A-Level Biology — Complete Specification

Eukaryotic vs prokaryotic

• Eukaryotic (animal, plant, fungi, protoctist): membrane-bound nucleus and organelles. Prokaryotic (bacteria): no nucleus (circular DNA free in the cytoplasm), no membrane-bound organelles, smaller 70S ribosomes, a murein cell wall, and sometimes plasmids, capsules and flagella.

Organelles (structure to function)

• Nucleus: holds DNA; the nucleolus makes ribosomes. Rough ER: ribosomes, protein synthesis/transport. Smooth ER: lipid synthesis. Golgi apparatus: modifies, packages and labels proteins into vesicles. Mitochondria: aerobic respiration/ATP (folded cristae, own DNA). Ribosomes (80S in eukaryotes): protein synthesis. Lysosomes: hydrolytic enzymes. Chloroplasts (plants): photosynthesis. Cell wall (cellulose in plants). Centrioles: spindle in cell division.
• The protein-production pathway: nucleus to rough ER to vesicle to Golgi to vesicle to cell-surface membrane (worth learning as a sequence).

Cell techniques

• Magnification = image size / actual size; watch unit conversions (1 mm = 1000 micrometres = 1,000,000 nm).
• Resolution = the smallest distance between two points seen as separate; limited by the wavelength used. Light microscope: living specimens, colour, low resolution (~200 nm). Electron microscope: far higher resolution - TEM (thin sections, internal detail, 2D) and SEM (surface, 3D); but specimens are dead and in a vacuum.
• Cell fractionation: homogenise in an ice-cold, isotonic, buffered solution (to protect organelles), filter, then ultracentrifuge - organelles separate by density (nuclei first, then mitochondria, then ER, then ribosomes).

Learn each organelle's structure-to-function link and always convert units carefully in magnification calculations.

Diagram