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Lebanese University Faculty of Agronomy Entrance Examination study materials
Lebanese University Faculty of Agronomy Entrance Examination study resources

Lebanese University Faculty of Agronomy Entrance Examination practice questions

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Question 1Biology (cell biology, plant and animal physiology, genetics, taxonomy)

Which plastid is specialized for storing starch in storage organs such as potato tubers?

  1. Amyloplast Correct answer.
  2. Elaioplast
  3. Chloroplast
  4. Chromoplast

Explanation

Amyloplasts are non-pigmented plastids that synthesize and store starch granules, which is why they are abundant in storage organs such as potato tubers. Chloroplasts carry out photosynthesis, chromoplasts accumulate carotenoid pigments, and elaioplasts store lipids.

Question 2Biology (cell biology, plant and animal physiology, genetics, taxonomy)

In a monohybrid cross between two pea plants heterozygous for a gene showing complete dominance (Tt × Tt), what phenotypic ratio is expected among the offspring?

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

Explanation

A Tt × Tt cross produces the genotypes TT, Tt, and tt in a 1:2:1 ratio; with complete dominance, both TT and Tt show the dominant phenotype, so the phenotypic ratio is 3 dominant : 1 recessive. The 1:2:1 ratio describes genotypes rather than phenotypes in this cross.

Question 3Biology (cell biology, plant and animal physiology, genetics, taxonomy)

Which mechanism is the main driving force that lifts water through the xylem of a tall tree during the day?

  1. Cohesion–tension transpiration pull Correct answer.
  2. Root pressure
  3. Phloem loading
  4. Xylem parenchyma pumping

Explanation

During transpiration, water evaporates from leaf mesophyll surfaces and the resulting tension is transmitted down continuous water columns in the xylem. Cohesion between water molecules and adhesion to xylem walls keep the columns intact, so this cohesion–tension mechanism is the main force lifting water in tall trees.

Question 4Biology (cell biology, plant and animal physiology, genetics, taxonomy)

In the mammalian nephron, the largest fraction of filtered glucose is reabsorbed in the:

  1. Proximal convoluted tubule Correct answer.
  2. Distal convoluted tubule
  3. Bowman's capsule
  4. Descending limb of the loop of Henle

Explanation

Filtered glucose is reabsorbed almost completely in the proximal convoluted tubule, where sodium–glucose cotransporters move it back into the blood. More distal segments normally reabsorb little or no glucose, so glucose appearing in final urine indicates that the filtered load exceeded proximal reabsorptive capacity.

Question 5Biology (cell biology, plant and animal physiology, genetics, taxonomy)

In botanical nomenclature, family names are formed by adding which suffix to the stem of a genus name?

  1. -eae
  2. -aceae Correct answer.
  3. -phyta
  4. -ales

Explanation

Botanical family names are formed from the stem of a genus name plus the suffix -aceae, for example Rosaceae from Rosa. The suffix -ales denotes an order, -phyta a division, and -eae a tribe.

Question 6Biology (cell biology, plant and animal physiology, genetics, taxonomy)

When a plant cell is placed in a hypertonic solution, its protoplast shrinks away from the cell wall. This phenomenon is called:

  1. Plasmolysis Correct answer.
  2. Guttation
  3. Imbibition
  4. Turgor

Explanation

In a hypertonic solution, water leaves the cell by osmosis, so the vacuole and protoplast shrink and pull away from the rigid cell wall; this is plasmolysis. Imbibition is water uptake by dry colloids, turgor is the inflated state in a hypotonic medium, and guttation is loss of liquid water through hydathodes.

Question 7Biology (cell biology, plant and animal physiology, genetics, taxonomy)

A man of blood group A (genotype IAi) and a woman of blood group B (genotype IBi) have children. What is the probability that a child will have blood group O?

  1. 1/4 Correct answer.
  2. 1
  3. 0
  4. 1/2

Explanation

The cross IAi × IBi gives four equally likely genotypes: IAIB (AB), IAi (A), IBi (B), and ii (O). Only the ii genotype produces blood group O, so the probability is 1/4.

