Unit 15 Question Bank
Format note: the TMU Biochemistry paper gives five suggested
answers (A–E), not four — these MCQs match that. Items tagged TMU 2019 or
TMU 2020/21 come from the real papers. Answers are verified against Harper's
Illustrated Biochemistry; the "marking schemes" in the source folder are other students'
answer sheets, not official, so they are never used as the authority.
0 / 25 answered
1Lipids are best defined as ( ).
Answer: A
Note what kind of definition this is. Every other class in the course is defined chemically; lipids are defined operationally — by their physical behaviour. They are “related more by their physical than by their chemical properties”. Option B defines fats only, a subclass.Harper's ch.21, p.212 · TMU Lecture 13 p.9
2A SIMPLE lipid is ( ).
Answer: B
Simple lipids are fats (esters with glycerol) and waxes (esters with higher molecular weight monohydric alcohols). Complex lipids contain groups in addition to an alcohol and fatty acids — a phosphate makes a phospholipid, a carbohydrate a glycolipid.Harper's ch.21, p.212
3Which of the following is classified as a PRECURSOR or DERIVED lipid?
Answer: C
Precursor and derived lipids include fatty acids, glycerol, steroids, other alcohols, fatty aldehydes, ketone bodies, hydrocarbons, lipid-soluble vitamins and hormones. Waxes are simple lipids; the rest listed are complex lipids.Harper's ch.21, p.212
4In the notation 18:2; Δ9,12, the numbers indicate ( ).
Answer: D
This is linoleic acid. The Δ system counts from the carboxyl group; the ω system counts from the methyl carbon and gives the ω-3 and ω-6 families. Both are examinable.Harper's ch.21 · TMU Lecture 13 p.18
5Which two fatty acids are the 16- and 18-carbon SATURATED acids?
Answer: E
Palmitic acid is 16 C, stearic acid 18 C, both saturated. Palmitoleic (16:1) and oleic (18:1) are monounsaturated; linoleic (18:2) and linolenic (18:3) polyunsaturated.TMU Lecture 13 p.16
6Most naturally occurring unsaturated fatty acids have ( ) double bonds.
Answer: A
In the cis configuration the acyl chains are on the same side of the double bond, producing a kink. Trans fatty acids arise largely from partial hydrogenation and are associated with elevated risk of cardiovascular disease.Harper's ch.21, p.213 · TMU Lecture 13 p.20
7The melting point of a fatty acid is strongly influenced by ( ).
Answer: B
Longer chain → higher melting point. More double bonds → LOWER melting point. The reason is packing: saturated chains form a zigzag pattern when extended and pack tightly, whereas a cis double bond kinks the chain and prevents close packing.Harper's ch.21, p.213 · TMU Lecture 13 p.32
8A trans fatty acid behaves more like a saturated fatty acid because ( ).
Answer: C
This is why trans fats are metabolically and clinically problematic. The named example is elaidic acid, 18:1;Δ9 trans — the trans isomer of oleic acid, identical in formula and opposite in effect, and associated with elevated cardiovascular risk.Harper's ch.21, p.213 · TMU Lecture 13 p.22
9Unsaturated fatty acids can be converted to saturated fatty acids by ( ).
Answer: D
The industrial process that hardens vegetable oils — and the process whose partial form generates trans fatty acids as a by-product.TMU Lecture 13 p.32
10Essential fatty acids cannot be synthesised by the body because mammals cannot ( ).
Answer: E
Hence the ω-3 and ω-6 families must be supplied in the diet: linoleic acid (18:2; ω-6) and α-linolenic acid (18:3; ω-3). This is also why the ω numbering system — which counts from the methyl end — is the one used in nutrition.Harper's ch.21, p.212
11Linoleic acid is essential largely because it gives rise to ( ).
Answer: A
Follow the chain: linoleic acid → arachidonic acid → prostaglandins, thromboxanes and leukotrienes → inflammation, platelet aggregation and bronchoconstriction. A dietary requirement at one end becomes, at the other, the reason aspirin works.Harper's ch.21, pp.212–213
12Eicosanoids are derived from ( ).
