Cell Biology
Cell Biology
This domain tests cell theory, prokaryotic and eukaryotic cells, plant and animal cell structures, organelles, membrane transport and cell division.
Core rules
- Cell theory: all living organisms are made of cells; the cell is the basic unit of life; new cells arise from pre-existing cells.
- Prokaryotic cells lack a true nucleus and membrane-bound organelles; eukaryotic cells have a nucleus and organelles.
- Plasma membrane is selectively permeable; diffusion moves substances down a concentration gradient, osmosis moves water across a semipermeable membrane, active transport uses energy.
- Mitochondria release energy by respiration; ribosomes make proteins; chloroplasts carry out photosynthesis; vacuoles store sap and maintain turgidity.
- Plant cells have a cellulose cell wall, chloroplasts and a large central vacuole; animal cells have centrioles and no cell wall.
- Mitosis produces two genetically identical diploid cells for growth and repair; meiosis produces four haploid gametes with variation.
Worked example
An onion peel cell is placed in concentrated sugar solution. Water leaves the vacuole by osmosis because the external solution is hypertonic, so the protoplast shrinks away from the cell wall. This is plasmolysis. If the cell is then placed in pure water, water enters by endosmosis and the cell becomes turgid, provided it is still living. The cell wall prevents bursting.
Traps
Do not say osmosis moves sugar; it moves water. A cell wall is not selectively permeable in the same way as the plasma membrane. Chloroplasts are not present in all plant cells, such as root cells. Mitosis does not halve chromosome number.