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JCE Biology practice questions

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Question 1Cell Biology and Cell Processes

A student observes a cell with a rigid outer layer, ribosomes, and a single circular chromosome, but no membrane-bound nucleus. Which cell type is described?

  1. Plant cell
  2. Animal cell
  3. Prokaryotic cell Correct answer.
  4. Fungal cell

Explanation

A cell lacking a membrane-bound nucleus and containing a single circular chromosome plus a rigid wall is prokaryotic, as in bacteria and archaea. Ribosomes are present in prokaryotes, so their presence does not contradict this classification.

Question 2Cell Biology and Cell Processes

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

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

Explanation

The inner mitochondrial membrane houses the electron transport chain and ATP synthase, which together carry out oxidative phosphorylation. The proton gradient across that membrane drives ATP production.

Question 3Cell Biology and Cell Processes

A protein destined for secretion is synthesized on ribosomes attached to the rough endoplasmic reticulum. Through which sequence of compartments does it normally pass?

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

Explanation

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

Question 4Cell Biology and Cell Processes

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

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

Explanation

DNA synthesis takes place during S phase, which lies between G1 and G2 in interphase. By the end of S phase each chromosome consists of two sister chromatids.

Question 5Cell Biology and Cell Processes

Which structural feature of the plasma membrane is best described by the fluid mosaic model?

  1. A phospholipid bilayer with embedded and peripheral proteins that can move laterally Correct answer.
  2. A static lattice of cholesterol molecules with no protein component
  3. A rigid bilayer of proteins with embedded phospholipids
  4. A single layer of phospholipids coated on both surfaces by carbohydrate

Explanation

The fluid mosaic model describes a phospholipid bilayer in which proteins are embedded or associated and can move laterally within the membrane. Membrane fluidity is influenced by lipid composition and cholesterol.

Question 6Cell Biology and Cell Processes

A molecule moves across the plasma membrane from a region of lower concentration to a region of higher concentration using ATP. Which transport mechanism is this?

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

Explanation

Movement against a concentration gradient requires energy, which defines active transport. When ATP is hydrolyzed directly by the transporter, as with the sodium-potassium pump, the process is primary active transport.

Question 7Cell Biology and Cell Processes

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

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

Explanation

Helicase breaks the hydrogen bonds between complementary base pairs, separating the strands so that replication machinery can access each template. The resulting fork moves along the duplex as synthesis proceeds.

Question 8Cell Biology and Cell Processes

Which statement correctly distinguishes prokaryotic from eukaryotic ribosomes?

  1. Prokaryotic ribosomes lack RNA and consist only of protein
  2. Both types are 70S but differ in RNA content
  3. Prokaryotic ribosomes are 80S while eukaryotic cytosolic ribosomes are 70S
  4. Prokaryotic ribosomes are 70S while eukaryotic cytosolic ribosomes are 80S Correct answer.

Explanation

Bacterial ribosomes are 70S, composed of 50S and 30S subunits, whereas eukaryotic cytosolic ribosomes are 80S, composed of 60S and 40S subunits. This difference underlies the selective action of several antibiotics.

Question 9Cell Biology and Cell Processes

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

  1. Water enters and the cell swells
  2. The cell maintains its volume unchanged
  3. Water leaves and the cell shrinks Correct answer.
  4. Solutes move out until equilibrium is reached

Explanation

In a hypertonic solution the external solute concentration is higher, so water potential outside is lower. Water leaves the cell by osmosis, causing shrinkage, which in animal cells can lead to crenation.

Question 10Cell Biology and Cell Processes

In the sodium-potassium pump, what is the stoichiometry of ions moved per ATP hydrolyzed?

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

Explanation

Each cycle of the pump exports three sodium ions and imports two potassium ions while hydrolyzing one ATP. This unequal exchange contributes to the negative resting membrane potential.

Question 11Cell Biology and Cell Processes

Which event occurs during anaphase of mitosis?

  1. Sister chromatids separate and move toward opposite poles Correct answer.
  2. The nuclear envelope reforms around each chromosome set
  3. Chromosomes align at the metaphase plate
  4. Chromosomes condense and the nuclear envelope breaks down

Explanation

Anaphase begins when cohesin is cleaved and sister chromatids are pulled apart by shortening kinetochore microtubules. Each chromatid becomes an independent chromosome moving toward its pole.

