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Lebanese Baccalaureate - Life Sciences (LS) study materials
Lebanese Baccalaureate - Life Sciences (LS) study resources

Lebanese Baccalaureate - Life Sciences (LS) practice questions

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Question 1Cell Biology and Cellular Physiology

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

  1. The mitochondrion Correct answer.
  2. The ribosome
  3. The lysosome
  4. The Golgi apparatus

Explanation

Mitochondria carry out oxidative phosphorylation, using the electron transport chain and ATP synthase to generate most of the cell's ATP. Ribosomes synthesize proteins, lysosomes degrade material, and the Golgi modifies and sorts proteins.

Question 2Cell Biology and Cellular Physiology

In the fluid mosaic model, the plasma membrane is best described as:

  1. A phospholipid bilayer with embedded proteins that can move laterally Correct answer.
  2. A static sheet of protein with fixed lipid patches
  3. A rigid layer of cellulose
  4. A double layer of proteins enclosing a lipid core

Explanation

The fluid mosaic model describes a phospholipid bilayer in which proteins are embedded and can move laterally within the plane of the membrane. This gives the membrane both fluidity and a mosaic distribution of components.

Question 3Cell Biology and Cellular Physiology

A red blood cell placed in distilled water swells and bursts. Which process explains this?

  1. Active transport of water into the cell
  2. Osmosis of water into the cell Correct answer.
  3. Endocytosis of the surrounding water
  4. Diffusion of solutes out of the cell

Explanation

Distilled water is hypotonic relative to the cytoplasm, so water moves by osmosis down its concentration gradient into the cell. The resulting water influx increases cell volume until the membrane ruptures.

Question 4Cell Biology and Cellular Physiology

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

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

Explanation

Transfer RNA molecules bind specific amino acids and deliver them to the ribosome, where their anticodons pair with mRNA codons. This ensures the correct amino acid is added to the growing polypeptide.

Question 5Cell Biology and Cellular Physiology

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

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

Explanation

DNA is duplicated during the S (synthesis) phase of interphase, so each chromosome becomes two sister chromatids. G1 and G2 are gap phases of growth, and M phase is when the duplicated DNA is segregated.

Question 6Cell Biology and Cellular Physiology

Which structural feature distinguishes prokaryotic cells from eukaryotic cells?

  1. Presence of a plasma membrane
  2. Presence of ribosomes
  3. Absence of a membrane-bound nucleus Correct answer.
  4. Presence of cytoplasm

Explanation

Prokaryotes lack a membrane-bound nucleus, so their DNA lies in a nucleoid region. Eukaryotic cells enclose their chromosomes within a nuclear envelope.

Question 7Cell Biology and Cellular Physiology

Enzymes increase the rate of a biochemical reaction primarily by:

  1. Raising the temperature of the medium
  2. Lowering the activation energy of the reaction Correct answer.
  3. Shifting the reaction away from equilibrium
  4. Increasing the energy released by the reaction

Explanation

Enzymes provide an alternative reaction pathway with a lower activation energy, so more substrate molecules can react per unit time. They do not change the overall free-energy change or the equilibrium position.

Question 8Cell Biology and Cellular Physiology

Which of the following is a function of the smooth endoplasmic reticulum?

  1. Assembly of ribosomal subunits
  2. Synthesis of lipids and detoxification Correct answer.
  3. Breakdown of hydrogen peroxide
  4. Replication of mitochondrial DNA

Explanation

The smooth endoplasmic reticulum lacks ribosomes and is involved in lipid and steroid synthesis and in detoxification reactions. The rough endoplasmic reticulum, studded with ribosomes, specializes in protein synthesis and processing.

Question 9Cell Biology and Cellular Physiology

In the sodium-potassium pump, what is moved across the membrane per cycle of the pump?

  1. Two sodium ions out and three potassium ions in
  2. Two sodium ions in and three potassium ions out
  3. Three potassium ions out and two sodium ions in
  4. Three sodium ions out and two potassium ions in Correct answer.

