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MSCE Biology practice questions

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Question 1Cell biology and cell physiology

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

  1. The lysosome
  2. The Golgi apparatus
  3. The mitochondrion Correct answer.
  4. The rough endoplasmic reticulum

Explanation

The inner mitochondrial membrane houses the electron transport chain and ATP synthase, which together carry out oxidative phosphorylation. Other organelles listed perform biosynthesis, modification, or degradation rather than bulk ATP production.

Question 2Cell biology and cell physiology

A cell is placed in a hypertonic solution. What is the expected immediate osmotic response of an animal cell?

  1. It swells and may lyse
  2. It remains unchanged in volume
  3. It shrinks as water leaves the cell Correct answer.
  4. It actively pumps water inward

Explanation

In a hypertonic environment the external solute concentration exceeds that inside the cell, so water moves out osmotically and the cell shrinks. Animal cells lack a cell wall, so shrinkage rather than turgor pressure results.

Question 3Cell biology and cell physiology

Which structural feature distinguishes prokaryotic cells from eukaryotic cells?

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

Explanation

Prokaryotes keep their genetic material in a nucleoid region without a nuclear envelope, whereas eukaryotes enclose DNA in a true nucleus. Plasma membranes, ribosomes, and DNA are shared by both cell types.

Question 4Cell biology and cell physiology

By which mechanism do small nonpolar molecules such as O2 and CO2 cross the lipid bilayer?

  1. Co-transport with sodium ions
  2. Primary active transport
  3. Receptor-mediated endocytosis
  4. Simple diffusion through the bilayer Correct answer.

Explanation

Small nonpolar gases dissolve in the hydrophobic core of the bilayer and diffuse down their concentration gradient without protein assistance or energy input. The other mechanisms require membrane proteins and, in most cases, energy or a coupling ion.

Question 5Cell biology and cell physiology

Which cytoskeletal element is the principal driver of cytokinesis in animal cells?

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

Explanation

A contractile ring of actin filaments and myosin II assembles at the cell equator and constricts to divide the cytoplasm. Microtubules guide chromosome movement earlier in mitosis but are not the main contractile machinery of the cleavage furrow.

Question 6Cell biology and cell physiology

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

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

Explanation

S phase (synthesis phase) is when the genome is duplicated so each daughter cell receives a complete chromosome set. G1 and G2 are gap phases of growth and checkpoint control, and M phase is mitosis plus cytokinesis.

Question 7Cell biology and cell physiology

Enzymes accelerate biochemical reactions primarily by which mechanism?

  1. Lowering the activation energy of the reaction Correct answer.
  2. Consuming the substrate irreversibly
  3. Raising the temperature of the cytosol
  4. Increasing the overall free-energy change of the reaction

Explanation

Enzymes stabilize the transition state and thereby reduce the activation energy barrier, increasing reaction rate without altering the equilibrium constant. They are not consumed and do not change the net free-energy difference between substrates and products.

Question 8Cell biology and cell physiology

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

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

Explanation

Hydrolysis of ATP's terminal phosphate bond releases free energy that drives transport, mechanical, and biosynthetic work directly. Glucose and pyruvate are fuel molecules whose energy must first be converted, and NADH is an electron carrier.

Question 9Cell biology and cell physiology

A cell is treated with ouabain, which inhibits the Na+/K+-ATPase. Which immediate consequence is most expected?

  1. Dissolution of the nuclear envelope
  2. Failure of ribosomes to translate mRNA
  3. Immediate block of mitochondrial ATP synthase
  4. Loss of the resting membrane potential gradient for Na+ and K+ Correct answer.

Explanation

The Na+/K+-ATPase maintains the steep Na+ and K+ gradients that underlie the resting membrane potential and secondary active transport. Inhibiting it causes these gradients to run down over time as ions leak across the membrane.

Question 10Cell biology and cell physiology

In the electron transport chain, what is the ultimate role of oxygen?

  1. It phosphorylates ADP directly
  2. It acts as the final electron acceptor, forming water Correct answer.
  3. It transports protons back into the matrix
  4. It donates electrons to complex I

Explanation

Oxygen accepts electrons at complex IV and combines with protons to form water, keeping the chain running. This maintains the proton gradient that ATP synthase uses, but oxygen itself does not phosphorylate ADP or transport protons.

Question 11Cell biology and cell physiology

Which statement best describes the function of the signal recognition particle (SRP) in protein sorting?

