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QCAA Biology EA study materials
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QCAA Biology EA practice questions

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Question 1Cells and multicellular organisms (Unit 3)

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

  1. Ribosome
  2. Golgi apparatus
  3. Lysosome
  4. Mitochondrion Correct answer.

Explanation

The inner mitochondrial membrane houses the electron transport chain and ATP synthase, which together carry out oxidative phosphorylation. Ribosomes synthesise proteins, the Golgi modifies and sorts them, and lysosomes carry out hydrolysis, so none of these generate ATP in this way.

Question 2Cells and multicellular organisms (Unit 3)

A cell is placed in a solution and its volume decreases over time. Which description of the external solution best explains this observation?

  1. Hypertonic, because solutes enter the cell by diffusion
  2. Isotonic, because water movement is balanced
  3. Hypertonic, because water leaves the cell by osmosis Correct answer.
  4. Hypotonic, because water enters the cell by osmosis

Explanation

Water moves by osmosis from a region of higher water potential to lower water potential. In a hypertonic solution the external water potential is lower, so water exits the cell and volume falls. Solute entry would not explain shrinking, and balanced movement in an isotonic solution would leave volume unchanged.

Question 3Cells and multicellular organisms (Unit 3)

Which statement correctly describes the structure of the fluid mosaic model of the plasma membrane?

  1. A double layer of cellulose with protein pores
  2. A phospholipid bilayer with embedded proteins and cholesterol Correct answer.
  3. A single layer of phospholipids coated with glycoproteins
  4. A rigid protein lattice enclosing a phospholipid core

Explanation

The fluid mosaic model describes a bilayer of phospholipids in which proteins are embedded or attached and cholesterol modulates fluidity. The hydrophobic tails face inward and the hydrophilic heads face the aqueous environments. Cellulose is a cell wall component, not a membrane component.

Question 4Cells and multicellular organisms (Unit 3)

Which process allows a large polar molecule such as glucose to enter a cell down its concentration gradient without direct ATP hydrolysis?

  1. Endocytosis of the molecule
  2. Facilitated diffusion through a carrier protein Correct answer.
  3. Simple diffusion through the bilayer
  4. Active transport by a proton pump

Explanation

Large polar molecules cannot cross the hydrophobic core by simple diffusion, so they require transport proteins. Facilitated diffusion uses carrier or channel proteins to move substances down their gradient without direct ATP hydrolysis. Endocytosis does not require a concentration gradient and pumps move substances against gradients.

Question 5Cells and multicellular organisms (Unit 3)

Which feature distinguishes prokaryotic cells from eukaryotic cells?

  1. Prokaryotic cells lack a membrane-bound nucleus Correct answer.
  2. Prokaryotic cells lack nucleic acids
  3. Prokaryotic cells lack a plasma membrane
  4. Prokaryotic cells lack ribosomes

Explanation

Prokaryotes keep their genetic material in a nucleoid region without a nuclear envelope, whereas eukaryotes enclose DNA in a nucleus. Prokaryotes still possess ribosomes, a plasma membrane and DNA, so those features cannot distinguish the two cell types.

Question 6Cells and multicellular organisms (Unit 3)

Which molecule carries the amino acid to the ribosome during translation?

  1. Transfer RNA Correct answer.
  2. DNA polymerase
  3. Messenger RNA
  4. Ribosomal RNA

Explanation

Transfer RNA has an anticodon that pairs with the mRNA codon and carries the corresponding amino acid to the ribosome. Messenger RNA carries the coding sequence and ribosomal RNA forms part of the ribosome, but neither delivers amino acids. DNA polymerase functions in replication.

Question 7Cells and multicellular organisms (Unit 3)

Cells of the small intestine lining are replaced frequently by mitosis. Which outcome is expected from a single mitotic division of one of these cells?

  1. Two daughter cells with double the chromosome number
  2. Four daughter cells with half the chromosome number
  3. Two daughter cells with the same chromosome number as the parent Correct answer.
  4. Four daughter cells with the same chromosome number

Explanation

Mitosis produces two genetically identical daughter cells that retain the diploid chromosome number of the parent cell. Halving of chromosome number and production of four cells are features of meiosis, which occurs in germline tissue rather than in the intestinal epithelium.

