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Abitur Biology practice questions

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Question 1Biological inquiry and interpretation of evidence

A researcher predicts that tomato plants given a nitrogen-enriched fertilizer will produce more fruit than plants given a standard fertilizer. In this study, the amount of fruit produced is best described as which of the following?

  1. The hypothesis
  2. A controlled variable
  3. The dependent variable Correct answer.
  4. The independent variable

Explanation

The dependent variable is the measured outcome that responds to the manipulation, which here is fruit yield. The fertilizer type is the factor the researcher deliberately changes, so it is the independent variable rather than the measured result.

Question 2Biological inquiry and interpretation of evidence

A student measures the mass of the same sample five times and obtains nearly identical values, but the balance was never zeroed and reads 2.0 g too high. How should these measurements be described?

  1. Both accurate and precise
  2. Precise but not accurate Correct answer.
  3. Accurate but not precise
  4. Neither accurate nor precise

Explanation

Precision refers to how closely repeated measurements agree with one another, while accuracy refers to how close they are to the true value. Because the readings cluster tightly but are systematically offset by the unzeroed balance, they are precise but inaccurate.

Question 3Biological inquiry and interpretation of evidence

Which of the following best distinguishes a scientific hypothesis from a scientific theory?

  1. A hypothesis is a testable proposed explanation, while a theory is a well-supported explanatory framework built from extensive evidence Correct answer.
  2. A hypothesis is always correct once tested, while a theory remains an untested guess
  3. A hypothesis is a broad framework, while a theory is a single testable prediction
  4. A hypothesis applies only to biology, while a theory applies only to physics

Explanation

In science, a hypothesis is a testable proposed explanation for a specific observation, whereas a theory is a broad, well-substantiated explanation supported by many lines of evidence. The common usage of theory as a mere guess is incorrect in scientific contexts.

Question 4Biological inquiry and interpretation of evidence

A scientist repeats an experiment several times and obtains consistent results. Which conclusion is most justified?

  1. The findings are reliable and reproducible under the tested conditions Correct answer.
  2. The results now apply to all organisms everywhere
  3. The hypothesis has been proven true beyond any doubt
  4. The experiment has no remaining sources of error

Explanation

Reproducible results indicate reliability, meaning the procedure yields consistent outcomes under the tested conditions. Science does not establish absolute proof, and replication does not eliminate error or guarantee universal applicability.

Question 5Biological inquiry and interpretation of evidence

A graph plots plant height on the y-axis against days of growth on the x-axis. Which statement correctly identifies the placement of these variables?

  1. Days of growth is the independent variable on the x-axis, and height is the dependent variable on the y-axis Correct answer.
  2. Both variables are dependent and should share the x-axis
  3. Days of growth is the dependent variable because it is measured over time
  4. Height is the independent variable on the y-axis, and days of growth is the dependent variable on the x-axis

Explanation

By convention, the independent variable is placed on the x-axis and the dependent variable on the y-axis. Height responds to elapsed time, so time is independent and height is dependent.

Question 6Biological inquiry and interpretation of evidence

A student notices that mold grows on bread left in a warm, humid place but not on bread stored in a dry, cool place. Which of the following is the most appropriate next step in the scientific process?

  1. Formulate a testable hypothesis and design a controlled experiment to test it Correct answer.
  2. Conclude immediately that warmth and moisture cause mold growth
  3. Publish the observation as a confirmed scientific law
  4. Discard the observation because it was not made in a laboratory

Explanation

An observation generates questions that should be framed as a testable hypothesis and examined through a controlled experiment. Reaching a firm causal conclusion from a single uncontrolled observation is premature.

Question 7Biochemistry

Which property of water is most directly responsible for its ability to moderate temperature in living organisms and large bodies of water?

  1. Its low specific heat
  2. Its inability to form hydrogen bonds
  3. Its nonpolar covalent bonds
  4. Its high specific heat Correct answer.

