sphingolipids

• Cholesterol

The plasma membrane is selectively permeable, meaning it controls which substances can enter and leave the cell.

C. Signaling

Some lipids or lipid-derived molecules function in cell signaling.

Examples:

• Prostaglandins

• Steroid hormones

• Other lipid-derived signaling molecules

D. Protection and insulation

Stored fat can:

• Cushion organs

• Provide thermal insulation

• Store energy for later use

E. Waterproofing

Waxes form protective, water-resistant coatings on:

• Plant leaves

• Plant stems

• Animal skin/hair

• Feathers

3. Structural Classification of Lipids

Lipids can be broadly divided into:

A. Lipids containing fatty acids

Examples:

1. Waxes

2. Triacylglycerols

3. Glycerophospholipids

4. Sphingolipids

5. Glycosphingolipids

6. Prostaglandins and related eicosanoids

B. Lipids that do not contain fatty acids as structural building blocks

The major example is:

• Steroids

4. Fatty Acids

A fatty acid consists of:

• A long hydrocarbon chain

• A terminal carboxylic acid group (-COOH)

General structure:

CH₃-(CH₂)n-COOH

The CH₃ end is called the:

Methyl end / omega (ω) end

The COOH end is called the:

Carboxyl end

Important characteristics

Most naturally occurring fatty acids:

• Have an even number of carbon atoms

• Are usually unbranched

• Have approximately 12–24 carbons, although shorter and longer fatty acids also exist

• Are poorly soluble in water because of their long nonpolar hydrocarbon chain

5. Saturated Fatty Acids

A saturated fatty acid contains:

NO carbon-carbon double bonds (C=C).

It contains only C-C single bonds.

Example:

Stearic acid

• 18 carbons

• 0 double bonds

• Formula: CH₃-(CH₂)₁₆-COOH

• Melting point ≈ 69°C

Common saturated fatty acids

Why do saturated fatty acids have higher melting points?

Their hydrocarbon chains are relatively straight.

Therefore, they can:

• Pack closely together

• Have stronger cumulative London dispersion forces

• Form more ordered structures

• Require more energy to separate

Therefore:

Better packing → stronger intermolecular attractions → higher melting point

Many saturated fats are therefore solid at room temperature.

6. Unsaturated Fatty Acids

An unsaturated fatty acid contains at least one:

C=C double bond

Monounsaturated

Contains one C=C.

Example:

Oleic acid

• 18 carbons

• 1 double bond

• 18:1

• Melting point ≈ 13–14°C

Polyunsaturated

Contains two or more C=C double bonds.

Examples:

• Linoleic acid → 18:2

• α-Linolenic acid → 18:3

• Arachidonic acid → 20:4

7. Cis and Trans Fatty Acids

Most naturally occurring unsaturated fatty acids have cis double bonds.

Cis

A cis double bond causes a bend or kink in the fatty acid chain.

This makes it harder for neighboring molecules to pack tightly.

Therefore:

More cis double bonds → more kinks → poorer packing → weaker intermolecular attraction → lower melting point

Trans

A trans double bond produces a straighter chain than a cis double bond.

Therefore, trans fatty acids can pack more similarly to saturated fatty acids.

Easy memory trick

CIS = CROOKED

TRANS = more straight

8. Effect of Double Bonds on Melting Point

For fatty acids with the same number of carbon atoms:

More cis double bonds → lower melting point

Example:

Stearic acid

18:0

69°C

↓

Oleic acid

18:1

≈14°C

↓

Linoleic acid

18:2

≈ -5 to -9°C

↓

α-Linolenic acid

18:3

≈ -17 to -11°C

The exact values can vary slightly depending on the reference/source.