Question 8Biology (cell biology, plant and animal physiology, genetics, taxonomy)

In C4 plants, the first stable product formed after CO2 fixation in the mesophyll cell is:

  1. Glucose
  2. Oxaloacetate Correct answer.
  3. Ribulose 1,5-bisphosphate
  4. 3-Phosphoglycerate

Explanation

In C4 plants, phosphoenolpyruvate carboxylase fixes CO2 in mesophyll cells to form the four-carbon acid oxaloacetate, which is then converted to malate. 3-Phosphoglycerate is the first stable product of the Calvin cycle in C3 plants, and ribulose 1,5-bisphosphate is the CO2 acceptor rather than a fixation product.

Question 9Biology (cell biology, plant and animal physiology, genetics, taxonomy)

An increase in circulating antidiuretic hormone (ADH) acts mainly on which nephron segment, and what is the resulting change in urine?

  1. Glomerulus - increased filtration rate
  2. Proximal convoluted tubule - more dilute urine
  3. Ascending limb of the loop of Henle - more dilute urine
  4. Collecting duct - more concentrated urine Correct answer.

Explanation

ADH binds receptors on principal cells of the collecting duct and promotes insertion of aquaporin-2 water channels, increasing water reabsorption and producing a smaller volume of more concentrated urine. The proximal tubule and ascending limb are not the main ADH targets, and ADH does not act on the glomerulus to raise filtration rate.

Question 10Biology (cell biology, plant and animal physiology, genetics, taxonomy)

In a large population at Hardy–Weinberg equilibrium, the frequency of a recessive allele is 0.2. What proportion of the individuals is expected to be heterozygous?

  1. 0.32 Correct answer.
  2. 0.04
  3. 0.64
  4. 0.80

Explanation

If q = 0.2, then p = 0.8, and the heterozygous frequency under Hardy–Weinberg equilibrium is 2pq = 2 × 0.8 × 0.2 = 0.32. The value 0.04 is q², the frequency of affected homozygotes, and 0.64 is p², the frequency of dominant homozygotes.

Question 11Biology (cell biology, plant and animal physiology, genetics, taxonomy)

During mitosis in a plant cell, which cytoskeletal structure forms the spindle fibers that separate sister chromatids?

  1. Intermediate filaments
  2. Microtubules Correct answer.
  3. Cellulose microfibrils
  4. Actin microfilaments

Explanation

Spindle fibers are polymers of tubulin, and microtubules are the cytoskeletal elements built from tubulin. They attach to kinetochores and move sister chromatids toward opposite poles during anaphase.

Question 12Biology (cell biology, plant and animal physiology, genetics, taxonomy)

Which cell wall component is characteristic of true fungi and absent from plant cell walls?

  1. Peptidoglycan
  2. Cellulose
  3. Chitin Correct answer.
  4. Lignin

Explanation

Fungal cell walls contain chitin, a polymer of N-acetylglucosamine. Plant cell walls contain cellulose, while peptidoglycan is characteristic of bacterial walls.

Question 13Biology (cell biology, plant and animal physiology, genetics, taxonomy)

Which pigment molecule donates the excited electron that passes to the primary electron acceptor in the reaction center of photosystem II?

  1. Chlorophyll b
  2. Phytochrome
  3. Carotenoid
  4. Chlorophyll a Correct answer.

Explanation

The reaction center of photosystem II contains chlorophyll a, specifically P680, which loses an excited electron to the primary acceptor. Chlorophyll b and carotenoids are accessory pigments that transfer absorbed energy to the reaction center.

Question 14Biology (cell biology, plant and animal physiology, genetics, taxonomy)

In the pressure-flow model, what directly drives bulk flow of sap from source to sink in the phloem?

  1. Capillary action within sieve tubes
  2. A pressure gradient generated by osmotic water movement Correct answer.
  3. Transpiration pull from the leaves
  4. Active pumping by companion cells along the entire pathway

Explanation

Sucrose loading at the source lowers water potential, so water enters the phloem and raises turgor pressure. Unloading at the sink lowers pressure, and this pressure gradient drives bulk flow through sieve tubes.

Question 15Biology (cell biology, plant and animal physiology, genetics, taxonomy)

During the cardiac cycle, closure of the atrioventricular valves occurs at the beginning of which phase?