  1. It adds mannose residues in the Golgi lumen
  2. It transports folded proteins through nuclear pores
  3. It binds signal peptides on nascent chains and targets the ribosome to the ER Correct answer.
  4. It cleaves signal sequences after translation is complete

Explanation

SRP recognizes the hydrophobic signal peptide emerging from the ribosome, pauses translation, and docks the complex at the SRP receptor on the ER membrane. This allows co-translational translocation into the ER lumen or membrane.

Question 12Cell biology and cell physiology

A mutation eliminates the G1/S checkpoint control. What is the most likely consequence for the cell?

  1. It enters S phase despite unfavorable conditions or damage Correct answer.
  2. It replicates damaged DNA without repair
  3. It fails to complete cytokinesis
  4. It arrests permanently in mitosis

Explanation

The G1/S checkpoint verifies cell size, nutrients, and DNA integrity before replication. Losing this control allows premature entry into S phase, permitting propagation of damage.

Question 13Cell biology and cell physiology

Which property of the lipid bilayer most directly restricts the free passage of large polar molecules such as glucose?

  1. The glycocalyx on the outer surface
  2. The presence of cholesterol in the bilayer
  3. The hydrophobic core of the fatty acid tails Correct answer.
  4. The fluidity of the membrane at 37 °C

Explanation

The hydrophobic interior formed by fatty acid tails repels polar and charged solutes, creating a permeability barrier. Glucose therefore requires specific transport proteins to cross.

Question 14Cell biology and cell physiology

During glycolysis, which step generates ATP by substrate-level phosphorylation?

  1. Conversion of glucose to glucose-6-phosphate
  2. Cleavage of fructose-1,6-bisphosphate
  3. Transfer of a phosphate from phosphoenolpyruvate to ADP Correct answer.
  4. Reduction of pyruvate to lactate

Explanation

Pyruvate kinase transfers the phosphate of phosphoenolpyruvate to ADP, forming ATP directly by substrate-level phosphorylation. The other listed steps either consume ATP or do not produce it.

Question 15Cell biology and cell physiology

A cell is shifted to a medium lacking extracellular Ca2+. Which process is most directly impaired?

  1. Exocytosis triggered by an intracellular calcium signal Correct answer.
  2. Replication of mitochondrial DNA
  3. Transcription of nuclear genes
  4. Assembly of ribosomal subunits

Explanation

Calcium influx or release raises cytosolic Ca2+, which triggers SNARE-mediated vesicle fusion with the plasma membrane during exocytosis. Removing external Ca2+ depletes this trigger for regulated secretion.

Question 16Cell biology and cell physiology

Which feature of the fluid mosaic model explains why membrane proteins can move laterally within the bilayer?

  1. Proteins are covalently anchored to the cytoskeleton at fixed positions
  2. The hydrophobic core is rigid and crystalline
  3. The bilayer behaves as a two-dimensional fluid Correct answer.
  4. Cholesterol immobilizes all proteins

Explanation

The model treats the membrane as a fluid two-dimensional solvent in which lipids and many proteins diffuse laterally. Anchoring or crowding can restrict motion, but the default state is lateral mobility.

Question 17Cell biology and cell physiology

Which of the following best describes the role of the spindle assembly checkpoint?

  1. It ensures that the nuclear envelope reforms after telophase
  2. It checks for DNA damage before S phase
  3. It verifies that all chromosomes are properly attached to spindle microtubules before anaphase Correct answer.
  4. It monitors nutrient availability before G1

Explanation

The spindle assembly checkpoint delays anaphase until every kinetochore is attached to microtubules from opposite poles and under tension. This prevents chromosome missegregation.

Question 18Cell biology and cell physiology

In a cell, a drug dissipates the proton gradient across the inner mitochondrial membrane without blocking electron transport. What is the expected effect?

  1. ATP synthesis decreases while electron transport continues Correct answer.
  2. ATP synthesis increases because protons return faster
  3. Electron transport halts immediately
  4. Glycolysis stops because glucose cannot enter

Explanation

ATP synthase requires a proton-motive force; uncoupling dissipates it, so ATP production falls even though electrons keep flowing (and oxygen consumption may rise). This is the classic uncoupler effect.

Question 19Cell biology and cell physiology

Which statement correctly distinguishes facilitated diffusion from active transport?