Main rule

More cis double bonds = more kinks = poorer packing = lower melting point

9. Common Unsaturated Fatty Acids

Palmitoleic acid

• 16 carbons

• 1 double bond

• 16:1

• Monounsaturated

Oleic acid

• 18 carbons

• 1 double bond

• 18:1

• Monounsaturated

• Abundant in olive oil

Linoleic acid

• 18 carbons

• 2 double bonds

• 18:2

• Polyunsaturated

• Common in many vegetable oils

α-Linolenic acid (ALA)

• 18 carbons

• 3 double bonds

• 18:3

• Omega-3 fatty acid

Arachidonic acid

• 20 carbons

• 4 double bonds

• 20:4

• Omega-6 fatty acid

• Important precursor for eicosanoids such as prostaglandins

10. Naming Fatty Acids

A useful notation is:

Number of carbons : number of double bonds

Examples:

Stearic acid

18:0

18 carbons, 0 double bonds

Oleic acid

18:1

18 carbons, 1 double bond

Linoleic acid

18:2

18 carbons, 2 double bonds

α-Linolenic acid

18:3

18 carbons, 3 double bonds

Arachidonic acid

20:4

20 carbons, 4 double bonds

11. Omega (ω) Fatty Acids

Omega numbering starts from the:

Methyl (CH₃) end

The position of the first double bond determines whether the fatty acid is omega-3, omega-6, etc.

Omega-3

The first double bond is at carbon 3 when counting from the methyl end.

Examples:

• α-Linolenic acid (ALA)

• EPA

• DHA

Omega-6

The first double bond is at carbon 6 when counting from the methyl end.

Examples:

• Linoleic acid

• Arachidonic acid

Memory trick

Omega = start counting from the CH₃ end.

Do NOT count from the COOH end when determining omega designation.

12. Essential Fatty Acids

Some fatty acids must be obtained from the diet because humans cannot introduce certain double bonds at particular positions.

The two major essential fatty acids are:

Linoleic acid (LA)

• Omega-6

• 18:2

α-Linolenic acid (ALA)

• Omega-3

• 18:3

Other important omega-3 fatty acids:

• EPA = eicosapentaenoic acid

• DHA = docosahexaenoic acid

13. Waxes

Waxes are esters formed from:

A long-chain fatty acid + a long-chain alcohol

General idea:

Fatty acid + alcohol → wax ester + water

Waxes are highly hydrophobic.

Functions

They can:

• Reduce water loss

• Protect surfaces

• Provide waterproofing

Examples

Beeswax

Produced by honeybees and used in:

• Candles

• Polishes

• Cosmetics

Carnauba wax

Obtained from the leaves of a Brazilian palm.

Used in:

• Polishes

• Coatings

• Some food/cosmetic applications

Jojoba wax

Obtained from the jojoba plant.

Used in:

• Cosmetics

• Skin products

• Hair products

Important correction

Your professor's note saying:

"waxes in sperm of male is caused by polyester shirts"

is not a correct biological explanation.

Also:

Waxes do not simply "start plastic."

Waxes and plastics are different classes of materials, although both are organic materials and can have long hydrocarbon-rich structures.

14. Triacylglycerols (Triglycerides)

A triacylglycerol (TAG) contains:

• 1 glycerol molecule

• 3 fatty acid molecules

Glycerol has three -OH groups.

Each -OH can react with the -COOH group of a fatty acid.

This produces:

• Three ester bonds

• Three molecules of water

Reaction

Glycerol + 3 fatty acids → Triacylglycerol + 3 H₂O

This is an esterification/condensation reaction.

Example

Glycerol + 3 stearic acids →

glyceryl tristearate (tristearin)

15. Fats vs Oils

Both fats and oils are mainly mixtures of triacylglycerols.

Fats

Usually:

• Solid at room temperature

• Higher proportion of saturated fatty acids

• Pack more tightly

• Higher melting points

Examples:

• Butter

• Beef tallow

• Some animal fats

Oils

Usually:

• Liquid at room temperature

• Higher proportion of unsaturated fatty acids

• More cis double bonds

• Poorer packing

• Lower melting points

Examples:

• Olive oil

• Canola oil

• Soybean oil

• Sunflower oil

Important correction

Do NOT memorize:

"Animal = saturated, plant = unsaturated"

as an absolute rule.