Explanation

Water has a high specific heat because hydrogen bonds must absorb or release substantial energy before water molecules change temperature. This allows water and water-rich organisms to resist large temperature swings, helping maintain homeostasis.

Question 8Cell structure, membranes and division

Which structure is found in a typical plant cell but NOT in a typical animal cell?

  1. Mitochondrion
  2. Nucleus
  3. Ribosome
  4. Cell wall made of cellulose Correct answer.

Explanation

Plant cells have a rigid cell wall composed primarily of cellulose that surrounds the plasma membrane, providing structural support. Animal cells lack a cell wall, relying on the cytoskeleton and extracellular matrix instead.

Question 9Genetics, inheritance and molecular information

Meiosis differs from mitosis in that meiosis produces

  1. Two genetically identical diploid cells
  2. One diploid cell with double the chromosomes
  3. Two haploid cells identical to the parent
  4. Four genetically varied haploid cells Correct answer.

Explanation

Meiosis consists of two divisions that produce four haploid gametes, each with half the chromosome number of the parent. Crossing over and independent assortment generate genetic variation, distinguishing meiosis from the identical-diploid products of mitosis.

Question 10Metabolism

An enzyme lowers the activation energy of a reaction. Which statement best describes how this affects the reaction?

  1. It increases the reaction rate without being consumed in the reaction Correct answer.
  2. It changes the overall free-energy difference between reactants and products
  3. It supplies the energy needed to make an endergonic reaction exergonic
  4. It permanently binds to the product to stop the reaction

Explanation

Enzymes are biological catalysts that lower activation energy, speeding up reactions without being consumed. They do not change the net free-energy difference (delta G) between reactants and products; they only make the reaction reach equilibrium faster.

Question 11Evolution

Which of the following is a necessary condition for natural selection to occur in a population?

  1. All individuals are genetically identical
  2. Heritable variation exists and affects survival or reproduction Correct answer.
  3. The environment never changes
  4. Every individual produces the same number of offspring

Explanation

Natural selection requires heritable variation among individuals and differences in reproductive success linked to that variation. Individuals with advantageous heritable traits leave more offspring, shifting the population's traits over generations.

Question 12Ecology

In an energy pyramid, only about 10 percent of the energy at one trophic level is transferred to the next. Most of the remaining energy is

  1. Converted directly into new biomass at the next level
  2. Lost as heat, largely through cellular respiration and metabolic processes Correct answer.
  3. Stored permanently in the soil
  4. Recycled back to producers as light energy

Explanation

At each trophic level, organisms use most of the energy they consume for metabolism and lose much of it as heat through cellular respiration. Only about 10 percent is stored as biomass available to the next level, which limits food chain length.

Question 13Neurobiology

What ionic mechanism is directly responsible for the rapid depolarizing phase of a neuronal action potential once the threshold potential (~ -55 mV) is reached?

  1. Opening of voltage-gated Na+ channels, causing rapid inward influx of Na+ down its electrochemical gradient Correct answer.
  2. Opening of voltage-gated K+ channels, causing rapid outward efflux of K+
  3. Active extrusion of 3 Na+ ions in exchange for 2 K+ ions by the Na+/K+ pump
  4. Opening of ligand-gated Cl- channels, causing hyperpolarizing influx of Cl-

Explanation

When depolarization reaches the threshold potential (~ -55 mV), voltage-gated sodium channels rapidly open. Sodium ions (Na+) rush into the axon down their steep electrical and chemical gradients, depolarizing the membrane potential to approximately +35 to +40 mV.

Question 14Physiology

Which feature allows a steroid hormone to pass directly through the plasma membrane?

  1. Its lipid (hydrophobic) nature lets it dissolve through the phospholipid bilayer Correct answer.
  2. Its large size lets it push proteins aside
  3. Its negative charge attracts it to the membrane interior
  4. Its attachment to a carbohydrate tag opens a channel

Explanation

Steroid hormones are lipid-soluble (hydrophobic), so they can dissolve through the hydrophobic interior of the phospholipid bilayer without a transport protein. Once inside, they typically bind intracellular receptors that act on the DNA to regulate gene expression.

