Cell Biology and the Organisation of Living Things
This domain tests whether you can link a structure to its function, distinguish plant from animal and prokaryotic from eukaryotic cells, explain how substances cross membranes, and place cells correctly into the levels of organisation.
Core cell structure
- Nucleus: holds DNA, controls the cell.
- Mitochondria: site of aerobic respiration, release energy.
- Ribosomes: protein synthesis; found in all cells, including bacteria.
- Chloroplasts: photosynthesis, contain chlorophyll; plant cells only.
- Cell wall: cellulose in plants, chitin in fungi, peptidoglycan in bacteria - support, never selectively permeable.
- Cell membrane: fluid mosaic of phospholipids and protein, selectively permeable, controls exchange.
- Vacuole: large permanent sap-filled vacuole in plant cells, gives turgor.
Movement across membranes
Diffusion moves particles down a concentration gradient without energy. Osmosis is diffusion of water through a selectively permeable membrane from dilute to concentrated solution. Active transport moves substances against the gradient and needs energy from respiration - this is how root hairs take up mineral ions.
Levels of organisation
Cell → tissue → organ → organ system → organism. Animal tissues: epithelial, connective, muscle, nervous. Plant tissues: epidermis, palisade mesophyll, xylem, phloem. A specialised cell's shape matches its job - a root hair cell is long and thin to increase surface area.
Worked example: magnification
A cell appears 40 mm long in a drawing; its real length is 50 µm. Convert first: 40 mm = 40 000 µm. Magnification = image size ÷ actual size = 40 000 ÷ 50 = ×800. Writing "800" without the × is accepted, but forgetting the unit conversion gives 0.8 and no marks - the conversion is the step the examiner is testing.
Traps
Swapping the magnification formula; forgetting that plant cells contain mitochondria as well as chloroplasts; claiming bacteria have mitochondria; treating the cell wall as the selectively permeable barrier; forgetting that active transport needs energy while osmosis does not.