Chapter 5: Lipid Structure and Function
🎯 Diagnostic: Test Your Starting Level 22 questions (2 per section). No prior reading required - see what you already know.
Aim to answer every question before checking. Missed questions point you to the sections you need most.
1. (5.1) Saturated fatty acids:
C. "Saturated" = saturated with hydrogen = no double bonds. Straight chains pack tightly (lard, butter = solids). Unsaturated chains have kinks at cis double bonds, pack poorly (olive oil = liquid).
2. (5.1) A cis double bond in a fatty acid:
B. Cis double bonds bend the chain ~30°, preventing the tight crystal-like packing of saturated chains. Fewer van der Waals contacts → lower melting point. Trans double bonds barely kink the chain and behave more like saturated fats (raising melting point and coronary risk).
3. (5.2) A triacylglycerol (triglyceride) consists of:
A. Glycerol has three -OH groups; each esterifies with a fatty acid carboxyl to form a triester. Releases 3 H2O. Triacylglycerols are the major storage form of fat in adipose tissue.
4. (5.2) Why do triglycerides store more energy per gram than carbohydrates?
D. Fats yield ~9 kcal/g vs. ~4 kcal/g for carbs. Two reasons: (1) hydrocarbon chains are more reduced than carbohydrate carbons (more electrons to hand to O2), and (2) fats are stored anhydrously, while glycogen carries ~2g water per 1g carb.
5. (5.3) Saponification is the base-catalyzed hydrolysis of:
B. NaOH or KOH hydrolyzes the ester bonds of a triglyceride, producing glycerol and three fatty acid carboxylates (soap). The carboxylate's amphipathic nature (polar head, long non-polar tail) is what makes soap dissolve grease in water.
6. (5.3) Soap molecules cluster in water to form:
C. Above the critical micelle concentration, amphipathic soap molecules spontaneously form micelles: heads out toward water, tails sequestered inside. Grease dissolves in the hydrophobic core, then is washed away with the micelle.
7. (5.4) A phospholipid (phosphoglyceride) structure consists of:
D. Phosphoglyceride = glycerol backbone with fatty acids on C1 and C2 (hydrophobic tails) and a phosphate + head group (choline, serine, ethanolamine, inositol) on C3 (hydrophilic head). Amphipathic = membrane builder.
8. (5.4) Why do phospholipids spontaneously form bilayers in water?
A. Two fatty acid tails are too bulky to form a micelle; they pack best as a two-layer sheet (bilayer) with tails shielded from water in the middle. This is the basis of every cell membrane.
9. (5.5) Sphingolipids are built on a backbone of:
B. Sphingosine is an 18-carbon amino alcohol. A fatty acid attached via an amide bond makes ceramide (the parent sphingolipid). Add phosphocholine = sphingomyelin (a phospholipid). Add sugars = cerebrosides (one sugar) or gangliosides (many sugars).
10. (5.5) Tay-Sachs disease results from deficient hexosaminidase A, which normally degrades:
C. Tay-Sachs is a sphingolipid storage disease: GM2 ganglioside piles up in lysosomes of neurons, causing neurodegeneration. One of several "lysosomal storage diseases" where a missing sphingolipid-degrading enzyme leads to accumulation.
11. (5.6) Cholesterol's role in animal cell membranes is to:
D. Cholesterol is a "bidirectional buffer" of membrane fluidity. At high temps, its rigid ring system restrains phospholipid motion. At low temps, it prevents phospholipids from packing too tightly and becoming crystalline. Also a precursor for steroid hormones, bile acids, and vitamin D.
12. (5.6) All steroid hormones share the same core structure of:
A. The sterane skeleton: 3 cyclohexane rings + 1 cyclopentane ring, fused. Cholesterol, testosterone, estradiol, cortisol, and aldosterone all share this scaffold. Decorations around the rings determine the hormone's identity and receptor.
