Cell: Structure and Function, Biomolecules and Cell Cycle
Domain 4: Cell - Structure and Function - what is tested
The exam tests prokaryotic versus eukaryotic organisation, organelle structure-function links, tests for biomolecules, enzyme action, and the phases of the cell cycle including mitosis and meiosis.
Core rules
- Prokaryotes lack a nucleus and membrane-bound organelles; ribosomes are 70S. Eukaryotic cytoplasmic ribosomes are 80S; mitochondrial and chloroplast ribosomes are 70S.
- Endomembrane system: nuclear envelope, ER, Golgi, lysosomes and vacuoles. Mitochondria and chloroplasts are semiautonomous and contain DNA.
- Cell wall: cellulose in plants, chitin in fungi, peptidoglycan in bacteria. No cell wall in animal cells.
- Biomolecule tests: starch with iodine (blue-black), reducing sugar with Benedict's (brick-red on heating), protein with Biuret (violet), lipid with Sudan III (red).
- Enzymes are mostly proteins; they lower activation energy and are affected by pH, temperature and substrate concentration.
- Cell cycle: interphase (G1, S, G2) then M phase. DNA doubles in S phase. Mitosis maintains chromosome number; meiosis halves it and creates variation.
Worked example
A cell shows chromosomes lined up singly at the equator, with spindle fibres attached to centromeres. Identify the stage and chromosome number if the parent cell has 2n = 8. The single row at the metaphase plate means metaphase of mitosis. Because mitosis preserves ploidy, each daughter cell will have 8 chromosomes; at anaphase the sister chromatids separate, so the cell temporarily has 16 chromatids moving to poles. If this were meiosis I, homologous pairs would line up, not single chromosomes.
Traps
Saying DNA doubles in G2; confusing 70S and 80S; calling lysosomes the site of protein synthesis; thinking meiosis produces identical cells; using iodine to test protein; forgetting that plant cells have both wall and vacuole.