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Bagrut Biology 5 Units study materials
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Bagrut Biology 5 Units practice questions

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Question 1Cell Biology: structure, organelles, membranes and transport

Which organelle is the primary site of ATP production by oxidative phosphorylation in a eukaryotic cell?

  1. The lysosome
  2. The Golgi apparatus
  3. The mitochondrion Correct answer.
  4. The smooth endoplasmic reticulum

Explanation

Oxidative phosphorylation depends on the electron transport chain and ATP synthase embedded in the inner mitochondrial membrane, which uses the proton gradient built during electron transfer to drive ATP synthesis. No other organelle listed carries an electron transport chain for this purpose.

Question 2Cell Biology: structure, organelles, membranes and transport

A cell biologist treats a tissue with a drug that depolymerizes actin filaments. Which process would be most directly impaired?

  1. Formation of the cleavage furrow during cytokinesis Correct answer.
  2. Translation of mRNA on the rough endoplasmic reticulum
  3. Assembly of the nuclear pore complex
  4. Sorting of hydrolases by the Golgi apparatus

Explanation

The contractile ring that pinches an animal cell in two is built from actin filaments together with myosin II, so depolymerizing actin directly blocks furrow ingression. The other processes depend on ribosomes, vesicle coats, or nucleoporins rather than on the actin cytoskeleton.

Question 3Cell Biology: structure, organelles, membranes and transport

According to the fluid mosaic model, which statement best describes the organization of the plasma membrane?

  1. A protein monolayer coated on both faces by a lipid film
  2. A lipid bilayer with embedded and peripheral proteins that can move laterally Correct answer.
  3. Alternating layers of protein and lipid stacked like a sandwich
  4. Proteins and lipids form a rigid, fixed lattice with little lateral movement

Explanation

The fluid mosaic model describes a fluid phospholipid bilayer in which integral proteins are embedded and peripheral proteins associate with the surfaces, with components free to diffuse laterally. Rigid lattices and sandwich or monolayer arrangements describe older or incorrect models.

Question 4Cell Biology: structure, organelles, membranes and transport

A root hair cell accumulates potassium ions to a concentration far above that of the surrounding soil solution. Which transport mechanism accounts for this accumulation?

  1. Active transport by a pump using ATP Correct answer.
  2. Simple diffusion through the lipid bilayer
  3. Facilitated diffusion through a channel down the gradient
  4. Osmosis through aquaporins

Explanation

Moving an ion against its electrochemical gradient requires energy, which is supplied by ATP-driven pumps such as the proton-ATPase that establishes the driving force and by coupled K+ transporters. Diffusion and channels only permit net movement down a gradient.

Question 5Cell Biology: structure, organelles, membranes and transport

Which structural feature of the phospholipid molecule is directly responsible for the bilayer forming spontaneously in water?

  1. The length and saturation of the fatty acid tails
  2. The glycerol backbone linking the two fatty acid tails
  3. The amphipathic nature of the molecule, with a hydrophilic head and hydrophobic tails Correct answer.
  4. The presence of a phosphate group that carries a positive charge at pH 7

Explanation

Phospholipids are amphipathic: the polar head interacts with water while the nonpolar tails avoid it, so the molecules self-assemble with tails inward and heads outward. This hydrophobic effect is the thermodynamic basis of bilayer formation.

Question 6Cell Biology: structure, organelles, membranes and transport

A protein destined for secretion is synthesized on the rough endoplasmic reticulum. Which sequence of compartments does it pass through on its way out of the cell?

  1. Rough ER, Golgi apparatus, secretory vesicle, plasma membrane Correct answer.
  2. Smooth ER, nucleus, Golgi apparatus, secretory vesicle
  3. Golgi apparatus, rough ER, lysosome, plasma membrane
  4. Rough ER, lysosome, Golgi apparatus, plasma membrane

Explanation

Secretory proteins enter the ER lumen, are carried by COPII vesicles to the Golgi for modification and sorting, and leave in secretory vesicles that fuse with the plasma membrane. The order of compartments is fixed by the directionality of vesicle traffic.