Question 8Cells and multicellular organisms (Unit 3)

Which tissue type lines body surfaces and forms glands?

  1. Epithelial tissue Correct answer.
  2. Connective tissue
  3. Nervous tissue
  4. Muscle tissue

Explanation

Epithelial tissue covers external and internal surfaces, forms protective linings and gives rise to glandular structures. Connective tissue supports and binds, muscle tissue contracts and nervous tissue conducts impulses, so none of these perform the lining and gland-forming role.

Question 9Cells and multicellular organisms (Unit 3)

An enzyme-catalysed reaction rate increases with temperature up to about 40 degrees Celsius and then falls sharply. Which explanation best accounts for the fall?

  1. The substrate concentration decreases at higher temperatures
  2. The activation energy increases at higher temperatures
  3. The enzyme's active site is denatured at higher temperatures Correct answer.
  4. The enzyme is diluted by condensation at higher temperatures

Explanation

Above an optimum temperature, increased kinetic energy disrupts hydrogen and ionic bonds that maintain enzyme tertiary structure. The active site loses its complementary shape, so substrate binding and catalysis decline. Substrate concentration and activation energy are not systematically altered by the temperature rise.

Question 10Cells and multicellular organisms (Unit 3)

A reaction rate continues to rise with substrate concentration but at a decreasing rate, approaching a plateau. Which interpretation is most consistent with these data?

  1. The enzyme changes its substrate specificity at high concentration
  2. The substrate begins to inhibit the enzyme at high concentration
  3. All active sites become saturated at high substrate concentration Correct answer.
  4. The enzyme is being consumed as the reaction proceeds

Explanation

When substrate concentration is high, enzyme active sites are occupied almost continuously, so adding more substrate cannot increase the rate further. This saturation produces a plateau corresponding to maximum velocity. Enzymes are catalysts and are not consumed, and the pattern is not explained by substrate inhibition or altered specificity.

Question 11Cells and multicellular organisms (Unit 3)

A competitive inhibitor is added to an enzyme-catalysed reaction. Which change is expected in the kinetic parameters?

  1. Maximum velocity decreases but the substrate concentration for half-maximal rate is unchanged
  2. Maximum velocity is unchanged but the substrate concentration needed for half-maximal rate increases Correct answer.
  3. Maximum velocity increases while substrate affinity decreases
  4. Both maximum velocity and substrate affinity increase

Explanation

A competitive inhibitor binds the active site reversibly, so its effect can be overcome by raising substrate concentration. The maximum velocity is therefore reached at high substrate concentration, but more substrate is needed to achieve half-maximal rate. Non-competitive inhibition instead lowers maximum velocity.

Question 12Cells and multicellular organisms (Unit 3)

Which statement best explains why the sodium-potassium pump is described as electrogenic?

  1. It generates ATP during each transport cycle
  2. It transports only positively charged ions
  3. It moves equal numbers of sodium and potassium ions
  4. It moves three sodium ions out for every two potassium ions in Correct answer.

Explanation

The pump exports three sodium ions and imports two potassium ions per cycle, producing a net outward movement of positive charge. This unequal stoichiometry contributes to the resting membrane potential, which is why the pump is called electrogenic. Equal exchange would be electrically neutral, and the pump consumes rather than produces ATP.

Question 13Cells and multicellular organisms (Unit 3)

A red blood cell is placed in distilled water. Which sequence of events is most likely?

  1. Solutes enter and the cell swells
  2. No net water movement occurs and the cell is stable
  3. Water enters by osmosis, the cell swells and may lyse Correct answer.
  4. Water leaves by osmosis and the cell shrinks

Explanation

Distilled water has a higher water potential than the cytoplasm, so water moves into the cell by osmosis. Because animal cells lack a rigid wall to resist expansion, the cell swells and may burst. Solute uptake and stable volume do not match placement in pure water.

Question 14Cells and multicellular organisms (Unit 3)

Which combination correctly describes the products of the light-dependent reactions used in the light-independent reactions of photosynthesis?

