2. Biomolecules — Proteins, Lipids, Carbohydrates and Enzymes

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CSS GSA — Paper 7, Part-I, Section II, Sub-topic 2: Biomolecules — Proteins, Lipids, Carbohydrates and Enzymes

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Past-paper weightage: high and rising. Long questions: cholesterol (2016, 2019), carbohydrate digestion and classification (2018, 2020, 2023), DNA/RNA (2022), enzymes (2023 Special), lipid diversity and membrane fluidity (2026). MCQs on proteins and enzymes appear almost every year (2016, 2018, 2020, 2024, 2025, 2026).

1. What Biomolecules Are

• Organic molecules made by living things, built mainly from C, H, O (plus N, P, S)
• Four big groups: carbohydrates, lipids, proteins, nucleic acids; most enzymes are proteins, with catalytic RNA (ribozymes) as an exception
• Large ones (polymers) are made by joining small units (monomers) through condensation (water removed) and broken down by hydrolysis (water added)

2. Carbohydrates

• Sugars and starches; general formula roughly Cₓ(H₂O)ᵧ; main and quickest energy source (≈ 4 kcal/g)
• Made by plants through photosynthesis

• Reducing sugars: all monosaccharides, maltose, lactose (give a positive Benedict's test)
• Non-reducing sugar: sucrose
• Functions: energy, storage (starch, glycogen), structure (cellulose, chitin), part of DNA/RNA (ribose, deoxyribose)

2.1 Digestion of carbohydrates — CSS 2018 Q3(c)

• Mouth: chewing increases surface area; salivary amylase begins starch hydrolysis into maltose and shorter dextrins.
• Stomach: acidic gastric juice inactivates salivary amylase; there is no significant new enzymatic carbohydrate digestion here.
• Duodenum: pancreatic amylase continues breaking starch into smaller sugars.
• Small-intestinal brush border: maltase (maltose → 2 glucose), sucrase (sucrose → glucose + fructose), lactase (lactose → glucose + galactose).
• Absorption: glucose and galactose use sodium-dependent transport; fructose uses facilitated diffusion. Monosaccharides enter intestinal capillaries and travel to the liver through the hepatic portal vein.
• Large intestine: undigested fibre is partly fermented by gut bacteria; remaining residue contributes to faeces.

Flow: Starch → amylase → maltose/short carbohydrates → brush-border enzymes → glucose, fructose and galactose → intestinal villi → portal blood.

Exam correction: Not all carbohydrates begin as polysaccharides; dietary sugars may already be mono- or disaccharides. The large intestine is not the principal site of carbohydrate digestion.

3. Proteins

• Long chains of amino acids joined by peptide bonds; a chain is a polypeptide
• 20 standard amino acids are commonly taught; 9 are essential for adults. Some others become conditionally essential during growth, illness or stress
• Provide ≈ 4 kcal/g

3.0 Amino acids — structure and nutritional classes

• Definition: amino acids are the building blocks of proteins, linked through peptide bonds; 20 standard amino acids are commonly used in human proteins.
• General structure: central alpha carbon bonded to an amino group (–NH₂), carboxyl group (–COOH), hydrogen and variable R side chain: H₂N–CH(R)–COOH. At physiological pH, many occur as zwitterions (NH₃⁺ and COO⁻).
• Essential (nine; must be supplied in adequate dietary amounts): histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine.
• Nonessential: body can synthesise them; examples alanine, asparagine, aspartate and glutamate.
• Conditionally essential: may be required from diet during growth, severe illness or stress; examples arginine, glutamine, cysteine and tyrosine. These categories overlap with nonessential amino acids under ordinary conditions.
• Functions: tissue growth and repair; enzymes and peptide hormones; transport and immunity; energy when necessary.

Remember: amino group = basic character; carboxyl group = acidic character; R group determines the identity and properties of an amino acid.