13. (5.7) Terpenes are built from repeating units of:
B. Terpenes are isoprene (C5H8) polymers: monoterpenes (C10, 2 isoprenes), sesquiterpenes (C15), diterpenes (C20), triterpenes (C30 - including squalene, the cholesterol precursor). Isoprenoid biosynthesis starts from acetyl-CoA via the mevalonate pathway.
14. (5.7) Terpenoids differ from terpenes in that terpenoids:
C. Terpenes are pure hydrocarbons; terpenoids have been oxidized (added -OH, C=O, etc.) giving them reactive functional groups. Vitamin A, menthol, and camphor are terpenoids.
15. (5.8) Waxes are primarily:
A. A wax is a single fatty acid esterified to a single long-chain alcohol. Very hydrophobic. Biological functions: waterproofing (plant cuticle, feathers, ear wax, bee hive).
16. (5.8) Which of the following is NOT a typical function of waxes?
D. Energy storage is triglycerides, not waxes. Waxes are for waterproofing and protection - very hydrophobic but not readily mobilized as fuel. The body cannot efficiently break them down for energy.
17. (5.9) Which lipoprotein has the highest protein-to-lipid ratio and carries cholesterol FROM tissues BACK to the liver?
B. HDL = highest density, most protein-rich. It scavenges cholesterol from peripheral tissues and returns it to the liver ("reverse cholesterol transport") - the "good" cholesterol. LDL delivers cholesterol to tissues; chylomicrons carry dietary fat; VLDL carries liver-made triglycerides.
18. (5.9) LDL is sometimes called "bad cholesterol" because:
C. LDL delivers cholesterol to peripheral tissues. When blood LDL is elevated (often due to diet or genetics - familial hypercholesterolemia is an LDL-receptor mutation), cholesterol accumulates in arterial walls, triggering inflammation and plaque formation.
19. (5.10) Prostaglandins are derived from:
D. Arachidonic acid (20:4, ω-6) is cleaved from membrane phospholipids by phospholipase A2, then oxidized by cyclooxygenase (COX) to prostaglandins or by lipoxygenase to leukotrienes. NSAIDs (aspirin, ibuprofen) inhibit COX, blocking prostaglandin synthesis.
20. (5.10) Aspirin's anti-inflammatory mechanism involves:
A. Aspirin acetylates a serine in COX-1 and COX-2, permanently inactivating them. Since platelets cannot synthesize new COX, a single aspirin dose disables platelet thromboxane production for the platelet's lifetime (~7 days) - which is why aspirin is used as an anti-clotting agent.
21. (5.11) Which vitamins are fat-soluble (and therefore stored in fatty tissues rather than excreted if consumed in excess)?
B. "ADEK" - the fat-soluble vitamins. They dissolve in dietary fat, require bile for absorption, and can reach toxic levels if over-supplemented (unlike water-soluble B and C, which are excreted in urine).
22. (5.11) Vitamin K is essential for:
C. Vitamin K cofactors a carboxylase that adds a second -COOH to glutamate residues on clotting factors, giving them Ca2+-binding sites needed for the coagulation cascade. Warfarin blocks vitamin K recycling - hence its use as an anticoagulant. Vitamin A = vision; D = calcium absorption; E = antioxidant.
Lipids are the insoluble molecules of the cell. They refuse to dissolve in water, which is exactly what makes them useful. Cell membranes are built from lipids because they exclude water. Fat is an energy reserve because it holds twice the energy per gram of carbohydrate. Steroid hormones travel through membranes because they are lipid-soluble.
The MCAT treats lipids as a long vocabulary chapter. Learn what each class of lipid looks like and what it does. Once you can recognize a structure, most questions answer themselves.
Lipids are Oily Building Blocks
Keep in mind that all lipids are defined by what they do NOT do: they do not dissolve in water. This trait comes from long hydrocarbon chains with minimal polar groups. The resulting molecules pack together through hydrophobic interactions. That is how membranes form, how fat droplets coalesce in fat cells, and how cholesterol tunes membrane fluidity.