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FY12CE Biology study materials
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FY12CE Biology practice questions

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Question 1Molecules and Cells

Which property of water allows insects such as water striders to move across the surface of a pond?

  1. Surface tension arising from cohesion between water molecules Correct answer.
  2. The high density of water compared with ice
  3. The ability of water to act as a solvent for gases
  4. The low specific heat capacity of liquid water

Explanation

Water molecules form hydrogen bonds with one another, producing cohesion that creates a surface tension strong enough to support small bodies. This property depends on hydrogen bonding rather than on heat capacity, density or solvent behaviour.

Question 2Molecules and Cells

A polypeptide is synthesised at the ribosome. Which bond forms between adjacent amino acids, and which reaction produces it?

  1. An ionic bond formed by oxidation
  2. A peptide bond formed by condensation Correct answer.
  3. A hydrogen bond formed by hydrolysis
  4. A phosphodiester bond formed by reduction

Explanation

Adjacent amino acids are joined by a peptide bond between the carboxyl group of one and the amino group of the next. This is a condensation reaction because a molecule of water is released.

Question 3Molecules and Cells

Which statement best describes the structure of the phospholipid bilayer of a cell membrane?

  1. A single layer of phospholipids with heads facing the cytoplasm
  2. A double layer with hydrophobic tails facing inward and hydrophilic heads facing outward Correct answer.
  3. A rigid lattice of cholesterol molecules with proteins embedded between them
  4. A double layer with hydrophobic heads facing outward and hydrophilic tails facing inward

Explanation

Phospholipids are amphipathic, so in a bilayer the hydrophobic fatty acid tails cluster away from water in the interior. The hydrophilic phosphate heads face the aqueous environments on both sides.

Question 4Molecules and Cells

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

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

Explanation

The inner mitochondrial membrane houses the electron transport chain and ATP synthase, which together generate most ATP by oxidative phosphorylation. The other organelles listed perform different roles in sorting, digestion or lipid synthesis.

Question 5Molecules and Cells

Which molecule carries the amino acid to the ribosome during translation and bears an anticodon?

  1. Messenger RNA
  2. Transfer RNA Correct answer.
  3. Deoxyribonucleic acid
  4. Ribosomal RNA

Explanation

Transfer RNA has an anticodon that base-pairs with the mRNA codon and carries the corresponding amino acid. Messenger RNA carries the coding sequence, and ribosomal RNA forms part of the ribosome itself.

Question 6Molecules and Cells

Which of the following correctly describes the role of NADH in cellular respiration?

  1. It accepts electrons and hydrogen at glycolysis and the Krebs cycle and donates them to the electron transport chain Correct answer.
  2. It transports oxygen from the cytoplasm into the mitochondrial matrix
  3. It hydrolyses ATP to drive the proton pumps of the inner membrane
  4. It donates electrons directly to pyruvate during the link reaction

Explanation

NADH is a reduced coenzyme that carries electrons and hydrogen removed during glycolysis and the Krebs cycle. It delivers these to the electron transport chain, where they drive proton pumping and ATP synthesis.

Question 7Molecules and Cells

In the light-dependent reactions of photosynthesis, which process directly generates the proton gradient used by ATP synthase?

  1. The oxidation of glucose in the cytoplasm
  2. The transfer of electrons along the thylakoid membrane and the resulting proton pumping Correct answer.
  3. The reduction of carbon dioxide by rubisco
  4. The hydrolysis of ATP in the stroma

Explanation

Electron transport along the thylakoid membrane moves protons into the thylakoid space, creating a gradient. Protons then flow back through ATP synthase, phosphorylating ADP to ATP.

Question 8Molecules and Cells

Which type of bond holds the two strands of a DNA double helix together?

  1. Covalent phosphodiester bonds
  2. Glycosidic bonds between deoxyribose units
  3. Peptide bonds between nitrogenous bases
  4. Hydrogen bonds between complementary bases Correct answer.

Explanation

The two strands are held by hydrogen bonds between complementary base pairs: adenine with thymine, and guanine with cytosine. Phosphodiester bonds instead link nucleotides within each strand.