Question 15Cell structure, membranes and division

Which cell structure contains the genetic information in a typical eukaryotic body cell?

  1. Ribosome
  2. Nucleus Correct answer.
  3. Lysosome
  4. Cell wall

Explanation

The nucleus contains the chromosomes of a typical eukaryotic body cell and is the main site where genetic information is stored and transcribed.

Question 16Cell structure, membranes and division

What is the main function of mitochondria in aerobic eukaryotic cells?

  1. Producing most ATP by cellular respiration Correct answer.
  2. Packaging proteins for secretion
  3. Breaking down DNA during replication
  4. Storing starch grains in chloroplasts

Explanation

Mitochondria carry out key steps of aerobic respiration, including the citric acid cycle and oxidative phosphorylation, producing most cellular ATP.

Question 17Cell structure, membranes and division

Why do phospholipids form bilayers in water?

  1. Their hydrophilic tails point toward water
  2. Their hydrophobic heads point toward water
  3. They have hydrophilic heads and hydrophobic tails Correct answer.
  4. They are covalently bonded into rigid sheets

Explanation

Phospholipids are amphipathic: polar heads interact with water, while nonpolar fatty acid tails avoid water and face inward, forming a bilayer.

Question 18Cell structure, membranes and division

A plant cell is placed in a solution with a much higher solute concentration than the cell sap. What will happen first by osmosis?

  1. Water enters the cell and turgor increases
  2. Water leaves the cell and the vacuole shrinks Correct answer.
  3. Solute leaves the solution and enters the nucleus
  4. The cell wall dissolves immediately

Explanation

Water moves from higher water potential inside the cell toward the more concentrated external solution, causing the vacuole and protoplast to shrink.

Question 19Cell structure, membranes and division

Which organelle is the main site of photosynthesis in plant cells?

  1. Chloroplast Correct answer.
  2. Mitochondrion
  3. Nucleus
  4. Golgi apparatus

Explanation

Chloroplasts contain chlorophyll and internal thylakoid membranes where light reactions occur, making them the main site of photosynthesis.

Question 20Cell structure, membranes and division

A plant cell undergoing plasmolysis has most likely been placed in which environment?

  1. A hypotonic solution
  2. A hypertonic solution Correct answer.
  3. Pure oxygen gas
  4. A solution with no solutes inside or outside

Explanation

Plasmolysis occurs when water leaves a plant cell in a hypertonic environment, causing the protoplast to pull away from the cell wall.

Question 21Cell structure, membranes and division

A cell has a water potential of -0.4 MPa and is placed in a solution with water potential -0.9 MPa. What is the predicted net movement of water?

  1. Into the cell because -0.9 is lower
  2. Out of the cell toward the lower water potential Correct answer.
  3. No net movement because both values are negative
  4. Into the cell because solute cannot affect water potential

Explanation

Water moves from higher water potential to lower water potential. Since -0.4 MPa is higher than -0.9 MPa, water moves out of the cell.

Question 22Cell structure, membranes and division

Red blood cells placed in distilled water swell and burst. Which explanation is best?

  1. Water enters by osmosis because the outside has higher water potential Correct answer.
  2. Salt leaves by active transport until the membrane breaks
  3. Water leaves because distilled water is hypertonic
  4. Hemoglobin replicates too quickly

Explanation

Distilled water is hypotonic relative to the cell interior, so water enters by osmosis, increasing volume until the membrane can rupture.

Question 23Metabolism

What usually happens to enzyme activity when temperature rises far above the enzyme's optimum?

  1. The active site may change shape and activity falls Correct answer.
  2. The enzyme becomes a carbohydrate
  3. The substrate permanently becomes the enzyme
  4. The reaction stops because activation energy becomes zero

Explanation

High temperatures can denature proteins, changing the active site's shape so the substrate no longer binds effectively and reaction rate decreases.

Question 24Metabolism

Which substances are direct reactants in photosynthesis?