  1. Isovolumetric ventricular relaxation
  2. Ventricular ejection
  3. Isovolumetric contraction Correct answer.
  4. Atrial systole

Explanation

When ventricular pressure rises above atrial pressure at the start of ventricular systole, the atrioventricular valves close. This brief phase before the semilunar valves open is called isovolumetric contraction.

Question 16Biology (cell biology, plant and animal physiology, genetics, taxonomy)

A female Drosophila heterozygous for a recessive X-linked allele is crossed with a male carrying the dominant wild-type allele. What proportion of their sons is expected to express the recessive phenotype?

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

Explanation

Sons inherit their only X chromosome from the mother, so each son has a 1/2 chance of receiving the recessive allele. The father's X-linked allele is passed only to daughters.

Question 17Biology (cell biology, plant and animal physiology, genetics, taxonomy)

In eukaryotic mitochondria, the proton gradient used by ATP synthase is established across which membrane?

  1. The plasma membrane
  2. The thylakoid membrane
  3. Inner mitochondrial membrane Correct answer.
  4. The outer mitochondrial envelope

Explanation

The electron transport chain pumps protons from the mitochondrial matrix into the intermembrane space across the inner membrane. ATP synthase uses the resulting proton-motive force as protons flow back into the matrix.

Question 18Biology (cell biology, plant and animal physiology, genetics, taxonomy)

Which hormone acts on osteoclasts to increase blood calcium when calcium levels fall?

  1. Calcitonin (thyrocalcitonin)
  2. Insulin
  3. Parathyroid hormone Correct answer.
  4. Aldosterone (mineralocorticoid)

Explanation

Parathyroid hormone is released when blood calcium is low and stimulates osteoclast activity, renal calcium reabsorption, and activation of vitamin D. Calcitonin has the opposite effect and lowers blood calcium.

Question 19Biology (cell biology, plant and animal physiology, genetics, taxonomy)

Two parents are carriers of an autosomal recessive condition. Their first child is unaffected. What is the probability that this unaffected child is a carrier?

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

Explanation

Among unaffected children of two carriers, the possible genotypes are AA, Aa, and Aa, because aa is excluded. Two of these three equally likely unaffected genotypes are heterozygous, so the probability is 2/3.

Question 20Biology (cell biology, plant and animal physiology, genetics, taxonomy)

A C3 leaf in bright light and warm, dry conditions closes its stomata. Which combination best explains the immediate effect on photosynthesis?

  1. CO2 supply decreases and photorespiration increases Correct answer.
  2. CO2 supply increases and photorespiration decreases sharply
  3. Light absorption stops and ATP synthesis increases
  4. Water loss increases and carboxylation increases

Explanation

Stomatal closure restricts CO2 entry, so the CO2 concentration inside the leaf falls. Rubisco then fixes O2 more often, increasing photorespiration and lowering net photosynthesis.

Question 21Biology (cell biology, plant and animal physiology, genetics, taxonomy)

Which membrane-bound organelle contains hydrolytic enzymes that operate at acidic pH and is responsible for degrading engulfed material in animal cells?

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

Explanation

Lysosomes are membrane-bound organelles that maintain an acidic lumen and contain hydrolases such as proteases, lipases, and nucleases. They fuse with endosomes or autophagosomes to digest engulfed material and worn-out organelles. The Golgi apparatus modifies and sorts proteins but is not the main acidic degradative compartment.

Question 22Biology (cell biology, plant and animal physiology, genetics, taxonomy)

Which plant hormone increases under water stress and promotes stomatal closure and seed dormancy?

  1. Cytokinin (CK)
  2. Auxin (IAA)
  3. Abscisic acid (ABA) Correct answer.
  4. Gibberellin (GA)

Explanation

Abscisic acid accumulates during water stress and triggers stomatal closure by promoting ion efflux from guard cells. It also maintains seed dormancy and inhibits premature germination. Auxin, gibberellin, and cytokinin mainly promote growth or developmental transitions rather than stress-induced dormancy.

Question 23Biology (cell biology, plant and animal physiology, genetics, taxonomy)

In the Linnaean taxonomic hierarchy, which rank lies immediately above the rank of class?