  1. NADH and carbon dioxide
  2. ATP and NADPH Correct answer.
  3. Glucose and oxygen
  4. ATP and oxygen

Explanation

The light-dependent reactions generate ATP and NADPH, which supply energy and reducing power for carbon fixation in the Calvin cycle. Oxygen is a by-product released to the atmosphere, and glucose is a product of the light-independent reactions rather than an input. NADH is associated mainly with respiration.

Question 15Cells and multicellular organisms (Unit 3)

In a cell undergoing meiosis, crossing over between homologous chromosomes occurs at which stage?

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

Explanation

Crossing over takes place during prophase I, when homologous chromosomes pair as bivalents and non-sister chromatids exchange segments. This recombination generates new allele combinations on the resulting chromosomes. Later stages separate chromosomes and chromatids but do not involve exchange between homologues.

Question 16Cells and multicellular organisms (Unit 3)

A cell with a diploid number of 24 undergoes meiosis. What is the chromosome number of each resulting gamete?

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

Explanation

Meiosis halves the chromosome number by separating homologous chromosomes, so each gamete receives one chromosome from each pair. For a diploid number of 24, each gamete therefore contains 12 chromosomes. Doubling or retaining the full number would not represent reduction division.

Question 17Cells and multicellular organisms (Unit 3)

Which feature of the mammalian lung increases the efficiency of gas exchange?

  1. A thick epithelial lining separating air and blood
  2. A large surface area provided by numerous alveoli Correct answer.
  3. A low concentration gradient of oxygen maintained across the epithelium
  4. A small blood supply to the alveolar walls

Explanation

Gas exchange is efficient when surface area is large, the diffusion distance is short and a steep concentration gradient is maintained. Numerous alveoli provide a very large surface area, and a rich blood supply helps sustain the gradient. Thick linings and low gradients would slow diffusion.

Question 18Cells and multicellular organisms (Unit 3)

Which description best explains how a hormone such as insulin can affect only certain target cells?

  1. The hormone enters all cells but is degraded only in target cells
  2. The hormone is present only in the vicinity of target cells
  3. Target cells have a lower metabolic rate than other cells
  4. Only target cells possess receptors that bind the hormone specifically Correct answer.

Explanation

Signalling depends on specific receptor proteins that bind the hormone with complementary shape. Cells lacking the receptor cannot respond even when the hormone is present. Hormones are often distributed widely in the blood, so proximity, metabolic rate or selective degradation do not explain specificity.

Question 19Cells and multicellular organisms (Unit 3)

A mutation changes a single base in a coding region so that a codon for one amino acid becomes a stop codon. Which consequence is most likely?

  1. Transcription fails to produce messenger RNA
  2. Translation terminates early, producing a shortened polypeptide Correct answer.
  3. The reading frame shifts and all later codons change
  4. The amino acid sequence is unchanged

Explanation

A change to a stop codon causes the ribosome to terminate translation prematurely, so a truncated polypeptide results. A frameshift requires insertion or deletion of a number of bases not divisible by three, and a silent mutation leaves the amino acid unchanged. Transcription can still occur because the change does not necessarily block RNA synthesis.

Question 20Cells and multicellular organisms (Unit 3)

Which statement best describes the role of cholesterol in animal cell membranes?

  1. It anchors ribosomes to the membrane surface
  2. It modulates membrane fluidity across different temperatures Correct answer.
  3. It forms channels for sodium ions
  4. It transports glucose across the bilayer

Explanation

Cholesterol sits between phospholipid molecules and restricts their movement at high temperatures while preventing tight packing at low temperatures. This buffers membrane fluidity against temperature change. Transport of glucose and ions depends on specific proteins, and ribosome attachment does not involve cholesterol.

Question 21Cells and multicellular organisms (Unit 3)

Two enzymes catalyse the same reaction but have different affinities for the substrate. Which measurement best compares their affinities?

  1. The total protein concentration in each preparation
  2. The substrate concentration at which each reaches half its maximum rate Correct answer.
  3. The temperature at which each enzyme denatures
  4. The maximum rate achieved by each enzyme

Explanation

The substrate concentration giving half-maximal velocity is inversely related to affinity, so a lower value indicates tighter substrate binding. Maximum rate reflects turnover capacity rather than affinity. Protein concentration and denaturation temperature do not directly measure binding affinity.

