I85PREP
⌘ K
Biology study materials
Biology study resources

Biology practice questions

One question at a time. Build your Biology understanding.

First 50 of 200 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1Molecular and Cellular Biology

Which molecule stores the genetic information that is directly translated into a polypeptide by the ribosome?

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

Explanation

Messenger RNA carries the coding sequence transcribed from DNA and is read codon by codon by the ribosome during translation. Ribosomal RNA is a structural and catalytic component of the ribosome, transfer RNA delivers amino acids, and small nuclear RNA functions in splicing.

Question 2Molecular and Cellular Biology

During which phase of the cell cycle does DNA replication occur?

  1. M phase
  2. G1 phase
  3. G2 phase
  4. S phase Correct answer.

Explanation

Replication of the genome takes place during S phase, which lies between the two gap phases. G1 and G2 are gap phases of growth and checkpoint control, and M phase is when chromosomes are segregated and the cell divides.

Question 3Molecular and Cellular Biology

A solution at pH 3 contains how many times more hydrogen ion concentration than a solution at pH 6?

  1. 1,000-fold Correct answer.
  2. 30-fold
  3. 3-fold
  4. 100-fold

Explanation

The pH scale is logarithmic, so each unit is a tenfold change in hydrogen ion concentration. A difference of three pH units therefore corresponds to 10 raised to the third power, a thousandfold difference.

Question 4Molecular and Cellular Biology

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

  1. Mitochondrion Correct answer.
  2. Golgi apparatus
  3. Lysosome
  4. Smooth endoplasmic reticulum

Explanation

The inner mitochondrial membrane houses the electron transport chain and ATP synthase, which together carry out oxidative phosphorylation. The other listed organelles handle protein modification, degradation, and lipid synthesis rather than chemiosmotic ATP production.

Question 5Molecular and Cellular Biology

Which bond links the phosphate group of one nucleotide to the sugar of the next nucleotide within a single DNA strand?

  1. Peptide bond
  2. Phosphodiester bond Correct answer.
  3. Glycosidic bond
  4. Hydrogen bond

Explanation

Adjacent nucleotides in a DNA strand are joined by phosphodiester bonds between the 3' hydroxyl of one sugar and the 5' phosphate of the next. Hydrogen bonds instead hold complementary bases together across the two strands.

Question 6Molecular and Cellular Biology

Which property of the plasma membrane is best described by the fluid mosaic model?

  1. Lipids are covalently anchored to the underlying cytoskeleton throughout
  2. Proteins diffuse laterally within a dynamic lipid bilayer Correct answer.
  3. The membrane is a single layer of phospholipids with no protein
  4. The bilayer is a rigid crystalline sheet with fixed proteins

Explanation

The fluid mosaic model holds that membrane proteins are embedded in or attached to a fluid lipid bilayer in which both lipids and proteins can move laterally. This dynamic arrangement allows processes such as receptor clustering and membrane fusion.

Question 7Molecular and Cellular Biology

A mutation changes a codon from UAC to UAA. What is the most likely consequence for the encoded protein?

  1. A shift in the reading frame
  2. Premature termination of translation Correct answer.
  3. Substitution of one amino acid for another
  4. No change because the codon is synonymous

Explanation

UAA is one of the three stop codons, so the ribosome will terminate translation at that point and release a truncated polypeptide. A single base change from a sense codon to a stop codon is called a nonsense mutation.

Question 8Molecular and Cellular Biology

Which enzyme unwinds the DNA double helix ahead of the replication fork during DNA replication?

  1. DNA ligase
  2. DNA helicase Correct answer.
  3. Topoisomerase
  4. DNA polymerase

Explanation

Helicase uses ATP hydrolysis to break the hydrogen bonds between complementary bases and separate the strands at the replication fork. Ligase seals nicks, polymerase extends new strands, and topoisomerase relieves supercoiling.

Question 9Molecular and Cellular Biology

In an enzyme-catalyzed reaction, what is the effect of a competitive inhibitor on the apparent Michaelis constant (Km) and maximum velocity (Vmax)?

