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TASC Biology Level 3 study materials
TASC Biology Level 3 study resources

TASC Biology Level 3 practice questions

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Question 1Molecules and Cells

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

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

Explanation

Mitochondria carry out oxidative phosphorylation, using the electron transport chain and ATP synthase to generate most of the cell's ATP. Ribosomes build proteins, the Golgi modifies and packages them, and lysosomes degrade material, so none of these organelles is the main ATP source.

Question 2Molecules and Cells

A solution surrounding a cell has a lower solute concentration than the cell's cytoplasm. How is that solution described?

  1. Hypertonic
  2. Hypotonic Correct answer.
  3. Isotonic
  4. Saturated

Explanation

Tonicity compares solute concentration outside the cell with that inside. A hypotonic solution has fewer solutes than the cytoplasm, so water tends to enter the cell by osmosis. Hypertonic means more solutes outside, and isotonic means equal concentrations.

Question 3Molecules and Cells

Which molecule stores the genetic instructions that are translated into proteins?

  1. ATP
  2. Cholesterol
  3. Glucose
  4. mRNA Correct answer.

Explanation

Messenger RNA carries a copy of a gene's sequence from DNA to the ribosome, where it directs the order of amino acids. ATP is an energy carrier, glucose is a fuel molecule, and cholesterol is a membrane lipid, so none of them carries protein-coding information.

Question 4Molecules and Cells

Which structural feature is shared by both plant and animal cell membranes?

  1. Chloroplasts
  2. A rigid cellulose wall
  3. A phospholipid bilayer with embedded proteins Correct answer.
  4. A single circular chromosome

Explanation

All biological membranes are built on a phospholipid bilayer studded with proteins, which regulates what enters and leaves the cell. Cellulose walls and chloroplasts occur in plant cells but not animal cells, and a single circular chromosome is typical of prokaryotes.

Question 5Molecules and Cells

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

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

Explanation

DNA synthesis takes place during S phase, which lies between the two gap phases. G1 is a growth phase before replication, G2 follows replication and prepares for division, and M phase is when the chromosomes are separated into daughter cells.

Question 6Molecules and Cells

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

  1. Helicase Correct answer.
  2. Ligase
  3. DNA polymerase
  4. Primase

Explanation

Helicase breaks the hydrogen bonds between complementary bases, separating the two strands so each can serve as a template. DNA polymerase adds nucleotides, ligase joins fragments, and primase lays down an RNA primer.

Question 7Molecules and Cells

Which type of bond links amino acids together in a polypeptide chain?

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

Explanation

A peptide bond forms between the carboxyl group of one amino acid and the amino group of the next, releasing water. Hydrogen and ionic bonds help stabilize the folded protein shape, while glycosidic bonds join sugars.

Question 8Molecules and Cells

In a eukaryotic cell, where does transcription of nuclear genes take place?

  1. In the cytoplasm
  2. In the mitochondrion
  3. In the nucleus Correct answer.
  4. On the rough endoplasmic reticulum

Explanation

Nuclear genes are transcribed into RNA inside the nucleus, and the RNA is then processed and exported to the cytoplasm for translation. Ribosomes on the rough endoplasmic reticulum and free in the cytoplasm carry out translation, not transcription of nuclear DNA.

Question 9Molecules and Cells

A red blood cell is placed in distilled water. What is the most likely outcome?

  1. The cell stays the same size
  2. The cell swells and may burst Correct answer.
  3. The cell shrinks as water leaves
  4. The cell dissolves its membrane proteins

Explanation

Distilled water is hypotonic relative to cytoplasm, so water enters the cell by osmosis faster than it leaves. Because an animal cell lacks a rigid wall to resist the pressure, it swells and can lyse.

Question 10Molecules and Cells

A mutation changes a codon from GAA to GAG. Both codons specify glutamic acid. How should this mutation be classified?

