Lipids Practice Questions

Correct answer: Hydrophobic or amphipathic nature

Lipids are defined by their hydrophobic or amphipathic character: they are insoluble in water but readily dissolve in nonpolar organic solvents such as chloroform, ether, and hexane. Some lipids (e.g., phospholipids) are amphipathic, with both polar and nonpolar regions.

Correct answer: A hydrocarbon chain with a terminal carboxyl group

Fatty acids consist of a long hydrocarbon chain (typically 12–20 carbons) with a terminal carboxyl group (–COOH). The hydrocarbon tail is hydrophobic, while the carboxyl head is weakly acidic and hydrophilic. This structure underlies their role in energy storage and membrane composition.

Correct answer: Unsaturated fatty acid

Unsaturated fatty acids contain one or more carbon–carbon double bonds (C=C) in the hydrocarbon chain. Monounsaturated fatty acids have one double bond (e.g., oleic acid), while polyunsaturated fatty acids have two or more (e.g., linoleic acid). These double bonds introduce kinks that reduce packing density and lower melting point, increasing membrane fluidity.

Correct answer: Stearic acid

Stearic acid (C18:0) is an 18-carbon fatty acid with no double bonds, making it fully saturated. Oleic acid (C18:1) is monounsaturated, linoleic acid (C18:2) is polyunsaturated, and arachidonic acid (C20:4) is a polyunsaturated essential fatty acid — all contain one or more double bonds.

Correct answer: Three fatty acids and glycerol

Triacylglycerols (triglycerides) consist of a glycerol backbone esterified with three fatty acid chains at positions sn-1, sn-2, and sn-3. They are the most efficient form of energy storage, yielding approximately 9 kcal/g — more than twice the energy density of carbohydrates or proteins.

Correct answer: Long-term energy storage

Triacylglycerols serve as the primary long-term energy reserve in animals, stored in adipose tissue. They are metabolized by lipolysis into fatty acids and glycerol, which then enter beta-oxidation and glycolysis respectively to yield ATP.

Correct answer: Phospholipids

Phospholipids are the principal structural lipids of biological membranes. Their amphipathic nature — hydrophilic phosphate head and hydrophobic fatty acid tails — drives spontaneous bilayer formation in aqueous environments, creating the selectively permeable barrier essential for cellular compartmentalization.

Correct answer: A phosphate group

Phospholipids replace one fatty acid of a triacylglycerol with a phosphate group linked to a polar head group (e.g., choline, ethanolamine, serine). This imparts an amphipathic character essential for bilayer formation, distinguishing them structurally and functionally from neutral triacylglycerols.

Correct answer: Phosphatidylcholine

Phosphatidylcholine is the most abundant phospholipid in most eukaryotic cell membranes, forming a major part of both membrane leaflets. Phosphatidylethanolamine is the second most common; cholesterol is a sterol (not a phospholipid); and triacylglycerol is a storage lipid not found in membranes.

Correct answer: Having both polar and nonpolar regions

Phospholipids are amphipathic: they have a hydrophilic (polar) phosphate head group and two hydrophobic (nonpolar) fatty acid tails. This dual character allows them to self-assemble into bilayers in aqueous environments, with tails shielded from water and heads exposed to it.

Correct answer: Four fused carbon rings

Steroids are characterized by a fused tetracyclic ring system: three six-membered cyclohexane rings (A, B, C) and one five-membered cyclopentane ring (D). This rigid planar backbone is shared by cholesterol, steroid hormones (estrogen, testosterone, cortisol), bile acids, and vitamin D.

Correct answer: Cholesterol

Cholesterol is the prototypical steroid in animals, featuring the four-fused-ring steroid skeleton with a hydroxyl group at C3 and a hydrocarbon side chain at C17. It is an essential component of animal cell membranes and the precursor for steroid hormones, bile acids, and vitamin D.

Correct answer: Regulating membrane fluidity

Cholesterol inserts between phospholipid molecules in the bilayer, moderating fluidity across temperature ranges: it prevents excessive rigidity at low temperatures and excessive fluidity at high temperatures. This 'fluidity buffer' role is critical for maintaining optimal membrane function in animals.

Correct answer: Cholesterol

Cholesterol is the direct precursor for all steroid hormones in animals, including glucocorticoids (cortisol), mineralocorticoids (aldosterone), and sex hormones (estrogen, progesterone, testosterone). Steroidogenesis begins with the cleavage of the cholesterol side chain in mitochondria. Cholesterol is also the precursor for bile acids and vitamin D.

Correct answer: Cannot be synthesized by the body

Essential fatty acids (EFAs) cannot be synthesized by humans because we lack the desaturase enzymes needed to introduce double bonds beyond C9. The two primary EFAs are linoleic acid (omega-6, C18:2) and alpha-linolenic acid (omega-3, C18:3). They must be obtained from dietary sources such as vegetable oils, nuts, and fish.

Correct answer: Linoleic acid

Linoleic acid is an essential fatty acid required in the diet. It is important for membrane structure and signaling molecules.

