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Castilla y León PAU Biology study materials
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Castilla y León PAU Biology practice questions

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Question 1Biological inquiry and interpretation of evidence

A researcher predicts that tomato plants given a nitrogen-enriched fertilizer will produce more fruit than plants given a standard fertilizer. In this study, the amount of fruit produced is best described as which of the following?

  1. The hypothesis
  2. A controlled variable
  3. The dependent variable Correct answer.
  4. The independent variable

Explanation

The dependent variable is the measured outcome that responds to the manipulation, which here is fruit yield. The fertilizer type is the factor the researcher deliberately changes, so it is the independent variable rather than the measured result.

Question 2Biological inquiry and interpretation of evidence

A student measures the mass of the same sample five times and obtains nearly identical values, but the balance was never zeroed and reads 2.0 g too high. How should these measurements be described?

  1. Both accurate and precise
  2. Precise but not accurate Correct answer.
  3. Accurate but not precise
  4. Neither accurate nor precise

Explanation

Precision refers to how closely repeated measurements agree with one another, while accuracy refers to how close they are to the true value. Because the readings cluster tightly but are systematically offset by the unzeroed balance, they are precise but inaccurate.

Question 3Biological inquiry and interpretation of evidence

Which of the following best distinguishes a scientific hypothesis from a scientific theory?

  1. A hypothesis is a testable proposed explanation, while a theory is a well-supported explanatory framework built from extensive evidence Correct answer.
  2. A hypothesis is always correct once tested, while a theory remains an untested guess
  3. A hypothesis is a broad framework, while a theory is a single testable prediction
  4. A hypothesis applies only to biology, while a theory applies only to physics

Explanation

In science, a hypothesis is a testable proposed explanation for a specific observation, whereas a theory is a broad, well-substantiated explanation supported by many lines of evidence. The common usage of theory as a mere guess is incorrect in scientific contexts.

Question 4Biological inquiry and interpretation of evidence

A scientist repeats an experiment several times and obtains consistent results. Which conclusion is most justified?

  1. The findings are reliable and reproducible under the tested conditions Correct answer.
  2. The results now apply to all organisms everywhere
  3. The hypothesis has been proven true beyond any doubt
  4. The experiment has no remaining sources of error

Explanation

Reproducible results indicate reliability, meaning the procedure yields consistent outcomes under the tested conditions. Science does not establish absolute proof, and replication does not eliminate error or guarantee universal applicability.

Question 5Biological inquiry and interpretation of evidence

A graph plots plant height on the y-axis against days of growth on the x-axis. Which statement correctly identifies the placement of these variables?

  1. Days of growth is the independent variable on the x-axis, and height is the dependent variable on the y-axis Correct answer.
  2. Both variables are dependent and should share the x-axis
  3. Days of growth is the dependent variable because it is measured over time
  4. Height is the independent variable on the y-axis, and days of growth is the dependent variable on the x-axis

Explanation

By convention, the independent variable is placed on the x-axis and the dependent variable on the y-axis. Height responds to elapsed time, so time is independent and height is dependent.

Question 6Biological inquiry and interpretation of evidence

A student notices that mold grows on bread left in a warm, humid place but not on bread stored in a dry, cool place. Which of the following is the most appropriate next step in the scientific process?

  1. Formulate a testable hypothesis and design a controlled experiment to test it Correct answer.
  2. Conclude immediately that warmth and moisture cause mold growth
  3. Publish the observation as a confirmed scientific law
  4. Discard the observation because it was not made in a laboratory

Explanation

An observation generates questions that should be framed as a testable hypothesis and examined through a controlled experiment. Reaching a firm causal conclusion from a single uncontrolled observation is premature.

Question 7Biochemistry

Which property of water is most directly responsible for its ability to moderate temperature in living organisms and large bodies of water?

  1. Its low specific heat
  2. Its inability to form hydrogen bonds
  3. Its nonpolar covalent bonds
  4. Its high specific heat Correct answer.

Explanation

Water has a high specific heat because hydrogen bonds must absorb or release substantial energy before water molecules change temperature. This allows water and water-rich organisms to resist large temperature swings, helping maintain homeostasis.