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

Explanation

A silent mutation changes the DNA sequence but not the amino acid, because the genetic code is redundant and multiple codons specify the same residue. A missense mutation swaps one amino acid for another, and a nonsense mutation creates a stop codon.

Question 11Molecules and Cells

An enzyme is most active near pH 2. Which enzyme is most likely being studied?

  1. Salivary amylase
  2. Pepsin Correct answer.
  3. Catalase
  4. DNA polymerase

Explanation

Pepsin digests proteins in the strongly acidic environment of the stomach, so its optimum pH is very low. Salivary amylase works near neutral pH in the mouth, and most other enzymes function near pH 7.

Question 12Molecules and Cells

A cell is treated with a drug that blocks ATP synthesis. Which transport process would be affected first?

  1. Osmosis of water
  2. Sodium-potassium pump activity Correct answer.
  3. Facilitated diffusion of glucose
  4. Simple diffusion of oxygen

Explanation

The sodium-potassium pump is active transport and directly hydrolyzes ATP to move ions against their gradients. Simple diffusion, osmosis, and facilitated diffusion are passive processes driven by concentration gradients and do not require ATP.

Question 13Molecules and Cells

Aerobic respiration produces far more ATP than anaerobic fermentation mainly because aerobic pathways do what?

  1. Use oxygen as a final electron acceptor to drive oxidative phosphorylation Correct answer.
  2. Convert glucose directly into ATP without intermediate steps
  3. Break down proteins instead of glucose
  4. Recycle NADH without an electron transport chain

Explanation

Oxygen accepts electrons at the end of the electron transport chain, allowing a proton gradient to power ATP synthase and yielding many ATP per glucose. Fermentation lacks this chain and merely regenerates NAD+ so glycolysis can continue.

Question 14Molecules and Cells

A plant cell with a solute potential of -0.6 MPa is placed in a solution with a solute potential of -0.2 MPa. Water potential is solute potential plus pressure potential, and the cell is initially at full turgor. Which direction will water move?

  1. No net movement, because pressure potential is zero outside
  2. Into the cell, because the external solution has higher water potential Correct answer.
  3. Into the cell, because the cell has more solutes
  4. Out of the cell, because the cell has a more negative solute potential

Explanation

Water moves from higher to lower water potential. The external solution at -0.2 MPa has a higher water potential than the cell interior at -0.6 MPa, so water enters the cell.

Question 15Molecules and Cells

During translation, which molecule delivers amino acids to the ribosome?

  1. mRNA
  2. tRNA Correct answer.
  3. DNA
  4. rRNA

Explanation

Transfer RNA carries a specific amino acid and pairs its anticodon with the mRNA codon in the ribosome. Ribosomal RNA forms the core of the ribosome, mRNA carries the coding sequence, and DNA stores the gene.

Question 16Molecules and Cells

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

  1. It moves three sodium ions out for every two potassium ions in, altering the membrane charge Correct answer.
  2. It transports only neutral molecules
  3. It moves equal numbers of positive and negative ions
  4. It generates ATP from ion gradients

Explanation

The pump exports three Na+ ions and imports two K+ ions per cycle, so each cycle removes a net positive charge from the cell interior. This contributes to the negative resting membrane potential, which is why the pump is called electrogenic.

Question 17Molecules and Cells

A cell in G2 phase is examined. Which condition is expected?

  1. DNA content is double the G1 amount Correct answer.
  2. The nuclear envelope has fully reformed around separated nuclei
  3. Chromosomes are aligned at the metaphase plate
  4. Sister chromatids have been pulled to opposite poles

Explanation

DNA replicates during S phase, so by G2 the cell contains twice the G1 DNA content. Chromosome alignment, nuclear envelope reformation, and chromatid separation are events of mitosis, which follows G2.

Question 18Molecules and Cells

A DNA template strand reads 3'-TAC GGA-5'. What mRNA sequence is transcribed from it?

  1. 5'-TAC GGA-3'
  2. 5'-ATG CCT-3'
  3. 5'-UAC GGA-3'
  4. 5'-AUG CCU-3' Correct answer.