Question 9Molecules and Cells

An enzyme-catalysed reaction is measured as substrate concentration increases while enzyme concentration is held constant. Which change is observed?

  1. The rate remains constant from the start regardless of substrate concentration
  2. The rate increases and then plateaus as active sites become saturated Correct answer.
  3. The rate decreases steadily because substrate inhibits the enzyme
  4. The rate increases indefinitely in direct proportion to substrate concentration

Explanation

At low substrate concentration the rate rises roughly linearly, but as active sites become occupied the rate levels off at a maximum. At saturation all active sites are working, so adding more substrate cannot increase the rate further.

Question 10Molecules and Cells

A competitive inhibitor is added to an enzyme-catalysed reaction. How does it affect the reaction compared with an uninhibited reaction?

  1. It binds the active site and its effect can be reduced by increasing substrate concentration Correct answer.
  2. It denatures the enzyme by breaking peptide bonds
  3. It lowers the activation energy so that the reaction proceeds faster
  4. It binds a site away from the active site and permanently changes the enzyme's shape

Explanation

A competitive inhibitor resembles the substrate and occupies the active site, so it competes for binding. Raising the substrate concentration increases the chance that substrate binds instead, reducing the inhibition.

Question 11Molecules and Cells

Why is the fluidity of a cell membrane reduced at low temperatures, and how does cholesterol counteract this?

  1. Phospholipids pack more closely when cold, and cholesterol disrupts the packing to maintain fluidity Correct answer.
  2. Water freezes within the bilayer, and cholesterol acts as an antifreeze solute
  3. Phospholipids become more mobile when cold, and cholesterol restricts their movement
  4. Proteins denature in the cold, and cholesterol replaces the denatured proteins

Explanation

At low temperatures the hydrophobic tails lose kinetic energy and pack together, making the membrane less fluid. Cholesterol sits between the phospholipids and prevents tight packing, helping to keep the membrane fluid over a range of temperatures.

Question 12Molecules and Cells

During glycolysis, one molecule of glucose is converted into two molecules of pyruvate. What is the net yield of ATP and NADH per glucose?

  1. 4 ATP and 4 NADH
  2. 4 ATP and 2 NADH
  3. 2 ATP and 2 NADH Correct answer.
  4. 2 ATP and 4 NADH

Explanation

Glycolysis uses two ATP to phosphorylate glucose and produces four ATP, giving a net gain of two ATP. Two NADH are also formed when the two triose phosphate molecules are oxidised.

Question 13Molecules and Cells

In the Krebs cycle, which statement correctly describes the fate of the carbon atoms entering as acetyl coenzyme A?

  1. They are released as carbon dioxide during the cycle Correct answer.
  2. They are converted directly into ATP without release
  3. They are incorporated into glucose during the cycle
  4. They remain in the cycle and are never lost

Explanation

Each acetyl group entering the Krebs cycle contains two carbons, and two molecules of carbon dioxide are released per turn. The carbon is therefore fully oxidised and leaves the cycle as gas.

Question 14Molecules and Cells

A mutation changes a single base in a gene but the resulting polypeptide has the same amino acid sequence. Which term best describes this mutation?

  1. A nonsense mutation
  2. A missense mutation
  3. A frameshift mutation
  4. A silent mutation Correct answer.

Explanation

Because the genetic code is degenerate, several codons can specify the same amino acid. A substitution that produces a codon for the same amino acid is therefore silent and leaves the protein unchanged.

Question 15Molecules and Cells

In a DNA molecule, adenine makes up 30 percent of the bases. What percentage of the bases is guanine?

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

Explanation

By complementary base pairing, the percentage of thymine equals that of adenine, so adenine plus thymine is 60 percent. The remaining 40 percent is shared equally by guanine and cytosine, giving 20 percent guanine.

Question 16Molecules and Cells

Which feature of the genetic code means that a single amino acid can be specified by more than one codon?

  1. The code is universal
  2. The code is non-overlapping
  3. The code is read in triplets
  4. The code is degenerate Correct answer.

Explanation

Degeneracy means that the 64 codons code for only 20 amino acids, so several codons can specify the same amino acid. Universality, non-overlap and triplet reading are different properties of the code.