3.1 Four levels of protein structure

• Heat or extreme pH can disrupt folding and denature a protein, often destroying its function. Ordinary denaturation generally preserves the primary peptide-bond sequence; hydrolysis breaks that sequence.

3.2 Functions

• Structural: keratin (hair, nails), collagen (skin, bone)
• Enzymes: pepsin, amylase
• Transport: haemoglobin carries oxygen
• Defence: antibodies
• Hormones: insulin, growth hormone
• Movement: actin and myosin in muscles

4. Lipids

• Fats, oils, waxes, steroids; insoluble in water, soluble in organic solvents
• Richest energy store: ≈ 9 kcal/g

4.1 Structural diversity of lipids

4.2 Saturated vs unsaturated fatty acids

• Hydrogenation adds hydrogen to double bonds and can harden vegetable oils into vanaspati. Partial hydrogenation can create trans fats. Dairy ghee is clarified butter, not hydrogenated vegetable oil

4.3 Lipids, membrane fluidity and signalling

• Membranes are a phospholipid bilayer: water-loving heads outside, water-hating tails inside
• Fluidity depends on the tails
  • More unsaturated tails → kinks → looser packing → more fluid
  • More saturated and longer tails → tighter packing → less fluid
  • Cholesterol acts as a buffer: stops membranes becoming too stiff in cold or too runny in heat
• Organisms adjust this: fish and plants in cold climates add more unsaturated fats to keep membranes working
• Signalling: fluid membranes let receptors move and meet partners; lipid rafts cluster signalling proteins; arachidonic acid is converted into prostaglandins (pain, inflammation); steroid hormones are themselves lipids

4.4 Cholesterol

• A steroid lipid needed for cell membranes, steroid hormones, vitamin D and bile
• Normal level: total under 200 mg/dL; LDL ideally under 100; HDL above 40 (men) and 50 (women)

4.5 How elevated LDL harms the heart — CSS 2019 Q2(b)

1. LDL particles enter the arterial wall and may become modified, promoting inflammation.
2. Immune cells take up lipids, forming foam cells and atherosclerotic plaques.
3. Plaques can narrow coronary arteries and reduce oxygen supply to heart muscle, causing angina.
4. Plaque rupture or erosion can trigger a clot that suddenly blocks an artery, causing a myocardial infarction (heart attack).

Clinical distinction: cholesterol is necessary for cell membranes, bile acids, vitamin D and steroid hormones; cardiovascular risk depends on the lipid profile and other risk factors, not simply the presence of cholesterol. Total cholesterol below 200 mg/dL and LDL below 100 mg/dL are commonly used general reference targets, not universal individual treatment goals. HDL is a risk marker, not a guarantee of protection.

5. Enzymes

• Biological catalysts, almost all proteins, that speed up reactions by lowering activation energy; not used up; highly specific
• Names usually end in "-ase" (lipase, sucrase); many need helpers: cofactors (metal ions like Zn²⁺, Mg²⁺) or coenzymes (often from B-vitamins)

5.0 Enzyme structure, helpers and catalytic cycle

• Active site: a three-dimensional pocket formed by amino-acid residues where the substrate binds and the reaction is catalysed.
• Specificity: the active site's shape and chemistry favour particular substrates and reactions.
• Cofactor: nonprotein helper, such as Mg²⁺ or Zn²⁺; coenzyme: organic helper, often vitamin-derived (e.g., NAD⁺).
• Apoenzyme: protein component without its required cofactor; holoenzyme: active enzyme–cofactor complex.
• Prosthetic group: tightly bound cofactor; cosubstrate: loosely bound coenzyme that participates transiently.
• Reaction: E + S ⇌ ES → E + P. Enzymes lower activation energy without changing the overall equilibrium and are regenerated after catalysis.
• Exception: some catalytic RNA molecules are ribozymes, so not every enzyme is a protein.

5.1 How they work

5.2 Seven enzyme classes (IUBMB; traditional texts list six)

The current EC system has seven classes. IUBMB EC 7 — Translocases.