Answer: B
“Eicosa” means twenty. They are very potent, short-lived chemical signals, comprising the prostanoids (prostaglandins, prostacyclins, thromboxanes), the leukotrienes and the lipoxins.Harper's ch.21, p.213 · TMU Lecture 13 p.25
13Prostaglandins are formed by ( ).
Answer: C
Thromboxanes have that cyclopentane ring interrupted by an oxygen atom — an oxane ring; leukotrienes and lipoxins are formed by the lipoxygenase pathway instead and are characterised by three or four conjugated double bonds respectively.Harper's ch.21, p.213
14Which eicosanoid is produced in platelets and causes arteriole contraction and platelet aggregation?
Answer: D
Thromboxane A₂. This is the target of low-dose aspirin. Compare prostaglandins — local hormones causing inflammation, smooth muscle contraction, sodium and water retention, platelet aggregation and gastric secretion — and leukotrienes, involved in allergic reactions, chemotaxis and bronchoconstriction.TMU Lecture 13 p.28
15Leukotrienes are clinically important chiefly because they ( ).
Answer: E
They are formed via the lipoxygenase pathway and are involved in allergic reactions, chemotaxis of white blood cells and inflammation. This is why leukotriene antagonists are asthma drugs.Harper's ch.21, p.213 · TMU Lecture 13 p.28
16In the name PGE₂, the subscript 2 indicates ( ).
Answer: A
Three different eicosanoic fatty acids give rise to three groups of eicosanoids — PG₁, PG₂, PG₃. The letter records the ring substituents: the “E” type has a keto group in position 9, the “F” type a hydroxyl. So PGE₂ decodes as: a prostaglandin, keto at position 9, derived from arachidonic acid.Harper's ch.21, p.213
17Triacylglycerols are a better fuel store than glycogen for two reasons. Which pair is correct?
Answer: B
Both matter, and the second matters more than students expect. Glycogen is stored hydrated; fat is stored essentially dry. An animal storing its energy reserve as glycogen could not move. It is also why liver glycogen lasts 12–18 hours while fat stores last weeks.TMU Lecture 13 p.13
18Phospholipids are defined as lipids containing, in addition to fatty acids and an alcohol, ( ).
Answer: C
They frequently also have nitrogen-containing bases such as choline. The alcohol is glycerol in glycerophospholipids, or sphingosine — which contains an amino group — in sphingophospholipids.Harper's ch.21, p.212 · TMU Lecture 13 p.36
19Glycolipids (glycosphingolipids) contain a fatty acid, a carbohydrate, and ( ).
Answer: D
The distinguishing feature of the sphingolipids is the alcohol: sphingosine rather than glycerol. Sphingomyelin is a sphingophospholipid; a glycolipid carries carbohydrate instead of phosphate.Harper's ch.21, p.212 · TMU Lecture 14 p.40
20Give the two main functions of phospholipids.
Answer: E
Note the second function — phospholipids are the reservoir from which arachidonic acid is released to make eicosanoids, which is how an inflammatory stimulus mobilises the substrate.TMU Lecture 13 p.39
21The major component of lung surfactant is ( ).
Answer: A
Lack of surfactant production in the lungs of infants causes respiratory distress syndrome. It works because a phospholipid is amphipathic: it sits at the alveolar air–water interface, lowering surface tension so the alveolus does not collapse on expiration.TMU Lecture 13 p.36
22A molecule described as amphipathic ( ).
Answer: B
This one property generates four structures you already know: micelles (fatty acids in water), bilayers (membranes), lipoproteins (a shell around a fat droplet) and surfactant (a film at an air–water interface).Harper's ch.21, p.211 · TMU Lecture 13 p.30
23Fatty acids in water form ( ).
Answer: C
Tails inward, polar heads outward — the simplest solution to the amphipathic problem. Phospholipids, sphingolipids and cholesterol instead form the bilayer that is the basic structure of biological membranes.TMU Lecture 13 p.30
24Non-polar lipids act as electrical insulators, which ( ).
