lab 3

Question 1

What is phosphatidylcholine (PC)?

Answer 1

A major membrane phospholipid composed of glycerol-3-phosphate with two fatty acids and a choline head group.

Question 2

Why does PC often appear as a smear on TLC?

Answer 2

PC contains many species with different combinations of saturated and unsaturated fatty acids, causing varied migration.

Question 3

What is the stationary phase in TLC for lipids?

Answer 3

Activated silica gel, which is polar.

Question 4

What is the mobile phase in this TLC experiment?

Answer 4

A non‑polar solvent mixture (chloroform:methanol:water).

Question 5

How does polarity affect TLC migration?

Answer 5

More non‑polar lipids travel farther with the non‑polar solvent; more polar lipids stick to the silica and move less.

Question 6

What is Rf and how is it calculated?

Answer 6

Rf = (distance traveled by compound) / (distance traveled by solvent front).

Question 7

Which lipid has the highest Rf in this TLC?

Answer 7

Cholesterol or related steroids (very non‑polar).

Question 8

Which lipid is most abundant in lecithin samples?

Answer 8

Phosphatidylcholine (PC).

Question 9

Which lipid is second most abundant in lecithin samples?

Answer 9

Phosphatidylethanolamine (PE).

Question 10

Why must TLC plates be heated before use?

Answer 10

To remove water molecules from silica that interfere with lipid migration.

Question 11

What does GC analyze in this experiment?

Answer 11

Fatty acid methyl esters (FAMEs) derived from phospholipids.

Question 12

Why must fatty acids be converted to methyl esters for GC?

Answer 12

FAMEs are more volatile and suitable for GC separation.

Question 13

What is saponification?

Answer 13

Breaking ester bonds in phospholipids with KOH in methanol to release fatty acids.

Question 14

What does retention time (RT) represent in GC?

Answer 14

The time a compound takes to pass through the GC column to the detector.

Question 15

How does carbon chain length affect GC retention time?

Answer 15

Longer chains are more hydrophobic, interact more with the column, and have longer retention times.

Question 16

How does degree of unsaturation affect GC retention time?

Answer 16

More double bonds reduce hydrophobic interactions, giving shorter retention times.

Question 17

Which fatty acid is most abundant in egg PC?

Answer 17

Palmitic acid (C16:0).

Question 18

Which fatty acid is most abundant in soy PC?

Answer 18

Linoleic acid (C18:2).

Question 19

Do egg and soy PC differ in fatty acid composition?

Answer 19

Yes. Egg PC is richer in saturated fatty acids, while soy PC contains more unsaturated fatty acids.

Question 20

Why do unsaturated fatty acids elute faster in GC?

Answer 20

Double bonds introduce bends that decrease hydrophobic interactions with the column.

Question 21

Why does cholesterol travel far on TLC?

Answer 21

It is very non‑polar and is strongly carried by the non‑polar solvent.

Question 22

What determines peak area in GC?

Answer 22

The quantity of each FAME in the sample.

Question 23

What is the purpose of comparing sample peaks to standards?

Answer 23

To identify fatty acids based on matching retention times.

Question 24

Why is PC more polar than cholesterol?

Answer 24

PC has a charged phosphate and choline head group that interacts strongly with silica.

Question 25

Why is PE more polar than PC?

Answer 25

PE has an amino group that forms stronger hydrogen bonds with the silica gel.