Question 8Cell structure, membranes and division

Which structure is found in a typical plant cell but NOT in a typical animal cell?

  1. Mitochondrion
  2. Nucleus
  3. Ribosome
  4. Cell wall made of cellulose Correct answer.

Explanation

Plant cells have a rigid cell wall composed primarily of cellulose that surrounds the plasma membrane, providing structural support. Animal cells lack a cell wall, relying on the cytoskeleton and extracellular matrix instead.

Question 9Genetics, inheritance and molecular information

Meiosis differs from mitosis in that meiosis produces

  1. Two genetically identical diploid cells
  2. One diploid cell with double the chromosomes
  3. Two haploid cells identical to the parent
  4. Four genetically varied haploid cells Correct answer.

Explanation

Meiosis consists of two divisions that produce four haploid gametes, each with half the chromosome number of the parent. Crossing over and independent assortment generate genetic variation, distinguishing meiosis from the identical-diploid products of mitosis.

Question 10Metabolism

An enzyme lowers the activation energy of a reaction. Which statement best describes how this affects the reaction?

  1. It increases the reaction rate without being consumed in the reaction Correct answer.
  2. It changes the overall free-energy difference between reactants and products
  3. It supplies the energy needed to make an endergonic reaction exergonic
  4. It permanently binds to the product to stop the reaction

Explanation

Enzymes are biological catalysts that lower activation energy, speeding up reactions without being consumed. They do not change the net free-energy difference (delta G) between reactants and products; they only make the reaction reach equilibrium faster.

Question 11Microbiology Immunology

Gram-positive bacteria differ structurally from Gram-negative bacteria in their cell wall composition. Which characteristic feature distinguishes Gram-positive bacterial cell walls upon Gram staining?

  1. Thick multi-layered peptidoglycan meshwork retaining crystal violet-iodine complex (purple) Correct answer.
  2. Thin single layer of peptidoglycan enclosed by an outer membrane containing lipopolysaccharide (LPS)
  3. Absence of peptidoglycan, composed entirely of chitin
  4. Lipid envelope derived from host plasma membrane

Explanation

Gram-positive bacteria possess a thick, multi-layered peptidoglycan (murein) cell wall rich in teichoic and lipoteichoic acids. This dense structure dehydrates during alcohol decolorization, trapping the crystal violet-iodine complex and staining purple. Gram-negative cell walls have thin peptidoglycan and an outer LPS membrane (staining pink with safranin).

Question 12Biotechnology

The polymerase chain reaction, or PCR, is mainly used to do what?

  1. Amplify a selected DNA sequence Correct answer.
  2. Destroy all proteins in a sample
  3. Make plant roots bend downward
  4. Measure animal breathing rate directly

Explanation

PCR uses primers, DNA polymerase, nucleotides, and temperature cycling to make many copies of a target DNA region. It is widely used in testing and research.

Question 13Biochemistry

Which physical property of water allows it to absorb or release large quantities of thermal energy with minimal change in its own temperature, buffering biological organisms against environmental thermal fluctuations?

  1. High specific heat capacity Correct answer.
  2. High surface tension
  3. Low latent heat of vaporization
  4. Maximum density at 0 °C

Explanation

Water possesses a high specific heat capacity (4.184 J/g·°C) due to its extensive intermolecular hydrogen-bonding network. Heat energy supplied to water is predominantly consumed in breaking hydrogen bonds before increasing the kinetic energy of individual water molecules, thus moderating internal body temperatures and aquatic habitats.

Question 14Biochemistry

When a red blood cell (erythrocyte) is placed in a hypertonic NaCl solution, water rapidly flows out of the cell across the plasma membrane. What is this cellular response termed in animal cell physiology?

  1. Crenation Correct answer.
  2. Plasmolysis
  3. Hemolysis
  4. Turgor

Explanation

In animal cells lacking a cell wall, placement in a hypertonic environment causes osmotic loss of intracellular water, leading to cell shrinkage and a spiky surface appearance known as crenation. In plant cells with rigid cell walls, the equivalent protoplast detachment is termed plasmolysis.