Answer: D
Fat also serves as a thermal insulator in subcutaneous tissue and around organs — but the electrical insulation is the one that explains saltatory conduction, and it is the less obvious of the two.Harper's ch.21, p.212
25Dietary supplementation with long-chain ω-3 fatty acids is believed to benefit ( ).
Answer: E
Harper's phrases it carefully as “believed to have beneficial effects” — note the hedge, which is appropriate for an association rather than a proven mechanism. Knowledge of lipid biochemistry is stated to be necessary for understanding obesity, diabetes mellitus and atherosclerosis.Harper's ch.21, p.212
1 Lipids — 3′+
A heterogeneous group of water-insoluble (hydrophobic) organic molecules that can be extracted from tissues by non-polar solvents.
They are related more by their physical than by their chemical properties, having in common that they are relatively insoluble in water and soluble in non-polar solvents such as ether and chloroform.
Main functions: fats are ideal storage molecules; cholesterol is the source of bile salts and steroid hormones; phospholipids are an essential component of cellular membranes. Fat also acts as a thermal insulator, and non-polar lipids as electrical insulators permitting rapid conduction along myelinated nerves.Harper's ch.21, p.212
They are related more by their physical than by their chemical properties, having in common that they are relatively insoluble in water and soluble in non-polar solvents such as ether and chloroform.
Main functions: fats are ideal storage molecules; cholesterol is the source of bile salts and steroid hormones; phospholipids are an essential component of cellular membranes. Fat also acts as a thermal insulator, and non-polar lipids as electrical insulators permitting rapid conduction along myelinated nerves.Harper's ch.21, p.212
2 Simple and complex lipids — 3′+
Simple lipids are esters of fatty acids with various alcohols: fats (esters with glycerol; oils are fats in the liquid state) and waxes (esters with higher molecular weight monohydric alcohols).
Complex lipids are esters of fatty acids containing groups in addition to an alcohol and one or more fatty acids: phospholipids (containing a phosphoric acid residue, often with a nitrogenous base; the alcohol is glycerol or sphingosine), glycolipids / glycosphingolipids (containing a fatty acid, sphingosine and carbohydrate) and others, including the lipoproteins.
Precursor and derived lipids include fatty acids, glycerol, steroids, ketone bodies, lipid-soluble vitamins and hormones.Harper's ch.21, p.212
Complex lipids are esters of fatty acids containing groups in addition to an alcohol and one or more fatty acids: phospholipids (containing a phosphoric acid residue, often with a nitrogenous base; the alcohol is glycerol or sphingosine), glycolipids / glycosphingolipids (containing a fatty acid, sphingosine and carbohydrate) and others, including the lipoproteins.
Precursor and derived lipids include fatty acids, glycerol, steroids, ketone bodies, lipid-soluble vitamins and hormones.Harper's ch.21, p.212
3 Essential fatty acid — 3′+
A fatty acid that cannot be synthesised by the body and must therefore be supplied in the diet.
The reason is a synthetic limitation: mammals cannot introduce double bonds beyond carbon 9 of the chain, so the ω-3 and ω-6 families cannot be made de novo.
The two are linoleic acid (18:2; Δ9,12 — ω-6), the precursor of arachidonic acid and hence of the eicosanoids, and α-linolenic acid (18:3; Δ9,12,15 — ω-3).Harper's ch.21, p.212
The reason is a synthetic limitation: mammals cannot introduce double bonds beyond carbon 9 of the chain, so the ω-3 and ω-6 families cannot be made de novo.
The two are linoleic acid (18:2; Δ9,12 — ω-6), the precursor of arachidonic acid and hence of the eicosanoids, and α-linolenic acid (18:3; Δ9,12,15 — ω-3).Harper's ch.21, p.212
4 Eicosanoids — 3′+
Very potent, short-lived chemical signals derived from eicosa- (20-carbon) polyunsaturated fatty acids — principally arachidonic acid.
They comprise the prostanoids — prostaglandins (PG), prostacyclins and thromboxanes (TX) — together with the leukotrienes (LT) and lipoxins.