Question 7Cell Biology: structure, organelles, membranes and transport

Which property of the cell membrane is best explained by the presence of cholesterol in animal cells?

  1. It provides a channel for the passage of small polar molecules
  2. It anchors the membrane to the cell wall
  3. It allows the membrane to remain fluid at low temperature while reducing excessive fluidity at high temperature Correct answer.
  4. It serves as the primary energy source for membrane pumps

Explanation

Cholesterol inserts between phospholipid tails and acts as a fluidity buffer: it prevents tight packing and crystallization at low temperature while restraining excessive movement at high temperature. It does not form channels or supply energy.

Question 8Cell Biology: structure, organelles, membranes and transport

An experiment shows that a solute enters a cell at a rate that increases with concentration but then levels off at a plateau, and the rate is reduced by a structural analogue of the solute. Which conclusion is supported?

  1. The solute crosses through a permanent membrane pore with unlimited capacity
  2. The solute crosses by simple diffusion through the lipid bilayer
  3. The solute is actively pumped out of the cell
  4. The solute crosses by a protein-mediated mechanism that can be saturated Correct answer.

Explanation

Saturation kinetics and competition by a structural analogue are the classic signatures of carrier- or channel-mediated transport, because the number of transport proteins is finite. Simple diffusion and unlimited pores would not plateau or show competitive inhibition.

Question 9Cell Biology: structure, organelles, membranes and transport

What is the main function of the nucleolus within the nucleus?

  1. Synthesis of the nuclear lamina proteins
  2. Replication of mitochondrial DNA
  3. Storage of hydrolytic enzymes
  4. Synthesis of ribosomal RNA and assembly of ribosomal subunits Correct answer.

Explanation

The nucleolus is the site of ribosomal RNA transcription and processing and of the assembly of ribosomal subunits from rRNA and ribosomal proteins. These subunits are then exported to the cytoplasm.

Question 10Cell Biology: structure, organelles, membranes and transport

A cell is placed in a hypertonic solution. What is the expected immediate response of an animal cell?

  1. Water leaves and the cell shrinks and may crenate Correct answer.
  2. The cell wall expands to counteract the osmotic gradient
  3. There is no net water movement because the membrane is impermeable to water
  4. Water enters and the cell swells and may lyse

Explanation

In a hypertonic solution the external solute concentration exceeds that inside the cell, so water moves out by osmosis and the cell loses volume. Animal cells lack a wall and can shrivel and crenate.

Question 11Cell Biology: structure, organelles, membranes and transport

Which feature distinguishes facilitated diffusion from simple diffusion across a plasma membrane?

  1. Facilitated diffusion transports only nonpolar gases
  2. Facilitated diffusion requires a membrane protein and shows specificity for the transported solute Correct answer.
  3. Facilitated diffusion always moves solutes against their concentration gradient
  4. Facilitated diffusion requires direct hydrolysis of ATP

Explanation

Facilitated diffusion uses specific integral membrane proteins, such as channels or carriers, to move solutes down their gradient, which gives it selectivity and saturation. It remains passive and does not consume ATP.

Question 12Cell Biology: structure, organelles, membranes and transport

Which organelle is responsible for the initial N-linked glycosylation of proteins?

  1. The lysosome
  2. The rough endoplasmic reticulum Correct answer.
  3. The peroxisome
  4. The smooth endoplasmic reticulum

Explanation

N-linked glycosylation begins in the rough ER lumen, where a preformed oligosaccharide is transferred to asparagine residues of the growing polypeptide. The Golgi then modifies these sugar chains further.

Question 13Cell Biology: structure, organelles, membranes and transport

A mutation removes the signal peptide from a protein that normally functions in the lysosome. What is the most likely consequence?

