Cell Structure and Function
Domain 3 covers cell structure, biomolecules and the cell cycle. Questions test organelle functions, membrane models, tests for biomolecules, enzyme action and the differences between mitosis and meiosis.
Core rules
- Prokaryotes lack a nucleus and membrane-bound organelles; they have 70S ribosomes, a nucleoid and often a cell wall. Eukaryotes have 80S ribosomes and organelles.
- The fluid mosaic model describes membranes as a phospholipid bilayer with embedded proteins and cholesterol in animal cells.
- Biomolecule tests: Benedict’s for reducing sugar, iodine for starch, Biuret for protein, emulsion test for lipid.
- Enzymes are proteins that lower activation energy; competitive inhibitors resemble substrate, non-competitive inhibitors bind elsewhere and change shape.
- Cell cycle: interphase (G1, S, G2) then M phase. Mitosis preserves chromosome number; meiosis halves it and creates variation by crossing over and independent assortment.
Worked example
A diploid cell has 2n = 24. How many chromosomes and DNA molecules are in each gamete after meiosis? Meiosis I separates homologous chromosomes; meiosis II separates sister chromatids. Each gamete receives n = 12 chromosomes. Before meiosis, DNA replicates, so each chromosome has two chromatids; after two divisions, each gamete has 12 chromosomes, each with one DNA molecule. The chromosome number is halved once, but DNA is replicated once and divided twice.
Traps
- Saying ribosomes are membrane-bound; they are not.
- Placing cellulose in animal cell walls; cellulose is plant, chitin is fungal.
- Confusing mitosis in haploid cells with meiosis; meiosis requires homologous chromosomes.
- Claiming enzymes are consumed; they are catalysts and are recovered.