Question 15Biochemistry

Which of the following monosaccharides is classified structural-chemically as a ketohexose?

  1. D-Fructose Correct answer.
  2. D-Glucose
  3. D-Galactose
  4. D-Ribose

Explanation

D-Fructose is a 6-carbon monosaccharide (hexose) containing a ketone functional group at carbon 2, classifying it as a ketohexose. D-Glucose and D-Galactose are aldohexoses, whereas D-Ribose is an aldopentose.

Question 16Biochemistry

Maltose is a disaccharide produced by starch hydrolysis. Which glycosidic linkage connects the two glucose units in maltose?

  1. alpha(1 -> 4) glycosidic bond Correct answer.
  2. beta(1 -> 4) glycosidic bond
  3. alpha(1 -> 2) glycosidic bond
  4. alpha(1 -> 6) glycosidic bond

Explanation

Maltose consists of two D-glucopyranose units linked by an alpha(1 -> 4) glycosidic bond formed via a condensation reaction between the anomeric hydroxyl of alpha-D-glucose (C1) and the C4 hydroxyl of another glucose molecule.

Question 17Biochemistry

Cellulose serves as a structural component in plant cell walls, whereas starch serves as energy storage. What structural feature accounts for the mechanical rigidity and resistance of cellulose to human digestive enzymes?

  1. Linear unbranched chains of D-glucose linked by beta(1 -> 4) glycosidic bonds forming microfibrils Correct answer.
  2. Branched polymer chains of D-glucose linked by alpha(1 -> 4) and alpha(1 -> 6) bonds
  3. Coiled helical chains of D-glucose linked exclusively by alpha(1 -> 4) bonds
  4. Heteropolysaccharide composition containing alternating units of N-acetylglucosamine

Explanation

Cellulose is composed of unbranched linear chains of D-glucose connected by beta(1 -> 4) glycosidic linkages. This geometry allows parallel glucose chains to form extensive interchain hydrogen bonds, creating insoluble microfibrils with high tensile strength that human alpha-amylases cannot hydrolyze.

Question 18Biochemistry

Which of the following lipid classes is non-saponifiable because it lacks fatty acid components connected by ester linkages?

  1. Steroids (e.g., cholesterol) Correct answer.
  2. Triacylglycerols (triglycerides)
  3. Glycerophospholipids (phosphatidylcholine)
  4. Sphingolipids (sphingomyelin)

Explanation

Steroids are non-saponifiable lipids derived from a tetracyclic cyclopentanoperhydrophenanthrene ring structure. Because they contain no esterified fatty acids, they do not undergo alkaline hydrolysis (saponification) to yield soap (fatty acid salts).

Question 19Biochemistry

Phospholipids self-assemble into lipid bilayers when introduced into an aqueous environment. What molecular property drives this spontaneous formation?

  1. Amphipathic structure with hydrophilic polar heads and hydrophobic non-polar fatty acid tails Correct answer.
  2. Complete hydrophobicity across all regions of the hydrocarbon skeleton
  3. High solubility in polar solvents mediated by ionic carboxylate groups
  4. Covalent cross-linking between adjacent fatty acid tails

Explanation

Phospholipids are amphipathic molecules containing a polar hydrophilic head (phosphate and attached group) and two non-polar hydrophobic tail regions (fatty acid chains). In water, hydrophobic interactions drive the non-polar tails inward away from water while hydrophilic heads face the aqueous environment, yielding a stable bilayer.

Question 20Biochemistry

Oleic acid is a monounsaturated 18-carbon fatty acid with a cis double bond at carbon 9. How does the presence of this cis double bond affect its physical melting point compared to stearic acid (saturated 18C)?

  1. It lowers the melting point by creating a kink in the hydrocarbon chain that disrupts intermolecular packing Correct answer.
  2. It increases the melting point by enhancing van der Waals packing density
  3. It elevates the melting point due to increased molecular weight
  4. It has no effect on physical melting temperature

Explanation

A cis double bond introduces a rigid bend (kink) in the fatty acid hydrocarbon chain. This prevents tight, parallel van der Waals packing among acyl chains, lowering the thermal energy required to disrupt the lattice and resulting in a lower melting point (liquid at room temperature).