Question 17Molecules and Cells

A plant cell is placed in a solution with a water potential higher than that of its cell contents. What is the most likely immediate result?

  1. Water enters the cell and the cell becomes turgid Correct answer.
  2. Water leaves the cell and the cell becomes plasmolysed
  3. Solutes move out of the cell until concentrations equalise
  4. The cell wall breaks because of the increased pressure

Explanation

Water moves by osmosis from a region of higher water potential to one of lower water potential. Since the external solution has the higher water potential, water enters the cell and the vacuole swells, making the cell turgid.

Question 18Molecules and Cells

A molecule moves across a membrane down its concentration gradient through a channel protein without the expenditure of ATP. Which process is described?

  1. Active transport
  2. Osmosis
  3. Endocytosis
  4. Facilitated diffusion Correct answer.

Explanation

Facilitated diffusion uses membrane proteins to allow polar or charged substances to cross down their concentration gradient passively. Because no energy is used, it is a form of passive transport.

Question 19Molecules and Cells

Which statement correctly compares the roles of the light-dependent and light-independent reactions of photosynthesis?

  1. The light-dependent reactions produce ATP and NADPH, which the light-independent reactions use to reduce carbon dioxide Correct answer.
  2. The light-dependent reactions fix carbon dioxide, and the light-independent reactions produce oxygen
  3. Both sets of reactions require light directly and occur only in the dark
  4. The light-independent reactions generate ATP, which the light-dependent reactions use to split water

Explanation

The light-dependent reactions capture light energy to make ATP and NADPH and release oxygen from water. The Calvin cycle then uses ATP and NADPH to reduce carbon dioxide into carbohydrate.

Question 20Molecules and Cells

A cell is treated with a drug that blocks the formation of the spindle apparatus. During which stage will the cell arrest?

  1. Cytokinesis
  2. Telophase
  3. Prophase Correct answer.
  4. G2 phase

Explanation

The spindle forms during prophase and is required to attach to chromosomes and move them during mitosis. If spindle formation is blocked, the cell cannot progress through the early stages of mitosis.

Question 21Molecules and Cells

An enzyme is heated to 80 degrees Celsius and then cooled to its optimum temperature. The reaction rate remains very low. Which explanation is most consistent with this observation?

  1. Cooling has permanently lowered the activation energy
  2. The substrate has been used up during heating
  3. The enzyme has been inhibited competitively by the heat
  4. The enzyme has been denatured, and the change in shape is irreversible Correct answer.

Explanation

High temperature breaks the hydrogen and ionic bonds maintaining the tertiary structure, unfolding the active site permanently. Cooling cannot restore the original conformation, so the rate stays low.

Question 22Molecules and Cells

A respiring cell is supplied with a substance that blocks the final electron acceptor of the electron transport chain. Which immediate consequence is expected?

  1. Electron transport, proton pumping and ATP synthesis by oxidative phosphorylation all stop Correct answer.
  2. The Krebs cycle accelerates to compensate for the loss of ATP
  3. Glycolysis stops immediately because glucose cannot be phosphorylated
  4. ATP synthase reverses to hydrolyse ATP while proton pumping continues normally

Explanation

If the final electron acceptor cannot accept electrons, the chain becomes blocked, so electron flow and proton pumping cease. Without a proton gradient, ATP synthase can no longer generate ATP by oxidative phosphorylation.

Question 23Molecules and Cells

A length of DNA is transcribed and the resulting mRNA is translated. A frameshift mutation caused by the insertion of two bases is most likely to have which effect?

  1. The mutation will have no effect because the code is degenerate
  2. Only the single amino acid at the insertion site will change
  3. All amino acids after the insertion site are likely to differ, and a premature stop codon may appear Correct answer.
  4. The mRNA will be unable to bind to the ribosome at all

Explanation

Inserting two bases shifts the reading frame, so every codon downstream of the insertion is read differently. This usually changes the entire downstream amino acid sequence and often creates a premature stop codon.

Question 24Molecules and Cells

In an experiment, isolated thylakoid membranes are illuminated in the presence of an artificial electron acceptor that removes electrons before they reach the final carrier. Which observation is most likely?