Question 12Cell Biology and Cell Processes

A mutation changes a codon from UAU to UAC. Both codons specify tyrosine. What type of mutation is this?

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

Explanation

Because the amino acid sequence is unchanged, the mutation is silent, reflecting the degeneracy of the genetic code. The protein product is expected to be identical in sequence.

Question 13Cell Biology and Cell Processes

During glycolysis, which net products are formed per molecule of glucose?

  1. 2 ATP, 2 NADH, and 2 pyruvate Correct answer.
  2. 1 ATP, 1 NADH, and 1 pyruvate
  3. 4 ATP, 4 NADH, and 2 pyruvate
  4. 2 ATP, 2 FADH2, and 2 acetyl-CoA

Explanation

Glycolysis consumes two ATP in its preparatory phase and produces four ATP by substrate-level phosphorylation, giving a net gain of two ATP. Two NADH and two pyruvate molecules are also generated per glucose.

Question 14Cell Biology and Cell Processes

Which statement best describes the role of the Golgi apparatus in protein processing?

  1. It modifies proteins with carbohydrate groups and sorts them to destinations Correct answer.
  2. It degrades misfolded proteins using proteases
  3. It synthesizes the polypeptide chain from mRNA
  4. It produces ATP for protein folding

Explanation

The Golgi receives proteins from the ER and carries out glycosylation and other modifications, then sorts them into vesicles for delivery to lysosomes, the plasma membrane, or secretion. Its stacked cisternae are organized from cis to trans faces.

Question 15Cell Biology and Cell Processes

A drug blocks the formation of the mitotic spindle. At which stage will a dividing cell most likely arrest?

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

Explanation

The spindle checkpoint operates during metaphase and delays anaphase until all chromosomes are properly attached to spindle microtubules. Without a spindle, chromosomes cannot align and the checkpoint keeps the cell arrested.

Question 16Cell Biology and Cell Processes

Which property of phospholipids is primarily responsible for their spontaneous assembly into bilayers in water?

  1. Their amphipathic nature, with hydrophilic heads and hydrophobic tails Correct answer.
  2. Their ability to form covalent bonds with water molecules
  3. Their rigid planar structure that stacks like sheets
  4. Their net positive charge at physiological pH

Explanation

Phospholipids have a hydrophilic phosphate head and hydrophobic hydrocarbon tails, so in water they arrange with tails inward and heads outward. This amphipathic behavior drives bilayer formation by hydrophobic effect.

Question 17Cell Biology and Cell Processes

A red blood cell is placed in distilled water. Which observation is expected?

  1. It remains unchanged because it is isotonic to water
  2. It swells and may lyse due to water uptake Correct answer.
  3. It actively pumps water out to maintain volume
  4. It shrinks due to water loss

Explanation

Distilled water is hypotonic relative to cytoplasm, so water enters the cell by osmosis. Lacking a cell wall, the red blood cell swells and eventually bursts, a process called hemolysis.

Question 18Cell Biology and Cell Processes

Which feature of the genetic code allows a single tRNA anticodon to recognize more than one codon?

  1. The code is universal across organisms
  2. The code is overlapping and reads two bases at a time
  3. The code is degenerate and wobble pairing occurs at the third codon position Correct answer.
  4. The code contains equal numbers of codons and amino acids

Explanation

Degeneracy means several codons specify the same amino acid, and wobble base pairing at the third codon position allows one tRNA to read multiple synonymous codons. This reduces the number of tRNA species needed.

Question 19Cell Biology and Cell Processes

A cell lacks functional lysosomes. Which process would be most directly impaired?

  1. Transport of glucose across the plasma membrane
  2. Formation of the mitotic spindle
  3. Synthesis of ribosomal RNA in the nucleolus
  4. Breakdown of engulfed bacteria and worn-out organelles Correct answer.

Explanation

Lysosomes contain acid hydrolases that degrade material taken up by endocytosis and components delivered by autophagy. Without them, intracellular digestion and organelle turnover would be severely impaired.