  1. Facilitated diffusion requires vesicle formation
  2. Facilitated diffusion moves solutes down their gradient without energy input Correct answer.
  3. Facilitated diffusion uses ATP; active transport does not
  4. Active transport can only move solutes down their gradient

Explanation

Facilitated diffusion uses membrane proteins to move solutes down their electrochemical gradient without energy expenditure. Active transport moves solutes against a gradient and requires energy, often from ATP hydrolysis.

Question 20Cell biology and cell physiology

A lysosomal enzyme is synthesized but lacks the mannose-6-phosphate tag. What is the most likely fate of the enzyme?

  1. It is degraded in the cytosol immediately after synthesis
  2. It is imported into the nucleus
  3. It is secreted outside the cell by default Correct answer.
  4. It is incorporated into the mitochondrial matrix

Explanation

Mannose-6-phosphate is the sorting signal that directs soluble hydrolases to endosomes and then lysosomes. Without it, the protein follows the default secretory pathway and is released from the cell.

Question 21Cell biology and cell physiology

A cell accumulates misfolded proteins in the ER lumen. Which response is most directly activated?

  1. Activation of caspases by death receptors
  2. The unfolded protein response, reducing translation and increasing chaperone expression Correct answer.
  3. The spindle assembly checkpoint
  4. Upregulation of the sodium-potassium pump

Explanation

ER stress sensors (IRE1, PERK, ATF6) trigger the unfolded protein response, which dampens translation and boosts chaperones and ER-associated degradation. This restores folding capacity or, if unresolved, promotes apoptosis.

Question 22Cell biology and cell physiology

In patch-clamp recordings, a channel's current-voltage relationship is linear and reverses at the Nernst potential for K+. Which conclusion is best supported?

  1. The channel is a nonselective cation channel
  2. The channel is a mechanosensitive channel
  3. The channel is an ATP-binding cassette transporter
  4. The channel is highly selective for K+ and ohmic under these conditions Correct answer.

Explanation

A reversal potential equal to the K+ equilibrium potential indicates high K+ selectivity, and a linear I-V curve indicates ohmic behavior over the tested range. Nonselective channels would reverse near 0 mV.

Question 23Cell biology and cell physiology

A dividing cell is treated with a drug that stabilizes microtubules and prevents their depolymerization. Which mitotic event is most directly blocked?

  1. Nuclear envelope breakdown
  2. Chromosome congression to the metaphase plate
  3. Anaphase chromosome separation Correct answer.
  4. Condensin-mediated chromosome condensation

Explanation

Anaphase depends on microtubule depolymerization and motor-driven shortening of kinetochore fibers to pull chromatids poleward. Stabilizing microtubules permits spindle formation but prevents the shortening needed for segregation.

Question 24Cell biology and cell physiology

A mutation in a G-protein-coupled receptor prevents GTP hydrolysis by its associated Gα subunit. What is the predicted signaling outcome?

  1. Signaling terminates rapidly after ligand binding
  2. The receptor is degraded in lysosomes immediately
  3. Ligand binding no longer occurs
  4. Signaling remains persistently active after stimulation Correct answer.

Explanation

GTP hydrolysis is the built-in timer that returns Gα to the inactive GDP state. If hydrolysis is blocked, Gα stays GTP-bound and keeps the effector active long after the ligand is removed.

Question 25Cell biology and cell physiology

Which observation would best support the conclusion that a vesicle-fusion event is SNARE-dependent?

  1. Fusion occurs only at 4 °C
  2. Fusion requires extracellular calcium influx
  3. Fusion still occurs after SNARE proteins are cleaved by a specific protease
  4. Fusion is abolished when SNARE proteins are cleaved by a specific protease Correct answer.

Explanation

If cleaving SNARE proteins with a specific protease abolishes fusion, the process depends on those proteins. A rescue experiment restoring SNARE function would further strengthen the conclusion.

Question 26Cell biology and cell physiology

Which cytoskeletal element is the principal structural component of microvilli at the apical surface of an intestinal epithelial cell?

  1. Microtubules
  2. Actin filaments Correct answer.
  3. Intermediate filaments
  4. Lamin filaments

Explanation

Microvilli are supported by bundles of actin filaments cross-linked by proteins such as villin and fimbrin, which anchor the core to the terminal web. Microtubules form the axoneme of cilia rather than microvilli, and intermediate filaments provide tensile strength without forming these core bundles.