Explanation

RNA polymerase builds mRNA complementary to the template, pairing A with U and C with G, and the mRNA is antiparallel to the template. Complementing TAC GGA gives AUG CCU, written 5' to 3'.

Question 19Molecules and Cells

Which property of the phospholipid bilayer directly allows small nonpolar molecules such as oxygen to cross membranes readily?

  1. The hydrophobic interior of the bilayer Correct answer.
  2. The presence of aquaporins
  3. The rigidity of cholesterol
  4. The negative charge on phosphate heads

Explanation

The fatty acid tails create a hydrophobic core that nonpolar gases dissolve into and diffuse across easily. Polar and charged substances are excluded by this core and require channels or carriers.

Question 20Molecules and Cells

A competitive inhibitor is added to an enzyme-catalyzed reaction. How can its effect be reversed?

  1. By changing the pH to the enzyme's optimum
  2. By lowering the temperature
  3. By increasing the substrate concentration Correct answer.
  4. By adding more enzyme inhibitor

Explanation

A competitive inhibitor binds the active site, competing with substrate. Raising substrate concentration increases the chance that substrate, rather than inhibitor, occupies the site, restoring reaction rate.

Question 21Molecules and Cells

A protein is synthesized with a signal peptide and enters the secretory pathway. Which sequence of organelles does it most likely follow?

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

Explanation

Proteins destined for secretion are translated on the rough ER, modified and sorted in the Golgi, packaged into vesicles, and released at the plasma membrane. The signal peptide directs the ribosome to the ER membrane.

Question 22Molecules and Cells

An enzyme-catalyzed reaction reaches a plateau as substrate concentration increases. What does this indicate?

  1. All active sites are saturated with substrate Correct answer.
  2. The substrate has been entirely consumed
  3. The reaction has become endergonic
  4. The enzyme is being permanently destroyed

Explanation

At high substrate concentration, every enzyme active site is occupied, so the rate levels off at the maximum velocity Vmax. Adding more substrate cannot increase the rate until product is removed or more enzyme is present.

Question 23Molecules and Cells

A frameshift mutation occurs near the start of a gene. Why is the resulting protein usually more severely affected than a single missense substitution?

  1. It shifts the reading frame, altering every downstream codon and often creating a premature stop Correct answer.
  2. It prevents transcription from starting
  3. It changes one codon but preserves the reading frame
  4. It always deletes the entire chromosome

Explanation

Inserting or deleting a number of nucleotides not divisible by three shifts the triplet reading frame, so all downstream codons are misread. This typically produces a truncated or nonfunctional protein, whereas a missense change alters only one residue.

Question 24Molecules and Cells

In the electron transport chain, what is the direct role of the proton gradient across the inner mitochondrial membrane?

  1. It oxidizes glucose to pyruvate
  2. It transports oxygen into the matrix
  3. It provides electrons to complex I
  4. It drives ATP synthase to phosphorylate ADP Correct answer.

Explanation

Electron transport pumps protons into the intermembrane space, creating an electrochemical gradient. Protons flow back through ATP synthase, and this flow powers the phosphorylation of ADP to ATP, a process called chemiosmosis.

Question 25Molecules and Cells

A cell is placed in a solution containing a nonpenetrating solute and is observed to maintain constant volume. Which conclusion is best supported?

  1. The solution is hypotonic to the cell
  2. The solution is isotonic to the cell Correct answer.
  3. The cell wall prevents all water movement
  4. The solute is actively pumped into the cell

Explanation

Constant volume indicates no net water movement, which occurs when the external solution has the same effective solute concentration as the cytoplasm. That condition defines an isotonic solution.

Question 26Molecules and Cells

Which organelle contains hydrolytic enzymes that digest worn-out organelles and macromolecules at acidic pH?