Question 20Cell Biology and Cell Processes

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

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

Explanation

Oxygen accepts electrons at complex IV and combines with protons to form water, keeping electrons flowing through the chain. Without oxygen as terminal acceptor, the chain and oxidative phosphorylation would halt.

Question 21Cell Biology and Cell Processes

A cell is treated with a drug that inhibits the enzyme responsible for removing RNA primers during DNA replication. Which outcome is most likely?

  1. DNA polymerase would incorporate RNA nucleotides instead of DNA nucleotides
  2. Helicase would be unable to unwind the double helix
  3. The DNA strands would remain permanently joined and unable to separate
  4. Okazaki fragments would remain unjoined, leaving nicks in the new strand Correct answer.

Explanation

RNA primers must be removed and replaced with DNA before ligase can seal the gaps between Okazaki fragments. If primer removal fails, the fragments cannot be joined and the lagging strand retains nicks.

Question 22Cell Biology and Cell Processes

A mutation in a gene encoding a Golgi-resident glycosyltransferase causes the enzyme to be secreted instead of retained. What is the most likely explanation?

  1. The enzyme gained a mitochondrial targeting sequence
  2. The enzyme lost the retention signal that targets it to the Golgi Correct answer.
  3. The enzyme lost its signal peptide, so it can no longer enter the ER
  4. The enzyme was unable to bind ATP for transport

Explanation

Golgi-resident proteins carry retention or retrieval signals that keep them in the correct cisterna; loss of such a signal allows default bulk flow to the plasma membrane. Secretion reflects failure of retention rather than failure of ER entry.

Question 23Cell Biology and Cell Processes

A cell has a mutation that prevents the phosphorylation of a key cyclin-dependent kinase substrate required for entry into S phase. Which checkpoint is most directly affected?

  1. The G1/S restriction point Correct answer.
  2. The cytokinesis checkpoint
  3. The DNA damage checkpoint in G2
  4. The spindle assembly checkpoint

Explanation

The G1/S transition is controlled by cyclin-dependent kinase activity that phosphorylates substrates licensing DNA replication. Blocking that phosphorylation prevents commitment to S phase at the restriction point.

Question 24Cell Biology and Cell Processes

A researcher measures a higher rate of oxygen consumption in mitochondria supplied with an uncoupling agent. What explains this observation?

  1. The proton gradient is increased, stimulating ATP synthase
  2. ATP synthase reverses and hydrolyzes ATP to maintain the gradient
  3. Electrons bypass complex IV and reduce oxygen directly in the matrix
  4. The proton gradient is dissipated, so electron transport accelerates but ATP synthesis falls Correct answer.

Explanation

Uncouplers carry protons across the inner membrane, short-circuiting the gradient that normally restrains electron transport. Respiration speeds up to restore proton pumping, but the energy is released as heat and ATP output decreases.

Question 25Cell Biology and Cell Processes

In a cell, a protein that normally resides in the cytosol is found inside the nucleus. Which change to the protein would best explain this relocalization?

  1. Addition of a nuclear localization signal Correct answer.
  2. Removal of all positively charged amino acids
  3. Addition of a hydrophobic transmembrane domain
  4. Addition of a mannose-6-phosphate tag

Explanation

Nuclear localization signals are short basic sequences recognized by importin, which mediates transport through the nuclear pore. Adding such a signal would redirect a cytosolic protein into the nucleus.

Question 26Cell Biology and Cell Processes

Which structural feature allows the rough endoplasmic reticulum to synthesize proteins that are inserted into membranes or secreted from the cell?

  1. Bound ribosomes attached to its cytosolic surface Correct answer.
  2. Cristae formed by infoldings of its membrane
  3. Peroxisomal enzymes within its lumen
  4. A proton gradient maintained across its membrane

Explanation

The rough endoplasmic reticulum is studded with ribosomes that translate proteins bearing a signal sequence, allowing the nascent polypeptide to be threaded into the ER lumen or membrane. This is the defining structural basis of its role in the secretory pathway.

Question 27Cell Biology and Cell Processes

A cell biologist treats a culture with a drug that depolymerizes actin filaments. Which cellular activity would be most directly disrupted?