It is a general trend, not a universal rule.

There are exceptions in both animals and plants.

16. Coconut Oil, Olive Oil, and Canola Oil

Coconut oil

Coconut oil contains a high proportion of saturated fatty acids, especially medium-chain saturated fatty acids.

Therefore it behaves differently from many vegetable oils.

Olive oil

Rich in:

Oleic acid (18:1)

Therefore it is predominantly monounsaturated.

Canola oil

Generally contains substantial amounts of:

• Monounsaturated fatty acids

• Polyunsaturated fatty acids

Important point

Calling an oil simply "healthy" or "unhealthy" based only on whether it is plant- or animal-derived is an oversimplification.

Nutrition depends on:

• Fatty-acid composition

• Amount consumed

• Overall diet

• Processing

• Individual health factors

17. Hydrogenation

Hydrogenation is the addition of hydrogen (H₂) across carbon-carbon double bonds.

A metal catalyst such as Ni can be used.

General reaction:

C=C + H₂ → C-C

Therefore:

Unsaturated fatty acid → more saturated fatty acid

Hydrogenation can increase:

• Saturation

• Melting point

• Solid character

This was historically used to make products such as:

• Shortening

• Margarine

18. Partial Hydrogenation and Trans Fats

During partial hydrogenation, not all double bonds are completely hydrogenated.

Some cis double bonds can undergo isomerization, producing trans double bonds.

Why are trans fats important?

Trans fats have structures that allow them to pack more like saturated fatty acids.

Dietary industrial trans fats are associated with increased cardiovascular disease risk.

They can:

• Increase LDL cholesterol

• Decrease HDL cholesterol

Important distinction

Complete hydrogenation generally removes the double bonds by converting them to single bonds.

Partial hydrogenation can produce trans fatty acids as a side product.

19. Cholesterol and Steroids

Steroids

Steroids have a characteristic:

Four fused carbon-ring structure

The steroid nucleus contains:

• 3 six-membered rings

• 1 five-membered ring

Important correction

Your professor's note says steroids have "aromatic rings."

That is incorrect.

The steroid nucleus is not defined by aromatic rings.

Examples of steroids/steroid-derived compounds

• Cholesterol

• Steroid hormones

• Bile acids/bile salts

20. Cholesterol

Cholesterol is a sterol, which is a type of steroid.

It is an important component of animal cell membranes.

Cholesterol also serves as a precursor for:

• Steroid hormones

• Bile acids/bile salts

• Vitamin D

Important correction

It is better to say:

Cholesterol is a steroid/sterol and is a precursor to several steroid-derived molecules.

Rather than saying:

"Cholesterol is derived from steroids."

21. Steroid Hormones

Examples include:

• Cortisol

• Aldosterone

• Estrogen

• Progesterone

• Testosterone

Steroid hormones are lipid-soluble and can cross cell membranes.

They bind to intracellular receptors and can influence gene expression.

22. Bile Salts

Bile salts are made from cholesterol by the liver.

They are stored/concentrated in the:

Gallbladder

and released into the small intestine.

Main function

Bile salts emulsify dietary fats.

They break large fat droplets into smaller droplets, increasing the surface area available to digestive enzymes such as lipase.

Important correction

Bile salts do not chemically "melt" fats.

Instead:

They emulsify fats.

23. Glycerophospholipids

Glycerophospholipids are major components of cell membranes.

They generally contain:

• Glycerol backbone

• 2 fatty acid tails

• Phosphate group

• Polar head group

General structure:

Glycerol + 2 fatty acids + phosphate + polar head group

They are amphipathic:

• Fatty acid tails → hydrophobic

• Phosphate/head group → hydrophilic

Because of this structure, they naturally form membrane bilayers in water.

Important correction

Your professor's note says:

"phosphate is positive"

This is incorrect.

A phosphate-containing head group generally carries negative charge or contributes to a zwitterionic/overall polar head group, depending on the specific phospholipid.

