SBI4U practice exam: Grade 12 Biology questions and exam rules

Each daily paper has 8 original questions worth 25 to 35 marks. The Grade 12 Biology (SBI4U) paper covers photosynthesis and respiration, gene expression and gene technology, homeostasis, nerves and populations; its inheritance and evolution questions are SBI3U topics. The Grade 11 Biology paper adds biomolecules, enzymes, membranes and protein synthesis, which belong to SBI4U's biochemistry and molecular genetics strands.

How the SBI4U exam counts in 2026-27

In Grade 12 science in Ontario, your final mark is now 65% classroom work, a 25% written exam, and 10% attendance and participation, with the exam written in your board's exam period. The prerequisite for SBI4U is Grade 11 university preparation biology (SBI3U). These figures come from the Ministry of Education's September 2026 update to Growing Success.

What's on the SBI4U exam: the strands

• Biochemistry: biological molecules and enzymes.
• Metabolic processes: cellular respiration and photosynthesis.
• Molecular genetics: DNA replication, protein synthesis and gene regulation.
• Homeostasis: feedback systems in the body.
• Population dynamics: how populations grow and interact.
• Scientific investigation skills and career exploration run through every unit.

SBI4U practice exam questions with answers

We wrote these for this page, one from each strand.

1. Biochemistry. A competitive inhibitor is added to an enzyme reaction. What happens to the rate, and how can it be overcome? Answer: the inhibitor resembles the substrate and occupies the active site, so fewer substrate molecules bind and the rate falls. Adding more substrate outcompetes the inhibitor and restores the maximum rate.

2. Metabolic processes. Where in a eukaryotic cell does each stage of aerobic respiration happen? Answer: glycolysis in the cytoplasm, the Krebs cycle in the mitochondrial matrix, and the electron transport chain on the inner mitochondrial membrane, where most of the ATP is made.

3. Molecular genetics. A DNA template strand reads 3′ TAC GGC ATT 5′. Give the mRNA and the amino acids it codes for. If the second codon's third base changed so the mRNA read CCA, what kind of mutation is that? Answer: the mRNA is 5′ AUG CCG UAA 3′, which codes for methionine, proline, then stop. CCA also codes for proline, so the protein doesn't change: a silent mutation. Check codons in the DNA Translator, then try a mutation question from our Genetics and DNA course.

4. Homeostasis. Explain how blood glucose returns to normal after a meal. Answer: the pancreas senses the rise and secretes insulin. Body cells then absorb more glucose from the blood, and liver cells convert the surplus to glycogen, so the level falls back. When glucose drops too low, glucagon reverses this. Each response undoes the change that caused it, which is negative feedback.

5. Population dynamics. A deer population of 500 grows by 8% a year with no limit. Estimate it after 5 years, and say what would eventually stop the growth. Answer: 500 × 1.08⁵ ≈ 735 deer. In reality, limited food, space, predators and disease set a carrying capacity, and growth slows into an S-shaped (logistic) curve. Model both in the Population Growth Calculator.

Use the daily papers as practice exams

1. Alternate the SBI4U and SBI3U papers, and work to the suggested time, 35 to 45 minutes.
2. Skip inheritance and evolution questions unless you are reviewing Grade 11.
3. Answer explain questions as a chain of cause and effect.
4. Mark each paper and note which strand each lost mark belongs to.

Common SBI4U mistakes

• Reading the DNA template as if it were the mRNA: pair each base, and use U instead of T.
• Putting glycolysis in the mitochondria.
• Saying enzymes are used up.
• Calling every feedback loop negative: some, such as during childbirth, are positive.
• Assuming populations grow forever: carrying capacity limits them.

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