Life Sciences: Cells, Ecosystems, Heredity, and Evolution
The Life Sciences domain covers molecules to organisms, ecosystems, heredity, and evolution. Grade 8 items ask you to explain cell processes, trace matter and energy through food webs, predict inheritance patterns, and use evidence to explain how populations change over time. Expect questions that connect structure to function and ask for evidence-based reasoning.
Core rules
- Cells are the basic unit of life. Photosynthesis uses light energy to make sugars from carbon dioxide and water; cellular respiration releases energy from sugars and produces carbon dioxide and water.
- In ecosystems, energy flows from producers to consumers and is lost as heat at each level; matter cycles through living and nonliving parts.
- Traits are influenced by genes and environment. Alleles can be dominant or recessive; a dominant allele is expressed when present, but it is not necessarily more common in a population.
- Natural selection acts on variation in a population over generations: individuals with traits that improve survival and reproduction in a given environment tend to leave more offspring, so those traits become more common.
Worked example
In pea plants, purple flowers (B) are dominant to white flowers (b). Cross two heterozygous purple plants: Bb × Bb.
Reasoning: each parent can pass B or b, so the Punnett square gives BB, Bb, Bb, bb. Genotypes: 25% BB, 50% Bb, 25% bb. Because B is dominant, BB and Bb are purple; only bb is white. Predicted phenotypes: 75% purple, 25% white. The 3:1 ratio is a prediction for many offspring, not a guarantee for exactly four seeds.
Traps that cost marks
- Saying a dominant trait is the most common trait in a population.
- Confusing energy flow, which is one-way with heat loss, with matter cycling.
- Claiming an individual organism “evolves” during its lifetime; populations evolve across generations.
- Treating an acquired trait, such as a scar, as inherited.