Lipids

Structure and function of fats, phospholipids, and steroids

🧈 Lipids

Overview

Lipids: Hydrophobic (nonpolar) biological molecules

Not true polymers (no repeating monomers)
Mostly composed of C, H, with some O
Functions: energy storage, membranes, signaling, insulation

Types of Lipids

1. Fats and Oils (Triglycerides)

Structure:

1 glycerol + 3 fatty acids
Linked by ester bonds (dehydration synthesis)

Fatty Acids:

Long hydrocarbon chains (C-C-C...)
Carboxyl group (-COOH) at one end

Saturated vs. Unsaturated:

| Saturated | Unsaturated | |-----------|-------------| | No C=C double bonds | One or more C=C double bonds | | Straight chains | Kinks at double bonds | | Pack tightly | Pack loosely | | Solid at room temp (fats) | Liquid at room temp (oils) | | Animal sources | Plant sources | | Higher melting point | Lower melting point |

Energy Storage:

More than 2× energy per gram vs. carbohydrates
Efficient long-term storage
Adipose tissue in animals

2. Phospholipids

Structure:

Glycerol + 2 fatty acids + phosphate group
Amphipathic: hydrophilic head + hydrophobic tails

Biological Role:

Cell membrane structure
Form bilayers in aqueous solution
Heads face water, tails face each other
Selectively permeable barrier

3. Steroids

Structure:

Four fused carbon rings
Different functional groups attached

Examples:

Cholesterol: membrane fluidity, precursor to other steroids
Sex hormones: testosterone, estrogen
Cortisol: stress hormone

Key Concepts

Lipids are hydrophobic (don't dissolve in water)
Triglycerides store energy efficiently
Saturated fats have no double bonds, pack tightly (solid)
Unsaturated fats have double bonds, don't pack well (liquid)
Phospholipids form cell membranes (bilayer structure)
Steroids have ring structure, various functions

📚 Practice Problems

1Problem 1medium

❓ Question:

Compare saturated and unsaturated fatty acids: (a) describe the structural difference, (b) explain how this affects physical properties, and (c) discuss health implications in human diet.

💡 Show Solution

Fatty Acid Comparison:

(a) Structural Differences:

Saturated Fatty Acids:

No C=C double bonds
All carbon-carbon bonds are single (C-C)
General formula: CH₃(CH₂)ₙCOOH
Maximum number of hydrogens ("saturated" with H)
Example: Palmitic acid (16:0), Stearic acid (18:0)

Unsaturated Fatty Acids:

One or more C=C double bonds
Monounsaturated: 1 double bond (oleic acid, 18:1)
Polyunsaturated: 2+ double bonds (linoleic acid, 18:2)
Each double bond creates a "kink" in the chain

(b) Physical Properties:

Saturated:

Straight chains pack tightly together
Strong van der Waals forces between molecules
Higher melting point → solid at room temperature
Examples: butter, lard, coconut oil

Unsaturated:

Kinked chains prevent tight packing
Weaker intermolecular forces
Lower melting point → liquid at room temperature
Examples: olive oil, fish oil, vegetable oils

(c) Health Implications:

Saturated Fats:

⚠️ Raise LDL ("bad") cholesterol
Associated with increased cardiovascular disease risk
Recommendation: limit to <10% of daily calories

Unsaturated Fats:

✓ Lower LDL cholesterol, raise HDL ("good") cholesterol
Omega-3 fatty acids (EPA, DHA): anti-inflammatory, heart-protective
Omega-6 fatty acids (linoleic): essential, but balance with omega-3
Recommendation: replace saturated with unsaturated fats

Trans Fats (Special Case):

Artificially saturated (hydrogenation)
Worst for health: raise LDL, lower HDL
Should be avoided completely

$\boxed{\text{Unsaturated (kinked) → liquid, healthier; Saturated (straight) → solid, limit intake}}$

2Problem 2medium

❓ Question:

Compare saturated and unsaturated fatty acids: (a) describe the structural difference, (b) explain how this affects physical properties, and (c) discuss health implications in human diet.