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

Explanation

Lysosomes are membrane-bound vesicles that maintain an acidic interior and hold hydrolases capable of breaking down proteins, lipids, and nucleic acids. They receive digestive enzymes from the Golgi and fuse with targets such as damaged organelles during autophagy. This degradative role distinguishes them from organelles that mainly modify or synthesize molecules.

Question 27Molecules and Cells

A scientist wants to observe the detailed internal structure of a mitochondrion, including its cristae. Which technique is most appropriate?

  1. Phase-contrast microscopy
  2. Transmission electron microscopy Correct answer.
  3. Light microscopy of living cells
  4. Fluorescence microscopy with a membrane dye

Explanation

Transmission electron microscopy uses a beam of electrons to resolve structures far smaller than the wavelength of visible light, so internal membrane folds such as cristae become visible. Light-based methods cannot resolve features at that scale. Fixation and sectioning also allow thin slices that reveal internal anatomy.

Question 28Molecules and Cells

Which molecule serves as the immediate energy currency that powers most cellular work?

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

Explanation

ATP couples exergonic reactions to endergonic ones by transferring its terminal phosphate group to target molecules. Hydrolysis of that phosphoanhydride bond releases usable free energy. Other molecules store energy but must first be converted into a proton gradient or ATP before directly driving most work.

Question 29Molecules and Cells

Which structural feature allows the plasma membrane to be described as a fluid mosaic?

  1. A rigid layer of cellulose outside the bilayer
  2. Proteins and lipids that can move laterally within the bilayer Correct answer.
  3. A continuous protein sheet covering the membrane surface
  4. Covalent cross-links that lock phospholipids in place

Explanation

The fluid mosaic model describes a phospholipid bilayer in which proteins are embedded and can drift laterally, giving the membrane a dynamic, patchwork appearance. Weak hydrophobic interactions permit this movement rather than fixed covalent locking. The mosaic quality comes from the varied proteins and lipids distributed throughout.

Question 30Molecules and Cells

A DNA molecule contains 30% adenine. What percentage of its bases is guanine?

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

Explanation

Chargaff's rules state that adenine pairs with thymine and guanine pairs with cytosine, so the percentages of A and T are equal, as are those of G and C. If A is 30%, T is also 30%, leaving 40% shared equally by G and C. Each of those therefore accounts for 20% of the bases.

Question 31Molecules and Cells

During DNA replication, which enzyme joins Okazaki fragments by forming phosphodiester bonds between them?

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

Explanation

DNA ligase seals the nicks between adjacent Okazaki fragments on the lagging strand by catalyzing phosphodiester bonds between the sugar-phosphate backbones. This step completes the continuous strand after primers are replaced. Other enzymes unwind, prime, or relieve supercoiling but do not seal fragment junctions.

Question 32Molecules and Cells

A tRNA molecule has the anticodon 5'-UGC-3'. Which amino acid does it carry, given that the relevant mRNA codon is read in the standard 5'-to-3' direction?

  1. Threonine, because the codon is ACG
  2. Alanine, because the codon is GCA Correct answer.
  3. Cysteine, because the codon is UGC
  4. Serine, because the codon is UCG

Explanation

Anticodons pair antiparallel with mRNA codons, so the codon matching 5'-UGC-3' is 5'-GCA-3'. The standard genetic code assigns GCA to alanine. Reversing the anticodon without accounting for antiparallel pairing leads to incorrect codons.

Question 33Molecules and Cells

A cell is treated with a drug that blocks the function of the Golgi apparatus. Which process is most directly impaired?

  1. Synthesis of the polypeptide chain on the ribosome
  2. Transcription of ribosomal RNA in the nucleolus
  3. Replication of mitochondrial DNA
  4. Modification and sorting of newly made secretory proteins Correct answer.

Explanation

The Golgi apparatus receives proteins from the endoplasmic reticulum and performs glycosylation, cleavage, and sorting before dispatching them to their destinations. Blocking it therefore disrupts post-translational processing and routing of secretory cargo. Ribosomal synthesis, rRNA transcription, and organelle DNA replication occur elsewhere.

