Topic 1.5 Notes – Lipids

1. What Lipids Are

Lipids are large biological molecules made mostly of carbon and hydrogen. That means lots of nonpolar C-H bonds.

Because they are nonpolar, lipids are:

• Hydrophobic → they do not mix with water
• Not true polymers like proteins or nucleic acids
• Built from hydrocarbon subcomponents (long chains or ring structures)

The overall behavior of a lipid comes from those hydrocarbon regions. Long chains and rings full of C-H bonds repel water, which is polar. That single fact explains most lipid functions.

When you see a question asking you to predict how a lipid behaves in water, always think:
Nonpolar structure → hydrophobic behavior.

That idea becomes clearer when we zoom in on fatty acids.

2. Fatty Acids and How Their Structure Affects Properties

A fatty acid has:

• A long hydrocarbon chain
• A terminal carboxyl group (-COOH)

They differ in:

• Chain length
• Number of double bonds

That second difference is huge for the AP exam.

Saturated Fatty Acids

• Only single bonds between carbons
• Straight hydrocarbon chains
• Pack tightly together
• Stronger intermolecular interactions
• Usually solid at room temperature (common in animal fats)

Straight chains stack neatly, like uncooked spaghetti in a box. Tight packing means less movement, which means solidity.

Unsaturated Fatty Acids

• At least one double bond
• Double bond creates a kink
• Cannot pack tightly
• Often liquid at room temperature (many plant oils)

The more double bonds, the more kinks.

Here’s the comparison clearly:

Trend to memorize:
More double bonds → more unsaturated → less packing → more fluid → lower melting point.

On tests, they love giving you a membrane with more unsaturated tails and asking what happens to fluidity. It increases.

3. Major Types of Lipids and Their Functions

Now connect structure to actual biological roles.

Fats (Triglycerides)

Structure:

• Glycerol + 3 fatty acids
• Long nonpolar tails

Functions:

• Long-term energy storage
• Insulation in mammals
• Cushioning organs
• Support cell function and transport fat-soluble vitamins (A, D, E, K)

Why so good for energy?

• Tons of C-H bonds → high potential energy
• Hydrophobic → stored without drawing in water (compact storage)

Carbohydrates are short-term energy. Fats are long-term.

Phospholipids

Structure:

• Glycerol
• 2 fatty acids
• Phosphate group

They are amphipathic:

• Polar, hydrophilic head
• Nonpolar, hydrophobic tails

In water, they self-assemble into bilayers, with hydrophilic heads facing the aqueous environments and hydrophobic tails tucked inside.

Phospholipid bilayer structure

This arrangement forms the plasma membrane and internal membranes. The hydrophobic interior acts as a barrier to many substances, allowing cells to maintain homeostasis.

If the fatty acid tails are more unsaturated, the membrane becomes more fluid. Organisms adjust this in response to temperature changes.

Steroids

Structure:

• Four fused carbon rings
• Mostly nonpolar

Functions:

• Many act as hormones
  • Regulate growth and development
  • Control energy metabolism
  • Maintain homeostasis

Because they are nonpolar, steroid hormones can pass directly through cell membranes and bind to intracellular receptors, often affecting gene expression.

Cholesterol

Cholesterol is a specific steroid.

Functions:

• Maintains membrane stability in animal cells
• Regulates membrane fluidity
• Precursor to other steroids

It fits between phospholipids in membranes. At low temperatures, it prevents the membrane from becoming too rigid. At high temperatures, it prevents it from becoming too fluid.

That balancing act shows up often in data-based questions.

4. How Lipid Structure Determines Biological Function

Everything ties back to structure:

• Many C-H bonds → high energy storage
• Hydrophobic tails → membrane formation
• Amphipathic structure → bilayer self-assembly
• Ring structure in steroids → membrane permeability + signaling
• Degree of unsaturation → membrane fluidity

When you see a structural change in a lipid, immediately predict the functional consequence. That’s how FRQs are often framed.