  1. The protein is translated on free ribosomes and remains in the cytosol Correct answer.
  2. The protein is secreted without entering the ER
  3. The protein is transported to the nucleus
  4. The protein is inserted into the inner mitochondrial membrane

Explanation

The signal peptide directs the ribosome to the rough ER and allows the polypeptide to enter the secretory pathway. Without it, translation is completed on free ribosomes and the protein stays in the cytosol, so it never reaches the lysosome.

Question 14Cell Biology: structure, organelles, membranes and transport

Which statement correctly describes the role of aquaporins?

  1. They anchor the cytoskeleton to the plasma membrane
  2. They allow rapid, passive movement of water across membranes Correct answer.
  3. They transport glucose against its gradient
  4. They pump sodium ions out of the cell

Explanation

Aquaporins are channel proteins that greatly increase the rate of passive water flow along an osmotic gradient without consuming ATP. They are highly selective for water, though some isoforms also pass small solutes such as glycerol.

Question 15Cell Biology: structure, organelles, membranes and transport

A plant cell is placed in a hypotonic solution. Which combination of pressures develops and prevents the cell from bursting?

  1. Low turgor pressure allowing the membrane to stretch freely
  2. High turgor pressure from the cell wall counteracting the osmotic water influx Correct answer.
  3. Complete loss of membrane permeability to water
  4. A negative hydrostatic pressure in the vacuole pulling water out

Explanation

Water enters the plant cell osmotically, but the rigid cell wall resists expansion and generates turgor pressure that opposes further water uptake. The result is a stable turgid cell rather than lysis.

Question 16Cell Biology: structure, organelles, membranes and transport

Which component of the cytoskeleton forms the tracks along which kinesin motors move secretory vesicles?

  1. The nuclear lamina
  2. Actin filaments
  3. Intermediate filaments
  4. Microtubules Correct answer.

Explanation

Kinesin motors move processively along microtubules, typically toward the plus end near the cell periphery, carrying vesicles and organelles. Actin filaments are used by myosin motors, while intermediate filaments are not motor tracks.

Question 17Cell Biology: structure, organelles, membranes and transport

Which observation would provide the strongest evidence that a membrane protein is an integral rather than a peripheral protein?

  1. The protein is found only in the cytosol fraction after cell fractionation
  2. The protein is released by changes in ionic strength alone
  3. The protein can be removed by chelating divalent cations
  4. The protein remains associated with the membrane after high-salt washes but is released by detergent Correct answer.

Explanation

Integral proteins are anchored in the hydrophobic core and can be extracted only by detergents that disrupt the bilayer, whereas peripheral proteins are held by electrostatic interactions and are released by salt or chelating agents. Resistance to salt and release by detergent is therefore diagnostic.

Question 18Cell Biology: structure, organelles, membranes and transport

What is the primary function of the Golgi apparatus?

  1. Transcription of ribosomal RNA
  2. Synthesis of ATP by the electron transport chain
  3. Degradation of worn-out organelles
  4. Modification, sorting and packaging of proteins and lipids for transport Correct answer.

Explanation

The Golgi apparatus receives cargo from the ER and modifies it, for example by further glycosylation, before sorting it into vesicles directed to the plasma membrane, lysosomes or secretion. It is a processing and distribution center.

Question 19Cell Biology: structure, organelles, membranes and transport

A cell is experimentally depleted of clathrin. Which process would be most directly affected?

  1. Passive diffusion of oxygen into the cell
  2. Receptor-mediated endocytosis of low-density lipoprotein Correct answer.
  3. Entry of water through aquaporins
  4. Import of proteins through nuclear pores

Explanation

Clathrin coats the cytoplasmic face of coated pits and vesicles and is required for receptor-mediated endocytosis of ligands such as LDL. Water movement, nuclear import and gas diffusion do not depend on clathrin.

Question 20Cell Biology: structure, organelles, membranes and transport

Which statement about the asymmetric distribution of phospholipids between the two leaflets of the plasma membrane is correct?