5.2a Enzyme action models — compare

Sequence: substrate approaches → enzyme–substrate complex forms → catalytic conversion → products released → enzyme reused.

5.3 Functions in organisms

• Digestion (amylase, pepsin, lipase)
• Energy release (respiration enzymes; ATP synthase)
• DNA copying and repair (DNA polymerase, ligase)
• Signalling (protein kinases)
• Detoxification in the liver
• Blood clotting (thrombin)

5.4 Factors and inhibition

• Temperature: many human enzymes work well near body temperature, but optima vary with enzyme and assay conditions; excessive heat can denature them
• pH: each has an optimum (pepsin ≈ 2; trypsin ≈ 8)
• Substrate and enzyme concentration
• Competitive inhibitor: blocks the active site; overcome by more substrate
• Non-competitive inhibitor: binds elsewhere and changes the shape; not overcome
• MCQ wording: carbonic anhydrase is the best answer among the listed 2025 options; avoid treating it as a universal speed record across all enzymes and conditions.

5.5 Enzyme inhibition and rate factors — detailed revision

Important nuance: noncompetitive inhibition is a special case of mixed inhibition; not all inhibitors binding outside the active site have unchanged Km. Heavy-metal ions may inhibit enzymes, but not every heavy-metal effect is irreversible.

Factors controlling activity

• Temperature: rising temperature initially speeds collisions; above an optimum, heat may unfold the enzyme and reduce activity. The optimum varies by enzyme.
• pH: changes ionisation of active-site residues and substrate; pepsin works best in acidic gastric conditions (around pH 2), while trypsin prefers alkaline intestinal conditions (around pH 8).
• Substrate concentration: rate rises at first, then approaches Vmax as active sites become saturated.
• Enzyme concentration: at abundant substrate, more enzyme generally increases reaction rate; limited substrate constrains this effect.
• Cofactor availability and inhibitors: absence of a required helper or presence of an inhibitor reduces activity.
• Covalent modification: phosphorylation/dephosphorylation can activate or inhibit enzymes; proteolytic cleavage activates some zymogens (pepsinogen → pepsin in acidic stomach conditions). Proteolytic activation is distinct from reversible phosphorylation.

Quick graphs to practise: temperature versus activity (rise and fall); pH versus activity (optimum peak); substrate concentration versus rate (saturation curve); enzyme concentration versus rate (approximately linear with excess substrate).

6. Nucleic Acids (DNA and RNA)

• Polymers of nucleotides (sugar + phosphate + nitrogen base)
• DNA: double helix (Watson and Crick, 1953), deoxyribose, bases A–T and G–C; stores genes
• RNA: single strand, ribose, uracil instead of thymine; mRNA, tRNA, rRNA carry out protein synthesis

7. MCQ Traps

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• Primary structure = amino acid sequence, not hydrogen bonds
• Proteins = polypeptides; nucleic acids = polynucleotides
• Main storage lipid = triglyceride; glycogen is a carbohydrate store
• Sucrose is the classic non-reducing sugar
• Most vegetable oils are rich in unsaturated fats; coconut/palm oils are important exceptions. Foods contain mixtures of fatty acids
• LDL = Lousy; HDL = Healthy
• Most enzymes are proteins; ribozymes are RNA exceptions. Catalysts are regenerated during the reaction.

8. Self-Test Questions

1. What is cholesterol? Give its importance, normal level and dangers of high levels.
2. Classify carbohydrates with examples.
3. Explain the four levels of protein structure.
4. Define enzymes; explain their types and functions.
5. How does fatty acid composition affect membrane fluidity?