Question 22Cells and multicellular organisms (Unit 3)

A cell is treated with a drug that blocks the electron transport chain. Which immediate change is most likely?

  1. The cell begins to synthesise glucose from carbon dioxide
  2. Carbon dioxide production by the Krebs cycle increases sharply
  3. The proton gradient across the inner mitochondrial membrane collapses and ATP synthesis falls Correct answer.
  4. Glycolysis stops immediately because it requires oxygen directly

Explanation

Electron transport pumps protons to build a gradient that drives ATP synthase, so blocking the chain dissipates the gradient and lowers ATP synthesis. Glycolysis does not require oxygen directly and can continue anaerobically, while the Krebs cycle slows when electron carriers cannot be reoxidised. Glucose synthesis is not a response to blocked electron transport.

Question 23Cells and multicellular organisms (Unit 3)

In an experiment, an enzyme is assayed with increasing substrate concentration in the presence of a fixed amount of a non-competitive inhibitor. Which outcome is expected?

  1. The maximum rate is reduced but the substrate concentration for half-maximal rate is unchanged Correct answer.
  2. The enzyme becomes permanently denatured and no reaction occurs
  3. Both the maximum rate and the substrate concentration for half-maximal rate increase
  4. The maximum rate is unchanged but more substrate is needed for half-maximal rate

Explanation

A non-competitive inhibitor binds at a site other than the active site and reduces the proportion of functional enzyme, lowering maximum velocity. Because it does not compete with substrate for the active site, raising substrate concentration cannot restore the rate, so the half-maximal substrate concentration is unchanged. Competitive inhibition instead raises that concentration without altering maximum velocity.

Question 24Cells and multicellular organisms (Unit 3)

A mutation deletes two nucleotides from the middle of a coding sequence. Which consequence is most likely?

  1. The reading frame shifts, altering every codon downstream of the deletion Correct answer.
  2. A single amino acid is substituted with no other change
  3. The polypeptide is unchanged because two bases are not significant
  4. Translation stops immediately at the deletion site

Explanation

Codons are read in non-overlapping triplets, so deleting two bases shifts the reading frame for all subsequent codons. This usually produces a substantially altered or truncated polypeptide. A single substitution affects one codon, and a deletion of two bases does not necessarily create an immediate stop codon.

Question 25Cells and multicellular organisms (Unit 3)

Which observation would provide the strongest evidence that a protein is involved in facilitated diffusion rather than active transport?

  1. Transport occurs only in living cells
  2. Transport is saturable at high substrate concentration
  3. Transport stops when the temperature is lowered
  4. Transport continues down the concentration gradient when ATP is depleted Correct answer.

Explanation

Active transport moves substances against a gradient and requires energy, so ATP depletion abolishes it. Continued movement down a gradient without ATP indicates facilitated diffusion through a protein. Saturability and temperature sensitivity occur for both carrier-mediated processes, so they do not discriminate between them.

Question 26Cells and multicellular organisms (Unit 3)

Which organelle is the primary site of ATP synthesis in eukaryotic cells?

  1. Lysosome
  2. Mitochondrion Correct answer.
  3. Ribosome
  4. Golgi apparatus

Explanation

Mitochondria house the electron transport chain and ATP synthase, which generate most of the cell's ATP through oxidative phosphorylation. Other organelles perform different roles in the cell.

Question 27Cells and multicellular organisms (Unit 3)

Which statement best describes the fluid mosaic model of the plasma membrane?

  1. A rigid layer of cellulose enclosing the cell
  2. A phospholipid bilayer with embedded proteins and cholesterol that can move laterally Correct answer.
  3. A double layer of proteins with a phospholipid core
  4. A static structure with proteins fixed in place

Explanation

The fluid mosaic model describes the membrane as a fluid phospholipid bilayer in which proteins, cholesterol and other components are embedded and can move laterally. This model accounts for membrane flexibility and selective permeability.

Question 28Cells and multicellular organisms (Unit 3)

Which process requires the hydrolysis of ATP to move substances against their concentration gradient?