Question 27Cell biology and cell physiology

A cell is placed in a hypotonic solution. Which sequence of events best describes the immediate osmotic response?

  1. Water enters and exits equally, so cell volume stays constant
  2. Water enters the cell, volume increases, and the membrane may stretch Correct answer.
  3. Solute diffuses out to equalize tonicity without water movement
  4. Water leaves the cell, volume decreases, and the cell shrinks

Explanation

In a hypotonic solution the extracellular solute concentration is lower, so the osmotic gradient drives water inward across the plasma membrane. The resulting increase in cell volume can stretch the membrane and, if severe, cause lysis.

Question 28Cell biology and cell physiology

A poison binds the F1 head of ATP synthase and blocks proton flow through its c-subunit ring. In an otherwise intact mitochondrion with adequate substrates and oxygen, which outcome is most expected?

  1. Substrate-level phosphorylation in the matrix stops immediately
  2. Electron transport accelerates because ATP synthase is bypassed
  3. The proton gradient collapses because electrons are rerouted to oxygen
  4. Electron transport slows because the proton gradient cannot be discharged Correct answer.

Explanation

Blocking proton translocation through ATP synthase prevents dissipation of the electrochemical gradient, so the gradient rises and back-pressure inhibits electron transport. Oxygen consumption therefore falls even though oxygen is present, a pattern known as respiratory control.

Question 29Cell biology and cell physiology

A researcher measures the rate of carrier-mediated glucose uptake across a plasma membrane as extracellular glucose rises. At high concentrations the rate plateaus. Which explanation best accounts for this plateau?

  1. Carriers are permanently inactivated by high glucose
  2. All carrier binding sites are saturated, so transport reaches Vmax Correct answer.
  3. Glucose begins to diffuse directly through the lipid bilayer
  4. The concentration gradient reverses and drives efflux

Explanation

Facilitated diffusion depends on a finite number of carrier proteins, so once every binding site is occupied the transport rate cannot increase further. This saturation kinetics distinguishes carrier-mediated transport from simple diffusion, which rises linearly with concentration.

Question 30Cell biology and cell physiology

In a cell-free system, a protein normally retained in the ER lumen is engineered so that its C-terminal KDEL sequence is deleted. Which fate is most consistent with current knowledge of retrieval signals?

  1. The protein is degraded in the cytosol immediately after synthesis
  2. The protein is secreted because it can no longer be retrieved from the Golgi Correct answer.
  3. The protein is directed to mitochondria because its ER signal is masked
  4. The protein is retained in the ER because KDEL is not required for retention

Explanation

KDEL is a retrieval signal recognized by KDEL receptors in the Golgi that return escaped ER luminal proteins via COPI vesicles. Removing it allows the protein to pass the Golgi and enter the secretory pathway by default.

Question 31Nutrition in plants and animals

Which structure in a plant root is the main site of water and mineral ion absorption?

  1. Xylem vessels
  2. Phloem sieve tubes
  3. Root hair cells Correct answer.
  4. Root cap cells

Explanation

Root hair cells are long, thin extensions of epidermal cells that greatly increase the surface area available for osmosis of water and active uptake of mineral ions from the soil solution. Xylem and phloem are transport tissues located deeper in the root, not the absorbing surface.

Question 32Nutrition in plants and animals

By which process does water move from soil into a root hair cell?

  1. Osmosis Correct answer.
  2. Diffusion of water through aquaporins against a gradient
  3. Active transport
  4. Phagocytosis

Explanation

Soil water contains a lower concentration of dissolved solutes than the cell sap of the root hair cell, so water moves down its water potential gradient through the partially permeable membrane by osmosis. This is passive and requires no energy from the plant.

Question 33Nutrition in plants and animals

Which feature of leaves is most directly an adaptation for efficient gas exchange during photosynthesis?

  1. Thick lignified walls in xylem vessels
  2. A waxy cuticle on the upper epidermis
  3. A large central vacuole in each mesophyll cell
  4. Air spaces between spongy mesophyll cells Correct answer.

Explanation

The network of air spaces in the spongy mesophyll allows carbon dioxide to diffuse rapidly through the leaf interior to reach photosynthesising cells, and oxygen to diffuse out. This internal air system shortens diffusion distances.

Question 34Nutrition in plants and animals

Which region of the mammalian digestive system is the principal site of absorption of digested nutrients?

