Cellular Structure, Transport Across Membranes and Cell Division
What this domain tests
Cell questions combine structure–function relationships, membrane transport and the differences between mitosis and meiosis. You should be able to label a cell, explain why each organelle suits its role and predict water movement.
Cell structure
- Eukaryotic cells have a nucleus and membrane-bound organelles; prokaryotes such as bacteria do not.
- Plant cells have a cellulose cell wall, a large permanent vacuole and chloroplasts; animal cells have none of these but may have centrioles.
- Mitochondria (cristae) carry out respiration; chloroplasts (grana, stroma) carry out photosynthesis; ribosomes make protein; the Golgi body modifies and packages; the membrane controls entry and exit.
Transport across membranes
- The membrane is a fluid mosaic of phospholipids and proteins and is selectively permeable.
- Diffusion moves particles down a concentration gradient. Osmosis is the movement of water from higher water potential to lower water potential across a semi-permeable membrane.
- Active transport uses ATP and carrier proteins to move substances against a gradient.
- In plant cells, water entry creates turgor pressure and supports the cell; water loss causes plasmolysis.
Cell division
- Mitosis produces two genetically identical diploid cells for growth, repair and asexual reproduction.
- Meiosis produces four haploid cells and generates variation through crossing over and independent assortment; it forms gametes.
Worked example
A plant cell is placed in 0.5 mol/L sucrose and its vacuole shrinks away from the cell wall. Reasoning: the external solution has a lower water potential than the cell, so water leaves by osmosis, turgor is lost and the protoplast pulls away from the wall - plasmolysis. In distilled water the same cell would become turgid, and the wall would prevent it bursting.
Traps
Saying water moves 'towards the higher water concentration'; omitting ATP from active transport; confusing the cell wall with the membrane; stating that mitosis produces four cells or that meiosis produces identical cells.