DOE National Science Bowl® · biology · 2027

Science Bowl biology practice questions, with the reasoning worked out.

37 original questions in the competition’s two formats — toss-ups you answer alone in seconds, bonuses your team reasons through together — across the six areas the biology category draws from. Each answer comes with the thinking that gets you there fast, because on a buzzer the reasoning is the speed.

Last updated September 24, 2026

How to use these. Cover the answer, read the stem once, and say your answer before five seconds are up. Then open the reasoning and check whether you got there by the same route — if you did not, that route is what to learn. Options are lettered W/X/Y/Z as in the competition. The calendar and what the category tests are on the Science Bowl biology page.

Cell & molecular biology

Toss-up · Multiple choiceWhich organelle is the principal site of lipid synthesis and detoxification of drugs in a liver cell?

W) Rough endoplasmic reticulum
X) Smooth endoplasmic reticulum
Y) Golgi apparatus
Z) Peroxisome

Answer: X — smooth endoplasmic reticulum

Rough ER makes secreted and membrane proteins; the Golgi modifies and ships; peroxisomes handle hydrogen peroxide. Lipid synthesis and cytochrome P450 detoxification are smooth-ER jobs — the liver’s abundant smooth ER is the tell.

Toss-up · Short answerWhat is the net gain of ATP from glycolysis per molecule of glucose?

Answer: 2 ATP

Four are made by substrate-level phosphorylation, two are spent in the investment phase. "Net" is the word the question hinges on; answer "4" and you have answered a different question.

Bonus · Short answerName the three stages of cellular respiration in order, and state where in a eukaryotic cell each occurs.

Answer: Glycolysis (cytosol), citric acid cycle (mitochondrial matrix), oxidative phosphorylation / electron transport chain (inner mitochondrial membrane)

Bonus questions reward the ordered list. Pyruvate oxidation is sometimes counted as a fourth step; if the question says three, give these three.

Toss-up · Multiple choiceA cell placed in a solution swells and bursts. Relative to the cell, the solution was:

W) Hypertonic
X) Isotonic
Y) Hypotonic
Z) Saturated

Answer: Y — hypotonic

Water moves toward the higher solute concentration. Bursting means water entered, so the outside had fewer solutes — hypotonic. Plant cells would become turgid instead because of the wall.

Toss-up · Short answerWhich enzyme unwinds the DNA double helix at the replication fork?

Answer: Helicase

Primase lays RNA primers, polymerase extends, ligase seals, topoisomerase relieves the supercoil ahead. "Unwinds" is helicase every time.

Bonus · Multiple choiceWhich of the following is NOT a post-transcriptional modification of eukaryotic pre-mRNA?

W) Addition of a 5′ cap
X) Addition of a poly-A tail
Y) Removal of introns
Z) Attachment of a signal peptide

Answer: Z — attachment of a signal peptide

The signal peptide is part of the translated protein, not the mRNA. Capping, tailing and splicing are the three RNA-processing steps.

Toss-up · Short answerDuring which phase of mitosis do sister chromatids separate and move to opposite poles?

Answer: Anaphase

Prophase condenses, metaphase aligns, anaphase separates, telophase reforms nuclei. The verb "separate" maps to anaphase.

Genetics

Toss-up · Short answerIn a cross between two heterozygotes for a single gene with complete dominance, what fraction of offspring is expected to show the recessive phenotype?

Answer: One quarter (1/4)

Aa × Aa gives 1 AA : 2 Aa : 1 aa. Only aa shows the recessive trait. Say the fraction, not the ratio — the question asked for a fraction.

Bonus · Short answerA dihybrid cross AaBb × AaBb, genes unlinked. Give the expected phenotypic ratio and the fraction of offspring that are homozygous recessive for both genes.

Answer: 9:3:3:1; 1/16

Independent assortment multiplies the monohybrid results: (3:1)(3:1) = 9:3:3:1, and aabb = 1/4 × 1/4 = 1/16.

Toss-up · Multiple choiceA trait appears in every generation and affected fathers pass it to all daughters but no sons. The most likely mode of inheritance is:

W) Autosomal dominant
X) Autosomal recessive
Y) X-linked dominant
Z) Y-linked

Answer: Y — X-linked dominant

Father-to-daughter-only transmission is the signature of the X chromosome; every generation affected points to dominance. Y-linked would go father to son.

Toss-up · Short answerWhat term describes a mutation that changes a codon into a stop codon?

Answer: Nonsense mutation

Missense swaps one amino acid for another; silent changes nothing; frameshift shifts the reading frame. A premature stop is nonsense.

Bonus · Multiple choiceIn the lac operon, lactose acts as an inducer by:

W) Binding RNA polymerase and increasing its affinity for the promoter
X) Binding the repressor and releasing it from the operator
Y) Methylating the operator
Z) Degrading the repressor protein

Answer: X — binding the repressor and releasing it from the operator

Allolactose (the question says lactose — accept it) binds the repressor, changing its shape so it leaves the operator and transcription can proceed. Nothing is degraded or methylated.

