Cell structure, division and membrane transport
Cell structure, division and membranes
This domain tests cell organelles and their functions, differences between plant, animal and bacterial cells, mitosis and its importance, and how substances move across membranes by diffusion, osmosis and active transport.
Core rules
- Nucleus contains DNA; mitochondria are the site of aerobic respiration; ribosomes make proteins; chloroplasts carry out photosynthesis; cell wall is cellulose in plants; vacuole stores sap; cell membrane controls entry and exit.
- Plant cells have a cell wall, chloroplasts and a large permanent vacuole; animal cells do not. Prokaryotes lack a nucleus and membrane-bound organelles.
- Mitosis produces two genetically identical diploid cells for growth, repair and asexual reproduction. Meiosis produces gametes with half the chromosome number.
- Diffusion is net movement of particles from high to low concentration; osmosis is water movement across a partially permeable membrane from higher to lower water potential; active transport uses energy to move substances against a gradient.
- Surface area to volume ratio falls as cells get larger, limiting diffusion.
Worked example
A drawing of a cell is 30 mm long. The actual cell is 0.05 mm long. Magnification = image size / actual size = 30 / 0.05 = ×600. Reasoning: both measurements must be in the same unit before dividing. If the drawing were 30 µm and actual 0.05 mm, convert 0.05 mm to 50 µm first, giving ×0.6, which signals an error because magnification should be greater than 1 for an enlarged drawing.
Traps
- Mixing up magnification and actual size.
- Forgetting to convert mm to µm (×1000).
- Saying osmosis moves solutes.
- Claiming mitosis makes gametes.
- Ignoring the partially permeable membrane in osmosis.