AP Biology Unit 1 Topic 1.4: Lipids — Fats, Phospholipids, Steroids, Saturated vs Unsaturated

Question 1

What are lipids and what makes them different from other macromolecules?

Accepted Answer

Lipids are a diverse group of hydrophobic molecules including fats, phospholipids, steroids, and waxes. Unlike carbohydrates, proteins, and nucleic acids, lipids are not built from a single repeating monomer, so they are not technically polymers. Their hydrophobic nature comes from long hydrocarbon chains.

Question 2

What are the components of a triglyceride?

Accepted Answer

A triglyceride, also called a fat, consists of one glycerol molecule bonded to three fatty acid chains. The bonds form through dehydration synthesis. Triglycerides are the main form in which animals store long-term energy because they are highly reduced and pack tightly without water.

Question 3

What is the difference between saturated and unsaturated fats?

Accepted Answer

Saturated fats have fatty acid chains with only single bonds between carbons, so each carbon is saturated with hydrogen atoms. The straight chains pack tightly, making these fats solid at room temperature. Unsaturated fats have one or more double bonds that introduce kinks, preventing tight packing and making them liquid at room temperature.

Question 4

Why do animals store fat instead of carbohydrates for long-term energy?

Accepted Answer

Fat stores about twice as much energy per gram as carbohydrates because the hydrocarbon chains in fats are highly reduced and contain many high-energy carbon-hydrogen bonds. Fat also stores without binding water, while glycogen requires water for storage. This makes fat a far more efficient long-term energy store.

Question 5

What is a phospholipid?

Accepted Answer

A phospholipid consists of a glycerol bonded to two fatty acid chains and one phosphate group, often with an additional polar group attached. The phosphate end is hydrophilic while the fatty acid tails are hydrophobic, making phospholipids amphipathic. This dual nature is what allows them to form membranes.

Question 6

How do phospholipids form a cell membrane?

Accepted Answer

In water, phospholipids spontaneously arrange into a bilayer with hydrophobic tails facing inward away from water and hydrophilic heads facing outward toward water on both sides. This phospholipid bilayer is the structural foundation of every cell membrane in every organism.

Question 7

What is the structure of a steroid?

Accepted Answer

Steroids are lipids characterized by four fused carbon rings rather than fatty acid chains. Different steroids differ in the functional groups attached to the ring system. This compact ring structure distinguishes them from other lipids while still keeping them hydrophobic.

Question 8

What is cholesterol and what does it do?

Accepted Answer

Cholesterol is a steroid found in animal cell membranes where it modulates membrane fluidity, keeping membranes from becoming too rigid in cold temperatures or too fluid in warm ones. It is also the precursor from which the body synthesizes steroid hormones like estrogen, testosterone, and cortisol.

Question 9

What are some examples of steroid hormones?

Accepted Answer

Major steroid hormones include estrogen and progesterone, which regulate female reproductive function; testosterone, which regulates male reproductive function; and cortisol, which regulates stress response and metabolism. All of these are synthesized from cholesterol.

Question 10

What is a trans fat and why is it considered unhealthy?

Accepted Answer

A trans fat is an unsaturated fat that has been chemically modified through hydrogenation to add hydrogen atoms across some double bonds, leaving others in an unnatural straight configuration. Trans fats raise harmful cholesterol levels and have been linked to heart disease, leading to bans in many countries.

Question 11

What are the main biological functions of lipids?

Accepted Answer

Lipids serve as long-term energy storage in fats, structural components of cell membranes as phospholipids, signaling molecules as steroid hormones, insulation and cushioning in animal tissues, and waterproof coatings as waxes on plant leaves and animal skin or feathers.

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