  1. Glucose and oxygen
  2. Carbon dioxide and water Correct answer.
  3. ATP and oxygen only
  4. Lactate and ethanol

Explanation

Photosynthesis uses carbon dioxide and water, with light energy, to produce carbohydrate and oxygen in chloroplasts.

Question 25Metabolism

Which process directly uses oxygen as the final electron acceptor in aerobic respiration?

  1. Glycolysis
  2. The Calvin cycle
  3. The electron transport chain Correct answer.
  4. DNA replication

Explanation

In aerobic respiration, oxygen accepts electrons at the end of the mitochondrial electron transport chain and combines with protons to form water.

Question 26Metabolism

An inhibitor binds away from an enzyme's active site and lowers activity by changing protein shape. Which type of inhibition is this most consistent with?

  1. Competitive inhibition
  2. Allosteric noncompetitive inhibition Correct answer.
  3. Substrate-level phosphorylation
  4. Semiconservative inhibition

Explanation

An inhibitor that binds outside the active site and changes enzyme conformation is acting allosterically, often described as noncompetitive inhibition.

Question 27Metabolism

Why can C4 plants have an advantage over C3 plants in hot, dry conditions?

  1. They concentrate carbon dioxide near Rubisco, reducing photorespiration Correct answer.
  2. They do not need ATP for carbon fixation
  3. They lack stomata entirely
  4. They perform respiration only at night

Explanation

C4 plants initially fix carbon in mesophyll cells and concentrate CO2 near Rubisco in bundle-sheath cells, reducing photorespiration when stomata are partly closed.

Question 28Metabolism

In chemiosmotic ATP formation, what directly drives ATP synthase?

  1. A proton gradient across a membrane Correct answer.
  2. The direct binding of oxygen to ATP
  3. A glucose molecule entering the nucleus
  4. The permanent destruction of NADH

Explanation

ATP synthase uses the potential energy stored in a proton gradient across a membrane to phosphorylate ADP to ATP.

Question 29Metabolism

During the Calvin cycle, which molecule is directly fixed into organic compounds?

  1. Oxygen
  2. Carbon dioxide Correct answer.
  3. Nitrogen gas
  4. Water vapor only

Explanation

The Calvin cycle incorporates carbon dioxide into organic molecules, beginning with carboxylation of RuBP by Rubisco.

Question 30Metabolism

What is the main purpose of fermentation in cells that cannot use oxygen for aerobic respiration?

  1. Regenerating NAD+ so glycolysis can continue Correct answer.
  2. Producing oxygen for mitochondria
  3. Converting DNA into glucose
  4. Stopping all ATP production

Explanation

Fermentation regenerates NAD+ from NADH, allowing glycolysis to continue producing a small amount of ATP without aerobic respiration.

Question 31Metabolism

If the inner mitochondrial membrane becomes freely permeable to protons, what happens to oxidative phosphorylation?

  1. ATP production by ATP synthase decreases Correct answer.
  2. ATP production immediately doubles
  3. DNA replication in the nucleus stops first
  4. The Calvin cycle begins in mitochondria

Explanation

A proton leak dissipates the proton gradient. Without that gradient, ATP synthase has less driving force, so oxidative ATP production falls.

Question 32Metabolism

In an experiment, algae are given CO2 labeled with carbon-14 for 5 seconds, then unlabeled CO2. The first labeled organic molecule detected is a three-carbon sugar phosphate. Which conclusion is most supported?

  1. CO2 is incorporated into Calvin-cycle intermediates before appearing in sucrose Correct answer.
  2. Oxygen from CO2 is immediately released as O2 in the light reactions
  3. The algae use only fermentation during illumination
  4. Carbon fixation occurs in mitochondrial cristae

Explanation

Short pulse labeling can reveal early carbon-fixation products. Label first appearing in a three-carbon sugar phosphate supports incorporation into Calvin-cycle intermediates before later products.