💡 Show Solution

Fatty Acid Comparison:

(a) Structural Differences:

Saturated Fatty Acids:

No C=C double bonds
All carbon-carbon bonds are single (C-C)
General formula: CH₃(CH₂)ₙCOOH
Maximum number of hydrogens ("saturated" with H)
Example: Palmitic acid (16:0), Stearic acid (18:0)

Unsaturated Fatty Acids:

One or more C=C double bonds
Monounsaturated: 1 double bond (oleic acid, 18:1)
Polyunsaturated: 2+ double bonds (linoleic acid, 18:2)
Each double bond creates a "kink" in the chain

(b) Physical Properties:

Saturated:

Straight chains pack tightly together
Strong van der Waals forces between molecules
Higher melting point → solid at room temperature
Examples: butter, lard, coconut oil

Unsaturated:

Kinked chains prevent tight packing
Weaker intermolecular forces
Lower melting point → liquid at room temperature
Examples: olive oil, fish oil, vegetable oils

(c) Health Implications:

Saturated Fats:

⚠️ Raise LDL ("bad") cholesterol
Associated with increased cardiovascular disease risk
Recommendation: limit to <10% of daily calories

Unsaturated Fats:

✓ Lower LDL cholesterol, raise HDL ("good") cholesterol
Omega-3 fatty acids (EPA, DHA): anti-inflammatory, heart-protective
Omega-6 fatty acids (linoleic): essential, but balance with omega-3
Recommendation: replace saturated with unsaturated fats

Trans Fats (Special Case):

Artificially saturated (hydrogenation)
Worst for health: raise LDL, lower HDL
Should be avoided completely

$\boxed{\text{Unsaturated (kinked) → liquid, healthier; Saturated (straight) → solid, limit intake}}$

3Problem 3medium

❓ Question:

Compare saturated and unsaturated fatty acids: (a) describe the structural difference, (b) explain how this affects physical properties, and (c) discuss health implications in human diet.

💡 Show Solution

Fatty Acid Comparison:

(a) Structural Differences:

Saturated Fatty Acids:

No C=C double bonds
All carbon-carbon bonds are single (C-C)
General formula: CH₃(CH₂)ₙCOOH
Maximum number of hydrogens ("saturated" with H)
Example: Palmitic acid (16:0), Stearic acid (18:0)

Unsaturated Fatty Acids:

One or more C=C double bonds
Monounsaturated: 1 double bond (oleic acid, 18:1)
Polyunsaturated: 2+ double bonds (linoleic acid, 18:2)
Each double bond creates a "kink" in the chain

(b) Physical Properties:

Saturated:

Straight chains pack tightly together
Strong van der Waals forces between molecules
Higher melting point → solid at room temperature
Examples: butter, lard, coconut oil

Unsaturated:

Kinked chains prevent tight packing
Weaker intermolecular forces
Lower melting point → liquid at room temperature
Examples: olive oil, fish oil, vegetable oils

(c) Health Implications:

Saturated Fats:

⚠️ Raise LDL ("bad") cholesterol
Associated with increased cardiovascular disease risk
Recommendation: limit to <10% of daily calories

Unsaturated Fats:

✓ Lower LDL cholesterol, raise HDL ("good") cholesterol
Omega-3 fatty acids (EPA, DHA): anti-inflammatory, heart-protective
Omega-6 fatty acids (linoleic): essential, but balance with omega-3
Recommendation: replace saturated with unsaturated fats

Trans Fats (Special Case):

Artificially saturated (hydrogenation)
Worst for health: raise LDL, lower HDL
Should be avoided completely

$\boxed{\text{Unsaturated (kinked) → liquid, healthier; Saturated (straight) → solid, limit intake}}$

4Problem 4medium

❓ Question:

Compare saturated and unsaturated fatty acids: (a) describe the structural difference, (b) explain how this affects physical properties, and (c) discuss health implications in human diet.