  1. Glycolipids are distributed symmetrically across both leaflets
  2. Phosphatidylserine is normally confined to the inner leaflet but flips outward during apoptosis Correct answer.
  3. Phosphatidylcholine is found exclusively in the outer leaflet under all conditions
  4. The two leaflets have identical lipid composition because lipids cannot flip

Explanation

Flippases and floppases maintain an asymmetric lipid distribution, keeping phosphatidylserine on the cytosolic side. During apoptosis, loss of this asymmetry exposes phosphatidylserine on the outer surface, which serves as an engulfment signal.

Question 21Cell Biology: structure, organelles, membranes and transport

Which structure connects adjacent plant cells and allows direct cytoplasmic continuity between them?

  1. Desmosomes
  2. Tight junctions
  3. Gap junctions
  4. Plasmodesmata Correct answer.

Explanation

Plasmodesmata are channels lined by plasma membrane that pass through the cell walls of plant cells and connect their cytoplasm, forming the symplast. The other junctions are characteristic of animal cells.

Question 22Cell Biology: structure, organelles, membranes and transport

In the Na+/K+-ATPase cycle, what is the net result of one complete cycle?

  1. Equal numbers of sodium and potassium ions are exchanged without ATP
  2. Three sodium ions leave the cell and two potassium ions enter, using one ATP Correct answer.
  3. Three sodium ions enter the cell and two potassium ions leave, using one ATP
  4. Two sodium ions leave the cell and three potassium ions enter, using one ATP

Explanation

Each cycle of the pump hydrolyzes one ATP and moves three Na+ out of the cell against their gradient while moving two K+ in. The unequal stoichiometry makes the pump electrogenic and contributes to the resting membrane potential.

Question 23Cell Biology: structure, organelles, membranes and transport

A scientist finds that a newly discovered single-celled organism lacks a nucleus and membrane-bound organelles but has a peptidoglycan cell wall. To which group does it most likely belong?

  1. Protozoa
  2. Archaea
  3. Fungi
  4. Bacteria Correct answer.

Explanation

Absence of a nucleus and membrane-bound organelles indicates a prokaryote, and peptidoglycan in the cell wall is a defining feature of bacteria. Archaea lack peptidoglycan, while fungi and protozoa are eukaryotes.

Question 24Cell Biology: structure, organelles, membranes and transport

Which feature of the inner mitochondrial membrane is essential for chemiosmotic ATP synthesis?

  1. The absence of integral membrane proteins
  2. Its direct continuity with the outer mitochondrial membrane
  3. Its high permeability to protons that allows free equilibration
  4. Its folding into cristae and its low permeability to protons Correct answer.

Explanation

The inner membrane is folded into cristae, which increase surface area for the electron transport chain, and it is relatively impermeable to protons so that a gradient can be maintained. Protons re-enter only through ATP synthase, coupling the gradient to ATP production.

Question 25Biochemistry: biological molecules, enzymes and metabolism

Which bond links two amino acids in a polypeptide chain, and between which atoms does it form?

  1. A phosphodiester bond, between the amino group and the carboxyl group
  2. A glycosidic bond, between two hydroxyl groups of the amino acids
  3. A hydrogen bond, between the side chains of two adjacent amino acids
  4. A peptide bond, between the carboxyl carbon of one amino acid and the amino nitrogen of the next Correct answer.

Explanation

During translation the carboxyl group of one amino acid reacts with the amino group of the next in a condensation reaction that releases water and forms an amide linkage called a peptide bond. This bond is always between the carbon of the carboxyl group and the nitrogen of the amino group, not between side chains.

Question 26Biochemistry: biological molecules, enzymes and metabolism

A solution of starch, a polysaccharide of glucose, is tested with iodine solution. What result is expected and why?

  1. A brick-red precipitate, because the iodine reduces the glucose monomers
  2. No colour change, because starch is a reducing sugar and requires Benedict's reagent
  3. A violet colour, because the iodine reacts with peptide bonds
  4. A blue-black colour, because iodine molecules become trapped inside the coiled helical structure of amylose Correct answer.