Prostaglandins act as local hormones in virtually every mammalian tissue, causing inflammation, smooth muscle contraction, sodium and water retention, platelet aggregation and gastric secretion. TXA₂, produced in platelets, causes arteriole contraction and platelet aggregation. Leukotrienes are involved in allergic reactions, chemotaxis of white blood cells and inflammation, and cause bronchoconstriction, playing a part in asthma.Harper's ch.21, p.213 · TMU Lecture 13 pp.25–28
They comprise the prostanoids — prostaglandins (PG), prostacyclins and thromboxanes (TX) — together with the leukotrienes (LT) and lipoxins.
Prostaglandins act as local hormones in virtually every mammalian tissue, causing inflammation, smooth muscle contraction, sodium and water retention, platelet aggregation and gastric secretion. TXA₂, produced in platelets, causes arteriole contraction and platelet aggregation. Leukotrienes are involved in allergic reactions, chemotaxis of white blood cells and inflammation, and cause bronchoconstriction, playing a part in asthma.Harper's ch.21, p.213 · TMU Lecture 13 pp.25–28
5 Amphipathic — 2′+
Having both hydrophobic and hydrophilic groups in the same molecule.
Fatty acids are amphipathic and form spherical micelles. The same property in phospholipids, sphingolipids and cholesterol enables them to form the basic structure of biological membranes, and it is also what allows phospholipid to act as lung surfactant at the alveolar air–water interface, and to form the outer monolayer of a lipoprotein.Harper's ch.21, p.211 · TMU Lecture 13 p.30
Fatty acids are amphipathic and form spherical micelles. The same property in phospholipids, sphingolipids and cholesterol enables them to form the basic structure of biological membranes, and it is also what allows phospholipid to act as lung surfactant at the alveolar air–water interface, and to form the outer monolayer of a lipoprotein.Harper's ch.21, p.211 · TMU Lecture 13 p.30
6 Phospholipids — 2′+
Lipids containing, in addition to fatty acids and an alcohol, a phosphoric acid residue, frequently with nitrogen-containing bases such as choline.
In glycerophospholipids the alcohol is glycerol; in sphingophospholipids it is sphingosine.
Functions: the major component of cellular membranes, and the store of some unsaturated fatty acids. Phosphatidylcholine is the major component of lung surfactant, whose lack in infants causes respiratory distress syndrome.Harper's ch.21, p.212 · TMU Lecture 13 pp.36, 39
In glycerophospholipids the alcohol is glycerol; in sphingophospholipids it is sphingosine.
Functions: the major component of cellular membranes, and the store of some unsaturated fatty acids. Phosphatidylcholine is the major component of lung surfactant, whose lack in infants causes respiratory distress syndrome.Harper's ch.21, p.212 · TMU Lecture 13 pp.36, 39
1
Outline briefly the classification of lipids and the structure of fatty acids.
5′ — 'outline briefly'
Definition
Lipids are a heterogeneous group of water-insoluble (hydrophobic) organic molecules that can be extracted from tissues by non-polar solvents, related more by their physical than by their chemical properties.
Classification
| Class | Definition | Members |
|---|---|---|
| Simple | Esters of fatty acids with various alcohols | Fats (with glycerol; oils are fats in the liquid state) · Waxes (with higher molecular weight monohydric alcohols) |
| Complex | Esters of fatty acids containing groups in addition to an alcohol and fatty acids | Phospholipids (a phosphoric acid residue, often with a nitrogenous base; alcohol = glycerol or sphingosine) · Glycolipids (fatty acid, sphingosine, carbohydrate) · others including lipoproteins |
| Precursor / derived | Building blocks and products | Fatty acids, glycerol, steroids, fatty aldehydes, ketone bodies, hydrocarbons, lipid-soluble vitamins, hormones |
Fatty acids
Unesterified fatty acids usually contain an even number of carbon atoms, and free fatty acid is also the transport form found in plasma.
- Saturated — no double bonds. Palmitic acid (16 C), stearic acid (18 C).