  1. Both apparent Km and Vmax decrease
  2. Apparent Km decreases while Vmax decreases
  3. Apparent Km increases while Vmax is unchanged Correct answer.
  4. Apparent Km is unchanged while Vmax decreases

Explanation

A competitive inhibitor binds the free enzyme at the active site and can be outcompeted by sufficiently high substrate concentration. This raises the substrate concentration needed to reach half-maximal velocity, increasing apparent Km, while Vmax is unchanged because the inhibitor is surmountable.

Question 10Molecular and Cellular Biology

Which statement correctly describes the role of the signal recognition particle (SRP) in protein sorting?

  1. It binds a signal peptide and pauses translation, targeting the ribosome to the endoplasmic reticulum Correct answer.
  2. It adds mannose residues to proteins within the Golgi lumen
  3. It transports fully folded cytosolic proteins into the nucleus through the nuclear pore
  4. It cleaves signal peptides from proteins after they enter the mitochondrial matrix

Explanation

SRP recognizes the hydrophobic signal peptide as it emerges from the ribosome, halts elongation, and docks the ribosome-SRP complex at the SRP receptor on the endoplasmic reticulum. Translation then resumes and the polypeptide is translocated into the ER lumen.

Question 11Molecular and Cellular Biology

A cell is treated with a drug that blocks the proton gradient across the inner mitochondrial membrane. Which outcome is most directly expected?

  1. The citric acid cycle is directly inhibited at citrate synthase
  2. NADH oxidation by complex I is enhanced
  3. ATP synthesis by ATP synthase decreases Correct answer.
  4. Glycolysis stops immediately

Explanation

ATP synthase uses the proton motive force to drive ATP production, so dissipating the gradient reduces ATP synthesis. Glycolysis and the citric acid cycle can continue in the short term, and electron transport may even slow because of feedback from a high proton gradient.

Question 12Molecular and Cellular Biology

Which feature distinguishes prokaryotic gene expression from eukaryotic gene expression?

  1. Prokaryotic genes are transcribed in the nucleus
  2. Transcription and translation can occur simultaneously in prokaryotes Correct answer.
  3. Prokaryotes use RNA polymerase while eukaryotes do not
  4. Prokaryotic mRNA requires capping and polyadenylation before translation

Explanation

Because prokaryotes lack a nuclear envelope, ribosomes can attach to an mRNA and begin translation while transcription is still underway. Eukaryotes separate transcription in the nucleus from translation in the cytoplasm and process mRNA with capping, splicing, and polyadenylation.

Question 13Molecular and Cellular Biology

During 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 occurs during prophase I, when homologous chromosomes are paired as bivalents and chiasmata form between nonsister chromatids. This recombination creates new combinations of alleles on the resulting chromosomes.

Question 14Molecular and Cellular Biology

Which process directly produces the ATP generated during glycolysis?

  1. Substrate-level phosphorylation Correct answer.
  2. Chemiosmosis across the inner mitochondrial membrane
  3. Oxidative phosphorylation
  4. Photophosphorylation

Explanation

Glycolysis generates ATP by transferring a phosphate group from a high-energy glycolytic intermediate directly to ADP, which is substrate-level phosphorylation. Oxidative and photophosphorylation rely on membrane gradients and are not part of glycolysis.

Question 15Molecular and Cellular Biology

A protein is synthesized with a nuclear localization signal. Where would it most likely accumulate?

  1. Nucleus Correct answer.
  2. Mitochondrial matrix
  3. Endoplasmic reticulum lumen
  4. Lysosome

Explanation

A nuclear localization signal is recognized by importins, which carry the cargo through the nuclear pore complex into the nucleus. Signals for mitochondrial, ER, and lysosomal destinations are distinct peptide sequences.

Question 16Molecular and Cellular Biology

Which of the following best explains why DNA polymerase requires a primer to begin synthesis?