Question 21Biochemistry

At physiological pH (~7.4), a standard amino acid lacking an ionizable side chain exists predominantly in which ionic state?

  1. Zwitterion (dipolar ion with protonated -NH3+ and deprotonated -COO-) Correct answer.
  2. Fully protonated cation (+1 charge)
  3. Fully deprotonated anion (-1 charge)
  4. Uncharged neutral non-polar molecule

Explanation

At physiological pH (~7.4), which is between the carboxyl pKa (~2.0) and amino pKa (~9.5), the alpha-carboxyl group loses a proton (-COO-) while the alpha-amino group retains a proton (-NH3+). The amino acid exists as a zwitterion with a net electrical charge of zero.

Question 22Biochemistry

Which of the following characteristics accurately describes the chemical nature of the peptide bond joining adjacent amino acid residues in proteins?

  1. Planar amide linkage with partial double-bond character that restricts free rotation around the C-N bond Correct answer.
  2. Flexible single covalent bond allowing unrestricted 360-degree rotation
  3. Weak non-covalent hydrogen bond easily broken by mild temperature increases
  4. Ionic interaction formed between positively and negatively charged side chains

Explanation

The peptide bond (-CO-NH-) is a covalent amide linkage formed by condensation. Resonance delocalization of electron pairs between the carbonyl oxygen and amide nitrogen imparts a ~40% double-bond character to the C-N bond, freezing the six backbone atoms in a rigid planar trans conformation.

Question 23Biochemistry

Secondary structure motifs in proteins, such as the alpha-helix and beta-pleated sheet, are stabilized primarily by which type of chemical bond?

  1. Hydrogen bonds between backbone carbonyl oxygens (-C=O) and amide nitrogens (-N-H) Correct answer.
  2. Disulfide bridges formed between cysteine sulfhydryl groups (-SH)
  3. Hydrophobic interactions among non-polar side chains
  4. Covalent peptide bonds linking amino acid alpha-carbons

Explanation

Protein secondary structure consists of localized folding patterns stabilized by hydrogen bonds between the electronegative carbonyl oxygen (-C=O) of one peptide bond and the amide hydrogen (-N-H) of another along the polypeptide backbone.

Question 24Biochemistry

When a protein undergoes thermal denaturation, it loses its biological activity. Which level of protein structural organization remains intact following denaturation?

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

Explanation

Denaturation disrupts non-covalent interactions (hydrogen bonds, hydrophobic interactions, ionic bonds) and disulfide links that maintain 2°, 3°, and 4° structures. The covalent peptide bonds forming the primary sequence remain unbroken.

Question 25Biochemistry

How do biological enzymes accelerate the rate of metabolic chemical reactions?

  1. By decreasing the activation energy (Ea) required to reach the transition state Correct answer.
  2. By shifting the chemical equilibrium constant (Keq) toward product formation
  3. By increasing the net free energy change (delta G) of the reaction
  4. By supplying external thermal energy to reactant molecules

Explanation

Enzymes act as biological catalysts by stabilizing the transition state of a reaction, thereby lowering the activation energy barrier (Ea). They accelerate both forward and reverse reaction rates without altering the thermodynamic equilibrium constant (Keq) or overall free energy change (delta G).

Question 26Biochemistry

In Michaelis-Menten enzyme kinetics, what does the constant Km represent?

  1. Substrate concentration at which the reaction velocity reaches half of Vmax Correct answer.
  2. Maximum reaction velocity achievable at saturating substrate concentration
  3. Number of substrate molecules converted per second per enzyme active site
  4. Equilibrium dissociation constant of the enzyme-product complex

Explanation

The Michaelis constant (Km) is defined as the specific substrate concentration [S] at which the initial reaction velocity (v0) is equal to half of the maximum velocity (1/2 Vmax). A low Km value indicates high substrate affinity for the enzyme active site.