  1. Order
  2. Kingdom
  3. Family
  4. Phylum Correct answer.

Explanation

The Linnaean hierarchy from broad to narrow is domain, kingdom, phylum, class, order, family, genus, and species. Therefore phylum is immediately above class, while order is immediately below class. Family is lower than order, and kingdom is above phylum.

Question 24Biology (cell biology, plant and animal physiology, genetics, taxonomy)

Two linked genes in maize are 12 map units apart. In a testcross of a coupling-phase heterozygote (AB/ab × ab/ab), 400 progeny are obtained. What is the expected number of recombinant offspring?

  1. 48 Correct answer.
  2. 24
  3. 96
  4. 12

Explanation

One map unit equals a 1% recombination frequency, so 12 map units predicts 12% recombinants among testcross progeny. Multiplying 0.12 by 400 gives 48 recombinant offspring. The coupling phase affects which recombinant classes appear, not the total recombination frequency.

Question 25Biology (cell biology, plant and animal physiology, genetics, taxonomy)

Winter wheat requires vernalization before it can flower. Which treatment satisfies this requirement?

  1. Cold treatment Correct answer.
  2. Gibberellin spray
  3. Long-day treatment
  4. Short-day treatment

Explanation

Vernalization is the promotion of flowering by prolonged cold exposure, typically experienced by winter annuals or biennials during winter. Winter wheat requires this cold period before it can transition to reproductive growth. Short days, long days, or gibberellin alone do not substitute for the vernalization requirement in this variety.

Question 26Biology (cell biology, plant and animal physiology, genetics, taxonomy)

During the rising phase of an action potential in a mammalian neuron, which ion movement directly produces depolarization?

  1. Sodium influx Correct answer.
  2. Chloride influx
  3. Potassium efflux
  4. Calcium efflux

Explanation

The rising phase of an action potential is caused by opening of voltage-gated sodium channels, allowing sodium ions to enter the axon down their electrochemical gradient. This inward current depolarizes the membrane toward threshold and then toward the sodium equilibrium potential. Potassium efflux mainly drives repolarization, and chloride or calcium movements do not directly produce this rapid depolarization.

Question 27Biology (cell biology, plant and animal physiology, genetics, taxonomy)

The Na+/K+ ATPase moves three sodium ions out and two potassium ions into the cell against their gradients. This transport is best classified as:

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

Explanation

The Na+/K+ ATPase directly hydrolyzes ATP to pump ions against their gradients, so it is primary active transport. Secondary active transport uses the gradient established by such a pump rather than ATP hydrolysis at the transporting protein itself. Simple and facilitated diffusion move solutes down gradients and cannot maintain the observed ion asymmetry.

Question 28Biology (cell biology, plant and animal physiology, genetics, taxonomy)

Two heterozygous plants (AaBb × AaBb) produce progeny with a 9:7 phenotypic ratio. Which genetic interaction best explains this ratio?

  1. Duplicate dominant epistasis
  2. Complementary action Correct answer.
  3. Dominant epistasis
  4. Additive polygenic inheritance

Explanation

A 9:7 ratio appears when the dominant allele at each of two loci is required for the wild-type phenotype, so only A_B_ progeny show the trait. This is complementary gene action, often described as duplicate recessive epistasis. Dominant epistasis gives 12:3:1, duplicate dominant epistasis gives 15:1, and additive polygenic inheritance gives continuous variation.

Question 29Biology (cell biology, plant and animal physiology, genetics, taxonomy)

In a C4 plant, after the initial fixation of CO2 in a mesophyll cell, the carbon is transported to the bundle-sheath cell mainly as which compound?

  1. Ribulose-1,5-bisphosphate
  2. Malate Correct answer.
  3. Sucrose
  4. 3-phosphoglycerate

Explanation

In C4 photosynthesis, PEP carboxylase fixes CO2 into oxaloacetate in mesophyll cells, and this is rapidly converted to malate. Malate moves through plasmodesmata to bundle-sheath cells, where decarboxylation releases CO2 for Rubisco. 3-phosphoglycerate and RuBP are Calvin cycle intermediates, while sucrose is a transport sugar.

Question 30Biology (cell biology, plant and animal physiology, genetics, taxonomy)

Which anatomical feature is characteristic of the stem of a monocotyledonous plant such as maize?