  1. Formation of the leading edge during cell migration Correct answer.
  2. Transport of vesicles from the ER to the Golgi
  3. Replication of mitochondrial DNA
  4. Movement of chromosomes along kinetochore microtubules

Explanation

Actin filaments drive protrusion of the leading edge during amoeboid and crawling cell migration, so depolymerizing actin directly blocks this process. The other listed activities depend on microtubules or on enzymes unrelated to the actin cytoskeleton.

Question 28Cell Biology and Cell Processes

In a cell, a protein that normally resides in the cytosol is found inside the nucleus. Which change to the protein would most plausibly explain this relocalization?

  1. Addition of a nuclear localization signal Correct answer.
  2. Addition of a mitochondrial targeting sequence
  3. Loss of an N-terminal signal peptide
  4. Gain of a hydrophobic transmembrane domain

Explanation

Proteins enter the nucleus through nuclear pore complexes only when they carry a nuclear localization signal that is recognized by importin receptors. Acquiring such a signal would redirect a normally cytosolic protein into the nucleus.

Question 29Cell Biology and Cell Processes

A researcher measures the rate of ATP synthesis in isolated mitochondria as the proton-motive force across the inner membrane is progressively increased. Which relationship is expected?

  1. ATP synthesis rate increases until another factor becomes limiting Correct answer.
  2. ATP synthesis rate rises only after the gradient is abolished
  3. ATP synthesis rate is unaffected by the gradient
  4. ATP synthesis rate decreases as the gradient rises

Explanation

ATP synthase uses the proton-motive force to drive phosphorylation, so a larger gradient supports a higher synthesis rate until substrate availability, enzyme capacity, or another factor limits the reaction. The relationship is therefore positive but saturating rather than absent or inverse.

Question 30Cell Biology and Cell Processes

A cell enters mitosis with chromosomes that have failed to complete DNA replication, leaving regions of single-stranded DNA. Which checkpoint response is most appropriate, and why?

  1. Arrest at the spindle assembly checkpoint, because kinetochores are unattached
  2. Proceed to cytokinesis, because the cleavage furrow forms normally
  3. Arrest at the G2/M checkpoint, because incomplete replication signals DNA damage Correct answer.
  4. Proceed to anaphase, because cohesion is released prematurely

Explanation

Unreplicated or damaged DNA activates the G2/M checkpoint through ATM/ATR signaling, halting entry into mitosis until replication is finished or damage is repaired. The spindle assembly checkpoint monitors kinetochore attachment and would not detect unreplicated DNA.

Question 31Nutrition in Plants and Animals

Which structure in a flowering plant is the main site of photosynthesis?

  1. Root hair cell
  2. Xylem vessel element
  3. Guard cell of the lower epidermis
  4. Palisade mesophyll cell Correct answer.

Explanation

Palisade mesophyll cells pack the upper part of the leaf with many chloroplasts and lie close to the air spaces and veins, so they carry out most photosynthesis. Root hair cells lack chloroplasts, and guard cells and xylem elements have other primary functions.

Question 32Nutrition in Plants and Animals

Which mineral ion deficiency most directly causes yellowing between the veins of older leaves because the ion is mobile in the phloem?

  1. Iron
  2. Magnesium Correct answer.
  3. Calcium
  4. Boron

Explanation

Magnesium is a central atom of chlorophyll and is mobile in the plant, so a shortage causes the plant to withdraw it from older leaves, which lose chlorophyll and turn yellow between the veins. The other three ions are largely immobile and their deficiency symptoms appear first in young tissue.

Question 33Nutrition in Plants and Animals

In a mammal, which process breaks large lipid droplets into smaller droplets before enzyme action?

  1. Emulsification by bile salts Correct answer.
  2. Denaturation by stomach acid
  3. Absorption by active transport
  4. Hydrolysis by lipase

Explanation

Bile salts are amphipathic and coat lipid droplets, breaking them into many smaller droplets. This emulsification increases the surface area available to lipase but does not itself hydrolyse any bonds.

Question 34Nutrition in Plants and Animals

A person's diet supplies adequate energy and protein but very little vitamin C. Which symptom is most likely to develop?