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

Explanation

Active transport uses energy from ATP hydrolysis to move ions or molecules against their electrochemical gradient. Passive processes such as diffusion and osmosis do not directly require ATP.

Question 29Cells and multicellular organisms (Unit 3)

Which stage of aerobic cellular respiration produces the most ATP per glucose molecule?

  1. Glycolysis
  2. The citric acid cycle
  3. Oxidative phosphorylation Correct answer.
  4. Pyruvate oxidation

Explanation

Oxidative phosphorylation, comprising the electron transport chain and chemiosmosis, generates the majority of ATP during aerobic respiration. The other stages produce relatively small amounts of ATP directly.

Question 30Cells and multicellular organisms (Unit 3)

In the light-dependent reactions of photosynthesis, which molecule is the final electron acceptor?

  1. NADP+ Correct answer.
  2. FAD
  3. Cytochrome c
  4. NAD+

Explanation

In the light-dependent reactions, electrons ultimately reduce NADP+ to NADPH, which carries reducing power to the Calvin cycle. NAD+ and FAD are used in cellular respiration, not as the final acceptor in photosynthesis.

Question 31Cells and multicellular organisms (Unit 3)

Which type of cell junction allows direct cytoplasmic communication between adjacent animal cells?

  1. Gap junction Correct answer.
  2. Desmosome
  3. Tight junction
  4. Plasmodesma

Explanation

Gap junctions form channels that connect the cytoplasm of adjacent animal cells, permitting the exchange of ions and small molecules. Tight junctions and desmosomes restrict or anchor cells, while plasmodesmata occur in plant cells.

Question 32Cells and multicellular organisms (Unit 3)

Which statement correctly describes the role of the Golgi apparatus?

  1. It contains enzymes that break down worn-out organelles.
  2. It generates ATP through oxidative phosphorylation.
  3. It synthesises ribosomal RNA.
  4. It modifies, sorts and packages proteins and lipids for transport. Correct answer.

Explanation

The Golgi apparatus receives proteins and lipids from the endoplasmic reticulum, modifies them, and packages them into vesicles for transport to their final destinations. It does not synthesise rRNA, degrade organelles, or produce ATP.

Question 33Cells and multicellular organisms (Unit 3)

Which of the following is a characteristic of prokaryotic cells?

  1. They possess a cytoskeleton made of microtubules.
  2. They lack a membrane-bound nucleus. Correct answer.
  3. They reproduce by mitosis.
  4. They contain mitochondria for ATP production.

Explanation

Prokaryotic cells do not have a membrane-bound nucleus; their genetic material is located in the nucleoid region. They also lack membrane-bound organelles such as mitochondria and reproduce by binary fission, not mitosis.

Question 34Cells and multicellular organisms (Unit 3)

A cell is placed in a hypertonic solution. What is the most likely immediate effect on an animal cell?

  1. There is no net movement of water.
  2. The cell wall prevents any change in cell volume.
  3. Water leaves the cell, causing it to shrink. Correct answer.
  4. Water enters the cell, causing it to swell and burst.

Explanation

In a hypertonic solution, the solute concentration outside the cell is higher than inside, so water moves out by osmosis and the cell shrinks. Animal cells lack a cell wall, so they cannot resist volume changes in the same way plant cells do.

Question 35Cells and multicellular organisms (Unit 3)

Which statement best explains why a cell's surface area to volume ratio limits cell size?

  1. As a cell grows, its surface area increases faster than its volume, causing excessive uptake of nutrients.
  2. As a cell grows, its volume increases faster than its surface area, reducing the efficiency of material exchange. Correct answer.
  3. A larger cell has a greater surface area to volume ratio, which slows metabolism.
  4. The ratio remains constant as cell size increases, so diffusion is unaffected.

Explanation

When a cell enlarges, its volume increases proportionally more than its surface area, so the surface area to volume ratio decreases. This reduces the relative area available for exchanging nutrients and wastes, limiting how large a cell can be.

Question 36Cells and multicellular organisms (Unit 3)

Which of the following is an example of active transport?