  1. Scattered vascular bundles Correct answer.
  2. Secondary growth from a vascular cambium
  3. Palisade mesophyll throughout the stem
  4. Vascular bundles arranged in a ring

Explanation

Monocot stems such as maize have scattered vascular bundles and lack a distinct ring arrangement of bundles. Dicot stems typically show vascular bundles in a ring and often undergo secondary growth from a vascular cambium. Palisade mesophyll is a leaf tissue, not a general stem feature.

Question 31Biology (cell biology, plant and animal physiology, genetics, taxonomy)

Which organelle receives proteins from the rough endoplasmic reticulum and modifies, sorts, and packages them for secretion or delivery to other compartments?

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

Explanation

The Golgi apparatus is a stack of flattened cisternae that modifies proteins received from the rough endoplasmic reticulum, attaches sugar groups, and sorts them into vesicles for secretion or delivery to other organelles. The rough endoplasmic reticulum synthesizes many of those proteins, but the Golgi carries out the later processing and sorting steps.

Question 32Biology (cell biology, plant and animal physiology, genetics, taxonomy)

In a tall tree, which process generates the negative pressure in leaf air spaces that pulls water upward through the xylem?

  1. Phloem loading at the source leaf
  2. Active transport of water by xylem parenchyma
  3. Transpiration from leaf mesophyll Correct answer.
  4. Root pressure generated by the endodermis

Explanation

Water evaporates from the moist walls of mesophyll cells into leaf air spaces, creating tension that is transmitted through the continuous water column in the xylem. Cohesion between water molecules allows this transpiration pull to draw water upward from the roots.

Question 33Biology (cell biology, plant and animal physiology, genetics, taxonomy)

Under the International Code of Nomenclature for algae, fungi, and plants, how is a scientific binomial written correctly?

  1. Genus and species both lowercase, inside quotation marks
  2. Genus lowercase, species capitalized, both underlined
  3. Both genus and species capitalized and italicized
  4. Genus capitalized, species lowercase, both italicized Correct answer.

Explanation

A binomial consists of the genus name, which is capitalized, followed by the species epithet, which begins with a lowercase letter; both are italicized in printed text. This convention distinguishes scientific names from common names and from higher taxonomic ranks.

Question 34Biology (cell biology, plant and animal physiology, genetics, taxonomy)

In snapdragons, a cross between a red-flowered homozygote and a white-flowered homozygote produces all pink F1 plants. When F1 plants are self-pollinated, what phenotypic ratio is expected in F2?

  1. 1 red : 2 pink : 1 white Correct answer.
  2. 1 red : 1 pink : 2 white
  3. 9 red : 7 pink
  4. 3 red : 1 white

Explanation

The pink F1 plants are heterozygous, and pink is the intermediate phenotype produced by incomplete dominance. Self-pollination of heterozygotes gives a 1:2:1 genotypic ratio, which here is also the phenotypic ratio: one red, two pink, and one white.

Question 35Biology (cell biology, plant and animal physiology, genetics, taxonomy)

What is the direct effect of increased CO2 concentration and lower pH in active skeletal muscle on the hemoglobin-oxygen dissociation curve?

  1. Left shift that raises O2 affinity
  2. Right shift that lowers O2 affinity Correct answer.
  3. No change, because CO2 is carried only by plasma proteins
  4. Right shift that raises O2 affinity

Explanation

Increased CO2 and lower pH reduce hemoglobin's affinity for oxygen, producing a rightward shift of the dissociation curve known as the Bohr effect. This shift promotes oxygen unloading where metabolic activity is high and CO2 is accumulating.

Question 36Biology (cell biology, plant and animal physiology, genetics, taxonomy)

Independent assortment of maternal and paternal homologues occurs during which meiotic stage?

  1. Metaphase I, when homologues align Correct answer.
  2. Anaphase II, as sister chromatids separate
  3. Prophase I, while homologues undergo synapsis
  4. Telophase I, after the first cytokinesis

Explanation

At metaphase I, homologous chromosome pairs orient randomly on the spindle, so the maternal and paternal homologues of different pairs assort independently. This random orientation is the physical basis of independent assortment.