24. Sphingolipids

Sphingolipids are built around a:

Sphingosine backbone

rather than glycerol.

A sphingolipid generally contains:

• Sphingosine

• Fatty acid

• Additional head group

The fatty acid is attached to sphingosine through an amide bond.

25. Sphingomyelin vs Glycosphingolipids

Sphingomyelin

Contains:

• Sphingosine

• Fatty acid

• Phosphate-containing head group

It is important in:

• Cell membranes

• Nervous tissue

• Myelin sheaths

Glycosphingolipids

Contain:

• Sphingosine

• Fatty acid

• One or more carbohydrate/sugar groups

They do not contain the phosphate-containing head group characteristic of sphingomyelin.

Memory trick

Sphingo + phosphate → sphingomyelin

Sphingo + sugar → glycosphingolipid

26. Prostaglandins

Prostaglandins belong to a larger family of lipid signaling molecules called:

Eicosanoids

They are derived from 20-carbon polyunsaturated fatty acids, especially arachidonic acid.

They contain a characteristic five-membered ring.

Functions

Prostaglandins participate in:

• Inflammation

• Pain

• Fever

• Blood vessel regulation

• Smooth muscle activity

• Reproductive processes

• Other physiological responses

Important correction

Prostaglandins are not simply "sexual hormones."

They are local lipid signaling molecules with many functions, including roles in reproduction.

27. Prostaglandins and Pain

Tissue injury can stimulate pathways that produce prostaglandins.

Prostaglandins can:

• Sensitize nociceptors

• Promote inflammatory responses

• Contribute to pain and fever

NSAIDs

Examples:

• Aspirin

• Ibuprofen

• Naproxen

These drugs inhibit cyclooxygenase (COX) enzymes, reducing prostaglandin synthesis.

Paracetamol / Acetaminophen

Acetaminophen also reduces prostaglandin production, particularly in the central nervous system, but its mechanism and anti-inflammatory action differ from classic NSAIDs.

So do not simply memorize:

"Paracetamol blocks all prostaglandins."

That is too broad.

28. Digestion of Fats

Dietary triacylglycerols are too large and hydrophobic to simply dissolve in the watery digestive tract.

Step 1: Emulsification

Bile salts help break large fat droplets into smaller droplets.

Step 2: Enzymatic digestion

Lipases hydrolyze ester bonds in triacylglycerols.

This produces products such as:

• Free fatty acids

• Monoacylglycerols

Step 3: Absorption

These products enter intestinal cells called:

Enterocytes

Step 4: Reassembly

Inside enterocytes, many fatty acids and monoacylglycerols are reassembled into triacylglycerols.

They are then packaged into chylomicrons and transported through the lymphatic system before entering the bloodstream.

29. Fat in Stool

Normally, most dietary fat is absorbed.

However, excessive fat in stool can occur when fat digestion or absorption is impaired.

This is called:

Steatorrhea

Possible signs include:

• Greasy/shiny stools

• Pale or bulky stools

• Difficult-to-flush stools

It can occur with problems involving:

• Pancreatic enzymes

• Bile

• Intestinal absorption

Important correction

Having gallstones does not automatically mean someone will have obvious fat in their stool. The effect depends on whether bile flow is significantly impaired.

30. Hydrolysis of Triacylglycerols

Hydrolysis is essentially the reverse of esterification.

Water is used to break ester bonds.

General reaction

Triacylglycerol + 3 H₂O → glycerol + 3 fatty acids

Hydrolysis can occur through:

• Acid catalysis

• Enzymatic action by lipases

31. Saponification

Saponification is the base hydrolysis of fats/oils.

A triacylglycerol reacts with a strong base such as:

• NaOH

• KOH

Products:

• Glycerol

• Fatty acid salts

The fatty acid salts are:

Soap

Example:

Triacylglycerol + 3 NaOH → glycerol + 3 sodium fatty-acid salts

32. How Soap Cleans

Soap molecules are amphiphilic.

