Biology (high school science MCAS)
What the MCAS Biology Test Asks
The Massachusetts MCAS high school Biology test measures the 2016 STE standards grouped into four topic areas: HS-LS1 molecules to organisms (cell structure, macromolecules, transport, photosynthesis and respiration, mitosis, homeostasis); HS-LS2 ecosystems (energy flow, cycling of matter, carrying capacity, biodiversity); HS-LS3 heredity (DNA, protein synthesis, meiosis, inheritance); and HS-LS4 evolution (natural selection, common ancestry, evidence). Items are multiple-choice questions and open-response items, some multi-part, that require a claim supported by data or biological reasoning.
Core Rules to Know
- Energy flows one way through ecosystems (sun → producers → consumers) and is lost as heat at each trophic level; matter cycles (carbon, nitrogen, water).
- Enzymes are proteins that lower activation energy; they are not consumed and are shape-specific.
- Transcription makes mRNA in the nucleus; translation builds protein at the ribosome.
- Mitosis produces genetically identical diploid cells; meiosis produces four haploid gametes with new allele combinations.
- Natural selection acts on existing heritable variation; individuals do not evolve, populations do.
Worked Example
Cross two heterozygous pea plants, Bb × Bb, where B = purple flowers (dominant) and b = white. Each parent makes B and b gametes in equal proportion. A 2×2 Punnett square gives BB, Bb, Bb, bb: genotype ratio 1:2:1 and phenotype ratio 3 purple : 1 white. Reasoning: the white phenotype appears only when both alleles are recessive, so the probability is 1/2 × 1/2 = 1/4. If a question asks for the probability that a purple offspring is BB, the answer is 1/3, because the bb class is excluded.
Traps That Cost Marks
- "Dominant" means expressed when present, not more common or stronger; a recessive allele can be the most frequent in a population.
- Do not say organisms "adapt" to need a trait; mutations arise randomly, and selection edits frequencies.
- Distinguish correlation from causation in graph questions, and quote actual data values in open-response answers.
- In energy pyramids, label the direction of arrows as energy flow, and never claim energy is recycled.