Explanation

Iodine fits into the helical coil of amylose and produces a characteristic blue-black complex, which is the standard qualitative test for starch. The colour arises from the physical trapping of iodine within the helix, not from a chemical reaction with glucose units.

Question 27Biochemistry: biological molecules, enzymes and metabolism

Which statement correctly describes the relationship between an enzyme and its substrate?

  1. The enzyme increases the activation energy so that only suitable substrates react
  2. The enzyme is consumed in the reaction and must be resynthesized each time
  3. The enzyme lowers the activation energy of the reaction by binding the substrate at its active site Correct answer.
  4. The enzyme changes the equilibrium constant of the reaction in favour of the products

Explanation

Enzymes are biological catalysts that speed reactions by providing an alternative pathway with a lower activation energy, achieved through binding substrates at the active site. They are not consumed and do not alter the thermodynamic equilibrium position, only the rate at which it is approached.

Question 28Biochemistry: biological molecules, enzymes and metabolism

Which property of water is mainly responsible for its ability to absorb large amounts of heat with only a small rise in temperature?

  1. Its ability to act as a solvent for many ionic substances
  2. Its high specific heat capacity, resulting from hydrogen bonding between molecules Correct answer.
  3. Its low molecular mass, which allows rapid movement of molecules
  4. Its low viscosity at body temperature

Explanation

Because each water molecule can form up to four hydrogen bonds, a large amount of energy is needed to raise its temperature, giving water a high specific heat capacity. This stabilizes the temperature of aquatic habitats and of organisms.

Question 29Biochemistry: biological molecules, enzymes and metabolism

In a reaction catalysed by a single enzyme, the rate is measured at increasing substrate concentrations while enzyme concentration is kept constant. Which description matches the typical result?

  1. The rate rises steeply at first and then levels off as the active sites become saturated Correct answer.
  2. The rate falls steadily as substrate concentration increases
  3. The rate stays constant regardless of substrate concentration
  4. The rate rises linearly without limit as substrate concentration increases

Explanation

At low substrate concentration the rate is approximately proportional to substrate concentration, but once all active sites are occupied the enzyme is saturated and the rate reaches a maximum. Adding more substrate then cannot increase the rate further.

Question 30Biochemistry: biological molecules, enzymes and metabolism

Which pair of molecules are both polymers built from nucleotide monomers?

  1. Insulin and haemoglobin
  2. Triglycerides and phospholipids
  3. DNA and RNA Correct answer.
  4. Starch and cellulose

Explanation

DNA and RNA are nucleic acids, both assembled from nucleotide monomers joined by phosphodiester bonds. Starch and cellulose are polysaccharides, proteins are polypeptides, and triglycerides and phospholipids are built from glycerol and fatty acids.

Question 31Biochemistry: biological molecules, enzymes and metabolism

A student boils a sample of the enzyme catalase before adding hydrogen peroxide. What is the most likely observation?

  1. The same amount of oxygen is produced, because enzymes are not affected by temperature
  2. More oxygen is produced, because heating supplies extra activation energy
  3. Little or no oxygen is produced, because boiling denatures the enzyme and destroys the shape of its active site Correct answer.
  4. Oxygen production stops only because hydrogen peroxide evaporates during boiling

Explanation

High temperature breaks the weak hydrogen and ionic bonds that maintain the tertiary structure of catalase, so the active site loses its complementary shape and the substrate can no longer bind. Denaturation is irreversible for most enzymes, so almost no gas is released.

Question 32Biochemistry: biological molecules, enzymes and metabolism

Which change is most likely to increase the rate of an enzyme-catalysed reaction when substrate is plentiful?

  1. Increasing the enzyme concentration, provided temperature and pH remain optimal Correct answer.
  2. Increasing the product concentration in the reaction mixture
  3. Adding a heavy metal ion that binds irreversibly to the active site
  4. Lowering the temperature well below the optimum

Explanation

When substrate is not limiting, the number of active sites available determines the rate, so adding more enzyme increases the number of substrate molecules converted per unit time. Temperature and pH must remain in the range where the enzyme retains its native conformation.