Correct answer: Geometry around the double bond

Trans fatty acids differ from cis fatty acids in the geometric configuration (geometry) around the double bond. In cis fats the hydrogen atoms are on the same side, producing a kinked chain; in trans fats they are on opposite sides, producing a straighter, more linear chain. This difference affects membrane fluidity and is linked to adverse health outcomes.

Correct answer: Plasma membranes of nerve cells

Glycolipids are especially abundant in the plasma membranes of nerve cells, where they play essential roles in cell recognition, signal transduction, and myelin sheath formation. Their sugar head groups face outward and contribute to the glycocalyx.

Correct answer: Steroids – hormonal regulation

Steroids include hormones that regulate physiological processes. Other lipid classes have different primary functions.

Correct answer: Are highly reduced

Lipids are highly reduced molecules with many C–H bonds. This allows them to release more energy upon oxidation.

Correct answer: A glycerol backbone esterified to three separate fatty acids

Triacylglycerols (fats) are formed by the esterification of three fatty acid chains to a single glycerol molecule, serving as the primary energy storage molecules in animals.

Correct answer: They are amphipathic, having both a polar head and non-polar tails

Phospholipids have a hydrophilic (water-loving) phosphate head and two hydrophobic (water-fearing) fatty acid tails. In water, the tails face inward to avoid water, forming a bilayer.

Correct answer: Cholesterol

Cholesterol is a vital sterol in animal membranes and acts as the chemical starting point for various steroidal compounds and fat-soluble vitamins.

Correct answer: Two or more double bonds between carbon atoms

Saturated fatty acids have no double bonds, monounsaturated have one, and polyunsaturated (PUFAs) have two or more double bonds.

Correct answer: Linolenic acid (18:3)

Linolenic acid — commonly called alpha-linolenic acid (ALA) — is an essential omega-3 fatty acid with three double bonds (18:3). Humans lack the desaturase enzymes needed to add double bonds near the omega-3 position, so it must come from dietary sources such as flaxseed and walnuts.

Correct answer: They introduce a 'kink' in the chain, preventing tight packing and lowering the melting point

Cis double bonds create a bend or kink. This prevents the molecules from stacking closely, which keeps the lipid in a liquid state (oil) at room temperature.

Correct answer: Sphingosine

Sphingomyelin belongs to the sphingolipids. It uses a sphingosine amino alcohol backbone instead of the glycerol backbone used by phosphoglycerides.

Correct answer: Preventing tight phospholipid packing at low temperatures and restricting excess movement at high temperatures

Cholesterol stabilizes the membrane. At high heat, it prevents it from becoming too fluid; at low heat, it prevents it from freezing/solidifying.

Correct answer: The alkaline hydrolysis of a triacylglycerol to produce glycerol and soap

Saponification involves treating fats with a strong base (like NaOH) to break the ester bonds, releasing glycerol and the salts of fatty acids (soap).

Correct answer: Cerebroside

Cerebrosides are neutral glycolipids consisting of a ceramide unit and a single sugar residue (usually galactose), vital for nerve insulation.

Correct answer: The degree of unsaturation (number of double bonds)

Iodine reacts with double bonds. A higher iodine number indicates a higher number of double bonds in the fat or oil.

Correct answer: Eicosanoids (Prostaglandins)

Eicosanoids, including prostaglandins, leukotrienes, and thromboxanes, are potent local signaling molecules derived from arachidonic acid.

Correct answer: A core of four fused carbon rings (cyclopentanoperhydrophenanthrene)

All steroids possess the same basic four-ring carbon skeleton, with different functional groups attached to distinguish specific molecules like estrogen or cholesterol.

Correct answer: The carbon atoms in lipids are in a more reduced state

Lipids are highly reduced (mostly C-H bonds). Oxidation releases energy; since they have less oxygen than sugars, they can be oxidized much further, releasing more energy.

Correct answer: Stearic acid

Stearic acid is an 18-carbon saturated fatty acid. The others listed are all unsaturated to varying degrees.

Correct answer: Phosphatidylcholine

Phosphatidylcholine (Lecithin) is the most abundant phospholipid in eukaryotic cell membranes and is essential for preventing alveolar collapse in the lungs.

Correct answer: Vitamin A (Retinol)

Vitamins A, D, E, and K are all fat-soluble (lipophilic) and are considered isoprenoid-derived lipids.

Correct answer: Steady oxidation of the unsaturated fatty acids

Oxidative rancidity occurs when oxygen attacks the double bonds in unsaturated fats, leading to the formation of volatile aldehydes and ketones.

Correct answer: An ester of a long-chain fatty acid with a long-chain monohydroxy alcohol

Waxes are simple lipids consisting of long-chain fatty acids esterified to long-chain alcohols. They serve as protective, water-repellent coatings.

Correct answer: Cardiolipin

Cardiolipin (diphosphatidylglycerol) is unique to the inner mitochondrial membrane and is critical for the optimal activity of enzymes involved in cellular respiration.