  1. It lacks the ability to proofread newly synthesized DNA
  2. It can only add nucleotides to a free 3' hydroxyl group Correct answer.
  3. It cannot hydrogen bond to the template strand without a primer
  4. It requires a primer to unwind the double helix

Explanation

DNA polymerases extend a preexisting strand by adding nucleotides to a 3' hydroxyl group, so they cannot initiate a chain de novo. Primase provides a short RNA primer with a free 3' hydroxyl that permits elongation.

Question 17Molecular and Cellular Biology

Which statement about the genetic code is accurate?

  1. The code differs substantially between bacteria and humans
  2. It is degenerate, meaning several codons can specify the same amino acid Correct answer.
  3. Stop codons also specify amino acids in the standard code
  4. Each codon specifies more than one amino acid

Explanation

Degeneracy means that most amino acids are encoded by more than one codon, which buffers some mutations. The code is otherwise nearly universal and each codon specifies a single amino acid or a stop signal.

Question 18Molecular and Cellular Biology

In a cell line, a mutation in a cyclin-dependent kinase makes it constitutively active. What is the most likely effect on cell cycle progression?

  1. Cells bypass the G1/S checkpoint and divide excessively Correct answer.
  2. Cells complete mitosis but fail cytokinesis
  3. Cells undergo apoptosis regardless of growth signals
  4. Cells arrest permanently in G1

Explanation

Cyclin-dependent kinases drive progression through checkpoints, so constitutive activity can override the requirement for growth signals at the G1/S transition. The result is unscheduled entry into S phase and excessive proliferation.

Question 19Molecular and Cellular Biology

Which interaction stabilizes the alpha-helical structure of a polypeptide?

  1. Disulfide bonds between cysteine side chains
  2. Hydrophobic interactions between aromatic side chains
  3. Hydrogen bonding between backbone carbonyl and amide groups Correct answer.
  4. Ionic bonds between terminal charges

Explanation

The alpha helix is stabilized by hydrogen bonds between the carbonyl oxygen of one residue and the amide nitrogen of a residue four positions away along the same backbone. Side chain interactions such as disulfide bonds and hydrophobic contacts contribute to tertiary structure.

Question 20Molecular and Cellular Biology

Which experimental result would best support the conclusion that a transcription factor directly activates a target gene?

  1. The factor is localized to the cytoplasm in dividing cells
  2. The factor is abundant in cells that express the target gene
  3. Chromatin immunoprecipitation shows factor binding at the promoter, and knockdown reduces target mRNA Correct answer.
  4. Overexpression of the factor increases total cellular protein

Explanation

Direct activation requires both physical occupancy of the regulatory region and a functional consequence on transcription. Combining occupancy by chromatin immunoprecipitation with reduced target mRNA after knockdown provides that causal evidence.

Question 21Molecular and Cellular Biology

A mutation in the gene encoding the signal recognition particle receptor would most directly impair which process?

  1. Sorting of proteins into the mitochondrial matrix
  2. Retrograde transport of proteins from the Golgi to the ER
  3. Transcription of ribosomal RNA in the nucleolus
  4. Co-translational translocation of proteins into the endoplasmic reticulum Correct answer.

Explanation

The SRP receptor on the ER membrane docks the SRP-ribosome complex, allowing the nascent chain to enter the translocon. Loss of receptor function blocks this targeting step and impairs co-translational translocation.

Question 22Molecular and Cellular Biology

In a mammalian cell, a nonsense mutation in the X-chromosome gene encoding an essential enzyme is present in a heterozygous female. Why might some cells express the enzyme normally while others do not?

  1. X-chromosome inactivation silences one allele randomly in each cell Correct answer.
  2. The wild-type allele is amplified in cells lacking the enzyme
  3. The mutated allele is selectively degraded only in the liver
  4. The mutation is corrected by somatic recombination in some cells

Explanation

In female mammals, one X chromosome is randomly inactivated in each cell early in development, so heterozygous cells express either the normal or the mutant allele. The mosaic pattern explains why some cells retain enzyme activity.