Question 37Biology (cell biology, plant and animal physiology, genetics, taxonomy)

In a root, what is the main consequence of the Casparian strip in the endodermis?

  1. It seals xylem conduits and blocks all water movement
  2. It forces water and solutes through a membrane Correct answer.
  3. It prevents root hairs from absorbing mineral ions
  4. It lets apoplastic water bypass the endodermal cells

Explanation

The Casparian strip is a suberin-rich band in the endodermal cell walls that blocks the apoplastic pathway. Water and dissolved minerals must therefore cross an endodermal plasma membrane to enter the stele, allowing the root to regulate which solutes are taken up.

Question 38Biology (cell biology, plant and animal physiology, genetics, taxonomy)

In the mammalian nephron, the bulk of filtered glucose is reabsorbed by which segment?

  1. Collecting duct
  2. Distal convoluted tubule
  3. Proximal convoluted tubule Correct answer.
  4. Descending limb of the loop of Henle

Explanation

Most filtered glucose is reabsorbed in the proximal convoluted tubule by sodium-dependent glucose cotransporters. Under normal conditions this recovery is nearly complete, so glucose does not normally appear in the final urine.

Question 39Biology (cell biology, plant and animal physiology, genetics, taxonomy)

A man affected by a mitochondrial DNA disorder has children with an unaffected woman. What is expected for their offspring?

  1. All daughters are carriers, while all sons are unaffected
  2. No child inherits it through sperm mitochondria Correct answer.
  3. All sons are affected, while all daughters are unaffected
  4. All children inherit the disorder because the father is affected

Explanation

Mitochondrial DNA is inherited essentially from the oocyte, because sperm mitochondria are normally excluded or degraded after fertilization. Therefore an affected father does not transmit his mitochondrial DNA disorder through sperm, and children of an unaffected mother are not expected to inherit the father's mitochondrial mutation.

Question 40Biology (cell biology, plant and animal physiology, genetics, taxonomy)

A desert cactus fixes CO2 at night and stores it as malate in vacuoles; during daytime stomata close. What is the main advantage of this CAM strategy?

  1. It eliminates the need for the light reactions
  2. It increases photorespiration by keeping stomata closed
  3. It prevents the Calvin cycle from operating during the day
  4. CO2 uptake with minimal water loss for daytime use Correct answer.

Explanation

CAM plants open stomata at night, when humidity is higher and water loss is lower, and store the fixed carbon as malate. During the day they close stomata and release CO2 from malate to the Calvin cycle, allowing photosynthesis while conserving water.

Question 41Biology (cell biology, plant and animal physiology, genetics, taxonomy)

Which organelle houses the enzymes of the citric acid cycle and carries out oxidative phosphorylation in an animal cell?

  1. Mitochondrion Correct answer.
  2. Rough endoplasmic reticulum
  3. Peroxisome
  4. Chloroplast

Explanation

The mitochondrion contains the citric acid cycle enzymes in its matrix and the electron transport chain on its inner membrane; oxidative phosphorylation there produces most cellular ATP. Chloroplasts perform photosynthesis, the rough endoplasmic reticulum modifies proteins, and peroxisomes carry out oxidative reactions such as fatty acid breakdown.

Question 42Biology (cell biology, plant and animal physiology, genetics, taxonomy)

Which mineral nutrient is a central component of the chlorophyll molecule and is therefore required for normal photosynthesis?

  1. Phosphorus
  2. Potassium
  3. Magnesium Correct answer.
  4. Calcium

Explanation

Magnesium occupies the center of the porphyrin ring of chlorophyll, so a deficiency reduces chlorophyll synthesis and causes chlorosis. Calcium, potassium, and phosphorus have other roles, such as cell-wall stability, osmoregulation, and energy transfer.

Question 43Biology (cell biology, plant and animal physiology, genetics, taxonomy)

Which group is characterized by chitinous cell walls, heterotrophic nutrition by absorption, and a body typically composed of hyphae?

  1. Fungi Correct answer.
  2. Mollusks
  3. Bryophytes
  4. Cyanobacteria

Explanation

Fungi have chitin in their cell walls and obtain nutrients by absorbing dissolved organic matter, often through a mycelium of hyphae. Bryophytes are photosynthetic plants, mollusks are animals, and cyanobacteria are prokaryotic photosynthesizers.