  1. Small intestine Correct answer.
  2. Stomach
  3. Large intestine
  4. Oesophagus

Explanation

The small intestine has a very large surface area created by villi and microvilli, a thin epithelium and a rich blood supply, making it the main site of absorption of sugars, amino acids and fatty acids. The other regions absorb little or none of these products.

Question 35Nutrition in plants and animals

Which enzyme digests starch into maltose in the human digestive system?

  1. Trypsin
  2. Pepsin
  3. Amylase Correct answer.
  4. Lipase

Explanation

Amylase hydrolyses the glycosidic bonds in starch, producing maltose; it is secreted in saliva and by the pancreas into the small intestine. The other listed enzymes act on lipids or proteins.

Question 36Nutrition in plants and animals

Which statement describes the role of bile in digestion?

  1. It emulsifies fats into small droplets, increasing the surface area for lipase. Correct answer.
  2. It absorbs glucose across the intestinal wall.
  3. It neutralises gastric acid and digests proteins.
  4. It hydrolyses triglycerides into fatty acids and glycerol.

Explanation

Bile salts emulsify large fat globules into many small droplets, which greatly increases the total surface area exposed to lipase and speeds fat digestion. Bile is not itself an enzyme, so it does not hydrolyse fats.

Question 37Nutrition in plants and animals

Which of the following is a macronutrient required by plants in relatively large amounts?

  1. Molybdenum
  2. Zinc
  3. Nitrate Correct answer.
  4. Copper

Explanation

Nitrate is a major source of nitrogen, which plants need in large quantities for making amino acids, proteins and nucleic acids. Zinc, copper and molybdenum are micronutrients required only in trace amounts.

Question 38Nutrition in plants and animals

Which structure increases the surface area of the mammalian small intestine for absorption?

  1. Villi Correct answer.
  2. Alveoli
  3. Nephrons
  4. Cilia

Explanation

Villi are finger-like projections of the intestinal lining, and their epithelial cells bear microvilli, together multiplying the absorptive surface many times. Alveoli, nephrons and cilia belong to other systems.

Question 39Nutrition in plants and animals

A plant is grown in a solution lacking magnesium ions. Which symptom is most likely to appear first?

  1. Brown necrotic spots on young leaves
  2. Purple discolouration of young leaves
  3. Wilting of the whole shoot
  4. Yellowing between the veins of older leaves Correct answer.

Explanation

Magnesium is a central component of chlorophyll, so deficiency reduces chlorophyll synthesis and causes chlorosis, a yellowing between veins. Because magnesium is mobile in the plant, it is relocated from older leaves to growing points, so older leaves show symptoms first.

Question 40Nutrition in plants and animals

Why does the rate of water uptake by a plant increase on a hot, dry, windy day?

  1. Photosynthesis produces more water that must be expelled.
  2. Transpiration from leaves increases, steepening the water potential gradient. Correct answer.
  3. Root hair cells become more permeable to mineral ions.
  4. Stomata close, reducing water loss from the leaf.

Explanation

Warm, dry, moving air increases the evaporation of water from mesophyll surfaces and its diffusion out of the leaf, raising the transpiration rate. This steepens the water potential gradient from soil to leaf, so more water is drawn up and absorbed by the roots.

Question 41Nutrition in plants and animals

A student measures the water potential of a root hair cell as -400 kPa and of the surrounding soil solution as -200 kPa. What is the expected direction of water movement?

  1. From the root hair cell into the soil
  2. From soil into the root hair cell Correct answer.
  3. No net movement because both values are negative
  4. Out of the cell until both reach -600 kPa

Explanation

Water moves from higher (less negative) water potential to lower (more negative) water potential. The soil at -200 kPa has the higher water potential, so water moves from soil into the cell at -400 kPa.

Question 42Nutrition in plants and animals

Which combination of features best explains how the ileum absorbs glucose efficiently?

  1. Thick muscle layer, mucus secretion and acidic pH
  2. Keratinised surface, low blood flow and thick walls
  3. Microvilli, thin epithelium and a dense capillary network Correct answer.
  4. Ciliated epithelium, cartilage rings and goblet cells

Explanation

Microvilli multiply the apical surface area, the thin epithelium shortens the diffusion distance, and the dense capillaries maintain a steep concentration gradient by carrying absorbed glucose away. Together these maximise the rate of absorption.

Question 43Nutrition in plants and animals

A person's gall bladder is removed and bile flows continuously into the small intestine. Which digestive problem is most likely?

