Cell Biology and Biochemistry: Structure, Molecules and Transport
What this domain tests
The Cell biology and biochemistry unit of the MATSEC Intermediate Biology examination asks you to link structure to function at cell level and to explain the chemistry that underpins life. Expect questions on prokaryotic and eukaryotic cell ultrastructure, organelle functions, biological molecules and their tests, enzyme action and membrane transport.
Core rules
- Prokaryotes lack membrane-bound organelles and a true nucleus; their DNA is circular and they may have plasmids, a capsule and 70S ribosomes. Eukaryotes have a nucleus, 80S ribosomes and membrane-bound organelles.
- Mitochondria and chloroplasts are double-membrane organelles; the inner mitochondrial membrane is folded into cristae, and chloroplasts contain thylakoids stacked into grana.
- Proteins are polymers of amino acids joined by peptide bonds; the biuret test gives a purple colour. Reducing sugars give a brick-red precipitate with Benedict's reagent on heating; starch gives blue-black with iodine.
- Enzymes are globular proteins that lower activation energy. Rate rises with temperature to an optimum, then falls as denaturation alters the active site.
- Diffusion, osmosis and active transport differ in whether they need energy and a carrier protein; water potential determines osmosis.
Worked example
A drawing of a cell is 40 mm wide and the magnification is ×2000. Actual width = image width ÷ magnification = 40 mm ÷ 2000 = 0.02 mm = 20 µm. Convert units before dividing, and remember that magnification increases image size but does not improve resolution.
Traps
Do not say an enzyme is 'killed' when denatured; it is inactivated because its tertiary structure changes. Do not confuse magnification with resolution. In osmosis, water moves from higher (less negative) water potential to lower (more negative) water potential.