  1. ATP synthesis increases because electrons bypass the chain
  2. Oxygen evolution stops completely because water can no longer be split
  3. Carbon dioxide fixation in the stroma increases directly
  4. Oxygen evolution and ATP synthesis may be uncoupled because the proton gradient is reduced Correct answer.

Explanation

If electrons are diverted before the end of the chain, fewer protons are pumped and the proton gradient is diminished. Water splitting can still supply electrons, so oxygen evolution may continue while ATP synthesis is reduced, uncoupling the two processes.

Question 25Molecules and Cells

A cell is placed in a solution whose water potential equals that of the cell contents. Which statement best describes the net movement of water and the cell's state?

  1. Water leaves the cell, causing it to shrink and become flaccid
  2. Water enters the cell until the cell wall bursts
  3. There is no net movement of water, and the cell remains at a constant volume Correct answer.
  4. Water enters and leaves equally, so the cell swells continuously

Explanation

When the external and internal water potentials are equal, the rates of water movement in both directions are equal. There is no net movement, so the cell maintains a constant volume.

Question 26Molecules and Cells

Which property of water allows it to act as a coolant in living organisms?

  1. Its high latent heat of vaporisation removes heat when it evaporates. Correct answer.
  2. Its low viscosity allows rapid flow through vessels.
  3. Its high specific heat capacity buffers temperature changes.
  4. Its solvent properties allow it to dissolve many ions.

Explanation

Water has a high latent heat of vaporisation because many hydrogen bonds must be broken for molecules to escape as gas. When water evaporates from a surface such as skin or a leaf, it absorbs a large amount of heat from the organism, producing a cooling effect. The other properties listed are important for water but do not directly explain its role as a coolant.

Question 27Molecules and Cells

Which statement correctly describes the structure of a phospholipid?

  1. A hydrophilic phosphate head and two hydrophobic fatty acid tails. Correct answer.
  2. A hydrophobic phosphate head and two hydrophilic fatty acid tails.
  3. A hydrophilic glycerol head and three hydrophobic fatty acid tails.
  4. A hydrophobic glycerol head and one hydrophilic fatty acid tail.

Explanation

A phospholipid consists of a glycerol backbone attached to a phosphate group and two fatty acid chains. The phosphate group is polar and therefore hydrophilic, while the fatty acid tails are non-polar and hydrophobic. This amphipathic nature causes phospholipids to form bilayers in aqueous environments.

Question 28Molecules and Cells

Which molecule is the universal energy currency of cells, releasing energy when its terminal phosphate bond is hydrolysed?

  1. NADPH
  2. ATP Correct answer.
  3. Glucose
  4. Pyruvate

Explanation

ATP (adenosine triphosphate) is hydrolysed to ADP and inorganic phosphate, releasing energy that can drive cellular processes. The bond between the second and third phosphate groups is easily broken, making ATP a convenient short-term energy carrier. Glucose and pyruvate store energy but are not directly used as the immediate currency.

Question 29Molecules and Cells

Which of the following is a function of the Golgi apparatus in eukaryotic cells?

  1. Transcription of DNA into mRNA.
  2. Modification and packaging of proteins for secretion. Correct answer.
  3. Breakdown of hydrogen peroxide by catalase.
  4. Synthesis of ATP through oxidative phosphorylation.

Explanation

The Golgi apparatus modifies proteins and lipids received from the endoplasmic reticulum, then packages them into vesicles for transport to their final destinations, including secretion. ATP synthesis occurs in mitochondria, transcription in the nucleus, and catalase is found in peroxisomes.

Question 30Molecules and Cells

Which type of RNA carries amino acids to the ribosome during translation?

  1. Transfer RNA Correct answer.
  2. Small nuclear RNA
  3. Messenger RNA
  4. Ribosomal RNA

Explanation

Transfer RNA (tRNA) molecules have an anticodon that pairs with the mRNA codon and an attachment site for a specific amino acid. They deliver the correct amino acid to the growing polypeptide chain. mRNA carries the genetic message, rRNA forms part of the ribosome, and snRNA is involved in splicing.

