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GTLE Biology practice questions

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Question 1Biology subject content knowledge

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

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

Explanation

The inner mitochondrial membrane houses the electron transport chain and ATP synthase, which together carry out oxidative phosphorylation. Ribosomes synthesize proteins, the Golgi modifies and sorts them, and lysosomes degrade material, so none of these generates most of the cell's ATP.

Question 2Biology subject content knowledge

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

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

Explanation

By complementary base pairing, adenine equals thymine, so thymine is also 30% and A+T together make 60%. The remaining 40% is split equally between guanine and cytosine, giving 20% guanine.

Question 3Biology subject content knowledge

During which phase of mitosis do sister chromatids separate and move toward opposite poles?

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

Explanation

Anaphase begins when cohesin is cleaved and sister chromatids are pulled apart by shortening kinetochore microtubules. Prophase involves condensation, metaphase alignment at the equator, and telophase nuclear re-formation.

Question 4Biology subject content knowledge

In a cross between two heterozygous individuals (Aa x Aa), what fraction of offspring is expected to show the recessive phenotype?

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

Explanation

The Punnett square yields genotypes in a 1 AA : 2 Aa : 1 aa ratio. Only the aa genotype expresses the recessive phenotype, which is one of four outcomes.

Question 5Biology subject content knowledge

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

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

Explanation

Helicase breaks the hydrogen bonds between complementary bases, separating the strands so replication can proceed. Polymerase extends new strands, ligase joins fragments, and primase lays down RNA primers.

Question 6Biology subject content knowledge

A protein is synthesized on the rough endoplasmic reticulum and destined for secretion. Which sequence best describes its route?

  1. Smooth ER to nucleus to Golgi to plasma membrane
  2. Rough ER to Golgi to secretory vesicle to plasma membrane Correct answer.
  3. Rough ER to lysosome to Golgi to plasma membrane
  4. Golgi to rough ER to lysosome to plasma membrane

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. The other routes misorder the compartments or use organelles not involved in secretion.

Question 7Biology subject content knowledge

Which statement correctly describes the role of NADH in cellular respiration?

  1. It donates phosphate groups directly to glucose during glycolysis.
  2. It transports pyruvate across the inner mitochondrial membrane.
  3. It is the final electron acceptor at the end of the electron transport chain.
  4. It carries electrons from glycolysis and the citric acid cycle to the electron transport chain. Correct answer.

Explanation

NADH is a reduced electron carrier produced in glycolysis and the citric acid cycle; it delivers electrons to complex I of the electron transport chain. Oxygen, not NADH, is the terminal electron acceptor.

Question 8Biology subject content knowledge

In photosynthesis, what is the primary role of the light reactions?

  1. To fix carbon dioxide into glucose
  2. To oxidize water into carbon dioxide
  3. To regenerate ribulose bisphosphate directly
  4. To produce ATP and NADPH for use in the Calvin cycle Correct answer.

Explanation

The light reactions convert light energy into chemical energy stored as ATP and NADPH, which the Calvin cycle then uses to reduce carbon dioxide. Carbon fixation itself occurs in the Calvin cycle, not the light reactions.

Question 9Biology subject content knowledge

Which of the following best explains why enzymes increase reaction rates?

  1. They change the equilibrium constant of the reaction.
  2. They lower the activation energy required for the reaction. Correct answer.
  3. They increase the free energy released by the reaction.
  4. They raise the temperature of the reaction mixture.

Explanation

Enzymes stabilize the transition state, lowering the activation energy barrier so more substrate molecules can react at a given temperature. They do not alter the overall free energy change or the equilibrium position.

Question 10Biology subject content knowledge

A mutation changes a single codon but the resulting protein has the same amino acid sequence. What type of mutation is this?

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

Explanation

A silent mutation alters a codon but, because of the redundancy of the genetic code, specifies the same amino acid. Missense changes the amino acid, nonsense creates a stop codon, and frameshift shifts the reading frame.

Question 11Biology subject content knowledge

Which of the following provides the best evidence that two species share a recent common ancestor?

  1. Similarities in behavior acquired by learning
  2. Similarities in homologous DNA sequences Correct answer.
  3. Similarities in body size
  4. Similarities in the environments they inhabit

Explanation

Shared DNA sequences in homologous genes reflect inheritance from a common ancestor and accumulate differences over time, providing a quantitative measure of relatedness. Environmental, size, and learned-behavior similarities can arise independently and are not reliable phylogenetic evidence.