  1. Night blindness
  2. Rickets
  3. Bleeding gums and poor wound healing Correct answer.
  4. Goitre

Explanation

Vitamin C is required for collagen synthesis, so a deficiency weakens connective tissue and causes scurvy, with bleeding gums and slow wound healing. The other symptoms belong to deficiencies of vitamin A, vitamin D and iodine respectively.

Question 35Nutrition in Plants and Animals

Which feature of the small intestine most increases the rate of absorption of glucose?

  1. Presence of circular muscle
  2. Presence of villi and microvilli Correct answer.
  3. Presence of goblet cells
  4. Presence of a serosa

Explanation

Villi and microvilli greatly expand the apical surface area of the intestinal epithelium, so more transport proteins are exposed and glucose uptake is faster. The other features serve mucus secretion, peristalsis and protection rather than absorption.

Question 36Nutrition in Plants and Animals

Which dental tissue is the hardest and covers the crown of a tooth?

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

Explanation

Enamel is a heavily mineralised tissue that forms the outer covering of the crown and resists abrasion during chewing. Dentine lies beneath it, cement covers the root, and pulp is the soft inner tissue.

Question 37Nutrition in Plants and Animals

Which statement describes the fate of the products of fat digestion during absorption?

  1. Glucose and amino acids are packaged into chylomicrons.
  2. Fatty acids are absorbed directly into the hepatic portal vein.
  3. Fatty acids and monoglycerides are reassembled into triglycerides inside epithelial cells. Correct answer.
  4. Monoglycerides are actively transported into the stomach.

Explanation

Fatty acids and monoglycerides enter the epithelial cells, are re-esterified into triglycerides, packaged with protein into chylomicrons and released into lacteals. Short-chain fatty acids can enter the blood, but the main route for long-chain products is lymphatic.

Question 38Nutrition in Plants and Animals

Which adaptation of a xerophyte reduces water loss without stopping photosynthesis completely?

  1. Complete loss of all stomata
  2. Absence of a cuticle
  3. Very large, thin leaves
  4. Sunken stomata in pits Correct answer.

Explanation

Sunken stomata create a humid microclimate in the pit, which lowers the diffusion gradient for water vapour and reduces transpiration while still allowing gas exchange. The other options either prevent gas exchange entirely or increase water loss.

Question 39Nutrition in Plants and Animals

A potted plant is kept in the dark for two days, then a leaf is tested with iodine solution. What is the expected result?

  1. The leaf turns blue-black because starch accumulated.
  2. The leaf decolourises because chlorophyll was destroyed.
  3. The leaf turns brick-red because reducing sugar is present.
  4. The leaf remains brown because little starch was formed. Correct answer.

Explanation

Without light, photosynthesis cannot proceed, so the leaf's existing starch is exported or respired and little new starch is made. Iodine solution gives a blue-black colour only where starch is present, so the leaf stays brown.

Question 40Nutrition in Plants and Animals

Which combination of environmental factors would most strongly limit the rate of photosynthesis in a well-watered crop on a bright, warm day?

  1. Low light intensity
  2. Waterlogged soil
  3. Low carbon dioxide concentration Correct answer.
  4. Low temperature

Explanation

With abundant light, warmth and water, the photosynthetic rate is usually limited by the supply of carbon dioxide to the chloroplasts. Raising carbon dioxide concentration would then increase the rate, so it is the limiting factor.

Question 41Nutrition in Plants and Animals

Which statement best explains why the rate of transpiration falls as relative humidity rises?

  1. The stomata close permanently at high humidity.
  2. Water molecules move through the xylem more slowly.
  3. The diffusion gradient for water vapour from leaf to air becomes smaller. Correct answer.
  4. The leaf surface temperature always drops sharply.

Explanation

Transpiration depends on a gradient between the humid air spaces inside the leaf and the surrounding air. When relative humidity rises, that gradient shrinks, so less water vapour diffuses out per unit time.

Question 42Nutrition in Plants and Animals

In an experiment, a leaf is destarched, partly covered with foil, exposed to light, then tested with iodine. What does this demonstrate?

  1. Chlorophyll is needed for starch formation
  2. Light is needed for starch formation Correct answer.
  3. Carbon dioxide is needed for starch formation
  4. Oxygen is produced during photosynthesis

Explanation

The covered region receives no light and stays brown with iodine, while the exposed region turns blue-black because starch has formed. The only variable is light, so the experiment shows that light is required for starch synthesis.