Explanation

The Na+/K+ ATPase exports three Na+ ions and imports two K+ ions per ATP hydrolyzed. This unequal exchange contributes to the negative resting membrane potential and consumes energy.

Question 10Cell Biology and Cellular Physiology

In a reaction catalyzed by an enzyme, adding a competitive inhibitor will:

  1. Irreversibly destroy the enzyme's active site
  2. Decrease the apparent affinity of the enzyme for substrate without changing maximum velocity Correct answer.
  3. Increase both substrate affinity and maximum velocity
  4. Decrease the maximum velocity without affecting substrate affinity

Explanation

A competitive inhibitor binds the active site and competes with substrate, so higher substrate concentrations are needed to reach the same rate, raising the apparent Km. Because it can be outcompeted, the maximum velocity is unchanged.

Question 11Cell Biology and Cellular Physiology

A cell is treated with a drug that blocks the electron transport chain. Which immediate consequence is expected?

  1. The cell can no longer perform any ATP synthesis
  2. Glycolysis stops completely and no ATP is made
  3. ATP production by oxidative phosphorylation falls sharply Correct answer.
  4. Oxygen consumption increases to compensate

Explanation

The electron transport chain establishes the proton gradient that drives ATP synthase, so blocking it sharply reduces oxidative phosphorylation. Glycolysis can still generate ATP by substrate-level phosphorylation, so some ATP production continues.

Question 12Cell Biology and Cellular Physiology

A protein destined for secretion is synthesized on ribosomes attached to the rough endoplasmic reticulum. Which sequence of compartments does it most likely follow?

  1. Rough ER, Golgi apparatus, secretory vesicle, plasma membrane Correct answer.
  2. Cytosol, mitochondrion, Golgi apparatus, plasma membrane
  3. Nucleus, smooth ER, peroxisome, plasma membrane
  4. Golgi apparatus, rough ER, lysosome, plasma membrane

Explanation

Secretory proteins enter the rough ER lumen, are transported in vesicles to the Golgi for modification and sorting, and are then packaged into secretory vesicles that fuse with the plasma membrane. This pathway is the standard secretory route.

Question 13Cell Biology and Cellular Physiology

During anaphase of mitosis, what happens to the chromosomes?

  1. Homologous chromosomes pair and exchange segments
  2. Sister chromatids are pulled toward opposite poles of the cell Correct answer.
  3. Chromosomes condense and become visible for the first time
  4. The nuclear envelope re-forms around each set of chromosomes

Explanation

In anaphase the sister chromatids separate at the centromere and are pulled to opposite poles by spindle fibers. This ensures each daughter cell receives one copy of each chromosome.

Question 14Cell Biology and Cellular Physiology

Which statement best describes the role of the cell membrane in maintaining homeostasis?

  1. It selectively controls the passage of substances, keeping the internal environment stable Correct answer.
  2. It allows all molecules to cross freely, equalizing internal and external conditions
  3. It prevents any exchange with the environment, isolating the cell completely
  4. It actively changes the cell's genetic information in response to the environment

Explanation

The plasma membrane is selectively permeable, using channels, pumps and transporters to regulate what enters and leaves. This regulation keeps the internal composition stable despite environmental changes.

Question 15Cell Biology and Cellular Physiology

A mutation changes a single base in a codon so that it now codes for a different amino acid. This is best described as:

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

Explanation

A missense mutation is a base substitution that changes one codon to a codon for a different amino acid. Silent mutations leave the amino acid unchanged, nonsense mutations create a stop codon, and frameshifts arise from insertions or deletions.

Question 16Cell Biology and Cellular Physiology

In the light-dependent reactions of photosynthesis, what is the main role of water?

  1. It accepts electrons at the end of the electron transport chain
  2. It serves as the final electron acceptor in place of NADP+
  3. It directly fixes carbon dioxide into glucose
  4. It supplies electrons to photosystem II and releases oxygen Correct answer.