💡 Show Solution

Fatty Acid Comparison:

(a) Structural Differences:

Saturated Fatty Acids:

No C=C double bonds
All carbon-carbon bonds are single (C-C)
General formula: CH₃(CH₂)ₙCOOH
Maximum number of hydrogens ("saturated" with H)
Example: Palmitic acid (16:0), Stearic acid (18:0)

Unsaturated Fatty Acids:

One or more C=C double bonds
Monounsaturated: 1 double bond (oleic acid, 18:1)
Polyunsaturated: 2+ double bonds (linoleic acid, 18:2)
Each double bond creates a "kink" in the chain

(b) Physical Properties:

Saturated:

Straight chains pack tightly together
Strong van der Waals forces between molecules
Higher melting point → solid at room temperature
Examples: butter, lard, coconut oil

Unsaturated:

Kinked chains prevent tight packing
Weaker intermolecular forces
Lower melting point → liquid at room temperature
Examples: olive oil, fish oil, vegetable oils

(c) Health Implications:

Saturated Fats:

⚠️ Raise LDL ("bad") cholesterol
Associated with increased cardiovascular disease risk
Recommendation: limit to <10% of daily calories

Unsaturated Fats:

✓ Lower LDL cholesterol, raise HDL ("good") cholesterol
Omega-3 fatty acids (EPA, DHA): anti-inflammatory, heart-protective
Omega-6 fatty acids (linoleic): essential, but balance with omega-3
Recommendation: replace saturated with unsaturated fats

Trans Fats (Special Case):

Artificially saturated (hydrogenation)
Worst for health: raise LDL, lower HDL
Should be avoided completely

$\boxed{\text{Unsaturated (kinked) → liquid, healthier; Saturated (straight) → solid, limit intake}}$

5Problem 5hard

❓ Question:

Draw and explain the structure of a phospholipid. Describe how phospholipids spontaneously form a bilayer in aqueous solution. What properties make this arrangement thermodynamically favorable?

💡 Show Solution

Phospholipid Structure:

Components:

Hydrophilic "head":
- Glycerol backbone
- Phosphate group (PO₄³⁻) - negatively charged
- Often additional molecule (choline, serine, ethanolamine)
- Polar → attracted to water
Hydrophobic "tails":
- Two fatty acid chains (usually 14-18 carbons)
- One typically saturated (straight)
- One typically unsaturated (kinked)
- Nonpolar → repelled by water

Structure Diagram:

        Choline
           |
    Phosphate group  ←  Hydrophilic head
           |              (polar, charged)
        Glycerol
          / \
    Fatty acid chains  ←  Hydrophobic tails
         |   |            (nonpolar)
         |   |
         |   | (kinked if unsaturated)

Bilayer Formation:

In aqueous solution, phospholipids spontaneously arrange into a bilayer:

Hydrophilic heads face outward toward water
Hydrophobic tails face inward, away from water
Forms continuous lipid bilayer membrane

Cross-section:

Water | ○○○○○○○○ | ← Heads (exterior)
      | ~~~~~~~~ |
      | ~~~~~~~~ | ← Tails (interior)
      | ~~~~~~~~ |
      | ~~~~~~~~ | ← Tails (interior)
      | ○○○○○○○○ | ← Heads (interior/exterior)
Water

Thermodynamic Favorability:

Entropy-driven process:

Hydrophobic effect:
- Water molecules form ordered "cages" around nonpolar tails
- Decreases entropy (unfavorable)
- Bilayer minimizes water-tail contact
- Releases ordered water → increases entropy ✓
Hydrogen bonding:
- Polar heads interact with water via H-bonds
- Maximizes favorable interactions
Van der Waals forces:
- Tails interact with each other in bilayer interior
- Stabilizes structure
Self-sealing:
- Exposed edges are unfavorable
- Bilayer spontaneously forms closed vesicles (liposomes)
- Minimizes edge effects

Gibbs Free Energy:

$\Delta G = \Delta H - T\Delta S$

ΔS is positive (entropy increases) → large negative TΔS term
ΔG < 0 → spontaneous process

$\boxed{\text{Bilayer formation is spontaneous due to hydrophobic effect (entropy-driven)}}$

Biological Significance:

Forms basis of all cell membranes
Selectively permeable barrier
Fluid mosaic model: proteins embedded in fluid lipid bilayer
~50% of membrane mass in typical cell

6Problem 6hard

❓ Question:

Draw and explain the structure of a phospholipid. Describe how phospholipids spontaneously form a bilayer in aqueous solution. What properties make this arrangement thermodynamically favorable?

