Biological Science: Cell Organization, Cell Division, Chemical Nature of the Gene, Bioenergetics, and Biosynthesis
This domain tests the cell as the unit of life: how it is organized, how it divides, how DNA encodes information, and how energy and matter flow through metabolism.
Cell organization
Prokaryotes lack a nucleus and membrane-bound organelles; eukaryotes compartmentalize. The plasma membrane is a fluid mosaic - a phospholipid bilayer with hydrophobic tails inward and proteins embedded. The endomembrane system runs ER → Golgi → vesicles. Mitochondria and chloroplasts have double membranes and their own DNA.
Cell division
The cell cycle is G1, S (DNA replication), G2, M, with checkpoints at G1/S and G2/M. Mitosis yields two genetically identical diploid cells. Meiosis yields four haploid cells: crossing over in prophase I and independent assortment at metaphase I generate variation; homologs separate in meiosis I, sister chromatids in meiosis II.
Chemical nature of the gene
DNA is antiparallel and double-stranded, with A–T and G–C pairs (G–C has three hydrogen bonds). Replication is semiconservative. The central dogma runs DNA → RNA → protein: transcription in the nucleus, translation on ribosomes with tRNA adapters reading three-base codons. The code is degenerate and nearly universal.
Bioenergetics and biosynthesis
Glycolysis in the cytosol nets 2 ATP and 2 NADH per glucose. Pyruvate oxidation and the Krebs cycle (mitochondrial matrix) supply NADH and FADH2 to oxidative phosphorylation on the inner membrane, where the electron transport chain builds a proton gradient and ATP synthase makes ATP by chemiosmosis. Aerobic yield is about 30–32 ATP; fermentation regenerates NAD+ and yields only 2. Photosynthesis reverses the flow: light reactions in thylakoids split water (releasing O2) and make ATP and NADPH; the Calvin cycle in the stroma uses RuBisCO to fix CO2.
Worked example
One glucose in a muscle cell without oxygen: glycolysis gives 2 ATP net plus 2 NADH, but the chain cannot reoxidize NADH, so pyruvate is reduced to lactate. Net yield 2 ATP versus roughly 30–32 aerobically. The reasoning: the large ATP tally comes from NADH and FADH2 feeding the chain, not from the cycle's direct GTP.
Traps
Crediting mitosis with generating variation; swapping transcription and replication; placing ATP synthase in the matrix rather than the inner membrane; claiming the Calvin cycle needs light directly; forgetting glycolysis is cytosolic.