Question 34Molecules and Cells

In glycolysis, one glucose molecule yields two pyruvate molecules along with a net gain of how many ATP and NADH molecules?

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

Explanation

Glycolysis invests two ATP during its preparatory phase and produces four ATP during its payoff phase, giving a net gain of two ATP. It also reduces two NAD+ to two NADH as glyceraldehyde-3-phosphate is oxidized. The gross ATP figure of four must not be confused with the net yield.

Question 35Molecules and Cells

A segment of mRNA has the sequence 5'-AUG GCA UAA-3'. How many amino acids does the resulting polypeptide contain?

  1. Three
  2. Zero
  3. One Correct answer.
  4. Two

Explanation

AUG is the start codon and specifies methionine, while UAA is a stop codon that terminates translation and does not encode an amino acid. Only the single codon between the start and stop signals contributes a residue. Translation therefore produces a one-amino-acid peptide.

Question 36Molecules and Cells

A cell is placed in a hypertonic solution. What happens to the cell, and why?

  1. It bursts because water enters faster than it exits
  2. It swells because solute enters by active transport
  3. It stays the same because solute concentrations are equal
  4. It shrinks because water leaves by osmosis Correct answer.

Explanation

In a hypertonic solution the outside solute concentration is higher, so water potential outside is lower and water moves out of the cell by osmosis. Loss of water reduces cell volume, causing shrinkage. Solute does not move to equalize concentrations in this scenario.

Question 37Molecules and Cells

An enzyme-catalyzed reaction is run at a temperature well above the enzyme's optimum. Why does the reaction rate fall sharply?

  1. The activation energy of the reaction increases permanently
  2. Substrate molecules lose their ability to collide
  3. The enzyme is converted into a substrate molecule
  4. The enzyme's active site changes shape as hydrogen bonds and other weak interactions break Correct answer.

Explanation

Excess heat disrupts the weak interactions that maintain secondary and tertiary structure, so the enzyme unfolds and its active site no longer fits the substrate. This denaturation is generally irreversible for many enzymes. The substrate itself is usually stable at these temperatures.

Question 38Molecules and Cells

A researcher measures the rate of photosynthesis in a plant exposed to increasing light intensity while carbon dioxide is held constant. The rate rises and then levels off. Which factor most likely limits the rate at the plateau?

  1. Oxygen concentration in the air
  2. Chlorophyll absorption spectrum
  3. Light intensity
  4. Carbon dioxide concentration or temperature Correct answer.

Explanation

When light is no longer limiting, the rate plateaus because another factor, typically carbon dioxide availability or temperature-dependent enzyme activity in the Calvin cycle, becomes limiting. Raising light further cannot increase the rate under those conditions. This illustrates the principle of the limiting factor.

Question 39Molecules and Cells

In the light-dependent reactions of photosynthesis, what is the direct source of the electrons that replace those lost by photosystem II?

  1. NADPH
  2. ATP synthase
  3. Plastoquinone
  4. Water Correct answer.

Explanation

Photosystem II splits water molecules through photolysis, releasing electrons that replenish the reaction center, along with protons and oxygen. This water-splitting step is the origin of the oxygen gas produced in photosynthesis. NADPH and plastoquinone are electron carriers downstream, not the ultimate source.

Question 40Molecules and Cells

A mutation deletes a single nucleotide in the middle of a coding sequence, and the resulting protein is truncated. Which explanation best accounts for this outcome?

  1. The deletion causes the ribosome to skip the mutated codon and continue normally
  2. The deletion removes one amino acid and leaves the rest of the protein intact
  3. The deletion creates a premature stop codon by shifting the reading frame Correct answer.
  4. The deletion converts the gene into a different allele without changing the reading frame

Explanation

A one-nucleotide deletion shifts every downstream codon into a new reading frame, so the sequence of amino acids after the mutation is completely altered. This frameshift often generates a premature stop codon, producing a shortened, nonfunctional protein. The effect extends far beyond the single deleted base.