  1. Poor emulsification of a large fatty meal, slowing fat digestion Correct answer.
  2. Inability to absorb water in the colon
  3. Complete failure to digest proteins
  4. Loss of starch digestion in the mouth

Explanation

Bile normally is stored and released in a concentrated burst when fatty food enters the duodenum. Without this, a large fatty meal is emulsified less effectively, so lipase has less surface area and fat digestion is slower.

Question 44Nutrition in plants and animals

Which process allows nitrate ions to be taken up by root hair cells even when the soil nitrate concentration is lower than that inside the cell?

  1. Active transport using energy from respiration Correct answer.
  2. Simple diffusion through the phospholipid bilayer
  3. Osmosis down a water potential gradient
  4. Transpiration pull in the xylem

Explanation

When soil ion concentration is lower than inside the cell, uptake must work against the concentration gradient, which requires active transport powered by ATP from respiration. This is why root uptake of minerals is reduced in waterlogged, oxygen-poor soils.

Question 45Nutrition in plants and animals

Which dental tissue is hardest and most highly mineralised, forming the outer crown of a tooth?

  1. Cementum
  2. Dentine
  3. Pulp
  4. Enamel Correct answer.

Explanation

Enamel is the hardest tissue in the body, composed mainly of calcium phosphate mineral, and it covers the crown of the tooth. It protects the softer dentine beneath it.

Question 46Nutrition in plants and animals

How does mechanical digestion in the mouth assist chemical digestion?

  1. Chewing kills bacteria that would digest the food.
  2. Chewing adds lipase to begin fat digestion.
  3. Chewing raises the pH of the mouth to activate pepsin.
  4. Chewing breaks food into smaller pieces, increasing the surface area for enzyme action. Correct answer.

Explanation

Chewing fragments food into smaller particles and mixes it with saliva, exposing a much greater surface area to salivary amylase. Enzyme-catalysed hydrolysis is faster when substrate surface area is larger.

Question 47Nutrition in plants and animals

Which statement correctly relates the fate of absorbed amino acids in the body?

  1. They are stored unchanged in adipose tissue as an energy reserve.
  2. They are converted directly into glucose by amylase.
  3. They are carried to the liver and used to synthesise proteins or deaminated. Correct answer.
  4. They are absorbed into lacteals as chylomicrons.

Explanation

Amino acids absorbed by the small intestine pass via the hepatic portal vein to the liver, where they are used for protein synthesis or deaminated, with the amino group converted to urea and the carbon skeleton used for energy or storage.

Question 48Nutrition in plants and animals

A plant is grown in waterlogged soil. Which explanation best accounts for the resulting yellowing of leaves?

  1. Oxygen lack in soil reduces active ion uptake, limiting nitrogen and magnesium for chlorophyll. Correct answer.
  2. Waterlogging increases transpiration, so leaves dry out and lose pigment.
  3. Excess water dilutes the chlorophyll inside chloroplasts.
  4. Waterlogging raises soil temperature, denaturing chlorophyll.

Explanation

Waterlogged soil has little oxygen, so root respiration and the ATP supply for active transport of mineral ions fall. Reduced uptake of nitrogen and magnesium limits chlorophyll synthesis, producing chlorosis.

Question 49Nutrition in plants and animals

Which feature of a xylem vessel is most important for maintaining a continuous water column under tension?

  1. Lignified walls that prevent collapse Correct answer.
  2. Sieve plates that regulate flow
  3. Companion cells that load sucrose
  4. Perforated end walls that allow cytoplasmic streaming

Explanation

Water in the xylem is under negative pressure (tension) during transpiration, which tends to pull the walls inward. Lignified thickening gives the vessels rigid walls that resist collapse and maintain the lumen.

Question 50Nutrition in plants and animals

Which adaptation of a leaf reduces water loss while still allowing carbon dioxide entry for photosynthesis?

  1. Large air spaces throughout the mesophyll
  2. Wide, thin leaves with stomata on both surfaces
  3. Stomata concentrated on the lower surface with a thick cuticle on the upper surface Correct answer.
  4. A single layer of epidermal cells with no cuticle

Explanation

Placing most stomata on the shaded lower surface and covering the upper surface with a thick waxy cuticle reduces evaporation from the sunlit side while still permitting gas exchange. This balances water conservation with CO2 uptake.