  1. Oxygen diffusing from the alveoli into the blood
  2. The sodium-potassium pump moving Na+ out of and K+ into a cell Correct answer.
  3. Glucose entering a red blood cell through a channel protein
  4. Water moving into a plant root hair cell by osmosis

Explanation

The sodium-potassium pump uses ATP to move sodium and potassium ions against their concentration gradients, which is active transport. The other options describe passive processes driven by concentration gradients.

Question 37Cells and multicellular organisms (Unit 3)

In an experiment, a plant cell is immersed in a solution and its protoplast shrinks away from the cell wall. What is this process called?

  1. Crenation
  2. Haemolysis
  3. Turgor pressure
  4. Plasmolysis Correct answer.

Explanation

Plasmolysis occurs when a plant cell loses water in a hypertonic solution, causing the protoplast to shrink away from the cell wall. Turgor pressure is the opposite state, while crenation and haemolysis refer to animal cells.

Question 38Cells and multicellular organisms (Unit 3)

Which component of the cell membrane is primarily responsible for maintaining membrane fluidity at low temperatures?

  1. Glycoproteins
  2. Integral proteins
  3. Phospholipid heads
  4. Cholesterol Correct answer.

Explanation

Cholesterol inserts between phospholipid molecules and prevents them from packing closely together at low temperatures, thus maintaining fluidity. It also reduces excessive fluidity at high temperatures.

Question 39Cells and multicellular organisms (Unit 3)

Which statement best describes the function of the smooth endoplasmic reticulum?

  1. It modifies and packages proteins for secretion.
  2. It is the site of ribosomal RNA synthesis.
  3. It synthesises lipids and detoxifies certain substances. Correct answer.
  4. It contains enzymes for intracellular digestion.

Explanation

The smooth endoplasmic reticulum lacks ribosomes and is involved in lipid synthesis, detoxification, and calcium ion storage. The rough endoplasmic reticulum and Golgi apparatus handle protein modification and packaging.

Question 40Cells and multicellular organisms (Unit 3)

A cell is treated with a drug that inhibits ATP synthase. Which immediate effect is most likely?

  1. The citric acid cycle stops completely.
  2. Glycolysis is immediately inhibited.
  3. ATP production by oxidative phosphorylation decreases. Correct answer.
  4. The electron transport chain stops completely.

Explanation

ATP synthase is responsible for generating ATP from ADP and Pi using the proton gradient. Inhibiting it directly reduces ATP production by oxidative phosphorylation, although the electron transport chain may continue until the proton gradient becomes excessive.

Question 41Cells and multicellular organisms (Unit 3)

Which of the following best explains how the light-independent reactions of photosynthesis are dependent on the light-dependent reactions?

  1. They require ATP and NADPH produced by the light-dependent reactions. Correct answer.
  2. They require water produced by the light-dependent reactions.
  3. They require oxygen produced by the light-dependent reactions.
  4. They require glucose produced by the light-dependent reactions.

Explanation

The Calvin cycle (light-independent reactions) uses ATP and NADPH generated by the light-dependent reactions to reduce carbon dioxide to carbohydrate. Oxygen, glucose, and water are not products of the light-dependent reactions used in the Calvin cycle.

Question 42Cells and multicellular organisms (Unit 3)

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

  1. It carries electrons to the electron transport chain. Correct answer.
  2. It directly phosphorylates ADP to form ATP.
  3. It is the final electron acceptor in the electron transport chain.
  4. It donates electrons to the Calvin cycle.

Explanation

NADH is an electron carrier that donates electrons to the electron transport chain, where they are used to pump protons and ultimately drive ATP synthesis. It does not donate to the Calvin cycle, act as the final acceptor, or directly phosphorylate ADP.

Question 43Cells and multicellular organisms (Unit 3)

Which type of microscopy would be most suitable for observing the detailed internal structure of a mitochondrion?

  1. Scanning electron microscopy
  2. Light microscopy
  3. Fluorescence microscopy
  4. Transmission electron microscopy Correct answer.

Explanation

Transmission electron microscopy provides high-resolution images of internal ultrastructure by transmitting electrons through a thin specimen. Light and fluorescence microscopy lack the resolution to resolve cristae, while scanning electron microscopy shows surface features.

Question 44Cells and multicellular organisms (Unit 3)

A mutation results in a protein that remains in the cytosol instead of being secreted. Which sequence is most likely affected?