Explanation

Water is split at photosystem II, donating electrons to replace those lost by the reaction center and releasing oxygen as a byproduct. NADP+ is the final electron acceptor of the light reactions.

Question 17Cell Biology and Cellular Physiology

A cell that lacks functional lysosomes would most directly be impaired in its ability to:

  1. Copy its DNA before division
  2. Translate messenger RNA into protein
  3. Synthesize ATP from glucose
  4. Digest and recycle intracellular macromolecules Correct answer.

Explanation

Lysosomes contain hydrolytic enzymes that break down macromolecules and recycle cellular material. Without them, digestion of engulfed material and autophagy are impaired.

Question 18Cell Biology and Cellular Physiology

Which observation best supports the conclusion that a transport process is active rather than passive?

  1. The substance moves from higher to lower concentration without energy input
  2. The rate of transport increases as the concentration gradient increases
  3. Transport stops when the temperature is lowered
  4. The substance moves from a region of lower concentration to one of higher concentration Correct answer.

Explanation

Movement against a concentration gradient requires energy input and is a defining feature of active transport. Passive transport always proceeds down the gradient and does not require metabolic energy.

Question 19Cell Biology and Cellular Physiology

During which stage of cellular respiration is most of the carbon dioxide produced?

  1. Lactic acid fermentation
  2. Glycolysis
  3. The Krebs cycle Correct answer.
  4. The electron transport chain

Explanation

The Krebs cycle releases carbon dioxide as acetyl-CoA is oxidized, producing NADH, FADH2 and ATP. Glycolysis and the electron transport chain do not release CO2.

Question 20Cell Biology and Cellular Physiology

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

  1. It stays the same because the solutes cannot move
  2. It swells because water enters the cell by osmosis
  3. It bursts because active transport pumps water inward
  4. It shrinks because water leaves the cell by osmosis Correct answer.

Explanation

In a hypertonic solution the external solute concentration is higher than inside the cell, so water moves out by osmosis. The loss of water reduces cell volume and causes shrinkage.

Question 21Cell Biology and Cellular Physiology

Which statement about the relationship between DNA, mRNA and protein is correct?

  1. mRNA is transcribed into DNA, which is translated into protein
  2. Protein is transcribed into mRNA, which is translated into DNA
  3. DNA is transcribed into mRNA, which is translated into protein Correct answer.
  4. DNA is translated into protein, which is transcribed into mRNA

Explanation

Gene expression follows the central dogma: DNA is transcribed into messenger RNA, and the mRNA is translated into a polypeptide. This order reflects the flow of genetic information in cells.

Question 22Cell Biology and Cellular Physiology

A student observes that a cell divides repeatedly without growing larger between divisions. Which phase of the cell cycle is most likely shortened?

  1. S phase
  2. G1 phase Correct answer.
  3. Prophase
  4. Telophase

Explanation

G1 is the main growth phase in which the cell increases in mass before committing to DNA replication. If the cell divides without growing, G1 is most likely shortened.

Question 23Cell Biology and Cellular Physiology

An enzyme is incubated with a noncompetitive inhibitor. Which change in kinetic parameters is expected?

  1. An increase in Km with no change in maximum velocity
  2. A decrease in maximum velocity with no change in Km Correct answer.
  3. No change in either maximum velocity or Km
  4. An increase in both maximum velocity and Km

Explanation

A noncompetitive inhibitor binds at a site other than the active site and reduces the number of functional enzyme molecules, lowering maximum velocity. Because substrate can still bind the remaining active enzymes normally, Km is essentially unchanged.

Question 24Cell Biology and Cellular Physiology

A mutation deletes two nucleotides from the coding region of a gene. What is the most likely consequence for the encoded protein?

  1. The mRNA is not transcribed from the gene
  2. Only one amino acid is replaced and the rest of the protein is unaffected
  3. The protein is synthesized normally because the genetic code is degenerate
  4. The reading frame shifts, usually producing a truncated or nonfunctional protein Correct answer.

