Lipids (Module 2 Lecture 3)

Question 1

List four groups of lipids important to human cells.

Answer 1

1. Fats or triacylglycerols (triglycerides)
2. Phospholipids (like lecithin)
3. Cholesterol and cholesterol esters
4. Fat-soluble vitamins (A, D, E, K)

Question 2

List five physiological functions of lipids.

Answer 2

1. Fuel molecules (free fatty acids)
2. Energy stores (triacylglycerols)
3. Barrier between aqueous compartments (membrane bilayers contain phospholipids, glycolipids, and cholesterol)
4. Hormones (steroid hormones are synthesized from cholesterol and prostaglandins and leukotrienes are made from arachidonic acid)
5. Intracellular second messengers (diacylglycerol, ceramide)

Question 3

The solubility of lipids DECREASES with increased what?

Answer 3

Chain length

Question 4

How many carbons long are dietary & membrane fatty acids usually?

Answer 4

16-20 carbons (although milk fat also contains short and medium chain fatty acids and certain neural and retinal membranes are particularly rich in very long chain polyunsaturated fatty acids with 22 or more carbons)

Question 5

How do double bonds affect fatty acid solubility and melting point?

Answer 5

1. INCREASES solubility

2. DECREASES melting point

Question 6

What are two common types of polyunsaturated fatty acids?

Answer 6

1. C18 (linoleic acid and alpha-linoleinic acid, which must be obtained from diet)
2. C20 (arachidonatic acid, which can be synthesized by elongation and desaturation of linoleate)

Question 7

Describe the structure, properties, and functions of triacylglycerols.

Answer 7

1. Used for storage of fatty acids
2. Contain three fatty acids esterified to a glycerol backbone
3. Completely non-polar and hydrophobic (separate out of aquous solution and form lipid droplets in cells)
4. Lipases hydrolyze triglycerides (found in cells and digestive tract)

Question 8

What is the major class of membrane lipids called?

Answer 8

Phospholipids

Question 9

What is the name for phospholipids with a glycerol backbone?

Answer 9

Phosphoglycerides (most phospholipids are in this form)

Question 10

What are the four common polar heads of phospholipids?

Answer 10

1. Choline
2. Ethanolamine
3. Serine
4. Inositol

Question 11

What is ethanolamine?

Answer 11

A two carbon molecule with a hydroxide substituent on one carbon and an amino substituent on the other

Question 12

What is choline?

Answer 12

An ethanolamine molecule with three methyls situated off of the now 4 degree aminel

Question 13

Describe two important features of phosphatidylethanolamine.

Answer 13

1. It’s amphipathic.

2. It’s a zwitterion (negative phosphate and positive amino group).

Question 14

Describe the structure of serine.

Answer 14

An amino acid with a hydroxylated methyl R group.

Question 15

Describe the structure of inositol.

Answer 15

A six-carbon cyclic ring with a hydroxyl substituent on every carbon.

Question 16

What happens to the hydroxyl groups on serine and inositol during phospholipid synthesis?

Answer 16

It is esterified (an R group replaces the H).

Question 17

Describe sphingolipids.

Answer 17

1. Structurally similar to glycerol attached to a long chain fatty acid, with one amino group instead of one of the hydroxyls.

Question 18

What is ceramide and how is it formed?

Answer 18

1. A sphingolipid
2. Formed by attachment of a fatty acid to the amino group (similar to diacylglycerol in size and properties due to the two long hydrocarbon chains)

Question 19

Describe the structure, function, and formation of sphingmyelin.

Answer 19

1. Structurally similar to phosphatidyl choline except the hydrocarbon chains are more saturated (which makes for vastly different properties on membrane domains)
2. Although present in all cells, it’s important in the myelin sheath surrounding nerve cell and is found in high levels there.
3. Formed by adding phosphate and choline to ceramide.

Question 20

Some sphingolipids are also what type of lipid and why?

Answer 20

Glycolipids because they are formed by addition of sugars to ceramics (which replaces the phosphocholine as the polar head group).

Question 21

What is a cerebroside?

Answer 21

A ceramide with one sugar (also classified as a glycolipid and sphingolipid)

Question 22

What is a ganlioside?

Answer 22

A ceramide with complex and sometimes branched carbohydrate chains (also classified as a glycolipid and sphingolipid)

Question 23

Describe the structure and function of cholesterol.

Answer 23

1. A lipid with four linked hydrocarbon rings with a hydroxyl group at one end and a (big fat) nonpolar hydrocarbon tail at the other.
2. An essential part of mammalian cell membranes and a precursor for steroid hormones like estrogen, testosterone, and cortisol.

Question 24

How and where is cholesterol used for storage?

Answer 24

1. Via cholesteryl esters, which are completely non polar molecules formed b esterification of a fatty acid to the hydroxyl group of cholesterol.
2. Steroidogenic cells store cholesteryl esters in cytoplasmic lipid droplets like triacylglycerols and neither is found in membranes.

Question 25

Describe the structure, location, and function of LDL.

Answer 25

1. LDL has an amphipathic surface with phospholipids, unesterified cholesterol, and a large protein known as apo B-100.
2. Found in plasma.
3. Low-density lipoprotein delivers cholesterol to cells and transports cholesteryl esters and triacylglycerols.

Question 26

Give two examples of vitamins that are also lipids.

Answer 26

1. Vitamin A derivatives have hormone-like effects on cell differentiation.
2. Both beta-carotene (provitamin A) and alpha-tocopherol (vitamin E) have anti-oxidant activity.