9. CSS Exam Perspective

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• Most repeated areas: cholesterol, carbohydrate classification and digestion, DNA vs RNA, enzymes
• New trend (2026): deeper biochemistry — lipid diversity, membrane fluidity, protein structure levels
• Likely 2027 angles: protein structure and denaturation, enzyme factors/inhibition, saturated vs unsaturated fats and heart health
• Tip: use one table per biomolecule and follow food through the gut for digestion answers

Syllabus completion — connecting the four biomolecule topics

One comparison to revise all four named items

Classification precision: oligosaccharides contain a small number of sugar units; disaccharides are one subgroup. Lipids do not all contain glycerol and three fatty acids: that description fits triglycerides. Phospholipids are amphipathic, with polar heads and nonpolar tails; cholesterol has a four-ring steroid skeleton.

Protein and lipid distinctions that improve answers

• Fibrous versus globular proteins: collagen and keratin support structures; haemoglobin and many enzymes have compact globular forms. Shape and amino-acid side chains explain function.
• Denaturation versus digestion: denaturation changes folding; proteolytic digestion hydrolyses peptide bonds. Do not say ordinary heating necessarily destroys the primary structure.
• Cis versus trans: cis double bonds usually bend fatty-acid chains; trans chains pack more like saturated chains. Foods contain mixtures, so “all vegetable oils are unsaturated” is too absolute.
• LDL and HDL: these are lipoprotein particles carrying cholesterol, not two chemically different cholesterol molecules.
• Membrane signalling: fatty-acid composition influences membrane packing and receptor organisation; some lipids also act directly as signals or signal precursors. Explain both mechanisms for the saved 2026 question.

Match the answer to the saved question

Additional practice—not past-paper questions: Why does enzyme activity plateau as substrate increases? Active sites become saturated. Why can cooling reduce activity without denaturing an enzyme? Molecular movement slows, but the folded structure may remain intact. Why is glycogen not a lipid? It is a glucose polymer and an animal carbohydrate store.

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10. Past-Paper Long Questions — Exam-Ready Answers

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These are exam-ready study answers for the questions recorded in this page. Year labels and wording follow the saved collection; they have not all been authenticated against original papers. Coverage splits are suggested planning guides, not official FPSC marking schemes. Use the marks printed on your paper. For combined questions without an explicit sub-split, divide space according to the tasks asked.

• CSS 2026 Q3(c): Discuss lipid structural diversity and evaluate how fatty-acid composition affects membrane fluidity and cell signalling. (5 marks)

  Suggested coverage: diversity 1.5; fluidity 2; signalling 1.5.

  Model answer

  Lipids are a diverse group of largely hydrophobic or amphipathic molecules involved in energy storage, membranes and signalling.

  Structural diversity: Triglycerides contain glycerol joined to three fatty acids and store energy. Typical glycerophospholipids have two fatty-acid tails and a polar phosphate-containing head, allowing bilayer formation. Waxes contain long-chain fatty acids linked to alcohols; steroids, including cholesterol, have four fused rings. Glycolipids carry carbohydrate groups.

  Membrane fluidity: Cis-unsaturated tails have bends that reduce close packing and generally increase fluidity. Saturated tails pack more closely; longer chains have stronger intermolecular interactions and generally reduce fluidity. Cholesterol restrains phospholipid movement at higher temperatures and limits tight packing at lower temperatures, buffering fluidity.

  Signalling: Membrane organisation affects receptor mobility, clustering and interaction with signalling proteins. Cholesterol-rich domains can organise some signalling complexes. Cleavage of membrane phospholipids generates messengers such as diacylglycerol; arachidonic acid supplies precursors for eicosanoids.

  Thus, lipid composition changes both the membrane’s physical properties and the availability and organisation of signalling molecules.

  Diagram: Compare straight saturated and bent cis-unsaturated tails; label polar heads and cholesterol.

• CSS 2023 Special Q5(c): Define enzymes and describe their types and functions. (5 marks)

  Suggested coverage: definition and action 1; classes with examples 2.5; functions 1.5.

  Model answer

  Enzymes are biological catalysts that accelerate reactions by lowering activation energy without being consumed. Most are proteins, but catalytic RNAs called ribozymes also exist. An active site binds suitable substrates; temperature, pH and inhibitors influence activity.

