Life Science
This domain tests core ideas about cells, heredity, evolution, and ecosystems, with emphasis on structure-function relationships and energy flow. Expect questions that ask you to predict outcomes, interpret food webs, and distinguish similar processes.
Cells and energy
- Cell theory states that all living things are made of cells, cells are the basic unit of life, and cells arise from existing cells.
- Photosynthesis occurs in chloroplasts: light energy makes glucose from carbon dioxide and water, releasing oxygen.
- Cellular respiration occurs mainly in mitochondria: energy is released from glucose, using oxygen and producing carbon dioxide and water.
- Homeostasis maintains stable internal conditions such as temperature and pH.
Heredity and evolution
- DNA contains genes; alleles are versions of a gene. A dominant allele masks a recessive allele in a heterozygote.
- Natural selection requires variation, heritability, and differential reproductive success.
- Energy flows one way through food webs, while matter cycles within ecosystems.
Worked example
In pea plants, tall (T) is dominant to short (t). Cross Tt x Tt. Each parent can pass T or t, so a 2x2 Punnett square gives TT, Tt, Tt, and tt. The genotype ratio is 1 TT : 2 Tt : 1 tt; the phenotype ratio is 3 tall : 1 short. The probability of a short offspring is 1/4, or 25%. Reasoning: alleles separate during gamete formation, combine at fertilization, and are then grouped by phenotype.
Traps that cost marks
- Saying dominant traits are more common; dominance concerns expression, not frequency.
- Mixing up mitosis (growth and repair, identical cells) and meiosis (gametes, genetic variation).
- Assuming energy increases at higher trophic levels; only about 10% typically transfers.
- Treating evolution as change in an individual; populations evolve across generations.
- Reversing the gases of photosynthesis and cellular respiration.