Question 27Biochemistry

A competitive enzyme inhibitor binds reversibly to the active site of an enzyme. How does a competitive inhibitor alter the kinetic parameters Vmax and Km?

  1. Increases apparent Km while leaving Vmax unchanged Correct answer.
  2. Decreases Vmax while leaving apparent Km unchanged
  3. Decreases both Vmax and apparent Km proportionally
  4. Increases both Vmax and apparent Km

Explanation

A competitive inhibitor competes directly with substrate for binding at the catalytic active site. High substrate concentrations outcompete the inhibitor, allowing the enzyme to achieve its original maximum velocity (Vmax unchanged), but requiring a higher substrate concentration to reach half-maximal speed (apparent Km increases).

Question 28Biochemistry

Which set of molecular features distinguishes RNA from DNA?

  1. Ribose pentose sugar, uracil pyrimidine base, and single-stranded structure Correct answer.
  2. Deoxyribose pentose sugar, thymine pyrimidine base, and double-stranded helix
  3. Ribose sugar, thymine base, and double-stranded helix
  4. Deoxyribose sugar, uracil base, and single-stranded structure

Explanation

RNA contains ribose (with a 2'-OH group), the pyrimidine base uracil (instead of thymine), and typically exists as single-stranded polynucleotide chains. DNA contains 2'-deoxyribose, thymine, and forms double-stranded helices.

Question 29Biochemistry

In double-stranded B-DNA, hydrogen bonding occurs between complementary nitrogenous bases across antiparallel strands. How many hydrogen bonds join a Cytosine-Guanine (C-G) base pair?

  1. 3 hydrogen bonds Correct answer.
  2. 2 hydrogen bonds
  3. 1 hydrogen bond
  4. 4 hydrogen bonds

Explanation

Cytosine pairs with Guanine via 3 hydrogen bonds, whereas Adenine pairs with Thymine via 2 hydrogen bonds. Consequently, DNA regions rich in C-G pairs display higher thermodynamic stability and higher denaturation melting temperatures (Tm).

Question 30Biochemistry

Nucleotides within a single polynucleotide strand of DNA or RNA are joined sequentially by which type of covalent linkage?

  1. 3',5'-phosphodiester bond Correct answer.
  2. N-glycosidic bond
  3. Peptide bond
  4. 2',3'-cyclic ester bond

Explanation

Nucleotides are linked covalently along a single strand by 3',5'-phosphodiester bonds. A phosphate group forms ester linkages connecting the 3'-hydroxyl group of one sugar ring to the 5'-hydroxyl group of the adjacent sugar.

Question 31Biochemistry

Which extracellular buffer system is primarily responsible for maintaining human blood plasma pH within the physiological range of 7.35 to 7.45?

  1. Carbonic acid / Bicarbonate buffer system (H2CO3 / HCO3-) Correct answer.
  2. Dihydrogen phosphate / Monohydrogen phosphate system (H2PO4- / HPO42-)
  3. Acetate / Acetic acid buffer system
  4. Ammonia / Ammonium ion buffer system

Explanation

The carbonic acid/bicarbonate buffer system regulates blood plasma pH. CO2 produced by tissues reacts with water via carbonic anhydrase to yield H2CO3, which dissociates into H+ and HCO3-. Respiratory ventilation and renal excretion adjust CO2 and HCO3- levels to buffer systemic pH.

Question 32Biochemistry

Inorganic mineral ions perform critical physiological roles in biological systems. Which ion acts as an essential cofactor for ATP hydrolysis reactions and stabilizes nucleic acid duplexes?

  1. Magnesium ion (Mg2+) Correct answer.
  2. Ferrous ion (Fe2+)
  3. Iodide ion (I-)
  4. Sodium ion (Na+)

Explanation

Magnesium ions (Mg2+) form coordinate complexes with ATP (Mg-ATP2-), which is the active substrate form required by kinases and ATPases. Mg2+ also neutralizes negative charges on DNA/RNA phosphate backbones, stabilizing secondary structures.

Question 33Biochemistry

Conjugated proteins (heteroproteins) consist of a simple polypeptide chain combined with a non-protein prosthetic group. Hemoglobin is an example of which conjugated protein class?