- Monounsaturated — one double bond. Palmitoleic 16:1;Δ9, oleic 18:1;Δ9.
- Polyunsaturated — two or more. Linoleic 18:2;Δ9,12, linolenic 18:3;Δ9,12,15, arachidonic acid.
- Eicosanoids — derived from 20-carbon polyenoic acids.
Nomenclature
The Δ system numbers from the carboxyl group; the ω system numbers from the methyl carbon, giving the ω-3 and ω-6 families. The ω designation matters physiologically because mammals cannot introduce double bonds beyond carbon 9, so the ω position identifies whether the body could have made the acid — the basis of the essential fatty acids, linoleic and α-linolenic.
Geometry and melting point
Most naturally occurring unsaturated fatty acids have cis double bonds, in which the acyl chains lie on the same side. Melting points are strongly influenced by the length and the degree of unsaturation of the hydrocarbon chain: saturated chains form an extended zigzag and pack tightly, giving a high melting point, whereas a cis double bond kinks the chain, prevents close packing and lowers it. Trans fatty acids remain nearly straight and behave like saturated ones; they are associated with elevated risk of cardiovascular disease. Unsaturated fatty acids can be converted to saturated ones by hydrogenation.
Marking guide: definition emphasising physical properties 1 · simple/complex/derived classification with members 1.5 · saturated and unsaturated fatty acids with named examples 1 · both numbering systems 0.5 · cis/trans and the packing explanation of melting point 1.
2
Elucidate the eicosanoids: their origin, classes and biological actions.
8′ — 'elucidate'
Definition and origin
Eicosanoids are very potent, short-lived chemical signals derived from eicosa- (20-carbon) polyunsaturated fatty acids, principally arachidonic acid. Arachidonic acid is itself derived from the essential fatty acid linoleic acid, and is stored esterified in membrane phospholipids, from which it is released on demand.
Classes
| Class | Structure | Pathway |
|---|---|---|
| Prostaglandins (PG) | Formed by cyclisation of the centre of the carbon chain to give a cyclopentane ring | Cyclooxygenase |
| Prostacyclins (PGI) and thromboxanes (TX) | In thromboxanes the cyclopentane ring is interrupted by an oxygen atom — an oxane ring | Cyclooxygenase |
| Leukotrienes (LT) and lipoxins (LX) | Characterised by three or four conjugated double bonds respectively | Lipoxygenase |
Nomenclature
Three different eicosanoic fatty acids give rise to three groups of eicosanoids, characterised by the number of double bonds in the side chains — PG₁, PG₂, PG₃. Different substituent groups attached to the rings give the series labelled A, B and so on: the “E” type has a keto group in position 9, whereas the “F” type has a hydroxyl group there. So PGE₂ reads as a prostaglandin with a keto group at position 9, derived from arachidonic acid.
Biological actions
- Prostaglandins exist in virtually every mammalian tissue, acting as local hormones. Their effects include inflammation, smooth muscle contraction, sodium and water retention, platelet aggregation and gastric secretion.
- Thromboxane A₂, produced in platelets, causes arteriole contraction and platelet aggregation.
- Leukotrienes are involved in allergic reactions, chemotaxis of white blood cells and inflammation, and cause bronchoconstriction, playing a part in asthma.
Why this matters pharmacologically
Read those three lines as three drug classes. Blocking prostaglandin synthesis produces an anti-inflammatory that also causes gastric ulceration, because the same mediator does both jobs. Blocking thromboxane gives an antiplatelet drug. Blocking leukotrienes gives an asthma drug. The side effects are not accidents — they are the pathway.
Note also that because arachidonic acid derives from the essential fatty acid linoleic acid, a dietary requirement at one end of the chain determines the availability of these mediators at the other.
Marking guide: definition as short-lived signals from 20-carbon PUFA, chiefly arachidonic acid 1.5 · three classes named with their structural features 2 · the cyclooxygenase/lipoxygenase distinction 1 · nomenclature — subscript and letter 1 · actions of PG, TX and LT 2 · one pharmacological or dietary link 0.5.