Question 23Molecular and Cellular Biology

An enzyme follows Michaelis-Menten kinetics. If substrate concentration equals the Km, what fraction of Vmax is achieved?

  1. One tenth
  2. One half Correct answer.
  3. Nine tenths
  4. One quarter

Explanation

The Michaelis-Menten equation reduces to v = Vmax multiplied by [S] divided by Km plus [S]. When substrate concentration equals Km, the ratio becomes one half, so the reaction proceeds at half of Vmax.

Question 24Molecular and Cellular Biology

Which observation would most strongly indicate that a membrane protein is a transmembrane protein rather than a peripheral membrane protein?

  1. The protein is released from membranes by treatment with high salt alone
  2. The protein is found only in the cytosol after cell fractionation
  3. The protein co-sediments with membranes at low ionic strength
  4. The protein remains associated with membranes after high-salt washes but is released by detergent Correct answer.

Explanation

Peripheral proteins associate through ionic and hydrogen bonds and are released by high salt, whereas integral membrane proteins require detergents to disrupt the lipid bilayer. Resistance to salt extraction with detergent-dependent release therefore indicates a transmembrane protein.

Question 25Molecular and Cellular Biology

A dividing cell is treated with a drug that stabilizes microtubules against depolymerization. Which mitotic process is most directly disrupted?

  1. Condensation of chromatin into chromosomes
  2. Formation of the nuclear envelope during telophase
  3. Replication of DNA during S phase
  4. Separation of sister chromatids by spindle shortening Correct answer.

Explanation

Anaphase chromosome movement depends on microtubule dynamics, including depolymerization at the kinetochore and spindle poles. Stabilizing microtubules prevents this shortening and blocks chromosome segregation.

Question 26Molecular and Cellular Biology

Which bond links the 3' hydroxyl group of one nucleotide to the 5' phosphate of the next nucleotide in a growing DNA strand?

  1. Phosphodiester bond Correct answer.
  2. Glycosidic bond
  3. Peptide bond
  4. Hydrogen bond

Explanation

DNA polymerases catalyze formation of a phosphodiester linkage between the 3'-OH of the growing chain and the 5'-phosphate of the incoming deoxynucleotide. This covalent backbone bond is what gives DNA its directional 5'-to-3' polarity. Hydrogen bonds instead hold complementary bases together across the two strands.

Question 27Molecular and Cellular Biology

A cell is shifted to a medium containing only heavy nitrogen (15N) and allowed to complete two rounds of replication. After density-gradient centrifugation of the extracted DNA, what band pattern is expected?

  1. A single heavy band
  2. One hybrid band and one light band
  3. Two hybrid bands
  4. One heavy band and one hybrid band Correct answer.

Explanation

Semiconservative replication means each new duplex keeps one parental strand. After the first round every molecule is hybrid (15N/14N); after the second round, half the molecules contain two 15N strands (heavy) and half retain one 14N strand (hybrid). No fully light DNA remains because the original 14N strands are diluted into hybrid molecules.

Question 28Molecular and Cellular Biology

Which type of RNA carries the amino acid to the ribosome and pairs its anticodon with the mRNA codon?

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

Explanation

Transfer RNA molecules are charged with specific amino acids by aminoacyl-tRNA synthetases and deliver them to the ribosome. The anticodon loop of each tRNA base-pairs with the complementary codon on mRNA, ensuring the correct amino acid is added. Ribosomal RNA is catalytic and structural within the ribosome, and mRNA carries the coding sequence.

Question 29Molecular and Cellular Biology

In eukaryotes, which modification is added to the 5' end of a nascent RNA polymerase II transcript and protects it from exonucleases?

  1. A 7-methylguanosine cap Correct answer.
  2. A ubiquitin chain
  3. An acetyl group
  4. A poly(A) tail

Explanation

Capping adds a 7-methylguanosine residue via an unusual 5'-5' triphosphate linkage shortly after transcription begins. The cap protects the transcript from 5' exonucleases and is later bound by translation initiation factors. Polyadenylation occurs at the 3' end and is a separate processing event.