Question 12Biology subject content knowledge

In a population at Hardy-Weinberg equilibrium, the frequency of the recessive allele is 0.2. What is the expected frequency of homozygous recessive individuals?

  1. 0.16
  2. 0.04 Correct answer.
  3. 0.32
  4. 0.80

Explanation

Homozygous recessive frequency is q squared. With q = 0.2, q squared equals 0.04. The value 0.32 is 2pq, the heterozygous frequency, and 0.16 is p squared.

Question 13Biology subject content knowledge

Which of the following is a density-dependent factor limiting population growth?

  1. A volcanic eruption
  2. Competition for nesting sites Correct answer.
  3. A sudden frost
  4. Seasonal drought

Explanation

Density-dependent factors intensify as population density rises; competition for limited nesting sites is an example. Volcanic eruptions, frosts, and droughts are density-independent because their impact does not depend on population size.

Question 14Biology subject content knowledge

Which organism is a primary producer in a food web?

  1. A rabbit eating grasses
  2. A hawk eating a snake
  3. A cyanobacterium carrying out photosynthesis Correct answer.
  4. A fungus decomposing leaf litter

Explanation

Primary producers synthesize organic compounds from inorganic sources, and a photosynthetic cyanobacterium does this using light energy. Fungi are decomposers, rabbits are primary consumers, and hawks are higher-level consumers.

Question 15Biology subject content knowledge

Which part of a neuron receives incoming signals and conducts graded potentials toward the cell body?

  1. Axon terminals
  2. Dendrites Correct answer.
  3. Myelin sheath
  4. Axon hillock

Explanation

Dendrites are the branched processes that receive synaptic input and generate graded potentials that spread toward the soma. The axon hillock integrates input and triggers action potentials, and terminals release neurotransmitter.

Question 16Biology subject content knowledge

In humans, which blood vessel carries oxygenated blood from the lungs to the heart?

  1. Pulmonary vein Correct answer.
  2. Superior vena cava
  3. Inferior vena cava
  4. Pulmonary artery

Explanation

The pulmonary veins return oxygenated blood from the lungs to the left atrium. The pulmonary artery carries deoxygenated blood to the lungs, and the venae cavae return deoxygenated systemic blood to the right atrium.

Question 17Biology subject content knowledge

Which hormone is released by the pancreas in response to low blood glucose?

  1. Insulin
  2. Glucagon Correct answer.
  3. Thyroxine
  4. Cortisol

Explanation

Alpha cells of the pancreatic islets secrete glucagon when blood glucose falls, promoting glycogen breakdown and gluconeogenesis. Insulin acts in the opposite direction, lowering blood glucose after a meal.

Question 18Biology subject content knowledge

A student finds that a plant wilted in a hypertonic solution. Which process best explains the observation?

  1. Water entered the cells by osmosis, increasing turgor pressure
  2. Water left the cells by osmosis, reducing turgor pressure Correct answer.
  3. Water moved out of the cells by active transport
  4. Solutes entered the cells by active transport, increasing turgor pressure

Explanation

In a hypertonic solution the external solute concentration is higher, so water moves out of the cells down its potential gradient by osmosis. Loss of water decreases turgor pressure, causing the tissue to wilt.

Question 19Biology subject content knowledge

Which feature distinguishes prokaryotic cells from eukaryotic cells?

  1. Prokaryotes lack a plasma membrane
  2. Prokaryotes lack ribosomes
  3. Prokaryotes lack nucleic acids
  4. Prokaryotes lack a membrane-bound nucleus Correct answer.

Explanation

Prokaryotes keep their DNA in a nucleoid region without a nuclear envelope, whereas eukaryotes enclose chromosomes in a nucleus. Prokaryotes still have ribosomes, a plasma membrane, and DNA.

Question 20Biology subject content knowledge

During which stage of aerobic respiration is most ATP produced?

  1. Glycolysis
  2. Oxidative phosphorylation Correct answer.
  3. The citric acid cycle
  4. Pyruvate oxidation

Explanation

Oxidative phosphorylation generates the majority of ATP through the proton gradient driving ATP synthase. Glycolysis and the citric acid cycle yield small amounts of substrate-level ATP and mostly electron carriers.