Question 33Metabolism

At 35 C and low CO2, Plant X has lower photorespiration and higher sugar production than Plant Y. Microscopy shows bundle-sheath cells rich in chloroplasts. What is the best interpretation?

  1. Plant X likely uses C4 photosynthesis Correct answer.
  2. Plant X lacks Rubisco completely
  3. Plant Y must be a nonphotosynthetic fungus
  4. Plant X produces ATP without light or respiration

Explanation

Low photorespiration, high productivity under warm low-CO2 conditions, and chloroplast-rich bundle-sheath cells are consistent with a C4 carbon-concentrating mechanism.

Question 34Metabolism

A toxin lets protons cross the inner mitochondrial membrane without passing through ATP synthase. Oxygen consumption rises, but ATP output falls. What is the toxin doing?

  1. Uncoupling electron transport from ATP synthesis Correct answer.
  2. Blocking glycolysis before pyruvate forms
  3. Inhibiting DNA ligase during replication
  4. Converting all NAD+ into glucose

Explanation

An uncoupler dissipates the proton gradient. Electron transport may continue or increase, but ATP synthase cannot capture the gradient efficiently.

Question 35Genetics, inheritance and molecular information

Which statement best distinguishes meiosis from mitosis?

  1. Meiosis produces genetically varied haploid cells Correct answer.
  2. Meiosis produces two identical diploid cells
  3. Mitosis halves chromosome number in gametes
  4. Mitosis always includes crossing over

Explanation

Meiosis reduces chromosome number and produces haploid cells that are genetically varied through recombination and independent assortment.

Question 36Genetics, inheritance and molecular information

An organism with alleles Aa for a trait is best described as what?

  1. Homozygous dominant
  2. Homozygous recessive
  3. Heterozygous Correct answer.
  4. Hemizygous for both alleles

Explanation

A heterozygous organism has two different alleles of a gene, such as A and a.

Question 37Genetics, inheritance and molecular information

In a simple dominant-recessive trait, what does a dominant allele do in a heterozygote?

  1. It is always more common in the population
  2. It is expressed in the phenotype when paired with a recessive allele Correct answer.
  3. It disappears from gametes during meiosis
  4. It changes every recessive allele into a dominant allele

Explanation

A dominant allele is expressed in the phenotype of a heterozygote, even when one recessive allele is also present.

Question 38Genetics, inheritance and molecular information

What is a gene mutation?

  1. A change in the nucleotide sequence of DNA Correct answer.
  2. The movement of water through a membrane
  3. The splitting of a population by a mountain range
  4. The folding of a protein into its final shape

Explanation

A gene mutation is a change in the DNA nucleotide sequence, such as a substitution, insertion, or deletion.

Question 39Genetics, inheritance and molecular information

What is PCR mainly used for in molecular biology?

  1. Amplifying a chosen DNA sequence Correct answer.
  2. Measuring the mass of a whole ecosystem
  3. Creating ATP in chloroplasts
  4. Counting all species in a forest by sight

Explanation

Polymerase chain reaction repeatedly copies a target DNA region, producing enough DNA for analysis such as gel electrophoresis or sequencing.

Question 40Genetics, inheritance and molecular information

Why does independent assortment in meiosis increase genetic variation?

  1. Homologous chromosome pairs align randomly relative to each other Correct answer.
  2. DNA polymerase intentionally copies every base incorrectly
  3. All chromosomes move to one gamete
  4. Dominant alleles destroy recessive alleles

Explanation

During metaphase I, each homologous pair can orient in either direction independently, producing different combinations of maternal and paternal chromosomes in gametes.

Question 41Genetics, inheritance and molecular information

In a cross AaBb x AaBb with independent assortment and complete dominance, what phenotypic ratio is expected for two traits?

  1. 1:1:1:1
  2. 3:1
  3. 9:3:3:1 Correct answer.
  4. 2:1

Explanation

A dihybrid cross of two heterozygotes with independent assortment and complete dominance gives the classic 9:3:3:1 phenotypic ratio.