Question 33Biochemistry: biological molecules, enzymes and metabolism

Which statement best explains why a competitive inhibitor reduces the reaction rate but the inhibition can be partly overcome by adding more substrate?

  1. The inhibitor resembles the substrate and binds to the active site, so more substrate competes for the same site Correct answer.
  2. The inhibitor binds to a site away from the active site and permanently changes the enzyme's shape
  3. The inhibitor destroys the substrate molecules before they can reach the enzyme
  4. The inhibitor increases the activation energy of the reaction regardless of substrate concentration

Explanation

A competitive inhibitor has a shape similar to the substrate and occupies the active site, preventing substrate binding. Raising substrate concentration increases the chance that substrate rather than inhibitor occupies the site, so the effect is reduced.

Question 34Biochemistry: biological molecules, enzymes and metabolism

Which feature of the phospholipid molecule makes it suitable as the main component of the cell membrane?

  1. A hydrophilic phosphate head and a hydrophobic fatty acid tail, allowing a bilayer to form in water Correct answer.
  2. A structure identical to a triglyceride, with three fatty acids attached to glycerol
  3. A single hydrophobic region along its whole length, allowing it to dissolve completely in water
  4. A charged glycerol backbone that binds water on both sides of the molecule

Explanation

Phospholipids are amphipathic: the phosphate-containing head interacts with water while the two fatty acid tails avoid it. In aqueous surroundings they spontaneously arrange into a bilayer with tails inward and heads facing the water on both surfaces.

Question 35Biochemistry: biological molecules, enzymes and metabolism

Two DNA samples are compared. Sample A contains 30% adenine. What percentage of guanine is expected in sample A, assuming standard complementary base pairing?

  1. 40%
  2. 30%
  3. 20% Correct answer.
  4. 70%

Explanation

If adenine is 30%, then thymine is also 30% because A pairs with T, so A plus T accounts for 60%. The remaining 40% is shared equally between guanine and cytosine, giving 20% guanine.

Question 36Biochemistry: biological molecules, enzymes and metabolism

A polypeptide is heated gently, then cooled slowly, and is found to regain its original biological activity. Which conclusion is best supported?

  1. The protein never lost its tertiary structure during heating
  2. The amino acid sequence was changed but the activity was restored by chance
  3. The primary structure was preserved, allowing the chain to refold into its functional tertiary shape Correct answer.
  4. The peptide bonds were hydrolysed during heating and then re-formed on cooling

Explanation

Gentle heating can disrupt the weak interactions maintaining tertiary structure while leaving the covalent peptide bonds and the amino acid sequence intact. Because the primary structure determines folding, the chain can renature when conditions return to normal.

Question 37Biochemistry: biological molecules, enzymes and metabolism

Which structural difference explains why cellulose is indigestible to humans while starch is digestible?

  1. Cellulose molecules are too small to be recognized by digestive enzymes
  2. Cellulose is made of fructose monomers, which humans cannot absorb
  3. Cellulose has beta-1,4 glycosidic bonds, which human digestive enzymes cannot hydrolyse Correct answer.
  4. Cellulose has alpha-1,4 bonds that are broken only in the stomach

Explanation

Both polymers are made of glucose, but the beta-1,4 linkages in cellulose produce straight chains with a different geometry. Human amylases are specific for alpha-1,4 bonds, so the beta linkages remain intact and cellulose passes through as dietary fibre.

Question 38Biochemistry: biological molecules, enzymes and metabolism

In an experiment, the rate of an enzyme reaction is measured at pH 4, 6, 7, 8 and 10. The rate is highest at pH 7 and very low at pH 4 and 10. Which explanation fits these results?

  1. pH has no effect on enzyme activity, so the differences are due to random error
  2. The enzyme is denatured only at pH 7, which is why the rate is highest there
  3. At pH 4 and 10 the substrate concentration is lower than at pH 7
  4. Changes in pH alter the ionization of amino acid side chains in the active site, reducing substrate binding away from the optimum Correct answer.