  1. Chromoprotein (Metalloprotein) Correct answer.
  2. Glycoprotein
  3. Lipoprotein
  4. Phosphoprotein

Explanation

Hemoglobin is a chromoprotein (and metalloprotein) containing globin polypeptide chains bound to iron-porphyrin prosthetic groups (heme), which impart color and bind oxygen reversibly.

Question 34Biochemistry

Fehling's test is used in biochemistry laboratories to detect reducing sugars. Which structural feature enables a monosaccharide or disaccharide to reduce Fehling's reagent (Cu2+ to Cu+ red precipitate)?

  1. Free hemiacetal or hemiketal group capable of opening into an active aldehyde or ketone Correct answer.
  2. Presence of a glycosidic bond between two anomeric carbon atoms
  3. Esterification of all hydroxyl groups by phosphate
  4. Intramolecular disulfide cross-linking

Explanation

Reducing sugars contain a free anomeric carbon atom (hemiacetal or hemiketal) that can open into an open-chain form with a free aldehyde or alpha-hydroxy-ketone group, reducing Cu2+ (blue) to Cu2O (red precipitate). In non-reducing sugars like sucrose, both anomeric carbons are locked in a glycosidic bond.

Question 35Biochemistry

Which class of bioactive lipid signaling molecules is synthesized from arachidonic acid (a 20-carbon polyunsaturated fatty acid) and mediates inflammatory responses, smooth muscle contraction, and blood clotting?

  1. Eicosanoids (e.g., prostaglandins and leukotrienes) Correct answer.
  2. Sphingolipids (e.g., ceramides)
  3. Terpenes (e.g., carotenoids)
  4. Steroids (e.g., aldosterone)

Explanation

Eicosanoids are 20-carbon signaling lipids synthesized from membrane-derived arachidonic acid via cyclooxygenase (COX) or lipoxygenase pathways. They include prostaglandins, thromboxanes, and leukotrienes, which act as short-lived autocrine/paracrine mediators of inflammation and pain.

Question 36Biochemistry

Allosteric enzymes exhibit non-Michaelis-Menten kinetics characterized by a sigmoidal velocity curve versus substrate concentration. What molecular mechanism produces this sigmoidal behavior?

  1. Cooperative conformational changes among multiple interacting catalytic subunits Correct answer.
  2. Covalent irreversible inactivation of active sites by heavy metal ions
  3. Simple competitive inhibition by reaction products
  4. Linear non-saturable passive binding

Explanation

Allosteric enzymes are multi-subunit proteins. Binding of substrate to one active site induces a conformational shift in adjacent subunits (cooperativity), increasing their binding affinity and producing a sigmoidal (S-shaped) saturation curve.

Question 37Biochemistry

Transfer RNA (tRNA) molecules adapt mRNA genetic codons into amino acid sequences. Where is the specific amino acid covalently attached on the tRNA cloverleaf structure?

  1. To the 3'-OH group of the conserved 5'-CCA-3' terminal sequence Correct answer.
  2. To the anticodon loop central triplet base
  3. To the modified D-loop dihydrouridine residue
  4. To the 5'-monophosphate terminus

Explanation

Aminoacyl-tRNA synthetases esterify a specific amino acid to the 3'-hydroxyl group of the adenosine residue at the invariant 3'-CCA terminus of the tRNA acceptor stem.

Question 38Cell structure, membranes and division

Which of the following cellular structures is present in eukaryotic cells but absent in prokaryotic bacterial cells?

  1. Membrane-bound nucleus enclosed by a double-membrane envelope Correct answer.
  2. 70S cytoplasmic ribosomes
  3. Circular double-stranded genomic DNA in a nucleoid region
  4. Plasma membrane composed of phospholipid bilayer

Explanation

Eukaryotic cells are defined by subcellular compartmentalization, including a membrane-bound nucleus containing genomic chromatin. Prokaryotes lack internal membrane-enclosed organelles and possess a non-enclosed nucleoid region.