Explanation

Deleting two nucleotides is not a multiple of three, so the reading frame shifts from the point of mutation onward. All downstream codons are read differently, usually introducing a premature stop codon and yielding a nonfunctional protein.

Question 25Cell Biology and Cellular Physiology

In an experiment, mitochondria are supplied with an uncoupler that makes the inner membrane permeable to protons. Which outcome is expected?

  1. ATP synthesis increases because protons enter more easily
  2. Electron transport stops immediately and oxygen consumption falls to zero
  3. The Krebs cycle stops because it requires a proton gradient
  4. Electron transport continues but ATP synthesis by ATP synthase decreases Correct answer.

Explanation

Uncouplers dissipate the proton gradient without blocking the electron transport chain, so electron flow and oxygen consumption continue. However, ATP synthase lacks the gradient needed to drive ATP production, so ATP synthesis falls.

Question 26Cell Biology and Cellular Physiology

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

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

Explanation

Mitochondria host the electron transport chain and ATP synthase, which generate most of the cell's ATP through oxidative phosphorylation. Other organelles perform different functions and do not produce ATP on this scale.

Question 27Cell Biology and Cellular Physiology

A red blood cell is placed in a solution of 0.9% NaCl. What is the expected effect on the cell?

  1. It swells and may burst.
  2. It remains unchanged in volume. Correct answer.
  3. It shrinks due to water loss.
  4. It actively pumps Na+ out to maintain size.

Explanation

A 0.9% NaCl solution is isotonic to red blood cells, meaning the solute concentration outside equals that inside. Therefore, there is no net movement of water across the membrane, and cell volume remains stable.

Question 28Cell Biology and Cellular Physiology

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

  1. A static bilayer of phospholipids with fixed protein positions.
  2. A double layer of proteins with a lipid core.
  3. A rigid layer of proteins with embedded lipids.
  4. A dynamic structure of lipids and proteins that can move laterally. Correct answer.

Explanation

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

Question 29Cell Biology and Cellular Physiology

A cell is treated with a drug that inhibits the enzyme responsible for adding the poly-A tail to mRNA. Which process is most directly affected?

  1. Export of mRNA from the nucleus to the cytoplasm. Correct answer.
  2. Translation of mRNA into protein at the ribosome.
  3. Splicing of introns from pre-mRNA.
  4. Transcription of DNA into pre-mRNA.

Explanation

The poly-A tail is added during mRNA processing in the nucleus and is required for mRNA stability and nuclear export. Without it, the mRNA cannot be efficiently transported to the cytoplasm for translation.

Question 30Cell Biology and Cellular Physiology

In a cell, a mutation in the gene encoding a cyclin protein prevents its degradation. What is the most likely consequence for the cell cycle?

  1. The cell will proceed through the cycle but skip S phase.
  2. The cell will undergo apoptosis immediately.
  3. The cell will progress to the next phase prematurely or in an uncontrolled manner. Correct answer.
  4. The cell will arrest in G1 phase.

Explanation

Cyclins regulate the cell cycle by activating cyclin-dependent kinases (CDKs) and must be degraded at specific points to allow progression. If a cyclin is not degraded, its associated CDK remains active, potentially causing premature or unregulated progression through the cell cycle.

Question 31Genetics and Molecular Biology

In a DNA molecule, adenine pairs with which base, and by how many hydrogen bonds?

  1. Guanine, with three hydrogen bonds
  2. Thymine, with two hydrogen bonds Correct answer.
  3. Cytosine, with two hydrogen bonds
  4. Uracil, with two hydrogen bonds

Explanation

Complementary base pairing in DNA joins adenine to thymine through two hydrogen bonds, while guanine pairs with cytosine through three hydrogen bonds. Uracil replaces thymine only in RNA.

Question 32Genetics and Molecular Biology

A cross between two heterozygous pea plants (Tt × Tt) produces offspring. What phenotypic ratio is expected for a trait showing complete dominance?