They contain:

• Hydrophobic hydrocarbon tail

• Hydrophilic ionic head

The hydrophobic tails interact with:

• Oils

• Grease

• Other nonpolar substances

The hydrophilic heads interact with water.

Soap molecules form structures called:

Micelles

The grease/oil becomes trapped inside the hydrophobic region of the micelle and can then be washed away with water.

Important correction

Soap does not work simply because:

"oil traps bacteria."

Its main cleaning mechanism involves emulsifying and suspending oils, dirt, and microorganisms so they can be removed with water.

33. Olestra

Olestra is a synthetic fat substitute.

It consists of sucrose esterified with multiple fatty acids.

Because of its structure, digestive enzymes have difficulty hydrolyzing it.

Therefore, it passes through the digestive tract largely undigested.

It was developed to provide:

• Fat-like texture

• Reduced absorption of calories from the fat substitute

34. Saturated vs Unsaturated Fatty Acids

Core concept

Structure determines physical properties.

This is one of the most important ideas to remember.

35. Chain Length and Melting Point

Another important trend:

Longer fatty acid chains generally have higher melting points, assuming comparable saturation and structure.

Longer chains have greater surface area for intermolecular interactions.

Therefore:

Longer chain → stronger cumulative dispersion forces → generally higher melting point

However, double bonds can dramatically lower the melting point, especially cis double bonds.

36. Why Unsaturated Fats Tend to Be Liquid

Cis double bonds create bends.

The molecules therefore cannot stack efficiently.

Think of it like this:

Saturated

────────────
────────────
────────────

Very organized packing.

Cis-unsaturated

────╲────
────╱────
───╲─────

The kinks make tight packing difficult.

Therefore:

More cis double bonds → lower melting point → more likely to be liquid at room temperature.

37. Fatty Acid Desaturation and Elongation

Cells can modify fatty acids enzymatically.

Desaturases

Desaturase enzymes introduce double bonds into fatty acids.

Therefore:

Desaturation → increases unsaturation

Elongases

Elongase enzymes add carbon units to increase fatty acid chain length.

Therefore:

Elongation → longer carbon chain

These pathways help cells produce different fatty acids required for:

• Membranes

• Signaling

• Metabolism

38. EPA and DHA

Omega-3 fatty acids include:

EPA

Eicosapentaenoic acid

• 20 carbons

• 5 double bonds

• 20:5

DHA

Docosahexaenoic acid

• 22 carbons

• 6 double bonds

• 22:6

DHA is particularly abundant in:

• Brain tissue

• Retina

Important correction

Do not memorize:

"DHA is the fastest to melt"

as a biological rule.

A more useful concept is:

Greater unsaturation generally lowers melting point.

39. Cholesterol, LDL, and HDL

These terms are often misunderstood.

LDL

Low-density lipoprotein

LDL transports cholesterol and other lipids from the liver toward peripheral tissues.

High levels of LDL cholesterol are associated with increased cardiovascular risk.

HDL

High-density lipoprotein

HDL participates in reverse cholesterol transport, helping transport excess cholesterol toward the liver.

Important correction

Your professor's note says:

"LDL should be processed by your body to become HDL."

This is not correct.

LDL does not simply turn into HDL.

They are different lipoprotein particles with different compositions and functions.

40. Metabolic Disease and Lipids

The body normally stores excess energy in adipose tissue as triacylglycerol.

Problems with:

• Insulin signaling

• Fat storage

• Lipid metabolism

• Energy balance

can cause abnormal amounts of fat to accumulate in inappropriate locations or circulate in the blood.

Diabetes

Diabetes involves problems with insulin secretion, insulin action, or both.

It is not always purely genetic, although genetics can strongly influence risk.

Lifestyle and environmental factors can also contribute, particularly in type 2 diabetes.

41. MODY

Your professor's note:

"MONGD / maturity onset of the glut diabetes"

is likely referring to:

MODY — Maturity-Onset Diabetes of the Young

MODY is a group of diabetes conditions caused by specific genetic mutations.

