Biomolecules — Practice Questions with Answers

Reducing sugars have a free (unblocked) anomeric carbon (aldehyde or ketone group) available, capable of being oxidised by mild oxidising agents like Tollen's or Fehling's reagent — example: glucose.

Non-reducing sugars have their anomeric carbons involved in glycosidic bond formation, blocking the free carbonyl group needed for the reducing reaction — example: sucrose, where both the glucose and fructose anomeric carbons are involved in the glycosidic linkage between them.

Fibrous proteins have elongated, thread-like structures with polypeptide chains running roughly parallel to each other, typically held together by hydrogen bonds and cross-links — example: keratin (found in hair, nails, wool), generally serving structural roles.

Globular proteins have polypeptide chains folded compactly into a roughly spherical shape, generally water-soluble — example: insulin (a hormone protein) or haemoglobin, generally serving functional/dynamic biological roles (like enzymes, hormones, and transport proteins).

Primary structure refers to the specific, precise linear sequence of amino acids joined together by peptide bonds, forming the polypeptide chain — this sequence is genetically determined and unique to each specific protein.

This primary sequence fundamentally determines all higher-order structures (secondary, tertiary, and quaternary) because the specific side chains (R-groups) of each amino acid, positioned at particular points along the chain according to this sequence, determine exactly where and how hydrogen bonds, disulfide bridges, hydrophobic interactions, and other stabilising forces can form — any change in this primary amino acid sequence can therefore substantially alter the protein's overall three-dimensional folding and, consequently, its biological function.

Sugar component: DNA contains deoxyribose (lacking an oxygen atom at the 2' carbon position); RNA contains ribose (with a hydroxyl group present at the 2' position).

Nitrogenous bases: Both DNA and RNA share adenine, guanine, and cytosine, but DNA specifically contains thymine, while RNA contains uracil in its place (uracil differs from thymine only by lacking a methyl group).

The lock-and-key model proposes that an enzyme's active site has a specific, precisely defined three-dimensional shape (the "lock"), which is complementary to and fits only a particular substrate molecule (the "key") — much as a specific key fits only its matching lock.

This model directly explains enzyme specificity, since only a substrate with the correct complementary shape and functional groups can properly bind within the enzyme's active site — this precise geometric/chemical matching requirement is why a given enzyme typically catalyses only one specific reaction (or a very narrow range of closely related reactions), rather than acting non-specifically on many different, unrelated substrates.

Vitamins are essential organic micronutrients that the human body cannot synthesise in sufficient quantities on its own (or, in some cases, cannot synthesise at all) — they must therefore be obtained regularly through diet, since they are required (often in small amounts) for various crucial metabolic functions.

Prolonged inadequate dietary intake of a specific vitamin leads to insufficient levels for the body's metabolic needs, resulting in a characteristic deficiency disease specific to that vitamin's particular biological role — example: Vitamin C deficiency causes scurvy (characterised by bleeding gums, poor wound healing, and connective tissue problems, since vitamin C is essential for collagen synthesis).

The anomeric carbon is the specific carbon atom (originally the carbonyl carbon in the open-chain form) that becomes a new stereocentre upon cyclisation of a sugar into its ring form, since it now bears an additional hydroxyl group (from the ring-forming reaction) alongside its other substituents.

Alpha (α) anomer: The hydroxyl group at this anomeric carbon points in the opposite direction (typically drawn "down," on the opposite face) relative to the reference group used to determine the sugar's overall D/L configuration.

Beta (β) anomer: The hydroxyl group at the anomeric carbon points in the same direction (typically drawn "up," on the same face) as this reference group.

Complementary base pairing refers to the specific, consistent pairing pattern observed between the nitrogenous bases on the two strands of the DNA double helix — adenine (A) always pairs specifically with thymine (T) via two hydrogen bonds, while guanine (G) always pairs specifically with cytosine (C) via three hydrogen bonds.

This specific, predictable pairing pattern is of fundamental biological significance because it directly enables accurate DNA replication (each original strand serves as a template, with the base-pairing rules ensuring an exact, faithful copy is generated) and also underlies the fundamental mechanism of genetic information transfer/expression more broadly.

Essential amino acids cannot be synthesised by the human body in sufficient quantities on its own, and must therefore be obtained directly through the diet — examples include leucine and lysine.

Non-essential amino acids can be adequately synthesised by the human body itself (typically from other available metabolic precursors), and therefore do not strictly need to be obtained directly from dietary sources — examples include alanine and glutamic acid, though adequate dietary intake is still generally beneficial.