Question 39Cell structure, membranes and division

According to the Fluid Mosaic Model (Singer & Nicolson) of cell membranes, how does cholesterol influence plasma membrane fluidity in mammalian animal cells at physiological body temperatures (~37 °C)?

  1. It decreases membrane fluidity by restricting excessive movement of phospholipid fatty acid chains Correct answer.
  2. It increases membrane fluidity by breaking all covalent glycerol bonds
  3. It completely freezes membrane lipids into a solid crystalline gel phase
  4. It dissolves all integral membrane transport proteins

Explanation

Cholesterol acts as a membrane fluidity buffer. At physiological warm temperatures (~37 °C), its rigid steroid ring interacts with phospholipid fatty acyl chains, restricting their lateral movement and reducing excessive membrane fluidity. At cool temperatures, it prevents tight packing, maintaining fluidity.

Question 40Cell structure, membranes and division

Which characteristic distinguishes facilitated diffusion from simple passive diffusion across a biological cell membrane?

  1. Facilitated diffusion exhibits transport saturation kinetics (Vmax) because it depends on specific membrane channel or carrier proteins Correct answer.
  2. Facilitated diffusion moves solute particles against their electrochemical concentration gradient
  3. Facilitated diffusion requires direct metabolic ATP hydrolysis
  4. Facilitated diffusion occurs exclusively in prokaryotic cells

Explanation

Both simple and facilitated diffusion are passive transport processes driven down concentration gradients without metabolic energy input. However, facilitated diffusion relies on protein transporters (channels or carriers), exhibiting substrate specificity and transport saturation at high solute concentrations (Vmax).

Question 41Cell structure, membranes and division

The plasma membrane Na+/K+-ATPase pump is an electrogenic primary active transporter. For each molecule of ATP hydrolyzed, what is the net stoichiometry and direction of ion transport?

  1. 3 Na+ ions pumped out of the cell and 2 K+ ions pumped into the cell Correct answer.
  2. 2 Na+ ions pumped out of the cell and 3 K+ ions pumped into the cell
  3. 3 Na+ ions pumped into the cell and 3 K+ ions pumped out of the cell
  4. 1 Na+ ion and 1 K+ ion co-transported out of the cell

Explanation

The Na+/K+-ATPase uses energy from ATP hydrolysis to pump 3 Na+ ions out of the cytoplasm into extracellular fluid and 2 K+ ions into the cytoplasm. This asymmetrical transport maintains resting membrane potential and intracellular osmotic volume.

Question 42Cell structure, membranes and division

Receptor-mediated endocytosis allows cells to internalize specific extracellular macromolecules (such as LDL particles). Which cytosolic coat protein polymerizes on the inner plasma membrane to form coated pits during this process?

  1. Clathrin Correct answer.
  2. Kinesin
  3. Actin
  4. Tubulin

Explanation

Receptor-mediated endocytosis relies on clathrin molecules assembling into a triskelion lattice structure on the cytosolic face of the membrane, forming a coated pit that invaginates and pinches off into a clathrin-coated vesicle.

Question 43Cell structure, membranes and division

Which organelle function is primarily associated with the smooth endoplasmic reticulum (SER) in eukaryotic cells?

  1. Synthesis of phospholipids, steroids, and detoxifying lipophilic drugs Correct answer.
  2. Synthesis of secretory proteins and membrane-bound enzymes on attached 80S ribosomes
  3. Synthesis of ATP via oxidative phosphorylation
  4. Modification and packaging of lysosomal enzymes into secretory vesicles

Explanation

The smooth endoplasmic reticulum (SER) lacks surface ribosomes and functions in lipid biosynthesis (phospholipids, cholesterol, steroid hormones), carbohydrate metabolism, drug/toxin detoxification (cytochrome P450 system), and sequestering intracellular Ca2+ ions.

Question 44Cell structure, membranes and division

Newly synthesized proteins destined for secretion exit the rough ER in transport vesicles and enter the Golgi apparatus. At which face of the Golgi complex do these incoming transport vesicles fuse?