  1. 9 dominant : 3 recessive
  2. 1 dominant : 2 intermediate : 1 recessive
  3. 3 dominant : 1 recessive Correct answer.
  4. 1 dominant : 1 recessive

Explanation

Two heterozygotes produce genotypic proportions 1 TT : 2 Tt : 1 tt. Because T is completely dominant, both TT and Tt show the dominant phenotype, giving 3 dominant to 1 recessive.

Question 33Genetics and Molecular Biology

What is the role of tRNA during translation?

  1. It forms the structural core of the ribosome
  2. It joins nucleotides to form the new mRNA strand
  3. It carries amino acids to the ribosome and pairs its anticodon with the mRNA codon Correct answer.
  4. It stores the genetic information that is copied into mRNA

Explanation

Transfer RNA molecules each carry a specific amino acid and expose an anticodon that base-pairs with the complementary codon on mRNA. This ensures amino acids are added in the order specified by the message.

Question 34Genetics and Molecular Biology

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

  1. Primase
  2. Helicase Correct answer.
  3. Ligase
  4. Polymerase

Explanation

Helicase breaks the hydrogen bonds between complementary bases, separating the two strands so replication can proceed. The other enzymes act later in the process.

Question 35Genetics and Molecular Biology

Which of the following describes the genotype of an individual that carries two identical alleles for a gene?

  1. Homozygous Correct answer.
  2. Hybrid
  3. Hemizygous
  4. Heterozygous

Explanation

A homozygous individual carries two identical alleles for the gene in question, such as TT or tt. Heterozygotes carry two different alleles.

Question 36Genetics and Molecular Biology

Which molecule is the immediate product of transcription in eukaryotic cells?

  1. A double-stranded DNA copy
  2. A pre-mRNA that must be processed Correct answer.
  3. A mature mRNA ready for translation
  4. A polypeptide chain

Explanation

RNA polymerase produces a primary transcript (pre-mRNA) that still contains introns. Processing by capping, polyadenylation and splicing converts it into mature mRNA.

Question 37Genetics and Molecular Biology

A human somatic cell normally contains how many chromosomes?

  1. 46 Correct answer.
  2. 23
  3. 44
  4. 92

Explanation

Human somatic cells are diploid, containing 23 pairs of chromosomes for a total of 46. Gametes are haploid and contain 23.

Question 38Genetics and Molecular Biology

A woman who is a carrier of an X-linked recessive disorder marries an unaffected man. What proportion of their sons is expected to be affected?

  1. All
  2. One quarter
  3. One half Correct answer.
  4. None

Explanation

Sons receive their single X chromosome from the mother. A carrier mother transmits the mutant allele to half of her sons, and because males have only one X, those sons express the disorder.

Question 39Genetics and Molecular Biology

A mutation changes a codon from GAA to GAG, and both codons specify glutamic acid. What type of mutation is this?

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

Explanation

Because the genetic code is degenerate, different codons can specify the same amino acid. A base change that leaves the amino acid sequence unchanged is a silent mutation.

Question 40Genetics and Molecular Biology

In a dihybrid cross between two individuals heterozygous for both genes (AaBb × AaBb) with independent assortment, what fraction of offspring is expected to be homozygous recessive for both traits?

  1. 1/16 Correct answer.
  2. 1/4
  3. 3/16
  4. 9/16

Explanation

Each gene behaves independently, and the probability of aa is 1/4 and of bb is 1/4. Multiplying independent probabilities gives 1/4 × 1/4 = 1/16 for the double homozygous recessive class.

Question 41Genetics and Molecular Biology

Which statement best describes the semiconservative nature of DNA replication?

  1. Only fragments of the parental DNA are randomly distributed to daughter molecules
  2. The parental double helix is conserved intact and a completely new helix is made
  3. Both strands of a daughter molecule are newly synthesized
  4. Each daughter molecule contains one parental strand and one newly synthesized strand Correct answer.