Although both starch and cellulose are polymers built entirely from glucose monomer units, they differ in the specific type of glycosidic linkage connecting these units — starch is composed of glucose units joined by alpha-glycosidic linkages, while cellulose is composed of glucose units joined by beta-glycosidic linkages.

This difference in linkage type causes starch to adopt a helical, coiled structure (which is more easily hydrolysed, and which human digestive enzymes are specifically adapted to break down for energy), while cellulose's beta-linkages produce long, straight, tightly hydrogen-bonded parallel chains (forming rigid, insoluble fibres well-suited for structural support in plant cell walls, but which human digestive enzymes cannot break down) — this single difference in linkage stereochemistry is precisely why these two glucose polymers have such dramatically different physical properties and biological functions.

Primary structure: The specific linear sequence of amino acids, held together by covalent peptide bonds.

Secondary structure: Regular, repeating local folding patterns (such as the alpha-helix or beta-pleated sheet), stabilised by hydrogen bonds between the backbone carbonyl and N-H groups of the polypeptide chain.

Tertiary structure: The overall, complete three-dimensional folded shape of a single polypeptide chain, stabilised by a variety of interactions among the amino acid side chains — including hydrogen bonds, disulfide bridges (covalent S-S bonds between cysteine residues), ionic (electrostatic) interactions, and hydrophobic interactions between non-polar side chains.

Quaternary structure: The specific spatial arrangement of multiple, separate polypeptide subunits assembling together into one larger, functional protein complex (only applicable to proteins made of more than one polypeptide chain), stabilised by similar non-covalent interactions (and sometimes disulfide bonds) between the different subunit surfaces.

Enzymes catalyse reactions by binding their specific substrate at the active site, forming an enzyme-substrate complex — this binding stabilises the reaction's transition state (the high-energy, unstable arrangement of atoms occurring partway through the reaction), effectively lowering the overall activation energy required for the reaction to proceed, thereby dramatically increasing the reaction rate without altering the reaction's overall thermodynamics (equilibrium position).

Effect of temperature: Enzyme activity generally increases with rising temperature initially (as with typical chemical reactions, due to increased molecular kinetic energy and collision frequency), reaching a characteristic optimal temperature (often around human body temperature, 37°C, for many human enzymes) where activity is maximal.

However, beyond this optimal temperature, enzyme activity sharply and often irreversibly declines, since the excessive thermal energy disrupts the enzyme's essential hydrogen bonds and other stabilising interactions, causing denaturation (loss of the specific three-dimensional shape required for proper substrate binding and catalytic function) — this characteristic temperature-activity profile (a rise followed by a sharp decline) is a defining, distinguishing feature of enzyme-catalysed reactions, quite different from simple chemical reactions which typically just continue increasing in rate with temperature.

Glucose, in its open-chain form ($CH_2OH-(CHOH)_4-CHO$), has 4 chiral (asymmetric) carbon centres — specifically carbons 2, 3, 4, and 5, each bonded to four different groups.

The maximum possible number of stereoisomers for a molecule with $n$ chiral centres is given by $2^n$:

Number of stereoisomers $= 2^4 = \mathbf{16}$

This accounts for all possible D- and L- aldohexose sugars (including glucose, mannose, galactose, and others), differing only in the specific spatial (R/S) configuration at each of these four chiral centres.

DNA replication begins with the separation (unwinding) of the two complementary strands of the parent DNA double helix, catalysed by the enzyme helicase, breaking the hydrogen bonds holding the two strands together.

Each of these two now-separated original strands then serves as a template — DNA polymerase enzyme synthesises a new, complementary strand alongside each original template strand, adding nucleotides according to the strict complementary base-pairing rules (A pairs with T, G pairs with C).

This process results in two complete daughter DNA double helices, each consisting of one original (parental) strand and one newly synthesised strand — since each daughter molecule retains exactly half of the original parent molecule (one intact original strand), this replication mechanism is termed "semi-conservative," as it conserves (retains) half of the original genetic material in each resulting daughter molecule, rather than either fully conserving the entire original molecule intact, or completely discarding it.

Amino acids contain both an acidic carboxyl group ($-COOH$) and a basic amino group ($-NH_2$) within the same molecule. In aqueous solution (and in the solid state), an internal acid-base proton transfer occurs — the carboxyl group donates its proton directly to the amino group, forming a zwitterion (dipolar ion) with a negatively charged carboxylate ($-COO^-$) and a positively charged ammonium ($-NH_3^+$) group simultaneously present on the same molecule, while the overall molecule remains net neutral.