Question 30Molecular and Cellular Biology

A frameshift mutation inserts a single nucleotide near the start of a coding sequence. Which consequence is most likely?

  1. No change because the code is degenerate
  2. A completely altered downstream amino acid sequence Correct answer.
  3. Premature termination at the same position as a silent mutation
  4. A single amino acid substitution only

Explanation

Inserting one nucleotide shifts the reading frame for every codon downstream of the insertion site. All subsequent amino acids are therefore translated from a different triplet register, usually producing a truncated or nonfunctional protein. Degeneracy buffers against substitutions, not against frame changes.

Question 31Molecular and Cellular Biology

Which organelle is the primary site of lipid and steroid synthesis in a eukaryotic cell?

  1. Rough endoplasmic reticulum
  2. Lysosome
  3. Smooth endoplasmic reticulum Correct answer.
  4. Golgi apparatus

Explanation

The smooth endoplasmic reticulum lacks bound ribosomes and is enriched in enzymes for lipid, phospholipid, and steroid biosynthesis. It also participates in detoxification and calcium storage. The rough ER specializes in synthesis of secretory and membrane proteins.

Question 32Molecular and Cellular Biology

Which cytoskeletal element is the principal driver of cleavage furrow constriction during cytokinesis in animal cells?

  1. Microtubules
  2. Nuclear lamins
  3. Intermediate filaments
  4. Actin filaments Correct answer.

Explanation

A contractile ring of actin filaments and myosin II assembles at the equatorial cortex and constricts to divide the cell. Microtubules position the spindle and signal furrow placement but do not form the constricting ring. Intermediate filaments provide mechanical strength rather than contraction.

Question 33Molecular and Cellular Biology

In the electron transport chain, which complex directly reduces molecular oxygen to water?

  1. Complex III (cytochrome bc1)
  2. Complex I (NADH dehydrogenase)
  3. Complex II (succinate dehydrogenase)
  4. Complex IV (cytochrome c oxidase) Correct answer.

Explanation

Complex IV transfers electrons from cytochrome c to oxygen, the terminal electron acceptor, producing water. This step also contributes to proton pumping across the inner membrane. Complexes I, II, and III pass electrons along the chain but do not reduce oxygen directly.

Question 34Molecular and Cellular Biology

Which statement best describes the chemiosmotic mechanism of ATP synthesis in mitochondria?

  1. Electrons are transferred directly to ADP to form ATP
  2. ATP is synthesized by substrate-level phosphorylation in the matrix
  3. ATP synthase pumps protons into the matrix using ATP hydrolysis
  4. Protons flow down their gradient through ATP synthase, driving rotation and catalysis Correct answer.

Explanation

The electron transport chain creates a proton gradient across the inner mitochondrial membrane. Protons return to the matrix through the F0 portion of ATP synthase, rotating the enzyme and driving conformational changes in F1 that synthesize ATP. The energy source is the electrochemical gradient, not direct electron transfer to ADP.

Question 35Molecular and Cellular Biology

Which molecule serves as the final electron acceptor in the mitochondrial electron transport chain?

  1. FAD
  2. Oxygen Correct answer.
  3. NAD+
  4. Pyruvate

Explanation

Oxygen accepts electrons at Complex IV and is reduced to water, allowing the chain to keep passing electrons. Without oxygen, the chain backs up and NADH cannot be reoxidized. NAD+ and FAD are electron carriers that donate to the chain, not terminal acceptors.

Question 36Molecular and Cellular Biology

In a cell, a drug makes the inner mitochondrial membrane freely permeable to protons. What is the most direct predicted effect?

  1. ATP synthesis falls while electron transport accelerates Correct answer.
  2. ATP synthesis increases because protons reach ATP synthase faster
  3. Glycolysis stops immediately because glucose cannot enter
  4. The citric acid cycle accelerates because NADH accumulates

Explanation

Uncoupling agents dissipate the proton gradient, so the proton-motive force driving ATP synthase is lost. Electron transport accelerates because the back-pressure of the gradient is relieved, but energy is released as heat rather than captured as ATP. This is the classic uncoupling effect.