💡 Show Solution

Phospholipid Structure:

Components:

Hydrophilic "head":
- Glycerol backbone
- Phosphate group (PO₄³⁻) - negatively charged
- Often additional molecule (choline, serine, ethanolamine)
- Polar → attracted to water
Hydrophobic "tails":
- Two fatty acid chains (usually 14-18 carbons)
- One typically saturated (straight)
- One typically unsaturated (kinked)
- Nonpolar → repelled by water

Structure Diagram:

        Choline
           |
    Phosphate group  ←  Hydrophilic head
           |              (polar, charged)
        Glycerol
          / \
    Fatty acid chains  ←  Hydrophobic tails
         |   |            (nonpolar)
         |   |
         |   | (kinked if unsaturated)

Bilayer Formation:

In aqueous solution, phospholipids spontaneously arrange into a bilayer:

Hydrophilic heads face outward toward water
Hydrophobic tails face inward, away from water
Forms continuous lipid bilayer membrane

Cross-section:

Water | ○○○○○○○○ | ← Heads (exterior)
      | ~~~~~~~~ |
      | ~~~~~~~~ | ← Tails (interior)
      | ~~~~~~~~ |
      | ~~~~~~~~ | ← Tails (interior)
      | ○○○○○○○○ | ← Heads (interior/exterior)
Water

Thermodynamic Favorability:

Entropy-driven process:

Hydrophobic effect:
- Water molecules form ordered "cages" around nonpolar tails
- Decreases entropy (unfavorable)
- Bilayer minimizes water-tail contact
- Releases ordered water → increases entropy ✓
Hydrogen bonding:
- Polar heads interact with water via H-bonds
- Maximizes favorable interactions
Van der Waals forces:
- Tails interact with each other in bilayer interior
- Stabilizes structure
Self-sealing:
- Exposed edges are unfavorable
- Bilayer spontaneously forms closed vesicles (liposomes)
- Minimizes edge effects

Gibbs Free Energy:

$\Delta G = \Delta H - T\Delta S$

ΔS is positive (entropy increases) → large negative TΔS term
ΔG < 0 → spontaneous process

$\boxed{\text{Bilayer formation is spontaneous due to hydrophobic effect (entropy-driven)}}$

Biological Significance:

Forms basis of all cell membranes
Selectively permeable barrier
Fluid mosaic model: proteins embedded in fluid lipid bilayer
~50% of membrane mass in typical cell

7Problem 7hard

❓ Question:

Draw and explain the structure of a phospholipid. Describe how phospholipids spontaneously form a bilayer in aqueous solution. What properties make this arrangement thermodynamically favorable?

💡 Show Solution

Phospholipid Structure:

Components:

Hydrophilic "head":
- Glycerol backbone
- Phosphate group (PO₄³⁻) - negatively charged
- Often additional molecule (choline, serine, ethanolamine)
- Polar → attracted to water
Hydrophobic "tails":
- Two fatty acid chains (usually 14-18 carbons)
- One typically saturated (straight)
- One typically unsaturated (kinked)
- Nonpolar → repelled by water

Structure Diagram:

        Choline
           |
    Phosphate group  ←  Hydrophilic head
           |              (polar, charged)
        Glycerol
          / \
    Fatty acid chains  ←  Hydrophobic tails
         |   |            (nonpolar)
         |   |
         |   | (kinked if unsaturated)

Bilayer Formation:

In aqueous solution, phospholipids spontaneously arrange into a bilayer:

Hydrophilic heads face outward toward water
Hydrophobic tails face inward, away from water
Forms continuous lipid bilayer membrane

Cross-section:

Water | ○○○○○○○○ | ← Heads (exterior)
      | ~~~~~~~~ |
      | ~~~~~~~~ | ← Tails (interior)
      | ~~~~~~~~ |
      | ~~~~~~~~ | ← Tails (interior)
      | ○○○○○○○○ | ← Heads (interior/exterior)
Water

Thermodynamic Favorability:

Entropy-driven process:

Hydrophobic effect:
- Water molecules form ordered "cages" around nonpolar tails
- Decreases entropy (unfavorable)
- Bilayer minimizes water-tail contact
- Releases ordered water → increases entropy ✓
Hydrogen bonding:
- Polar heads interact with water via H-bonds
- Maximizes favorable interactions
Van der Waals forces:
- Tails interact with each other in bilayer interior
- Stabilizes structure
Self-sealing:
- Exposed edges are unfavorable
- Bilayer spontaneously forms closed vesicles (liposomes)
- Minimizes edge effects

Gibbs Free Energy:

$\Delta G = \Delta H - T\Delta S$

ΔS is positive (entropy increases) → large negative TΔS term
ΔG < 0 → spontaneous process

$\boxed{\text{Bilayer formation is spontaneous due to hydrophobic effect (entropy-driven)}}$

Biological Significance:

Forms basis of all cell membranes
Selectively permeable barrier
Fluid mosaic model: proteins embedded in fluid lipid bilayer
~50% of membrane mass in typical cell

8Problem 8hard

❓ Question:

Draw and explain the structure of a phospholipid. Describe how phospholipids spontaneously form a bilayer in aqueous solution. What properties make this arrangement thermodynamically favorable?

💡 Show Solution

Phospholipid Structure:

Components:

Hydrophilic "head":
- Glycerol backbone
- Phosphate group (PO₄³⁻) - negatively charged
- Often additional molecule (choline, serine, ethanolamine)
- Polar → attracted to water
Hydrophobic "tails":
- Two fatty acid chains (usually 14-18 carbons)
- One typically saturated (straight)
- One typically unsaturated (kinked)
- Nonpolar → repelled by water

Structure Diagram:

        Choline
           |
    Phosphate group  ←  Hydrophilic head
           |              (polar, charged)
        Glycerol
          / \
    Fatty acid chains  ←  Hydrophobic tails
         |   |            (nonpolar)
         |   |
         |   | (kinked if unsaturated)

Bilayer Formation:

In aqueous solution, phospholipids spontaneously arrange into a bilayer:

Hydrophilic heads face outward toward water
Hydrophobic tails face inward, away from water
Forms continuous lipid bilayer membrane

Cross-section:

Water | ○○○○○○○○ | ← Heads (exterior)
      | ~~~~~~~~ |
      | ~~~~~~~~ | ← Tails (interior)
      | ~~~~~~~~ |
      | ~~~~~~~~ | ← Tails (interior)
      | ○○○○○○○○ | ← Heads (interior/exterior)
Water

Thermodynamic Favorability:

Entropy-driven process:

Hydrophobic effect:
- Water molecules form ordered "cages" around nonpolar tails
- Decreases entropy (unfavorable)
- Bilayer minimizes water-tail contact
- Releases ordered water → increases entropy ✓
Hydrogen bonding:
- Polar heads interact with water via H-bonds
- Maximizes favorable interactions
Van der Waals forces:
- Tails interact with each other in bilayer interior
- Stabilizes structure
Self-sealing:
- Exposed edges are unfavorable
- Bilayer spontaneously forms closed vesicles (liposomes)
- Minimizes edge effects

Gibbs Free Energy:

$\Delta G = \Delta H - T\Delta S$

ΔS is positive (entropy increases) → large negative TΔS term
ΔG < 0 → spontaneous process

$\boxed{\text{Bilayer formation is spontaneous due to hydrophobic effect (entropy-driven)}}$

Biological Significance:

Forms basis of all cell membranes
Selectively permeable barrier
Fluid mosaic model: proteins embedded in fluid lipid bilayer
~50% of membrane mass in typical cell

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