Question 43Nutrition in Plants and Animals

Which enzyme begins the chemical digestion of starch in the human digestive system?

  1. Pepsin
  2. Trypsin
  3. Lipase
  4. Salivary amylase Correct answer.

Explanation

Salivary amylase hydrolyses glycosidic bonds in starch to maltose in the mouth, and pancreatic amylase continues this in the small intestine. Pepsin and trypsin act on proteins, and lipase acts on lipids.

Question 44Nutrition in Plants and Animals

A patient's stomach produces insufficient hydrochloric acid. Which consequence is most direct?

  1. Amylase stops hydrolysing starch
  2. Bile salts are not produced
  3. Glucose absorption in the ileum fails
  4. Pepsinogen is converted to pepsin less efficiently Correct answer.

Explanation

Hydrochloric acid provides the low pH that converts inactive pepsinogen into active pepsin and maintains the acidic environment pepsin needs. Bile, amylase and glucose absorption are not directly dependent on stomach acid.

Question 45Nutrition in Plants and Animals

Which structural feature of the mammalian gut is most important for increasing the surface area for absorption?

  1. Villi projecting into the lumen Correct answer.
  2. A short, straight intestine
  3. A thick muscularis externa
  4. A keratinised epithelium

Explanation

Villi are finger-like projections that, together with microvilli, multiply the apical surface area of the intestinal epithelium. A thick muscle layer aids movement, a short gut reduces time and area, and keratinisation would block absorption.

Question 46Nutrition in Plants and Animals

Which molecule is the final electron acceptor in the electron transport chain of aerobic respiration?

  1. NAD+
  2. Carbon dioxide
  3. Pyruvate
  4. Oxygen Correct answer.

Explanation

Oxygen accepts electrons and protons at the end of the chain, forming water and allowing the chain to keep running. NAD+ is an electron carrier that is reduced earlier, pyruvate is a substrate, and carbon dioxide is a waste product.

Question 47Nutrition in Plants and Animals

Why must the diet of a mammal contain essential amino acids?

  1. They are converted directly into vitamins
  2. The body cannot synthesise their carbon skeletons at a sufficient rate Correct answer.
  3. They cannot be absorbed from the gut
  4. They are needed only as an energy source

Explanation

Essential amino acids cannot be made in adequate amounts from other nitrogen-containing compounds, so they must be supplied in the diet. They are used mainly to build proteins, and they are absorbed normally like other amino acids.

Question 48Nutrition in Plants and Animals

Which observation supports the conclusion that a plant has a deficiency of a mobile mineral ion?

  1. Symptoms appear first in young leaves
  2. Symptoms appear first in older leaves Correct answer.
  3. Symptoms appear only in flowers
  4. Symptoms appear only in the roots

Explanation

A mobile ion can be withdrawn from older tissues and redirected to growing points, so the older leaves show deficiency symptoms first. Immobile ions cannot be redistributed, so their shortage affects young tissue first.

Question 49Nutrition in Plants and Animals

Which process in the nitrogen cycle converts ammonium ions into nitrite and then nitrate?

  1. Nitrogen fixation
  2. Denitrification
  3. Ammonification
  4. Nitrification Correct answer.

Explanation

Nitrification is carried out by nitrifying bacteria, which oxidise ammonium to nitrite and then nitrite to nitrate, the form plants absorb most readily. The other processes fix atmospheric nitrogen, reduce nitrate to nitrogen gas, or release ammonium from organic matter.

Question 50Nutrition in Plants and Animals

A plant is grown in a solution lacking nitrate but supplied with ammonium. Which process becomes the main route for incorporating nitrogen into amino acids?

  1. Denitrification in the roots supplies nitrate
  2. Nitrogen fixation in root nodules increases
  3. Reduction of nitrate to ammonia is bypassed, and ammonium is assimilated directly Correct answer.
  4. Phosphorus replaces nitrogen in proteins

Explanation

Amino acid synthesis normally requires ammonia, which is produced by reducing absorbed nitrate. When ammonium is supplied directly, that reduction step is unnecessary and the ammonium can be assimilated into organic compounds.