Explanation

In semiconservative replication the two parental strands separate and each serves as a template, so every resulting double helix has one old and one new strand. This was demonstrated experimentally by Meselson and Stahl.

Question 42Genetics and Molecular Biology

Restriction enzymes are widely used in genetic engineering because they

  1. join DNA fragments together
  2. separate proteins according to size
  3. cut DNA at specific recognition sequences Correct answer.
  4. synthesize RNA from a DNA template

Explanation

Restriction endonucleases recognize specific palindromic sequences and cleave the DNA at or near those sites, producing defined fragments. This specificity makes them essential tools for cloning.

Question 43Genetics and Molecular Biology

In humans, a gene located on the Y chromosome is passed from a father to

  1. all of his daughters only
  2. all of his sons only Correct answer.
  3. half of his sons and half of his daughters
  4. all of his children

Explanation

Sons inherit the Y chromosome from their father, while daughters inherit his X chromosome. Therefore a Y-linked gene is transmitted to all sons and to no daughters.

Question 44Genetics and Molecular Biology

A blood group test shows a person has both A and B antigens on the surface of red blood cells. What is the person's blood group and which antibodies are present in the plasma?

  1. Group O, with both anti-A and anti-B antibodies
  2. Group A, with anti-B antibodies
  3. Group B, with anti-A antibodies
  4. Group AB, with no anti-A or anti-B antibodies Correct answer.

Explanation

The ABO blood group is determined by the presence of A and B antigens, which are codominant. A person with both antigens has group AB and, because the antibodies target antigens not present on their own cells, produces neither anti-A nor anti-B.

Question 45Genetics and Molecular Biology

During which stage of meiosis does crossing over between homologous chromosomes occur?

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

Explanation

Crossing over occurs in prophase I, when homologous chromosomes pair as bivalents and exchange segments between non-sister chromatids. This recombination increases genetic variation.

Question 46Genetics and Molecular Biology

The lac operon in bacteria is expressed at high levels only when

  1. neither glucose nor lactose is present
  2. both glucose and lactose are present
  3. glucose is present and lactose is absent
  4. lactose is present and glucose is absent Correct answer.

Explanation

Lactose inactivates the repressor so transcription can occur, while the absence of glucose raises cAMP levels and allows the CAP activator to bind. Maximal expression therefore requires lactose present and glucose absent.

Question 47Genetics and Molecular Biology

A couple, both with normal vision, has a color-blind son. Color blindness is X-linked recessive. What is the most likely explanation?

  1. A new mutation occurred in the son's somatic cells
  2. The father carries the mutant allele on his X chromosome
  3. The son inherited the mutant allele from his father's Y chromosome
  4. The mother carries the mutant allele on one of her X chromosomes Correct answer.

Explanation

A son receives his X chromosome from his mother and his Y chromosome from his father. Since the father is unaffected he cannot transmit a mutant X, so the mother must be a heterozygous carrier.

Question 48Genetics and Molecular Biology

Which technique separates DNA fragments of different sizes on the basis of their migration through a gel under an electric field?

  1. Polymerase chain reaction
  2. Gel electrophoresis Correct answer.
  3. Centrifugation
  4. Chromatography

Explanation

In gel electrophoresis, negatively charged DNA fragments migrate toward the positive electrode, and smaller fragments move farther through the gel than larger ones. This allows fragments to be separated by size.

Question 49Genetics and Molecular Biology

What is the role of the promoter region of a gene?

  1. It codes for the amino acids of the protein
  2. It is the site where introns are removed
  3. It signals the end of transcription
  4. It is a binding site for RNA polymerase and transcription factors Correct answer.

Explanation

The promoter lies upstream of the coding region and provides a recognition site where RNA polymerase and transcription factors assemble to initiate transcription. It is not translated into protein.

Question 50Genetics and Molecular Biology

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

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

Explanation

The AB father can transmit either the A or the B allele, while the O mother transmits only the O allele. Children therefore receive either A and O (group A) or B and O (group B).