It is different from typical type 1 and type 2 diabetes.

42. Fatty Acids and Glucose

A common statement in your professor's notes was:

"Fats can become sugar."

This needs an important correction.

Glycerol

The glycerol portion of a triacylglycerol can contribute to glucose production through gluconeogenesis.

Fatty acids

Most fatty acids are broken down by β-oxidation to produce acetyl-CoA.

In humans, acetyl-CoA from even-chain fatty acids cannot be converted into net glucose.

Therefore:

Glycerol can contribute to glucose production, but most fatty acids cannot be converted into net glucose.

43. Hydrophobic Interactions and London Dispersion Forces

The hydrocarbon portions of lipids are nonpolar.

Water molecules prefer interacting with other polar molecules and hydrogen-bonding partners.

Hydrophobic molecules therefore tend to cluster together in water.

This contributes to the formation of:

• Lipid droplets

• Membrane bilayers

• Micelles

London dispersion forces

Individual London dispersion forces are weak.

However:

Many weak interactions occurring simultaneously can produce significant overall attraction.

This helps explain why long hydrocarbon chains can interact strongly enough to affect melting points.

44. Double Bonds and Molecular Shape

A carbon-carbon double bond is more rigid than a single bond because rotation around the double bond is restricted.

Therefore:

C=C → restricted rotation → more rigid region

A cis double bond introduces a bend in the chain.

A trans double bond produces a more extended shape.

45. Single, Double, and Triple Bonds

Alkane

Contains only single C-C bonds.

Alkene

Contains at least one C=C double bond.

Alkyne

Contains at least one C≡C triple bond.

For biological lipids, C=C double bonds are much more common than carbon-carbon triple bonds.

46. Important Corrections to the Lecture Notes

❌ "Lipids are hydrocarbons."

Correct:

Lipids are mostly hydrophobic or amphipathic biomolecules. Many contain carbon and hydrogen but can also contain oxygen, phosphorus, nitrogen, etc.

❌ "The plasma membrane does not let water in."

Correct:

The lipid bilayer is selectively permeable. Small amounts of water can cross the membrane, and specialized proteins called aquaporins facilitate rapid water movement.

❌ "Triglycerides can be converted back to glucose."

Correct:

The glycerol portion can contribute to glucose production. Most fatty acids cannot produce net glucose in humans.

❌ "Steroids contain aromatic rings."

Correct:

Steroids contain a characteristic four-ring fused steroid nucleus. These rings are not simply described as aromatic rings.

❌ "Three aromatic rings + one pentameric ring."

Correct:

Three six-membered rings + one five-membered ring.

❌ "Phosphate is positive."

Correct:

Phosphate groups are generally negatively charged or contribute to a polar/charged head group.

❌ "Prostaglandins are sexual hormones."

Correct:

Prostaglandins are local lipid signaling molecules/eicosanoids. They do have important reproductive functions but are not classified simply as sex hormones.

❌ "Bile salts melt fats."

Correct:

Bile salts emulsify fats, increasing surface area for lipase.

❌ "LDL becomes HDL."

Correct:

LDL and HDL are different lipoprotein particles with different roles.

❌ "Animal fat is always saturated and plant oil is always unsaturated."

Correct:

This is a general trend, not an absolute rule.

❌ "Unsaturated fats are healthy because they don't aggregate."

Better explanation:

Cis unsaturated fatty acids generally pack less efficiently and have different effects on membrane properties and lipid metabolism. Their health effects depend on the specific fatty acid and the overall dietary context.

❌ "Saturated fat hardens in your stomach."

Correct:

The melting point is a physical property determined by molecular structure and temperature. Body temperature does not mean dietary saturated fat literally "hardens in your stomach."

❌ "Fat causes atherosclerosis because it stays in the blood and hardens."

Correct:

Atherosclerosis involves complex processes including lipoprotein retention in arterial walls, inflammation, oxidation/modification of lipoproteins, and plaque formation. It is much more complicated than fat simply hardening inside blood vessels.