Toss-up · Short answerIn PCR, what is the purpose of the annealing step?

Answer: To allow the primers to base-pair (anneal) to the single-stranded template DNA

Denaturation separates strands (~95 °C), annealing lets primers bind (~50–65 °C), extension lets the polymerase copy (~72 °C). Name the primers in your answer.

Toss-up · Multiple choiceIn a population in Hardy–Weinberg equilibrium the frequency of the recessive allele is 0.2. What fraction of the population is heterozygous?

W) 0.04
X) 0.16
Y) 0.32
Z) 0.64

Answer: Y — 0.32

2pq with q = 0.2 and p = 0.8: 2 × 0.8 × 0.2 = 0.32. The trap answers are q² (0.04) and p² (0.64).

Human & animal physiology

Toss-up · Short answerWhich hormone, released by the posterior pituitary, increases water reabsorption in the kidney’s collecting duct?

Answer: Antidiuretic hormone (ADH, vasopressin)

Aldosterone also increases water retention but acts via sodium and comes from the adrenal cortex. "Posterior pituitary" plus "collecting duct" is ADH.

Toss-up · Multiple choiceWhich blood vessel carries oxygen-poor blood?

W) Pulmonary vein
X) Aorta
Y) Pulmonary artery
Z) Coronary artery

Answer: Y — pulmonary artery

Arteries carry blood away from the heart; the pulmonary artery carries deoxygenated blood to the lungs, the one exception to "arteries carry oxygenated blood". The pulmonary vein is the mirror exception.

Bonus · Short answerTrace the path of a nerve impulse across a chemical synapse in four steps.

Answer: Action potential reaches the axon terminal → voltage-gated Ca²⁺ channels open and Ca²⁺ enters → synaptic vesicles fuse and release neurotransmitter into the cleft → neurotransmitter binds postsynaptic receptors, opening ion channels

The step teams drop is the calcium influx; without it there is no vesicle fusion. Four steps means four — do not stop at three.

Toss-up · Short answerWhich part of the nephron is the primary site of glucose reabsorption?

Answer: Proximal convoluted tubule

Roughly all filtered glucose is recovered in the proximal tubule by secondary active transport. The loop of Henle concentrates; the distal tubule fine-tunes ions.

Toss-up · Multiple choiceWhich class of immune cell matures in the thymus?

W) B lymphocytes
X) T lymphocytes
Y) Macrophages
Z) Neutrophils

Answer: X — T lymphocytes

B cells mature in the bone marrow (B for bone marrow is a coincidence that works in mammals); T cells mature in the thymus. Macrophages and neutrophils are innate cells from marrow.

Bonus · Multiple choiceThe Bohr effect describes the shift of the oxygen–haemoglobin dissociation curve to the right when:

W) pH rises
X) CO₂ falls
Y) Temperature falls
Z) pH falls

Answer: Z — pH falls

Active tissue is acidic, warm and CO₂-rich; all three push the curve right so haemoglobin unloads oxygen where it is needed. Options W, X and Y each describe the opposite condition.

Toss-up · Short answerWhat is the name of the pacemaker region of the human heart?

Answer: Sinoatrial (SA) node

The SA node in the right atrium initiates the beat; the atrioventricular node delays it; the bundle of His and Purkinje fibres conduct it to the ventricles.

Plant biology

Toss-up · Multiple choiceIn C4 plants, the first stable product of carbon fixation is:

W) 3-phosphoglycerate
X) Oxaloacetate
Y) Ribulose bisphosphate
Z) Glyceraldehyde-3-phosphate

Answer: X — oxaloacetate

PEP carboxylase fixes CO₂ into the four-carbon oxaloacetate in mesophyll cells; 3-PGA is the C3 first product. "C4" literally counts the carbons in the answer.

Toss-up · Short answerWhich plant hormone is primarily responsible for fruit ripening?

Answer: Ethylene

Auxin drives elongation and apical dominance, gibberellin stem growth and germination, cytokinin cell division, abscisic acid dormancy and stomatal closure. Ripening — and the "one bad apple" effect — is ethylene, a gas.

Bonus · Short answerExplain, in terms of water potential, why water moves from soil into a root hair cell.

Answer: The root cell has a lower (more negative) water potential than the soil solution because its cytoplasm and vacuole contain more solutes; water moves by osmosis down the water-potential gradient, from higher (soil) to lower (cell)

The bonus wants the mechanism, not the fact. Say "lower water potential" for the cell and "down the gradient" — the graders listen for both.

Toss-up · Short answerWhich tissue transports sugars from leaves to the rest of the plant?

Answer: Phloem

Xylem carries water and minerals up from roots; phloem carries sugars from source to sink in either direction. "Sugars" is the cue.

Toss-up · Multiple choiceStomata typically close in response to which hormone during water stress?