Question 41Genetics and Heredity

In a certain plant, purple flowers (P) are dominant to white flowers (p). A heterozygous purple plant is crossed with a white plant. What fraction of the offspring is expected to have white flowers?

  1. One quarter
  2. Zero
  3. Three quarters
  4. One half Correct answer.

Explanation

A heterozygous purple plant has genotype Pp and makes gametes P and p in equal numbers. The white parent is pp and contributes only p. Offspring are half Pp (purple) and half pp (white), so one half are white.

Question 42Genetics and Heredity

Which molecule carries the genetic instructions from the nucleus to the ribosome for protein synthesis?

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

Explanation

Messenger RNA is transcribed from DNA in the nucleus and carries the coded sequence to the ribosome. Transfer RNA brings amino acids, and ribosomal RNA forms part of the ribosome itself.

Question 43Genetics and Heredity

A human body cell contains 46 chromosomes. How many chromosomes are present in a normal human egg cell?

  1. 23 Correct answer.
  2. 46
  3. 92
  4. 12

Explanation

Gametes are produced by meiosis, which halves the chromosome number. A normal human egg is haploid and carries 23 chromosomes, one from each homologous pair.

Question 44Genetics and Heredity

A man with type AB blood and a woman with type O blood have children. Which blood types are possible in their offspring?

  1. Type A or type B only Correct answer.
  2. Type O only
  3. Type A, B, AB, or O
  4. Type AB or type O only

Explanation

The AB father passes on either an A allele or a B allele, while the O mother can pass on only an O allele. Offspring are therefore AO (type A) or BO (type B), and neither AB nor O is possible.

Question 45Genetics and Heredity

What is the name of the structure that holds two sister chromatids together before they separate during cell division?

  1. Centromere Correct answer.
  2. Nucleolus
  3. Centriole
  4. Chromatin

Explanation

The centromere is the constricted region where the two sister chromatids of a replicated chromosome are joined. Spindle fibers attach there to pull chromatids apart.

Question 46Genetics and Heredity

In a pedigree, a trait appears in every generation and affects about half of the offspring of an affected parent. Which pattern of inheritance is most consistent with these observations?

  1. X-linked recessive
  2. Autosomal recessive
  3. Mitochondrial
  4. Autosomal dominant Correct answer.

Explanation

A dominant allele is expressed in heterozygotes, so an affected parent passes the trait to roughly half of the children in each generation. Recessive traits typically skip generations, and mitochondrial traits pass only through mothers.

Question 47Genetics and Heredity

Which of the following describes the phenotype of an organism?

  1. Its observable traits Correct answer.
  2. Its complete set of alleles
  3. The order of bases in one gene
  4. The number of chromosomes it has

Explanation

Phenotype refers to the observable characteristics of an organism, such as color or height. Genotype, by contrast, is the underlying combination of alleles.

Question 48Genetics and Heredity

A DNA strand has the base sequence TAC. What is the sequence of the complementary DNA strand?

  1. ATG Correct answer.
  2. TAC
  3. GTA
  4. AUG

Explanation

In DNA, adenine pairs with thymine and cytosine pairs with guanine. Pairing each base of TAC with its complement gives ATG.

Question 49Genetics and Heredity

Two heterozygous tall pea plants (Tt) are crossed. What is the expected ratio of tall to short offspring?

  1. 1 tall to 3 short
  2. All tall
  3. 3 tall to 1 short Correct answer.
  4. 1 tall to 1 short

Explanation

A Tt by Tt cross yields genotypes TT, Tt, Tt, and tt in equal proportions. Because T is dominant, three of the four offspring are tall and one is short.

Question 50Genetics and Heredity

During which phase of meiosis do homologous chromosomes separate from each other?

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

Explanation

Homologous chromosomes pair during prophase I and are pulled to opposite poles in anaphase I. Sister chromatids separate later, in anaphase II.