Question 21Biology subject content knowledge

In a dihybrid cross of two heterozygotes (AaBb x AaBb) with independent assortment, what proportion of offspring is expected to be homozygous recessive at both loci?

  1. 1/16 Correct answer.
  2. 9/16
  3. 1/8
  4. 1/4

Explanation

Each locus independently gives a 1/4 chance of homozygosity for the recessive alleles. Multiplying the independent probabilities gives 1/4 times 1/4, equal to 1/16.

Question 22Biology subject content knowledge

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 will be affected?

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

Explanation

A son inherits his X chromosome from his mother and his Y from his father. The carrier mother transmits the mutant allele half the time, so each son has a 1/2 chance of being affected.

Question 23Biology subject content knowledge

Which component of a DNA nucleotide distinguishes the four bases found in DNA?

  1. The nitrogenous base Correct answer.
  2. The hydrogen bond
  3. The phosphate group
  4. The deoxyribose sugar

Explanation

A DNA nucleotide consists of deoxyribose, a phosphate group, and a nitrogenous base; only the base varies among adenine, thymine, guanine, and cytosine. The sugar and phosphate backbone are the same throughout the strand.

Question 24Biology subject content knowledge

A population of insects develops resistance to an insecticide after several generations. Which mechanism best explains this outcome?

  1. Natural selection favoring individuals with pre-existing resistance alleles Correct answer.
  2. Directed mutation induced by the insecticide in each individual
  3. Horizontal gene transfer from the insecticide to the insects
  4. Acclimation of individual insects to the chemical during their lifetimes

Explanation

Resistance alleles arise by mutation independently of the insecticide; the chemical then acts as a selective pressure that increases the frequency of resistant genotypes. Acquired acclimation is not inherited and cannot explain heritable population change.

Question 25Biology subject content knowledge

Which structure in the human respiratory system is the primary site of gas exchange?

  1. Alveoli Correct answer.
  2. Larynx
  3. Bronchi
  4. Trachea

Explanation

Alveoli provide a thin, moist, highly vascularized surface where oxygen and carbon dioxide diffuse between air and blood. The bronchi, trachea, and larynx conduct air but have limited exchange surface.

Question 26Biology subject content knowledge

Which property of water allows insects such as water striders to move across the surface of a pond without breaking through?

  1. Ability of water to act as a solvent for gases
  2. Surface tension arising from hydrogen bonding between water molecules Correct answer.
  3. High density of water compared with air
  4. Low specific heat capacity of liquid water

Explanation

Water molecules cohere through hydrogen bonds, and molecules at the air interface experience unbalanced inward attraction that creates a cohesive surface film. This surface tension supports small objects whose weight is less than the force the film can resist. The other listed properties are real features of water but do not explain flotation on the surface film.

Question 27Biology subject content knowledge

A cell is placed in a solution and its volume decreases as water leaves the cell. Which description of the surrounding solution is correct?

  1. Hypertonic, because the solution contains no dissolved solutes
  2. Hypertonic, because solute concentration outside the cell exceeds that inside Correct answer.
  3. Hypotonic, because solute concentration outside the cell is lower than inside
  4. Isotonic, because net water movement is zero

Explanation

Water moves by osmosis from a region of lower solute concentration to a region of higher solute concentration. If the cell loses water, the external solution must have the higher solute concentration and is therefore hypertonic. A hypotonic solution would cause the cell to gain water, and an isotonic solution would produce no net change.

Question 28Biology subject content knowledge

Which molecule serves as the immediate energy donor for the power stroke of a myosin head during muscle contraction?

  1. ATP Correct answer.
  2. NADH
  3. Glucose
  4. Creatine phosphate

Explanation

Myosin heads hydrolyze bound ATP to ADP and inorganic phosphate, and the energy released drives the conformational change that moves actin filaments. ATP binding also detaches myosin from actin, allowing repeated cycles. Glucose and creatine phosphate are energy stores or reservoirs that ultimately regenerate ATP rather than powering the stroke directly.

Question 29Biology subject content knowledge

Which level of protein structure is defined by the linear order of amino acids joined by peptide bonds?

