Question 1 of 10
The image shows the human respiratory system. Which structure is the primary site of gas exchange? 
ALarynx
BBronchi
CAlveoli
DTrachea
Explanation
Gas exchange (O₂ ↔ CO₂) happens at the alveoli, where their thin walls allow gases to diffuse between air and capillary blood.
Question 2 of 10
In the food web below, what would happen if all the frogs were removed? 
AInsect population increases and snake population decreases
BSnake population increases
CNothing changes
DAll organisms die
Explanation
📌 If frogs are removed:
• Insects (grasshoppers) they ate would INCREASE (no predator)
• Snakes that ate frogs would DECREASE (less food)
• This demonstrates how removing one species affects the entire food web.
• Insects (grasshoppers) they ate would INCREASE (no predator)
• Snakes that ate frogs would DECREASE (less food)
• This demonstrates how removing one species affects the entire food web.
Question 3 of 10
Why does the left ventricle have a thicker muscular wall than the right ventricle?
AIt is in direct contact with the lungs
BIt receives more deoxygenated blood from the body
CIt contains valves that the right ventricle lacks
DIt must pump blood at higher pressure to reach the entire body
Explanation
The left ventricle pumps oxygenated blood through the *systemic circuit* — every tissue in the body — which requires high pressure. The right ventricle only needs to push blood a short distance to the lungs (pulmonary circuit), so its wall is thinner. Distractors: the left ventricle pumps *oxygenated* blood, not deoxygenated. Both ventricles have valves (AV and semilunar). The right ventricle, not the left, is closer to the lungs.
Question 4 of 10
Which class of biomolecule is built from nucleotide monomers?
ALipids
BCarbohydrates
CProteins
DNucleic acids (DNA and RNA)
Explanation
Nucleic acids (DNA, RNA) are polymers of nucleotide monomers — each nucleotide is a phosphate + sugar + nitrogenous base. Distractors: Proteins are polymers of *amino acids*. Carbohydrates are polymers of *monosaccharides* (e.g., glucose). Lipids are not true polymers — they assemble from fatty acids and glycerol but don't repeat a single monomer.
Question 5 of 10
Which pair of processes drives the most direct exchange of CO₂ between living organisms and the atmosphere?
APhotosynthesis (absorbs CO₂) and cellular respiration (releases CO₂)
BNitrogen fixation (absorbs CO₂) and decomposition (releases O₂)
CPhotosynthesis (releases CO₂) and cellular respiration (absorbs CO₂)
DTranspiration (releases CO₂) and condensation (absorbs CO₂)
Explanation
Photosynthesis consumes atmospheric CO₂ to build glucose (CO₂ → sugar), while cellular respiration breaks glucose back down to CO₂ and water. Together they form the short-term carbon cycle between the biosphere and atmosphere. Distractor B reverses the direction. Transpiration moves water vapor, not CO₂. Nitrogen fixation moves nitrogen, not carbon.
Question 6 of 10
Which monomer–macromolecule pairing is CORRECT?
AAmino acids → proteins
BGlucose → lipids
CNucleotides → carbohydrates
DFatty acids → nucleic acids
Explanation
Amino acids are the monomers that polymerize via peptide bonds to form proteins. Distractors: Glucose is a monosaccharide — it builds *carbohydrates* (starch, glycogen), not lipids. Nucleotides build *nucleic acids* (DNA, RNA), not carbohydrates. Fatty acids combine with glycerol to form *lipids*, not nucleic acids.
Question 7 of 10
Use the Punnett square to determine the probability of offspring with the homozygous recessive (aa) genotype.
A3/4 (75%)
B0
C1/2 (50%)
D1/4 (25%)
Explanation
Aa × Aa cross: AA, Aa, Aa, aa. Only one box (aa) shows the homozygous recessive: 1/4 = 25%.
Question 8 of 10
For AA × aa, what is the genotype of all offspring?
AAll aa
BAll Aa
CAll AA
D1 AA : 2 Aa : 1 aa
Explanation
Each parent contributes a different homozygous allele → 100% Aa heterozygous.
Question 9 of 10
Lipids store roughly twice as much energy per gram as carbohydrates. Which property of lipid molecules best explains this?
AThey contain more nitrogen atoms than carbohydrates
BThey form longer polymer chains than carbohydrates
CThey are insoluble in water, so they take up less space when stored
DLong hydrocarbon chains contain many C–H bonds that can be oxidized to release energy
Explanation
Lipid fatty-acid tails are long chains of carbon–hydrogen bonds. Oxidizing each C–H bond releases energy, so the more C–H bonds per gram, the more energy is stored. Carbohydrates have fewer C–H bonds per carbon because oxygen atoms are already attached. Distractors: water insolubility relates to *where* lipids are stored, not how much energy they hold. Lipids aren't true polymers. Lipids contain almost no nitrogen — that's a feature of proteins and nucleic acids.
Question 10 of 10
The DNA double helix is shown. Which base pairs are correctly bonded?
AAll bases pair with each other freely
BA pairs with C, and T pairs with G
CA pairs with G, and T pairs with C
DA pairs with T, and G pairs with C
Explanation
In DNA, complementary base pairing is strict: Adenine (A) – Thymine (T) via 2 hydrogen bonds; Guanine (G) – Cytosine (C) via 3 hydrogen bonds.