Question 37Molecular and Cellular Biology

Which feature of the genetic code allows a single tRNA species to recognize more than one codon?

  1. Wobble base pairing at the third codon position Correct answer.
  2. Proofreading by DNA polymerase
  3. Ribosomal translocation
  4. Alternative splicing of the mRNA

Explanation

The third base of many codons can pair loosely with the first base of the anticodon, a phenomenon called wobble. This flexibility lets fewer tRNA species cover all 61 sense codons. Proofreading, splicing, and translocation are unrelated to codon-anticodon recognition.

Question 38Molecular and Cellular Biology

Which level of protein structure is defined by the three-dimensional arrangement of a single polypeptide chain, including its domains?

  1. Primary structure
  2. Secondary structure
  3. Quaternary structure
  4. Tertiary structure Correct answer.

Explanation

Tertiary structure describes the overall three-dimensional fold of one polypeptide, stabilized by hydrophobic interactions, hydrogen bonds, ionic bonds, and disulfide bridges. Primary structure is the amino acid sequence, secondary structure comprises local motifs such as helices and sheets, and quaternary structure involves multiple subunits.

Question 39Molecular and Cellular Biology

A missense mutation replaces a charged surface residue with a hydrophobic residue in a folded cytosolic protein. What is the most likely structural consequence?

  1. Conversion of the protein into a nucleic acid
  2. No effect because only the primary sequence is altered
  3. Enhanced solubility because hydrophobic residues repel water
  4. Misfolding and aggregation due to burial of an unfavorable hydrophobic group Correct answer.

Explanation

Charged residues on the protein surface usually interact with water, while hydrophobic residues are buried in the core. Placing a hydrophobic side chain at the surface exposes it to water, promoting misfolding, aggregation, or degradation by quality-control machinery. A single substitution can therefore destabilize the fold.

Question 40Molecular and Cellular Biology

Which protein complex mediates the recognition and degradation of ubiquitin-tagged proteins in the cytosol?

  1. The nuclear pore complex
  2. The spliceosome
  3. The 26S proteasome Correct answer.
  4. The ribosome

Explanation

The 26S proteasome recognizes polyubiquitin chains and translocates the tagged protein into its proteolytic core, where it is degraded into short peptides. This pathway is the main route for selective cytosolic protein turnover. The ribosome synthesizes proteins and the spliceosome processes RNA.

Question 41Molecular and Cellular Biology

Which phase of the cell cycle is characterized by DNA replication?

  1. M phase
  2. G2 phase
  3. G1 phase
  4. S phase Correct answer.

Explanation

S phase is the synthesis phase, during which the genome is duplicated so each daughter cell receives a complete copy. G1 and G2 are gap phases for growth and checkpoint control, and M phase is mitosis and cytokinesis. DNA content doubles specifically in S phase.

Question 42Molecular and Cellular Biology

A cell lacking functional p53 protein is exposed to ionizing radiation that damages DNA. What is the most likely outcome?

  1. DNA damage will be repaired faster because p53 normally inhibits repair
  2. The cell may proceed through the cell cycle despite unrepaired damage Correct answer.
  3. The cell will immediately undergo apoptosis in every case
  4. The cell will arrest permanently in G1 regardless of damage

Explanation

p53 acts as a checkpoint transcription factor that induces cell-cycle arrest or apoptosis when DNA is damaged. Without functional p53, the G1/S checkpoint is compromised, allowing replication of damaged DNA and accumulation of mutations. Loss of p53 is common in many cancers.

Question 43Molecular and Cellular Biology

Which process converts a primary transcript into a mature mRNA by removing intervening sequences?

  1. Polyadenylation
  2. Splicing Correct answer.
  3. Translation
  4. Capping

Explanation

Splicing removes introns and joins exons, and it is carried out by the spliceosome in the nucleus of eukaryotic cells. Capping and polyadenylation are separate processing steps that modify the ends of the transcript. Translation uses the mature mRNA to synthesize protein in the cytoplasm.