Question 31Molecules and Cells

In the light-independent reactions of photosynthesis, which molecule is the source of carbon for the synthesis of triose phosphate?

  1. Carbon dioxide Correct answer.
  2. Pyruvate
  3. Oxygen
  4. Glucose

Explanation

The light-independent reactions (Calvin cycle) use carbon dioxide as the carbon source. CO2 combines with ribulose bisphosphate (RuBP) in a reaction catalysed by rubisco, eventually producing triose phosphate. Glucose and pyruvate are products or intermediates of respiration, and oxygen is not a carbon source.

Question 32Molecules and Cells

Which of the following correctly describes the role of NADP+ in photosynthesis?

  1. It donates electrons to the electron transport chain.
  2. It is the final electron acceptor in the electron transport chain.
  3. It accepts electrons and protons to become NADPH. Correct answer.
  4. It phosphorylates ADP to form ATP.

Explanation

In the light-dependent reactions, NADP+ acts as an electron acceptor. It accepts electrons from the electron transport chain and protons from the stroma to form NADPH, which is then used in the Calvin cycle. NADP+ does not donate electrons to the chain, nor does it phosphorylate ADP.

Question 33Molecules and Cells

Which of the following is a structural difference between DNA and RNA?

  1. DNA contains ribose, while RNA contains deoxyribose.
  2. DNA is usually single-stranded, while RNA is double-stranded.
  3. DNA contains uracil, while RNA contains thymine.
  4. DNA contains deoxyribose, while RNA contains ribose. Correct answer.

Explanation

DNA contains the sugar deoxyribose, whereas RNA contains ribose. This difference affects stability and function. DNA also contains thymine while RNA contains uracil, and DNA is typically double-stranded while RNA is usually single-stranded.

Question 34Molecules and Cells

A student investigates the effect of pH on the activity of salivary amylase. At which pH would the enzyme be expected to show maximum activity?

  1. pH 2
  2. pH 7 Correct answer.
  3. pH 10
  4. pH 4

Explanation

Salivary amylase works in the mouth, where the pH is close to neutral (around pH 7). Each enzyme has an optimum pH at which its active site is best complementary to the substrate. Extremes of pH can denature the enzyme by disrupting ionic and hydrogen bonds.

Question 35Molecules and Cells

Which statement best explains why a reaction catalysed by an enzyme is faster than the same reaction without the enzyme?

  1. The enzyme is consumed during the reaction, providing additional energy.
  2. The enzyme changes the equilibrium constant of the reaction.
  3. The enzyme increases the activation energy of the reaction.
  4. The enzyme lowers the activation energy of the reaction. Correct answer.

Explanation

Enzymes speed up reactions by lowering the activation energy, which is the energy barrier that must be overcome for the reaction to occur. They do not alter the overall free-energy change or the equilibrium position, and they are not consumed. This allows reactions to proceed at a faster rate at normal cellular temperatures.

Question 36Molecules and Cells

A segment of a DNA molecule has the base sequence 5'-ATGCCG-3'. What is the sequence of the complementary DNA strand?

  1. 3'-ATGCCG-5'
  2. 5'-TACGGC-3'
  3. 5'-UACGGC-3'
  4. 3'-TACGGC-5' Correct answer.

Explanation

DNA strands are antiparallel, so the complementary strand runs in the opposite direction. Adenine pairs with thymine and guanine pairs with cytosine. The complementary sequence to 5'-ATGCCG-3' is therefore 3'-TACGGC-5'. Option 0 has the correct bases but the wrong orientation.

Question 37Molecules and Cells

In the genetic code, a codon consists of how many nucleotides, and how many codons specify amino acids?

  1. Three nucleotides; 64 codons specify amino acids.
  2. Two nucleotides; 16 codons specify amino acids.
  3. Four nucleotides; 64 codons specify amino acids.
  4. Three nucleotides; 61 codons specify amino acids. Correct answer.

Explanation

A codon is a triplet of three nucleotides. There are 64 possible codons, but three of these (UAA, UAG, UGA) are stop codons and do not code for amino acids. Therefore, 61 codons specify amino acids, with some amino acids encoded by multiple codons (degeneracy).