W) Auxin
X) Gibberellin
Y) Abscisic acid
Z) Cytokinin

Answer: Y — abscisic acid

ABA triggers guard-cell ion efflux, water leaves the guard cells and the pore closes. It is also the dormancy hormone — "stress hormone" is the mnemonic.

Ecology & evolution

Toss-up · Short answerWhat term describes the maximum population size an environment can sustain indefinitely?

Answer: Carrying capacity (K)

Logistic growth levels off at K; exponential growth has no K. "Sustain indefinitely" is the definition.

Toss-up · Multiple choiceRoughly what percentage of energy is transferred from one trophic level to the next?

W) 1%
X) 10%
Y) 50%
Z) 90%

Answer: X — 10%

The ten-percent rule: most energy is lost as heat and in unconsumed or undigested biomass. It is why food chains are short.

Bonus · Short answerDistinguish primary from secondary succession and give one example of each.

Answer: Primary succession begins on newly exposed substrate with no soil (e.g. bare rock after a lava flow or a retreating glacier); secondary succession begins where soil remains after a disturbance (e.g. an abandoned field or a forest after fire)

The dividing line is soil. Give the example with the definition — bonuses score the parts separately.

Toss-up · Short answerWhich type of natural selection favours both extremes of a trait over the intermediate?

Answer: Disruptive (diversifying) selection

Directional shifts the mean one way; stabilising favours the middle; disruptive favours the tails. "Both extremes" is the giveaway.

Toss-up · Multiple choiceTwo species that evolve similar traits independently because they occupy similar niches illustrate:

W) Divergent evolution
X) Convergent evolution
Y) Coevolution
Z) Adaptive radiation

Answer: X — convergent evolution

Independent origin of similar traits (analogous structures) is convergence — dolphins and sharks. Divergence is one ancestor splitting; radiation is many species from one; coevolution is reciprocal.

Bonus · Short answerA population of 500 individuals has 50 births and 20 deaths in a year, with no migration. State the per-capita growth rate for that year.

Answer: 0.06 (or 6% per year)

(b − d) / N = (50 − 20) / 500 = 30/500 = 0.06. Give the number; "6%" is accepted if the question allows percent.

Classification & diversity

Toss-up · Multiple choiceWhich of the following is a prokaryote?

W) Amoeba
X) Yeast
Y) Cyanobacterium
Z) Paramecium

Answer: Y — cyanobacterium

Amoeba and Paramecium are protists, yeast is a fungus — all eukaryotes. Cyanobacteria are photosynthetic bacteria; "-bacterium" gives it away.

Toss-up · Short answerWhich animal phylum is characterised by a water vascular system and tube feet?

Answer: Echinodermata

Sea stars, urchins and sea cucumbers. The water vascular system is unique to the phylum; radial symmetry as adults is the other cue.

Bonus · Short answerList the taxonomic ranks from kingdom to species in order.

Answer: Kingdom, phylum, class, order, family, genus, species

Seven ranks, in order. Domain sits above kingdom and is usually not asked for in this form; do not add it unless the question says "from domain".

Toss-up · Multiple choiceViruses are not classified as living organisms mainly because they:

W) Lack nucleic acid
X) Cannot reproduce independently of a host cell
Y) Are smaller than bacteria
Z) Have no protein

Answer: X — cannot reproduce independently of a host cell

Viruses have nucleic acid and a protein coat; size is not a criterion for life. Obligate intracellular replication is the reason.

Toss-up · Short answerWhich group of fungi includes yeasts and the morels, and reproduces sexually via a sac-like structure?

Answer: Ascomycota (sac fungi)

Ascus = sac. Basidiomycota make club-shaped basidia (mushrooms); Zygomycota make zygospores (bread moulds).

Questions about the questions

What is the difference between a toss-up and a bonus question in Science Bowl?

A toss-up is answered by one player with no conferring — the first correct buzz wins and a wrong early buzz costs the team. A bonus follows a correct toss-up, allows the whole team to confer for a short time, and asks for something a step deeper: a mechanism, a calculation or an ordered list.

Are these the official DOE Science Bowl questions?

No. Every question here is original, written in the DOE format so that practice transfers. The DOE publishes its own sample sets; use those for format familiarity and these for the reasoning.

How should I practise Science Bowl biology?

Read the reasoning, not just the answer, until you can predict the answer from the first half of the stem. Then drill toss-ups against a clock — five seconds from stem to answer — and practise bonuses aloud, in the ordered form the graders expect.

Which biology areas come up most in Science Bowl?

Cell and molecular biology, genetics and human physiology carry most of the biology questions in recent sample sets and rounds; plants, ecology and classification are lighter but frequent enough to decide close rounds.

Independent coaching. Not affiliated with, authorized by, or endorsed by the U.S. Department of Energy or the National Science Bowl®.

Where students go next

Most students preparing for one of these are eligible for the others. The routes overlap more than they look like they do.