Question 44Molecular and Cellular Biology

A bacterial operon is controlled by a repressor that binds the operator only when bound by a small molecule. This is an example of which type of regulation?

  1. Negative inducible control Correct answer.
  2. Positive repressible control
  3. Negative repressible control
  4. Positive inducible control

Explanation

The repressor inhibits transcription when bound to the operator, making regulation negative. Because the small molecule prevents repressor binding and thus turns expression on, the system is inducible. The classic example is the lactose operon, where allolactose relieves repression.

Question 45Molecular and Cellular Biology

Which chromatin modification is generally associated with transcriptional activation?

  1. Histone deacetylation by HDACs
  2. DNA methylation at CpG islands
  3. Acetylation of histone lysine residues Correct answer.
  4. Trimethylation of histone H3 lysine 9

Explanation

Histone acetyltransferases add acetyl groups to lysine residues, neutralizing positive charge and loosening chromatin for transcription. H3K9 trimethylation and CpG methylation are typically associated with repression. HDACs remove acetyl groups and promote compaction.

Question 46Molecular and Cellular Biology

Which feature of the plasma membrane is best described by the fluid mosaic model?

  1. Membrane proteins are immobile and fixed in place
  2. Proteins and lipids diffuse laterally within a bilayer Correct answer.
  3. The bilayer is composed entirely of phospholipids with no proteins
  4. The membrane is a rigid crystalline sheet

Explanation

The fluid mosaic model describes a fluid phospholipid bilayer in which proteins are embedded and can move laterally. This dynamic organization underlies processes such as receptor clustering and membrane fusion. The membrane is not a static or protein-free structure.

Question 47Molecular and Cellular Biology

A red blood cell is placed in a solution that is hypertonic relative to its cytoplasm. What is the expected osmotic response?

  1. Solute diffuses out until the cell bursts
  2. No net water movement occurs
  3. Water enters the cell and it swells
  4. Water leaves the cell and it shrinks Correct answer.

Explanation

In a hypertonic solution the external solute concentration is higher, so water flows out of the cell by osmosis. The cell loses volume and shrinks, a process called crenation in red blood cells. Water moves toward the region of higher solute concentration.

Question 48Molecular and Cellular Biology

Which transport mechanism moves solutes against their concentration gradient using energy from ATP?

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

Explanation

Primary active transport directly couples ATP hydrolysis to the movement of ions or molecules against their electrochemical gradient, as the Na+/K+ ATPase does. Facilitated diffusion and simple diffusion are passive and proceed down gradients. Osmosis specifically refers to water movement.

Question 49Molecular and Cellular Biology

In a signaling pathway, a ligand binds a G protein-coupled receptor and triggers exchange of GDP for GTP on the alpha subunit. What happens next?

  1. The receptor is immediately degraded by the proteasome
  2. The alpha subunit hydrolyzes GTP before dissociating
  3. The GTP-bound alpha subunit dissociates from the beta-gamma complex and modulates effectors Correct answer.
  4. The beta-gamma complex enters the nucleus to bind DNA directly

Explanation

Activation of a GPCR catalyzes GDP/GTP exchange on G-alpha, and the GTP-bound alpha subunit separates from beta-gamma. Both the alpha subunit and the beta-gamma complex can then regulate downstream effectors such as adenylyl cyclase. GTP hydrolysis occurs later and terminates signaling.

Question 50Molecular and Cellular Biology

Which second messenger is produced by adenylyl cyclase and activates protein kinase A?

  1. Cyclic AMP Correct answer.
  2. Diacylglycerol
  3. Inositol trisphosphate
  4. Calcium ions

Explanation

Adenylyl cyclase converts ATP into cyclic AMP, which binds the regulatory subunits of protein kinase A and releases its active catalytic subunits. Diacylglycerol and IP3 are generated by phospholipase C from PIP2 and act in a different branch of signaling. Calcium is an ion, not the product of adenylyl cyclase.