  Class	Reaction and example
  Oxidoreductases	Electron transfer; dehydrogenases
  Transferases	Transfer functional groups; kinases
  Hydrolases	Break bonds using water; lipase
  Lyases	Non-hydrolytic bond cleavage/addition; decarboxylases
  Isomerases	Rearrange atoms within a molecule; phosphoglucose isomerase
  Ligases	Join molecules coupled to energy use; DNA ligase
  Translocases	Move ions or molecules across membranes; sodium–potassium ATPase

  Functions: Amylase, pepsin and lipase digest food. Respiratory enzymes support ATP production. DNA polymerases and ligases enable genome replication and repair. Kinases regulate signalling, and enzyme cascades contribute to blood clotting.

  Revision note: Older books list six classes; the current classification includes translocases as class seven. IUBMB enzyme classification.

• CSS 2016 Q5(b), 2019 Q2(b): Cholesterol—importance, blood level and cardiovascular danger. (5-mark topic variants)

  Suggested coverage: definition/functions 2; level qualification 1; harmful mechanism 2.

  Model answer

  Cholesterol is a sterol lipid synthesised mainly in the liver and also obtained from animal foods. Because it is poorly soluble in water, lipoproteins carry it in blood.

  Importance: It helps regulate cell-membrane properties and is a precursor of steroid hormones, bile acids and vitamin D.

  Blood levels: A commonly quoted adult desirable total-cholesterol value is below 200 mg/dL. This is a population reference, not a universal treatment target; LDL goals depend on overall cardiovascular risk. LDL carries cholesterol to tissues, while HDL participates in its return towards the liver.

  Why excessive LDL is harmful: LDL-containing particles can accumulate in arterial walls and promote inflammation and atherosclerotic plaque formation. Plaques may narrow coronary arteries, reducing oxygen supply and causing angina. Plaque rupture can trigger a clot that blocks blood flow, producing a myocardial infarction. Similar disease in cerebral arteries can cause stroke.

  Genetics, diets high in saturated or trans fats, smoking and some metabolic disorders influence risk. Prevention includes appropriate diet, physical activity, tobacco avoidance and risk-based medical management.

  Exam focus: Explain plaque rupture and thrombosis; “fat blocks the heart” is insufficient.

• CSS 2020 Q3(b): Define carbohydrates and classify them with examples. (5 marks)

  Suggested coverage: definition 1; three classes with examples and functions 4.

  Model answer

  Carbohydrates are organic compounds containing carbon, hydrogen and oxygen. Many approximately follow the formula Cₓ(H₂O)ᵧ, although this is not universal. Their units are linked by glycosidic bonds.

  Class	Structure	Examples and roles
  Monosaccharides	Single sugar units; cannot be hydrolysed into simpler carbohydrates	Glucose supplies energy; ribose and deoxyribose occur in nucleic acids
  Oligosaccharides	Short chains, commonly 2–10 sugar units	Disaccharides include sucrose (glucose + fructose), lactose (glucose + galactose) and maltose (two glucose units)
  Polysaccharides	Long chains of monosaccharides	Starch stores glucose in plants; glycogen stores it in animals; cellulose supports plant walls

  Functions: Carbohydrates provide respiratory fuel, energy reserves and structural support. Cell-surface carbohydrates also contribute to recognition. Humans do not digest cellulose enzymatically, but it contributes dietary fibre.

  Diagram cue: one sugar unit → short chain → long chain; name the glycosidic link.

• CSS 2018 Q3(c): Define carbohydrates and describe their digestion. (5-mark variant)

  Suggested coverage: definition 1; sites, enzymes, products and absorption 4.

  Model answer

  Carbohydrates include sugars and polymers such as starch; digestible carbohydrates provide an important source of glucose.

