Cell Biology and Biochemistry
What you must know
Cell theory states that all living organisms are made of cells, that cells arise from pre-existing cells, and that the cell is the basic unit of life. Candidates must compare prokaryotic and eukaryotic cells (prokaryotes: no nucleus, 70S ribosomes, circular DNA, plasmids, capsule, flagella; eukaryotes: membrane-bound organelles, linear chromosomes, 80S ribosomes) and state the functions of the nucleus, mitochondrion, chloroplast, ribosome, rough and smooth ER, Golgi apparatus, lysosome, vacuole and cell membrane. The fluid-mosaic model explains membrane behaviour: phospholipid bilayer, intrinsic and extrinsic proteins, cholesterol and glycolipids.
Biochemistry: condensation joins monomers with the loss of water; hydrolysis reverses it. Know the building blocks of carbohydrates, proteins (amino acids joined by peptide bonds), lipids and nucleic acids, and the food tests (Benedict's, iodine, Biuret, emulsion). Enzymes are biological catalysts whose activity depends on temperature and pH; above the optimum the active site is denatured.
Traps and a worked example
Common traps: assuming only plant cells have cell walls (fungi and bacteria do too); confusing magnification with resolution; describing an enzyme as 'killed' by heat rather than denatured; using the word osmosis for solutes rather than water.
Worked example: a cell 50 micrometres across is drawn 20 mm wide. Magnification = image size divided by actual size = 20 mm divided by 0.05 mm = x400. Rule of thumb: I = A x M, so rearrange the triangle before substituting, and always convert units first.