Question 44Biology (cell biology, plant and animal physiology, genetics, taxonomy)

During ventricular systole in the mammalian heart, which valve prevents blood from flowing back from the left ventricle into the left atrium?

  1. Aortic valve
  2. Mitral valve Correct answer.
  3. Tricuspid valve
  4. Pulmonary valve

Explanation

The mitral, or bicuspid, valve lies between the left atrium and left ventricle and closes when ventricular pressure exceeds atrial pressure, preventing backflow. The aortic and pulmonary valves guard the exits from the ventricles, and the tricuspid valve separates the right atrium and right ventricle.

Question 45Biology (cell biology, plant and animal physiology, genetics, taxonomy)

A woman who is a carrier of an X-linked recessive disorder has children with an unaffected man. What is the probability that a son of this couple is affected by the disorder?

  1. 1/4
  2. 0
  3. 1
  4. 1/2 Correct answer.

Explanation

A carrier woman has one normal X chromosome and one X chromosome carrying the recessive allele. A son inherits one X from his mother, so half of her sons receive the affected X and are affected; the father contributes a Y and does not influence the son's X-linked genotype.

Question 46Biology (cell biology, plant and animal physiology, genetics, taxonomy)

In the phloem of an angiosperm, which living cell type conducts sugars and lacks a nucleus at maturity?

  1. Phloem companion cell
  2. Xylem tracheid
  3. Sieve tube element Correct answer.
  4. Xylem vessel element

Explanation

Sieve tube elements are the conducting cells of angiosperm phloem; they lose their nucleus at maturity and rely on companion cells for metabolic support. Tracheids and vessel elements are dead xylem cells that conduct water, while companion cells retain nuclei and regulate the sieve tubes.

Question 47Biology (cell biology, plant and animal physiology, genetics, taxonomy)

A cultured animal cell is treated with a drug that blocks DNA synthesis. The cell would arrest before completing which phase of the cell cycle?

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

Explanation

DNA synthesis defines S phase; blocking it prevents the cell from completing S phase and reaching G2. G1 precedes S phase, G2 follows S phase, and M phase is mitosis, which requires prior DNA replication.

Question 48Biology (cell biology, plant and animal physiology, genetics, taxonomy)

In a diploid population at Hardy-Weinberg equilibrium, the frequency of a recessive allele is 0.2. What proportion of the population is expected to be heterozygous?

  1. 0.64
  2. 0.32 Correct answer.
  3. 0.04
  4. 0.16

Explanation

If q = 0.2, then p = 0.8, and the heterozygous frequency is 2pq = 2 × 0.8 × 0.2 = 0.32. The values 0.04 and 0.64 are q² and p², respectively, and 0.16 is not the heterozygous frequency.

Question 49Biology (cell biology, plant and animal physiology, genetics, taxonomy)

Cyclic electron flow around photosystem I in chloroplasts generates ATP without producing NADPH. What is the main physiological advantage of this pathway?

  1. It splits water at photosystem II to release oxygen.
  2. It raises the ATP/NADPH ratio for the Calvin cycle. Correct answer.
  3. It fixes CO2 into malate for nighttime storage.
  4. It triggers stomatal closure via reactive oxygen species.

Explanation

Cyclic electron flow returns electrons from photosystem I to the cytochrome b6f complex, pumping protons and producing ATP while bypassing NADP+ reduction. This adjusts the ATP/NADPH ratio to meet the Calvin cycle's demand, especially when linear electron flow produces relatively more NADPH.

Question 50Biology (cell biology, plant and animal physiology, genetics, taxonomy)

Two independently assorting genes interact to determine seed color, and a dihybrid cross produces progeny in a 12:3:1 phenotypic ratio. Which type of gene interaction does this ratio indicate?

  1. Dominant epistasis Correct answer.
  2. Recessive epistasis
  3. Incomplete dominance
  4. Complementary gene action

Explanation

A 12:3:1 ratio occurs when a dominant allele at one locus masks the expression of both alleles at a second locus, which is dominant epistasis. Recessive epistasis gives 9:3:4, complementary action gives 9:7, and incomplete dominance produces intermediate phenotypes rather than this dihybrid ratio.