  1. Mouth: Chewing increases surface area. Salivary amylase begins starch hydrolysis, forming maltose and shorter carbohydrate chains.
  2. Stomach: Salivary action may continue briefly inside a food bolus, but gastric acidity subsequently inactivates amylase. The stomach secretes no major carbohydrate-digesting enzyme.
  3. Small intestine: Pancreatic amylase continues starch digestion. Brush-border maltase, sucrase and lactase convert disaccharides into absorbable monosaccharides.
  4. Absorption: Glucose, galactose and fructose enter intestinal epithelial cells and then villus blood capillaries. Portal blood carries them to the liver for processing, storage or distribution.
  5. Residue: Undigested carbohydrate, including much fibre, reaches the colon; some is fermented by microbes.

  Flow: starch → smaller sugars → monosaccharides → intestinal blood → liver.

  The endpoint is absorbable monosaccharides, not absorption of intact starch.

• CSS 2023 Q5(b): Digestion of carbohydrates, proteins and fats. (5-mark variant; verify exact original wording)

  Suggested coverage: brief definition 0.5; balanced three-nutrient comparison 3.5; absorption 1.

  Model answer

  Digestion breaks large food molecules into smaller absorbable products through mechanical processing and enzymatic hydrolysis.

  Nutrient	Main sites and agents	Products
  Carbohydrates	Salivary amylase in mouth; pancreatic amylase and brush-border enzymes in small intestine	Glucose, galactose and fructose
  Proteins	Pepsin in acidic stomach; pancreatic proteases and intestinal peptidases in small intestine	Mainly amino acids, with small peptides absorbed and further processed
  Fats	Bile emulsification in duodenum; mainly pancreatic lipase in small intestine	Mainly free fatty acids and monoglycerides

  Bile is not an enzyme: it increases the fat surface available to lipase. Pancreatic bicarbonate neutralises incoming gastric acid, enabling intestinal enzymes to function.

  Absorption: Monosaccharides and amino acids enter villus capillaries and portal blood. Most long-chain fat products are reassembled into triglycerides and packaged as chylomicrons, entering lacteals before reaching the blood.

  Diagram cue: Label mouth, stomach and duodenum with their relevant enzymes; finish with villus capillary versus lacteal.

• CSS 2022 Q3(c): What do you understand by DNA and RNA? (5 marks)

  Suggested coverage: definitions/shared units 1; comparison 3; information flow 1.

  Model answer

  DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are nucleic acids made of nucleotides. Each nucleotide contains a sugar, phosphate and nitrogenous base. Phosphodiester bonds join the sugar–phosphate backbone.

  Feature	DNA	RNA
  Sugar	Deoxyribose	Ribose
  Bases	A, T, G, C	A, U, G, C
  Typical structure	Two antiparallel complementary strands forming a double helix	Usually single-stranded, often folded
  Eukaryotic location	Mainly nucleus; also mitochondria and chloroplasts	Nucleus and cytoplasm, including ribosomes
  Main role	Long-term storage and transmission of genetic information	Gene expression, regulation and some catalytic functions

  In DNA, A pairs with T and G with C. Messenger RNA carries a transcribed sequence to ribosomes; transfer RNA delivers amino acids; ribosomal RNA forms the structural and catalytic core of ribosomes.

  Information flow: DNA → transcription → RNA → translation → protein. RNA is not merely a temporary copy: different RNAs perform distinct tasks.

11. Past-Paper MCQ Quiz (FPSC)

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Real FPSC GSA MCQs on this topic, newest first. Answers and explanations are worked out.

1. (CSS 2026) The primary structure of a protein is determined by its:

• (A) Amino acid sequence (B) Hydrogen bonding (C) Ionic interactions (D) Disulfide bridges
• ✅ Answer: (A) Amino acid sequence — primary structure is simply the order of amino acids. Hydrogen bonds shape the secondary level; ionic bonds and disulphide bridges hold the tertiary level.

2. (CSS 2026) The major storage lipid in animals is:

• (A) Steroid (B) Terpene (C) Glycogen (D) Wax
• ✅ Answer: the question has no correct option as written. The major storage lipid in animals is triglyceride (fat), stored in adipose tissue. Glycogen is the animal storage carbohydrate; steroids have structural/signalling roles, and wax is protective. Flag the item as flawed rather than learning glycogen as a lipid.