  1. Secondary structure
  2. Tertiary structure
  3. Quaternary structure
  4. Primary structure Correct answer.

Explanation

Primary structure is the sequence of amino acids in a polypeptide chain, held together by covalent peptide bonds. Secondary structure refers to local folding patterns such as alpha helices and beta sheets, tertiary structure to the overall three-dimensional shape of one chain, and quaternary structure to the assembly of multiple chains.

Question 30Biology subject content knowledge

Which event occurs during anaphase of mitosis?

  1. Sister chromatids are pulled to opposite poles of the cell Correct answer.
  2. The nuclear envelope re-forms around each set of chromosomes
  3. Homologous chromosomes pair and exchange segments
  4. Chromosomes condense and the nuclear envelope breaks down

Explanation

Anaphase begins when cohesin is cleaved and sister chromatids separate, after which spindle fibers draw them toward opposite poles. Chromosome condensation and nuclear envelope breakdown occur in prophase, and envelope re-formation occurs in telophase. Homologous pairing and crossing over are features of meiosis I, not mitosis.

Question 31Biology subject content knowledge

Which structure in a flowering plant produces the male gametophyte that will eventually form a pollen tube?

  1. The pollen grain Correct answer.
  2. The receptacle
  3. The ovule
  4. The stigma

Explanation

A pollen grain is the immature male gametophyte and, after landing on a compatible stigma, germinates to produce a pollen tube carrying sperm cells. The ovule houses the female gametophyte, the stigma is the receptive surface for pollen, and the receptacle supports the flower parts.

Question 32Biology subject content knowledge

In a pedigree, a trait appears in every generation and affects roughly equal numbers of males and females. Which mode of inheritance is most consistent with this pattern?

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

Explanation

An autosomal dominant allele is expressed in heterozygotes, so an affected parent can transmit the trait to about half of the offspring in each generation. Equal sex ratios are expected because the gene lies on an autosome. Autosomal recessive traits often skip generations, X-linked recessive traits affect males far more often, and Y-linked traits appear only in males.

Question 33Biology subject content knowledge

Which gas exchange surface is found in insects and consists of a network of air-filled tubes that deliver oxygen directly to tissues?

  1. Alveoli
  2. Tracheal system Correct answer.
  3. Book lungs
  4. Gill filaments

Explanation

Insects use a tracheal system in which spiracles open to branching tracheae and fine tracheoles that carry air close to every cell. This direct delivery of oxygen reduces the need for respiratory pigments in many insects. Gills, alveoli, and book lungs are respiratory structures of other animal groups.

Question 34Biology subject content knowledge

A researcher treats a culture of eukaryotic cells with a drug that blocks the function of the sodium-potassium pump. What immediate effect is expected on the resting membrane potential?

  1. The membrane potential will become more negative as potassium accumulates inside
  2. The membrane potential will reverse instantly to a positive value
  3. The membrane potential will remain unchanged because the pump does not move charge
  4. The membrane potential will gradually depolarize as ion gradients dissipate Correct answer.

Explanation

The sodium-potassium pump uses ATP to export three sodium ions and import two potassium ions, maintaining the gradients that set the resting potential. If the pump stops, the gradients slowly run down, potassium leaks out less effectively, and the interior becomes less negative. The change is gradual rather than instantaneous because ion gradients take time to dissipate.

Question 35Biology subject content knowledge

A particular enzyme has a Km of 5 mM for its substrate. At a substrate concentration of 5 mM, what fraction of the maximum reaction rate is observed under Michaelis-Menten conditions?

  1. One quarter
  2. All of it
  3. Three quarters
  4. One half Correct answer.

Explanation

Km is defined as the substrate concentration at which the reaction proceeds at half of Vmax. Because the given concentration equals Km, the velocity is exactly half of the maximum. Higher concentrations approach Vmax asymptotically, while lower concentrations give proportionally smaller fractions.

Question 36Biology subject content knowledge

A segment of DNA has the template strand sequence 3'-TACGGA-5'. Which mRNA sequence is transcribed from it?

  1. 5'-AUGGCA-3'
  2. 5'-AUGCCU-3' Correct answer.
  3. 5'-UACGGA-3'
  4. 5'-TACGGA-3'

Explanation

RNA polymerase reads the template strand in the 3' to 5' direction and builds RNA complementary and antiparallel to it, substituting uracil for thymine. Pairing T with A, A with U, C with G, and G with C across the template 3'-TACGGA-5' yields 5'-AUGCCU-3'. The other options either keep thymine, ignore complementarity, or mispair a base.

