Cells and Multicellular Organisms
Core concepts
Candidates must understand the plasma membrane as a fluid mosaic of phospholipids and proteins, and how its selective permeability underpins transport. Distinguish passive processes (simple diffusion, facilitated diffusion, osmosis) from active transport requiring ATP, and recognise endocytosis/exocytosis as bulk transport. Know the organelles and their functions - mitochondria (aerobic respiration), chloroplasts (photosynthesis), ribosomes (protein synthesis), rough and smooth ER, Golgi apparatus, lysosomes and the nucleus - and relate structure to function.
Cell theory, the differences between prokaryotes and eukaryotes, and the compartmentalisation of eukaryotic cells are frequent targets. Understand the cell cycle, mitosis and its stages, and how uncontrolled division relates to cancer. For multicellular organisms, grasp how specialised cells form tissues, organs and systems, and how surface-area-to-volume ratio constrains cell size and exchange.
Traps
- Osmosis moves water down its own potential gradient, not solute; students often invert the direction.
- Facilitated diffusion uses proteins but needs no ATP - do not label it active.
- Mitosis produces genetically identical diploid cells; meiosis produces haploid gametes with variation.
- SA:V ratio decreases as cells enlarge, limiting diffusion efficiency.
Worked example
A cell with sides 2 µm has SA:V = (6×4)/(8) = 3; doubling to 4 µm gives (6×16)/(64) = 1.5. The larger cell has half the relative surface area, so exchange becomes limiting - a rule of thumb: as linear dimension doubles, SA:V halves.