3. (CSS 2025) The fastest enzyme is:

• (A) Pepsin (B) DNA polymerase (C) Carbonic anhydrase (D) None of these
• ✅ Answer: (C) Carbonic anhydrase — the best answer among these options; carbonic anhydrase rapidly catalyses CO₂ hydration. “Fastest” depends on the enzyme and measurement conditions, so it is not an absolute ranking.

4. (CSS 2024) The protein molecule is made of:

• (A) Alkaloids (B) Nucleosides (C) Short range chain of amino acids (D) Long range chain of amino acids
• ✅ Answer: (D) Long range chain of amino acids — proteins are long polypeptides (often hundreds of amino acids); short chains are just peptides. Nucleosides belong to nucleic acids.

5. (CSS 2020) Which of the following is an unsaturated fat?

• (A) Cheese (B) Butter (C) Vegetable oil (D) None of these
• ✅ Answer: (C) Vegetable oil — many vegetable oils are rich in cis-unsaturated fatty acids. Foods contain mixtures; palm and coconut oils are relatively saturated, so plant origin alone does not establish unsaturation.

6. (CSS 2020) What is a non-reducing sugar?

• (A) Maltose (B) Sucrose (C) Glucose (D) None of these
• ✅ Answer: (B) Sucrose — the reducing ends of glucose and fructose are locked in the bond between them, so it fails Benedict's test. Glucose and maltose are reducing sugars.

7. (CSS 2018) Enzymes in human bodies are:

• (A) Proteins (B) Viruses (C) Bacteria (D) None of these
• ✅ Answer: (A) Proteins — nearly all enzymes are globular proteins (a few RNA "ribozymes" are exceptions).

8. (CSS 2016, repeated 2018) Proteins are made up of:

• (A) Polynucleotide (B) Polypeptide (C) Oxyacetylene (D) None of these
• ✅ Answer: (B) Polypeptide — chains of amino acids joined by peptide bonds. Polynucleotides make DNA/RNA; oxyacetylene is a welding gas mixture.

Figure and quick distinctions — building blocks versus bonds

Schematic for learning the table: a bond is the connection; a building block is the unit connected. Enzymes are catalysts, not an extra class of repeating monomer.

flowchart TD
 A["Monosaccharides"] -->|"glycosidic bonds"| B["Polysaccharide"]
 C["Amino acids"] -->|"peptide bonds"| D["Polypeptide"]
 E["Glycerol and three fatty acids"] -->|"three ester bonds"| F["Triglyceride"]
 G["Nucleotides"] -->|"phosphodiester bonds"| H["DNA or RNA chain"]

• Condensation/hydrolysis: in the introductory model, joining units releases water; hydrolysis uses water to break a bond. Actual cellular biosynthesis can use activated intermediates.
• DNA bond distinction: phosphodiester bonds run along each sugar–phosphate backbone; hydrogen bonds connect complementary bases between strands. A–T has two hydrogen bonds; G–C has three.
• Lipid distinction: a triglyceride stores energy; a typical glycerophospholipid has two fatty-acid tails plus a phosphate-containing head and forms membranes; cholesterol has four fused rings.
• Food tests: Benedict's test with heating detects reducing sugars; iodine detects starch; Biuret detects peptide bonds; the ethanol-emulsion test detects lipids. These are supporting practice facts, not newly attributed past-paper questions.
• Energy figures: digestible carbohydrate ≈4 kcal/g, protein ≈4 kcal/g and fat ≈9 kcal/g; these are rounded nutritional conversion factors.

Sources: OpenStax lipids, OpenStax proteins.

Cholesterol reference: Total cholesterol below 200 mg/dL is a common desirable adult reference; individual LDL goals are risk-based. MedlinePlus cholesterol levels.