Question 37Biology subject content knowledge

During which phase of meiosis does independent assortment of nonhomologous chromosomes occur?

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

Explanation

In metaphase I, homologous chromosome pairs align at the spindle equator, and the orientation of each pair is random with respect to the others. This random orientation at metaphase I is the physical basis of independent assortment. Meiosis II separates sister chromatids and does not reshuffle nonhomologous chromosomes.

Question 38Biology subject content knowledge

A bacterial population is grown in a medium containing glucose and lactose. Glucose is present, but the lac operon is expressed at a high level. Which condition best explains this observation?

  1. Cyclic AMP levels are low because glucose is abundant
  2. The CAP protein is bound but the repressor is active
  3. The lac repressor is absent or unable to bind the operator Correct answer.
  4. Lactose is absent from the medium

Explanation

High lac expression despite glucose requires both relief of repression and activation. If the repressor cannot bind the operator, transcription can proceed even when glucose keeps cyclic AMP and CAP activation low. The presence of lactose alone would not overcome repression, and low cyclic AMP would normally reduce expression.

Question 39Biology subject content knowledge

Two true-breeding plants, one with red flowers and one with white flowers, are crossed and all the offspring have pink flowers. When the pink offspring are crossed among themselves, the next generation shows a 1:2:1 ratio of red to pink to white. Which inheritance pattern does this illustrate?

  1. Complete dominance
  2. Incomplete dominance Correct answer.
  3. Codominance
  4. Sex-linked inheritance

Explanation

In incomplete dominance, the heterozygote shows an intermediate phenotype, so a cross of two heterozygotes yields a 1:2:1 phenotypic ratio matching the genotypic ratio. Complete dominance would give a 3:1 ratio with two phenotypes. Codominance would produce both parental phenotypes together in the heterozygote rather than a blended color.

Question 40Biology subject content knowledge

Which feature of the genetic code allows a single base substitution to sometimes leave the amino acid sequence unchanged?

  1. The code being read in triplets
  2. The presence of start and stop codons
  3. The antiparallel nature of DNA
  4. Degeneracy of the code Correct answer.

Explanation

Degeneracy means that most amino acids are specified by more than one codon, often differing at the third position. A substitution can therefore produce a synonymous codon and a silent mutation. Triplet reading and start or stop codons do not by themselves permit silent changes, and antiparallel DNA concerns strand orientation.

Question 41Biology subject content knowledge

A population of birds on an island has beaks of intermediate size, and individuals with very small or very large beaks survive poorly. Which type of selection is acting on beak size?

  1. Artificial selection
  2. Disruptive selection
  3. Directional selection
  4. Stabilizing selection Correct answer.

Explanation

Stabilizing selection favors intermediate phenotypes and reduces variation around the mean, which matches the poor survival of both extremes. Directional selection would shift the mean toward one extreme, and disruptive selection would favor both extremes over the middle. Artificial selection requires deliberate human choice of breeding individuals.

Question 42Biology subject content knowledge

In a food chain, only about ten percent of the energy at one trophic level is transferred to the next. Which statement best explains the fate of the remaining energy?

  1. It is transferred to decomposers as usable energy for predators
  2. It is stored indefinitely in the bodies of consumers
  3. It is lost mainly as heat through respiration and in waste Correct answer.
  4. It is converted entirely into biomass of the next level

Explanation

At each trophic level, organisms use much of the assimilated energy for metabolism and release it as heat during respiration, while some is lost in feces and excretions. Only a small fraction becomes new biomass available to the next level. This is why food chains rarely have many trophic levels.

Question 43Biology subject content knowledge

Which statement best describes the role of the loop of Henle in the mammalian kidney?

  1. It establishes a concentration gradient in the medulla that allows water reabsorption Correct answer.
  2. It secretes hydrogen ions to regulate blood pH
  3. It reabsorbs most glucose and amino acids from the filtrate
  4. It filters blood plasma under pressure to form the initial filtrate

Explanation

The loop of Henle creates a countercurrent multiplier that makes the medullary interstitium increasingly salty. This osmotic gradient allows water to be reabsorbed from the collecting duct, concentrating the urine. Filtration occurs at the glomerulus, and most nutrient reabsorption occurs in the proximal tubule.