❌ "If you exercise, you can melt stearic acid."

Correct:

Your body does not need to physically melt dietary fat before metabolizing it. Digestion chemically breaks down triglycerides, and the resulting molecules are metabolized enzymatically.

❌ "Soap traps bacteria because bacteria are in oil."

Correct:

Soap is amphiphilic and forms micelles that help remove oils, dirt, and microorganisms from surfaces.

❌ "All molecules eventually reach global thermal equilibrium, therefore this is called GTE."

Correct:

Thermodynamic systems tend toward equilibrium under appropriate conditions, but "Global Thermal Equilibrium" is not an appropriate explanation for why a dead body cools. A body cools because heat transfers from the warmer body to the cooler environment until thermal equilibrium is approached.

47. HIGH-YIELD EXAM CONCEPTS

If you need to prioritize what to memorize, focus on these:

1. Saturated vs unsaturated

Saturated = no C=C

Unsaturated = one or more C=C

2. Cis vs trans

Cis = kink

Trans = more straight

3. More cis double bonds

More cis double bonds → more kinks → poorer packing → lower melting point

4. Chain length

Longer fatty acid chain → generally higher melting point

5. Fat vs oil

Fats → usually more saturated → usually solid

Oils → usually more unsaturated → usually liquid

6. Triacylglycerol

Glycerol + 3 fatty acids

7. Glycerophospholipid

Glycerol + 2 fatty acids + phosphate + polar head group

8. Sphingolipid

Sphingosine backbone

9. Glycosphingolipid

Sphingosine + fatty acid + sugar

10. Sphingomyelin

Sphingosine + fatty acid + phosphate-containing head group

11. Wax

Long-chain fatty acid + long-chain alcohol

12. Steroid

Four fused rings

3 six-membered + 1 five-membered

13. Cholesterol

Important for:

• Membranes

• Steroid hormone synthesis

• Bile acid synthesis

• Vitamin D synthesis

14. Bile salts

Emulsify fats

15. Lipase

Digests triglycerides/triacylglycerols

16. Saponification

Fat + strong base → glycerol + soap

17. Hydrogenation

Adds H₂ across C=C

18. Partial hydrogenation

Can produce:

cis → trans fatty acids

19. Omega numbering

Count from the:

CH₃ / methyl end

20. Essential fatty acids

Linoleic acid = omega-6

α-Linolenic acid = omega-3

21. Eicosanoids

20-carbon fatty-acid-derived signaling molecules.

Examples:

• Prostaglandins

• Thromboxanes

• Leukotrienes

48. SUPER-SHORT MEMORY MAP

LIPIDS

Fatty-acid-containing
→ Waxes
→ Triacylglycerols
→ Glycerophospholipids
→ Sphingolipids
→ Glycosphingolipids
→ Eicosanoids

Steroid-related
→ Cholesterol
→ Steroid hormones
→ Bile acids/salts

FATTY ACIDS

Saturated
→ No C=C
→ Straight
→ Tight packing
→ Higher melting point

Unsaturated
→ Has C=C
→ Cis = kink
→ Poor packing
→ Lower melting point

OMEGA

Count from:

CH₃ → → → COOH

ω-3 = first double bond at carbon 3

ω-6 = first double bond at carbon 6

TRIACYLGLYCEROL

Glycerol + 3 fatty acids

↓ hydrolysis

Glycerol + fatty acids

PHOSPHOLIPID

Polar head + hydrophobic tails

↓

Membrane bilayer

SPHINGOLIPIDS

Sphingosine backbone

Sphingomyelin → phosphate-containing head

Glycosphingolipid → sugar-containing head

STEROIDS

4 fused rings

Cholesterol → precursor for:

• Steroid hormones

• Bile acids

• Vitamin D

PROSTAGLANDINS

Arachidonic acid → COX → prostaglandins

Prostaglandins → inflammation + pain + fever + other functions

NSAIDs → inhibit COX → ↓ prostaglandin synthesis