Explanation

Each enzyme has an optimum pH at which the charges on the active site residues favour substrate binding and catalysis. Away from the optimum, altered ionization and eventual denaturation reduce activity, explaining the low rates at pH 4 and 10.

Question 39Biochemistry: biological molecules, enzymes and metabolism

Which description correctly matches a hydrolysis reaction of a biological polymer?

  1. A triglyceride is broken into glycerol and fatty acids by the addition of water Correct answer.
  2. Amino acids are joined into a polypeptide with the release of water
  3. Glucose molecules are joined to form glycogen by the addition of water
  4. Two nucleotides are joined with the addition of water to form a dinucleotide

Explanation

Hydrolysis uses water to cleave covalent bonds in polymers, and the breakdown of a triglyceride into glycerol and fatty acids is a classic example. The opposite process, joining monomers, is condensation, which releases water.

Question 40Biochemistry: biological molecules, enzymes and metabolism

A patient's cells lack a functional enzyme in the pathway that converts galactose to glucose. Which consequence is most directly expected when galactose is consumed?

  1. Galactose is stored directly as glycogen without any chemical change
  2. The absence of the enzyme increases the rate of the whole pathway
  3. Galactose is converted to glucose faster because the pathway is short-circuited
  4. Galactose accumulates in the cells because the blocked step prevents its conversion to a usable metabolite Correct answer.

Explanation

A metabolic pathway proceeds through sequential enzyme-catalysed steps, and blocking one step causes its substrate to accumulate while downstream products are depleted. In this case galactose cannot be converted to glucose and builds up, which is the biochemical basis of galactosemia.

Question 41Biochemistry: biological molecules, enzymes and metabolism

An enzyme is added to a reaction that has already reached equilibrium. What happens to the concentrations of substrates and products?

  1. They remain unchanged, because the enzyme speeds the approach to equilibrium but does not shift its position Correct answer.
  2. The product concentration decreases as the enzyme catalyses the reverse reaction only
  3. The equilibrium constant increases, favouring more product
  4. The substrate concentration decreases until all substrate is converted to product

Explanation

A catalyst accelerates both the forward and reverse reactions equally, so equilibrium is reached faster but the final ratio of substrate to product is unchanged. The equilibrium constant depends on the free energy difference between substrates and products, not on the enzyme.

Question 42Biochemistry: biological molecules, enzymes and metabolism

A non-competitive inhibitor is added to an enzyme-catalysed reaction. Which observation is expected when substrate concentration is increased greatly?

  1. The enzyme's affinity for substrate increases, lowering the substrate concentration needed for half-maximal rate
  2. The maximum reaction rate increases above the uninhibited value
  3. The inhibition disappears completely, because substrate outcompetes the inhibitor at the active site
  4. The maximum reaction rate remains lower than without the inhibitor, because inhibition is not relieved by more substrate Correct answer.

Explanation

A non-competitive inhibitor binds at a site other than the active site and changes the enzyme's conformation, so adding more substrate cannot displace it. The result is a reduced maximum rate that persists even at high substrate concentration.

Question 43Biochemistry: biological molecules, enzymes and metabolism

Which statement best explains the specificity of an enzyme for a particular substrate?

  1. The substrate changes its shape to match any enzyme it encounters
  2. The active site has a shape and charge distribution complementary to the substrate, so only molecules that fit can form an enzyme-substrate complex Correct answer.
  3. Specificity depends only on the temperature at which the reaction occurs
  4. The enzyme has a random surface that binds any molecule with similar molecular mass

Explanation

The active site is a small region with a precise three-dimensional arrangement of amino acid side chains, giving it a complementary shape and chemistry to its substrate. Molecules that cannot form the necessary interactions fail to bind productively, which is the basis of specificity.

Question 44Biochemistry: biological molecules, enzymes and metabolism

A cell is treated with a drug that blocks the electron transport chain. Which immediate metabolic consequence is expected?