  1. Cis face (forming face) Correct answer.
  2. Trans face (maturing face)
  3. Medial cisternae
  4. Trans-Golgi network (TGN) exit portal

Explanation

The Golgi apparatus possesses structural polarity. Vesicles budding from the rough ER arrive at and fuse with the convex cis face (entry/forming face). Proteins progress through medial cisternae for post-translational modification and exit at the concave trans face.

Question 45Cell structure, membranes and division

Lysosomes maintain an acidic internal lumen (pH ~4.5 to 5.0) optimal for acid hydrolases. How is this low luminal pH actively maintained?

  1. Vacuolar V-type H+-ATPase proton pumps in the lysosomal membrane actively transport H+ into the lumen Correct answer.
  2. Passive sodium-potassium exchange channels release hydroxyl ions
  3. Passive diffusion of carbonic acid across membrane pores
  4. Direct uptake of lactic acid from the cytosol

Explanation

Lysosomal membranes contain vacuolar-type H+-ATPases (V-ATPase) that hydrolyze ATP to actively pump protons (H+) from the neutral cytosol (pH 7.2) into the lysosomal lumen, establishing an acidic environment (pH ~4.8) required for enzymatic hydrolysis.

Question 46Cell structure, membranes and division

Peroxisomes generate toxic hydrogen peroxide (H2O2) during metabolic beta-oxidation of long-chain fatty acids. Which enzyme inside peroxisomes decomposes H2O2 into harmless water and oxygen?

  1. Catalase Correct answer.
  2. Superoxide dismutase
  3. Peroxidase
  4. Amylase

Explanation

Peroxisomes contain high concentrations of catalase, a heme-containing enzyme that rapidly converts toxic hydrogen peroxide (2 H2O2) produced by oxidases into water (2 H2O) and molecular oxygen (O2).

Question 47Cell structure, membranes and division

Where are the enzyme complexes of the electron transport chain and ATP synthase localized within eukaryotic mitochondria?

  1. Inner mitochondrial membrane (cristae) Correct answer.
  2. Mitochondrial matrix
  3. Outer mitochondrial membrane
  4. Intermembrane space

Explanation

The inner mitochondrial membrane is highly folded into cristae to maximize surface area. It houses respiratory Complexes I-IV, electron carriers (ubiquinone, cytochrome c), and F0F1-ATP synthase complexes.

Question 48Cell structure, membranes and division

In plant chloroplasts, where do the light-dependent reactions of photosynthesis take place?

  1. Thylakoid membranes Correct answer.
  2. Stroma
  3. Outer chloroplast membrane
  4. Intermembrane space

Explanation

Light-dependent reactions occur in the thylakoid membranes (organized in stacks called grana), where photosystems I and II, electron transport chains, and ATP synthase are embedded. The light-independent Calvin cycle occurs in the fluid stroma.

Question 49Cell structure, membranes and division

Which piece of scientific evidence strongly supports Lynn Margulis's Endosymbiotic Theory regarding the evolutionary origin of mitochondria and chloroplasts?

  1. Both organelles possess circular double-stranded DNA genomes and 70S prokaryotic-like ribosomes Correct answer.
  2. Both organelles contain 80S eukaryotic ribosomes and linear chromosomes bound by histones
  3. Both organelles can survive indefinitely outside living host cells
  4. Both organelles are synthesized de novo by the cell's Golgi apparatus

Explanation

Mitochondria and chloroplasts contain their own circular DNA genomes (devoid of histones), 70S ribosomes (inhibited by bacterial antibiotics like chloramphenicol), and divide by binary fission, providing evidence for their endosymbiotic bacterial origin.

Question 50Cell structure, membranes and division

Which cytoskeletal filament type is composed of polymerized alpha- and beta-tubulin dimers and serves as structural tracks for intracellular vesicle transport by kinesin and dynein motor proteins?

  1. Microtubules Correct answer.
  2. Microfilaments (actin filaments)
  3. Intermediate filaments (keratin)
  4. Myosin filaments

Explanation

Microtubules are hollow cylindrical tubes (25 nm diameter) composed of polymerized alpha- and beta-tubulin heterodimers. They maintain cell shape, form the mitotic spindle during cell division, and act as structural tracks for kinesin and dynein motor proteins.