Question 44Biology subject content knowledge

A cell is treated with a drug that prevents the formation of the mitotic spindle. At which point will dividing cells most likely arrest?

  1. Metaphase, because chromosomes cannot align at the equator Correct answer.
  2. S phase, because DNA cannot replicate
  3. Prophase, because chromosomes cannot condense
  4. Telophase, because the nuclear envelope cannot re-form

Explanation

Spindle microtubules attach to kinetochores and align chromosomes at the metaphase plate before the spindle assembly checkpoint permits anaphase. Without a spindle, chromosomes cannot achieve proper alignment and the cell arrests before sister chromatids separate. Chromosome condensation, DNA replication, and nuclear envelope re-formation do not depend on spindle formation.

Question 45Biology subject content knowledge

Which observation would provide the strongest evidence that a trait in a population is influenced by environmental rather than genetic variation?

  1. Closely related individuals raised together show similar trait values
  2. The trait varies among populations that differ genetically
  3. Genetically similar individuals raised in different environments show different trait values Correct answer.
  4. The trait shows a high heritability estimate in a controlled study

Explanation

If genetically similar organisms express different phenotypes when raised under different conditions, the environment must contribute to the variation. Similarity among relatives raised together can reflect shared genes or shared environment, and high heritability indicates genetic contribution. Variation among genetically distinct populations is consistent with genetic differences.

Question 46Biology subject content knowledge

A single gene mutation causes a protein to fold incorrectly and be degraded, yet the organism shows several unrelated symptoms. Which concept best explains this outcome?

  1. Pleiotropy, because one gene affects multiple phenotypic traits Correct answer.
  2. Epistasis, because one gene masks another
  3. Codominance, because both alleles are expressed
  4. Polygenic inheritance, because many genes affect one trait

Explanation

Pleiotropy occurs when a single gene product influences several distinct traits, often because the protein functions in multiple tissues or pathways. Here one mutation produces multiple symptoms. Polygenic inheritance and epistasis involve interactions among multiple genes, and codominance concerns allele expression within a heterozygote.

Question 47Biology subject content knowledge

A population of 200 individuals has 32 individuals expressing a recessive phenotype controlled by a single locus. Assuming Hardy-Weinberg equilibrium, what is the frequency of the dominant allele?

  1. 0.4
  2. 0.2
  3. 0.6 Correct answer.
  4. 0.8

Explanation

The recessive phenotype frequency q squared equals 32 divided by 200, which is 0.16, so q equals 0.4. The dominant allele frequency p equals 1 minus q, giving 0.6. The other values correspond to q or to misapplied calculations.

Question 48Biology subject content knowledge

Two species of lizards live in the same desert, but one is active at dawn and the other at midday. Which term best describes this difference?

  1. Competitive exclusion
  2. Temporal niche partitioning Correct answer.
  3. Mutualism
  4. Primary succession

Explanation

Using the same habitat at different times reduces direct competition for resources and is a form of temporal niche partitioning. Competitive exclusion would occur if one species eliminated the other from shared resource use. Mutualism and primary succession describe beneficial interactions and colonization of bare substrates, respectively.

Question 49Biology subject content knowledge

An enzyme catalyzes a reaction in which a substrate is converted to product. Which change would increase the rate of product formation without altering the enzyme's Vmax?

  1. A decrease in substrate concentration
  2. An increase in the activation energy
  3. A decrease in Km that increases substrate affinity Correct answer.
  4. An increase in enzyme concentration

Explanation

Km is inversely related to the affinity of an enzyme for its substrate, so a lower Km means the enzyme reaches half-maximal velocity at a lower substrate concentration. This raises the reaction rate at subsaturating substrate levels while leaving Vmax unchanged. Increasing enzyme concentration raises Vmax, and raising activation energy slows the reaction.

Question 50Biology subject content knowledge

In a controlled cross, a female heterozygous for two independently assorting genes is mated with a male homozygous recessive for both genes. What proportion of offspring is expected to show both recessive phenotypes?

  1. Three quarters
  2. None
  3. One half
  4. One quarter Correct answer.

Explanation

This is a testcross in which the heterozygous parent produces four gamete types in equal proportions, and the recessive parent contributes only recessive alleles. Only the gamete carrying both recessive alleles yields offspring expressing both recessive traits, so the expected proportion is one quarter. Independent assortment makes each gene pair segregate with a one-half probability.