  1. The poly-A tail that stabilises the mRNA
  2. The promoter region that initiates transcription
  3. The signal peptide that directs the protein to the endoplasmic reticulum Correct answer.
  4. The stop codon that terminates translation

Explanation

Secretory proteins have an N-terminal signal peptide that directs the ribosome to the rough endoplasmic reticulum. If this signal is mutated, the protein is translated in the cytosol and remains there.

Question 45Cells and multicellular organisms (Unit 3)

Which observation would provide the strongest evidence that a substance enters a cell by simple diffusion rather than facilitated diffusion?

  1. Uptake is reduced when ATP synthesis is inhibited.
  2. The rate of uptake increases with concentration but plateaus at high concentrations.
  3. The rate of uptake is directly proportional to the concentration gradient and is not saturable. Correct answer.
  4. Uptake is inhibited by a structural analogue of the substance.

Explanation

Simple diffusion is not saturable and its rate is directly proportional to the concentration gradient. Facilitated diffusion shows saturation kinetics because it relies on a limited number of transport proteins, and can be inhibited by analogues.

Question 46Cells and multicellular organisms (Unit 3)

A cell is placed in a solution containing a substance that inhibits the sodium-potassium pump. Which immediate effect is most likely?

  1. The resting membrane potential will gradually depolarise. Correct answer.
  2. Intracellular sodium concentration will decrease.
  3. Potassium will rapidly enter the cell.
  4. The cell will immediately swell and burst.

Explanation

The sodium-potassium pump maintains the resting membrane potential by exporting three Na+ and importing two K+ per ATP. Inhibiting it allows ion gradients to dissipate, leading to gradual depolarisation.

Question 47Cells and multicellular organisms (Unit 3)

In an experiment, a cell is exposed to a drug that increases the permeability of the inner mitochondrial membrane to protons. Which outcome is most likely?

  1. Glycolysis is inhibited.
  2. ATP synthesis increases because more protons flow through ATP synthase.
  3. ATP synthesis decreases because the proton gradient is dissipated. Correct answer.
  4. The electron transport chain stops immediately.

Explanation

Uncoupling agents increase proton permeability, allowing protons to leak across the inner membrane without passing through ATP synthase. This dissipates the proton gradient and reduces ATP synthesis, even though the electron transport chain may continue.

Question 48Cells and multicellular organisms (Unit 3)

A researcher observes that a particular protein is found in the cytoplasm and also in the nucleus of the same cell. Which statement best explains this observation?

  1. The protein is too large to pass through nuclear pores and remains in both compartments.
  2. The protein is synthesised in the nucleus and then moves to the cytoplasm.
  3. The protein is a transmembrane protein that spans the nuclear envelope.
  4. The protein contains a nuclear localisation signal that allows it to be transported into the nucleus. Correct answer.

Explanation

Proteins destined for the nucleus carry a nuclear localisation signal that directs their transport through nuclear pores. Proteins are synthesised in the cytoplasm, not the nucleus, and large proteins require active transport through pores.

Question 49Cells and multicellular organisms (Unit 3)

Which of the following best explains why the rate of photosynthesis increases with light intensity up to a plateau?

  1. At high light intensity, chlorophyll is destroyed by photo-oxidation.
  2. At high light intensity, the stomata close, preventing oxygen release.
  3. At high light intensity, the light-dependent reactions are inhibited by excess ATP.
  4. At high light intensity, another factor such as carbon dioxide concentration becomes limiting. Correct answer.

Explanation

The initial increase is due to light being the limiting factor. At the plateau, light is no longer limiting and another factor, such as carbon dioxide concentration or temperature, limits the rate.

Question 50Cells and multicellular organisms (Unit 3)

A cell is treated with a drug that prevents the assembly of microtubules. Which process is most directly affected?

  1. Replication of DNA during S phase
  2. Synthesis of proteins on ribosomes
  3. Movement of water across the membrane
  4. Separation of chromosomes during mitosis Correct answer.

Explanation

Microtubules form the mitotic spindle, which is required for chromosome separation during mitosis. DNA replication, protein synthesis, and osmosis do not directly depend on microtubule assembly.