Lipids Flashcards – AQA A-Level Biology - (7402)

What molecules form a triglyceride?

One glycerol molecule and three fatty acid molecules.

What is the general formula of a fatty acid, and what does its R group represent?

$\ce{RCOOH}$, where $R$ is the hydrocarbon group.

What happens when a glycerol hydroxyl group reacts with a fatty acid carboxyl group?

A condensation reaction forms an ester bond and releases water.

How many ester bonds and water molecules are formed when one triglyceride is made?

Three ester bonds and three water molecules.

Why are triglycerides predominantly non-polar?

Because of their hydrocarbon chains.

How do triglycerides interact with water?

They are hydrophobic and insoluble in water.

How do triglycerides differ from phospholipids in their hydrophilic head?

Triglycerides lack a phosphate-containing hydrophilic head.

What do the parts of the fatty acid formula $\ce{RCOOH}$ represent?

$\ce{R}$ is the hydrocarbon group; $\ce{COOH}$ is the carboxyl group.

How do saturated and unsaturated fatty acids differ in their hydrocarbon-chain bonds?

Saturated: no carbon–carbon double bonds. Unsaturated: one or more carbon–carbon double bonds.

How does fatty acid saturation relate to the number of hydrogen atoms in the chain?

A saturated chain carries the maximum possible number of hydrogen atoms; an unsaturated chain carries fewer.

Does the carbon–oxygen double bond in a fatty acid’s carboxyl group make it unsaturated?

No. Unsaturation depends on carbon–carbon double bonds in the hydrocarbon chain.

How does a phospholipid’s structure differ from a triglyceride’s?

Both have a glycerol-based structure, but a phospholipid has two fatty acid tails and a phosphate-containing group in place of the triglyceride’s third fatty acid.

How do the head and tails of a phospholipid differ in polarity and interaction with water?

The phosphate-containing head is polar and hydrophilic; the hydrocarbon tails are non-polar and hydrophobic.

What does it mean to say that a phospholipid is amphipathic?

Different parts of the molecule interact differently with water.

How are phospholipids arranged in a bilayer in water?

The hydrophilic heads face the water, while the hydrophobic tails point away from it.

Why do triglycerides not form the same bilayer arrangement as phospholipids?

They lack the phosphate-containing hydrophilic head.

What does the emulsion test detect?

Lipids in a sample.

How is the emulsion test performed on a sample?

Shake the sample with ethanol, then add the ethanol mixture to water.

What is the role of ethanol in the emulsion test?

It dissolves any lipid present in the sample.

What do positive and negative emulsion test results look like?

• Positive: a milky-white, cloudy emulsion.
• Negative: a clear mixture.

Why does a positive emulsion test become cloudy?

Lipid is insoluble in water, so it forms tiny suspended droplets that scatter light.