Cell Structure and Function
Cell Structure and Function treats the cell as a working machine: membranes, organelles, the cell cycle, and the biomolecules and enzymes that run them. Expect organelle–function matching, diagram labelling, and numericals on chromosome and DNA content through mitosis and meiosis.
Membranes and organelles
- Fluid mosaic model (Singer and Nicolson): lipid bilayer with integral and peripheral proteins, semi-permeable. Transport by diffusion, osmosis, facilitated diffusion and active transport (Na+/K+ ATPase).
- Prokaryote: 70S ribosome, no nucleus, mesosomes. Eukaryote: 80S ribosome, membrane-bound organelles.
- Mitochondrion - oxidative phosphorylation, own DNA; chloroplast - photosynthesis; lysosome - acid hydrolases; Golgi - glycosylation and packaging; rough ER - protein synthesis; smooth ER - lipid synthesis.
Cell cycle and division
- Interphase: G1, S (DNA doubles, chromosome number unchanged), G2; then M phase.
- Mitosis: prophase, metaphase (chromosomes at the equator), anaphase (centromeres split), telophase. Meiosis I is reductional, meiosis II equational; crossing over occurs in pachytene.
- Significance: mitosis for growth and repair, meiosis for variation and gamete formation.
Worked example
A root-tip cell has 2n = 8. At mitotic metaphase it has 8 chromosomes, 16 chromatids and 4C DNA. After telophase each daughter cell has 8 chromosomes and 2C DNA. If a pollen mother cell with 2n = 8 undergoes meiosis, each of the four products has 4 chromosomes and 1C DNA. Reasoning: S phase doubles DNA, not chromosome number; meiosis halves both.
Traps that cost marks
- Saying chromosomes double in S phase; DNA doubles, chromosome number stays constant.
- Confusing lysosomal acid hydrolases with peroxisomal catalase.
- Writing "70S ribosome" as 70 subunits - S refers to sedimentation coefficient.