Biology: Cells, Information, Energy, and Populations
Biology on this exam asks you to connect structures to functions and to reason quantitatively about inheritance and populations. Core topics are cell membranes, enzymes, energy transformations, gene expression, homeostasis, and ecological interactions.
Core concepts
- Membranes are selectively permeable. Diffusion moves substances down a concentration gradient; osmosis is water movement across a membrane toward higher solute concentration.
- Enzymes lower activation energy and are sensitive to temperature and pH. Denaturation changes shape and reduces activity; an enzyme's effect on reaction rate is not the same as changing the equilibrium position.
- Cellular respiration converts chemical energy in glucose to ATP; photosynthesis captures light energy and stores it in organic molecules. Both involve electron transport and proton gradients.
- DNA is transcribed to mRNA, which is translated to protein. Alleles segregate during meiosis, and independent assortment applies to genes on different chromosomes.
- Homeostasis uses negative feedback: a deviation triggers a response that reduces the deviation.
Worked example
A recessive disorder affects 1 in 10,000 people in a population at Hardy-Weinberg equilibrium. Estimate the carrier frequency.
Let $q^2=0.0001$, so $q=0.01$ and $p=0.99$. Carriers are heterozygous, with frequency $2pq=2(0.99)(0.01)=0.0198$, about 2%. This assumes random mating, no selection, no mutation, no migration, and a large population; real populations may deviate.
Traps that cost marks
- Saying a dominant allele is necessarily more common; dominance describes expression, not frequency.
- Confusing diffusion with active transport, which requires energy.
- Mixing transcription and translation locations or templates.
- Treating energy transfer between trophic levels as 100%; typically only about 10% passes to the next level.
- Forgetting that enzymes affect rates, not the final equilibrium.