HKDSE Biology cheat sheet

HKDSE Biology - Core Ideas, Skills and Exam Conventions

• Levels of organisation: organelle → cell → tissue → organ → organ system → organism; each level adds emergent properties not predictable from the level below alone.

• Cell theory: all organisms are composed of cells, the cell is the basic unit of life, and cells arise from pre-existing cells by cell division.

• Prokaryotic vs eukaryotic: prokaryotes lack a membrane-bound nucleus and membrane-bound organelles (mitochondria, ER, chloroplasts) and have 70S ribosomes; eukaryotes have a true nucleus, 80S ribosomes and membrane-bound organelles.

• Magnification = image size ÷ actual size; keep units consistent (convert µm to mm by ×1000). Actual size = image size ÷ magnification.

• Resolution (resolving power) is the minimum distance at which two points remain distinguishable - a separate property from magnification; an electron microscope has far higher resolution than a light microscope.

• Biological molecules and their tests: reducing sugars by Benedict's test (blue → brick-red on heating), starch by iodine solution (brown → blue-black), protein by Biuret test (blue → violet), lipid by emulsion test (white emulsion with water) or grease spot.

• Enzymes are biological catalysts (mostly proteins) that lower activation energy; they are specific, and activity is affected by temperature (denaturation above optimum), pH and substrate concentration.

• Movement across membranes: diffusion (passive, down a concentration gradient), osmosis (water across a partially permeable membrane from higher to lower water potential), active transport (against the gradient, requires ATP), and phagocytosis/endocytosis for bulk transport.

• Practical skills assessed in Paper 1 and the SBA: designing controlled experiments, identifying independent/dependent/controlled variables, drawing labelled diagrams and tables, plotting graphs with units, and evaluating sources of error and limitations.