  1. ATP production increases because the cell switches entirely to fermentation
  2. NADH accumulates because it cannot be reoxidized, and ATP production by oxidative phosphorylation falls Correct answer.
  3. Oxygen consumption increases as the cell tries to compensate for the block
  4. Glycolysis stops immediately because glucose can no longer be phosphorylated

Explanation

The electron transport chain reoxidizes NADH and FADH2 while pumping protons to drive ATP synthase. If the chain is blocked, NADH cannot be reoxidized and oxidative phosphorylation stops, so NADH accumulates and ATP yield drops sharply.

Question 45Genetics: inheritance, DNA, gene expression and genetic engineering

In a dihybrid cross between two heterozygous individuals (AaBb × AaBb) for two genes on different chromosomes showing complete dominance, what phenotypic ratio is expected among the offspring?

  1. 1:2:1
  2. 3:1
  3. 9:3:3:1 Correct answer.
  4. 1:1:1:1

Explanation

When two genes assort independently, each heterozygous parent produces four gamete types in equal frequency, and combining them yields the classic 9:3:3:1 phenotypic ratio. This ratio depends on complete dominance at both loci and on independent assortment.

Question 46Genetics: inheritance, DNA, gene expression and genetic engineering

A woman who is a carrier of an X-linked recessive disorder (X^A X^a) has children with an unaffected man (X^A Y). What proportion of their sons is expected to be affected?

  1. 100%
  2. 25%
  3. 0%
  4. 50% Correct answer.

Explanation

Sons inherit their single X chromosome from the mother, and a carrier mother transmits X^a to half of her sons. Because males are hemizygous, any son receiving X^a will express the recessive disorder.

Question 47Genetics: inheritance, DNA, gene expression and genetic engineering

Which statement describes the role of DNA ligase in DNA replication?

  1. It removes mismatched bases after replication.
  2. It separates the two parental strands by breaking hydrogen bonds.
  3. It joins Okazaki fragments by forming phosphodiester bonds. Correct answer.
  4. It synthesizes RNA primers on the lagging strand.

Explanation

DNA ligase seals the nicks between adjacent Okazaki fragments on the lagging strand by catalyzing phosphodiester bonds between the sugar-phosphate backbones. This completes the continuous sugar-phosphate chain.

Question 48Genetics: inheritance, DNA, gene expression and genetic engineering

During transcription in a eukaryotic cell, which enzyme is responsible for synthesizing the mRNA molecule?

  1. Ribosome
  2. RNA polymerase Correct answer.
  3. Reverse transcriptase
  4. DNA polymerase

Explanation

RNA polymerase binds the promoter and polymerizes ribonucleotides complementary to the DNA template strand, producing an RNA transcript. It reads the template in the 3' to 5' direction and builds RNA 5' to 3'.

Question 49Genetics: inheritance, DNA, gene expression and genetic engineering

A mutation changes a single DNA base but the amino acid sequence of the resulting protein is unchanged. Which term best describes this mutation?

  1. Frameshift mutation
  2. Silent mutation Correct answer.
  3. Missense mutation
  4. Nonsense mutation

Explanation

A silent mutation alters a codon but, because the genetic code is degenerate, the new codon still specifies the same amino acid. The protein sequence is therefore unchanged.

Question 50Genetics: inheritance, DNA, gene expression and genetic engineering

Which of the following correctly describes the structure of a nucleotide in DNA?

  1. A deoxyribose sugar bonded to two nitrogenous bases
  2. Two phosphate groups and a single base
  3. A deoxyribose sugar, a phosphate group, and a nitrogenous base Correct answer.
  4. A ribose sugar, a phosphate group, and an amino acid

Explanation

A DNA nucleotide consists of one deoxyribose sugar, one phosphate group, and one nitrogenous base (adenine, thymine, guanine, or cytosine). Nucleotides are linked into a polymer through the sugar-phosphate backbone.