Question 38Molecules and Cells

Which statement correctly describes the behaviour of chromosomes during metaphase of mitosis?

  1. The nuclear envelope reforms around the separated chromatids.
  2. Chromosomes line up individually along the metaphase plate. Correct answer.
  3. Homologous chromosomes pair and form bivalents.
  4. Sister chromatids separate and move to opposite poles.

Explanation

In metaphase of mitosis, individual chromosomes (each consisting of two sister chromatids) align along the equator of the cell, called the metaphase plate. Homologous pairing occurs in meiosis I, sister chromatid separation occurs in anaphase, and nuclear envelope reformation occurs in telophase.

Question 39Molecules and Cells

A cell with a diploid number of 24 undergoes meiosis. What is the number of chromosomes in each daughter cell at the end of meiosis II?

  1. 24
  2. 48
  3. 12 Correct answer.
  4. 6

Explanation

Meiosis halves the chromosome number. Starting with a diploid cell of 24 chromosomes, meiosis I separates homologous chromosomes, producing two haploid cells with 12 chromosomes each. Meiosis II separates sister chromatids, resulting in four haploid cells, each with 12 chromosomes.

Question 40Molecules and Cells

Which of the following best explains why the rate of diffusion of a gas across a respiratory surface increases when the surface area is larger?

  1. A larger surface area increases the concentration gradient across the surface.
  2. A larger surface area provides more space for gas molecules to cross per unit time. Correct answer.
  3. A larger surface area increases the temperature of the gas.
  4. A larger surface area decreases the diffusion distance.

Explanation

According to Fick's law, the rate of diffusion is directly proportional to the surface area. A larger surface area allows more gas molecules to cross the membrane per unit time, increasing the overall rate. The concentration gradient and diffusion distance are independent of surface area.

Question 41Molecules and Cells

A plant cell with a water potential of -500 kPa is placed in a solution with a water potential of -200 kPa. Which statement describes the net movement of water?

  1. Water moves out of the cell by osmosis.
  2. Water moves into the cell by active transport.
  3. There is no net movement of water.
  4. Water moves into the cell by osmosis. Correct answer.

Explanation

Water moves by osmosis from a region of higher water potential (less negative) to a region of lower water potential (more negative). The solution has a higher water potential (-200 kPa) than the cell (-500 kPa), so water enters the cell. Active transport is not involved in osmosis.

Question 42Molecules and Cells

Which of the following is the correct sequence of events in the light-dependent reactions of photosynthesis?

  1. Electron transport, light absorption, reduction of NADP+, photolysis of water.
  2. Photolysis of water, reduction of NADP+, light absorption, electron transport.
  3. Reduction of NADP+, photolysis of water, light absorption, electron transport.
  4. Light absorption by chlorophyll, photolysis of water, electron transport, reduction of NADP+. Correct answer.

Explanation

Light energy is absorbed by chlorophyll, exciting electrons. This drives electron transport, and the electrons lost from photosystem II are replaced by electrons from the photolysis of water. The electrons ultimately reduce NADP+ to NADPH, and the proton gradient drives ATP synthesis. The other sequences place these events in the wrong order.

Question 43Molecules and Cells

In an experiment, a plant is exposed to radioactive carbon dioxide (14CO2) for a short period, and then the radioactive carbon is tracked. Which compound is the first stable product to contain the radioactive carbon?

  1. Glycerate 3-phosphate (GP) Correct answer.
  2. Ribulose bisphosphate (RuBP)
  3. Glucose
  4. Triose phosphate (TP)

Explanation

In the Calvin cycle, 14CO2 combines with RuBP to form an unstable six-carbon intermediate that immediately splits into two molecules of glycerate 3-phosphate (GP). Therefore, GP is the first stable product to contain the radioactive carbon. RuBP is the carbon acceptor, and triose phosphate and glucose are formed later.

Question 44Molecules and Cells

A mutation in a gene results in a protein with a different amino acid sequence but the same number of amino acids. Which type of mutation is most likely responsible?