This zwitterionic structure means amino acids effectively behave somewhat like ionic compounds (possessing both a full positive and full negative charge center within each molecule) — the resulting strong electrostatic (ionic-like) attractions between neighbouring zwitterionic molecules in the solid crystal lattice require considerably more thermal energy to overcome compared to the weaker intermolecular forces (like typical hydrogen bonding alone) present in simple carboxylic acids or amines lacking this dual-charge zwitterionic character — this is precisely why amino acids characteristically show notably high melting points, often decomposing before actually melting.

Fat-soluble vitamins (vitamins A, D, E, and K) are non-polar or only weakly polar molecules, soluble in lipids/fats rather than water.

Water-soluble vitamins (vitamin C and the B-complex vitamins) are polar molecules, readily soluble in aqueous (water-based) environments like blood plasma.

Implication: Fat-soluble vitamins can be stored in significant quantities within the body's fatty tissues and liver for extended periods, meaning regular daily intake isn't strictly necessary, but this storage capability also creates a risk of toxic accumulation (hypervitaminosis) if consumed in excessive quantities over time. Water-soluble vitamins, by contrast, are generally not stored significantly in the body (excess amounts are typically readily excreted in urine) — this means they generally need to be consumed more regularly/frequently through the diet to avoid deficiency, but excessive intake is comparatively less likely to cause serious toxicity, since the excess is efficiently eliminated rather than accumulated.

A nucleotide, the fundamental repeating monomer unit of nucleic acids (DNA and RNA), consists of three essential covalently-linked components:

1. A nitrogenous base: One of the five possible bases (adenine, guanine, cytosine, thymine, or uracil), attached to the sugar.

2. A pentose sugar: Either deoxyribose (in DNA) or ribose (in RNA), forming the structural core connecting the base and phosphate group.

3. A phosphate group: Attached to the sugar (typically at the 5' carbon position), which is essential for forming the phosphodiester bonds that link successive nucleotides together into the long nucleic acid polymer chain (the "backbone" of DNA/RNA).

Cellulose is composed of glucose units connected by beta-1,4-glycosidic linkages. Human digestive systems produce only enzymes (like amylase) capable of hydrolysing alpha-glycosidic linkages (as found in starch) — humans lack the specific enzyme (cellulase) required to break the beta-glycosidic bonds present in cellulose.

Ruminant animals (like cows), by contrast, host large populations of specialised symbiotic microorganisms within their multi-chambered stomachs (particularly the rumen) — these microorganisms do produce cellulase enzymes, capable of hydrolysing cellulose's beta-glycosidic linkages, breaking it down into glucose units that can then be absorbed and utilised for energy by the ruminant host.

This fundamental difference in available digestive enzymes (specifically, the presence or absence of cellulase, whether from the animal itself or from symbiotic gut microorganisms) is precisely why humans cannot digest cellulose (it passes through as indigestible dietary fibre), while ruminant animals can effectively utilise it as a significant nutritional energy source.

The isoelectric point (pI) is the specific pH value at which a given amino acid exists predominantly in its neutral zwitterionic form, carrying no net overall electrical charge (the positive charge on the ammonium group exactly balances the negative charge on the carboxylate group).

Significance for electrophoresis: Electrophoresis separates charged molecules based on their movement through an electric field, with the direction and rate of movement depending on the molecule's net charge. At its isoelectric point, since the amino acid carries no net charge, it will not migrate towards either electrode under an applied electric field.

At a pH below the isoelectric point, the amino acid carries an overall net positive charge (favouring the protonated ammonium form) and migrates towards the negative electrode (cathode); at a pH above the isoelectric point, it carries an overall net negative charge (favouring the deprotonated carboxylate form) and migrates towards the positive electrode (anode) — this pH-dependent charge behaviour, centred around each amino acid's specific isoelectric point, is the fundamental basis for using electrophoresis to separate and analyse mixtures of different amino acids or proteins.

mRNA (messenger RNA): Carries the genetic coding information transcribed directly from DNA in the nucleus out to the ribosomes in the cytoplasm, serving as the direct template/blueprint that specifies the exact amino acid sequence to be assembled during protein synthesis (translation).

tRNA (transfer RNA): Acts as an adaptor molecule, each specific tRNA recognising a particular three-nucleotide codon on the mRNA (via its complementary anticodon) and carrying/delivering the correspondingly specific amino acid to the ribosome for incorporation into the growing polypeptide chain.

rRNA (ribosomal RNA): A major structural and functional component of the ribosome itself (the molecular machine where protein synthesis actually takes place), playing a crucial catalytic role in forming the peptide bonds between successive amino acids as the polypeptide chain is assembled according to the mRNA template.

Together, these three distinct RNA types work in coordinated fashion to accurately translate the genetic information encoded in DNA into the specific, correctly ordered amino acid sequence of a functional protein.