Question 42Genetics, inheritance and molecular information

A healthy couple has an affected child with a rare disorder. The trait appears in males and females equally and can skip generations. Which inheritance pattern is most likely?

  1. Autosomal recessive Correct answer.
  2. Y-linked
  3. Mitochondrial only
  4. Autosomal dominant with full penetrance

Explanation

Unaffected carrier parents can have an affected child if the disorder is autosomal recessive. Equal sex distribution supports an autosomal pattern.

Question 43Genetics, inheritance and molecular information

A recessive allele causing red-green color blindness is on the X chromosome. Why are males more often affected?

  1. Males usually have only one X chromosome, so one recessive allele can be expressed Correct answer.
  2. Males cannot inherit X chromosomes from mothers
  3. The allele is dominant in females but recessive in males
  4. Females do not have retinal cells

Explanation

Because males are typically XY, a recessive allele on their single X chromosome is not masked by a second X allele.

Question 44Genetics, inheritance and molecular information

How can DNA methylation near a promoter affect gene expression in eukaryotic cells?

  1. It often reduces transcription by making the region less accessible Correct answer.
  2. It changes every cytosine into thymine immediately
  3. It always increases translation speed at ribosomes
  4. It removes the gene from the chromosome

Explanation

Promoter methylation is commonly associated with reduced transcription because it can inhibit transcription-factor binding and promote compact chromatin.

Question 45Genetics, inheritance and molecular information

What is the role of a transcription factor?

  1. Binding DNA or regulatory proteins to influence transcription Correct answer.
  2. Carrying amino acids to a ribosome as tRNA
  3. Digesting cellulose in plant cell walls
  4. Separating sister chromatids at anaphase

Explanation

Transcription factors regulate gene expression by binding DNA regulatory sequences or interacting with other proteins involved in transcription.

Question 46Genetics, inheritance and molecular information

A point mutation changes a codon but the same amino acid is inserted during translation. What type of mutation is this?

  1. Silent mutation Correct answer.
  2. Frameshift mutation
  3. Nonsense mutation
  4. Chromosome deletion

Explanation

Because the genetic code is redundant, some base substitutions do not change the amino acid sequence; these are silent mutations.

Question 47Genetics, inheritance and molecular information

In gel electrophoresis of DNA fragments, which fragments usually travel farthest through the gel in a fixed time?

  1. Small fragments Correct answer.
  2. Largest fragments
  3. Fragments with no charge only
  4. Fragments still inside intact chromosomes

Explanation

Smaller DNA fragments pass through gel pores more easily and therefore migrate farther than larger fragments under the same conditions.

Question 48Genetics, inheritance and molecular information

Why does PCR use repeated temperature cycles?

  1. To separate DNA strands, allow primers to bind, and extend new strands Correct answer.
  2. To make ribosomes translate DNA directly
  3. To denature all polymerase permanently in the first step
  4. To sort proteins by size in a gel

Explanation

PCR cycles through denaturation, primer annealing, and extension so a target DNA region is copied repeatedly.

Question 49Genetics, inheritance and molecular information

In a testcross for linked genes, parental phenotypes total 820 offspring and recombinant phenotypes total 180 offspring. What is the approximate map distance between the genes?

  1. 9 cM
  2. 18 cM Correct answer.
  3. 41 cM
  4. 82 cM

Explanation

Recombination frequency is recombinant offspring divided by total offspring: 180/(820+180) = 0.18, so the map distance is about 18 cM.

Question 50Genetics, inheritance and molecular information

A dihybrid cross yields a 9:3:4 phenotypic ratio. Which explanation best fits this modified Mendelian ratio?

  1. Recessive epistasis, where one homozygous recessive locus masks the other gene Correct answer.
  2. Independent assortment with no gene interaction
  3. A simple monohybrid cross with complete dominance
  4. Y-linked inheritance in females

Explanation

A 9:3:4 ratio is a classic sign of recessive epistasis, where homozygosity for one recessive allele masks expression at another locus.