  1. A silent mutation
  2. A frameshift mutation
  3. A nonsense mutation
  4. A missense mutation Correct answer.

Explanation

A missense mutation changes a single base, resulting in a different codon that specifies a different amino acid, but the length of the polypeptide remains the same. A silent mutation does not change the amino acid, a nonsense mutation introduces a stop codon and shortens the protein, and a frameshift alters the reading frame and usually changes many amino acids and often the length.

Question 45Molecules and Cells

Which of the following correctly describes the role of the enzyme DNA helicase during DNA replication?

  1. It joins Okazaki fragments together.
  2. It adds nucleotides to the growing DNA strand.
  3. It unwinds the double helix by breaking hydrogen bonds between base pairs. Correct answer.
  4. It removes RNA primers and replaces them with DNA.

Explanation

DNA helicase unwinds the double helix by breaking the hydrogen bonds between complementary base pairs, separating the two strands to expose the template. DNA ligase joins Okazaki fragments, DNA polymerase adds nucleotides, and other enzymes remove RNA primers. Helicase does not synthesise or join DNA.

Question 46Molecules and Cells

A cell is treated with a drug that inhibits the enzyme responsible for the formation of the mitotic spindle. At which stage of mitosis would the cell most likely arrest?

  1. Prophase
  2. Metaphase Correct answer.
  3. Telophase
  4. Anaphase

Explanation

The mitotic spindle is required for the alignment of chromosomes at the metaphase plate and for their subsequent separation. If spindle formation is inhibited, chromosomes cannot align properly, and the cell arrests in metaphase due to the spindle assembly checkpoint. Prophase, anaphase, and telophase depend on a functional spindle but the primary arrest occurs at metaphase.

Question 47Molecules and Cells

Which statement best explains why the rate of enzyme-catalysed reactions increases with temperature up to an optimum?

  1. Higher temperature changes the shape of the active site to fit the substrate better.
  2. Higher temperature increases the concentration of the substrate.
  3. Higher temperature increases the kinetic energy of molecules, leading to more frequent successful collisions. Correct answer.
  4. Higher temperature decreases the activation energy of the reaction.

Explanation

As temperature rises, molecules gain kinetic energy and move faster, so enzyme and substrate molecules collide more frequently and with greater energy. This increases the rate of reaction until the optimum temperature is reached. Above the optimum, denaturation reduces activity. Temperature does not lower activation energy or increase substrate concentration.

Question 48Molecules and Cells

In a cell, a protein destined for secretion is synthesised by ribosomes attached to the rough endoplasmic reticulum. Which pathway correctly describes its transport to the outside of the cell?

  1. Rough ER → Golgi apparatus → secretory vesicle → plasma membrane Correct answer.
  2. Rough ER → smooth ER → Golgi apparatus → nucleus
  3. Cytoplasm → Golgi apparatus → rough ER → secretory vesicle
  4. Golgi apparatus → rough ER → lysosome → plasma membrane

Explanation

Proteins synthesised on the rough ER enter the ER lumen, are transported in vesicles to the Golgi apparatus for modification, then packaged into secretory vesicles that fuse with the plasma membrane to release the protein. The other pathways list organelles in the wrong order or include destinations that are not part of secretion.

Question 49Molecules and Cells

A cell is placed in a hypertonic solution. Which change is most likely to occur in an animal cell?

  1. The cell will remain unchanged.
  2. The cell will swell and burst.
  3. The cell will actively pump water out.
  4. The cell will shrink and shrivel. Correct answer.

Explanation

In a hypertonic solution, the external water potential is lower than that inside the cell. Water moves out of the cell by osmosis, causing the cell to lose volume and shrivel. Animal cells lack a cell wall, so they do not become turgid; they shrink. Active pumping of water is not a normal mechanism.

Question 50Molecules and Cells

In an experiment on photosynthesis, a plant is supplied with water containing radioactive oxygen (H2^18O). Where would the radioactive oxygen first appear?

  1. In the glucose produced
  2. In the oxygen gas released Correct answer.
  3. In the carbon dioxide produced
  4. In the NADPH produced

Explanation

The oxygen released during photosynthesis comes from the photolysis of water, not from carbon dioxide. When water is labelled with ^18O, the radioactive oxygen is released as ^18O2 gas. The oxygen atoms in glucose come from carbon dioxide, not water.