ZIMSEC A Level · Biology · Form 5
Eukaryotic and prokaryotic cells
- Eukaryotic cells have a nucleus and membrane-bound organelles: nucleus, rough and smooth ER, Golgi body, mitochondria, lysosomes, ribosomes (80S), centrioles (animals), chloroplasts and a cellulose wall (plants).
- Prokaryotic cells (bacteria) have no nucleus (circular DNA in the cytoplasm), plasmids, 70S ribosomes, a peptidoglycan (murein) wall, and sometimes a capsule and flagella.
Microscopy
Light microscopes resolve about 200 nm; electron microscopes about 0.5–1 nm (TEM for internal detail, SEM for surfaces). Magnification = image size ÷ actual size.
The fluid mosaic model
A phospholipid bilayer with proteins floating in it; cholesterol regulates fluidity; glycoproteins and glycolipids act in cell recognition.
Transport across membranes
- Simple diffusion (small non-polar molecules), facilitated diffusion (channel and carrier proteins), osmosis (water, down a water potential gradient), active transport (carrier proteins using ATP, against a gradient), endocytosis and exocytosis (bulk transport in vesicles).
- Water potential (ψ): pure water is 0; solutions are negative. Water moves from higher (less negative) to lower (more negative) ψ.
Exam tip: Use the term “water potential” rather than “concentration of water” at A Level.
Check yourself
- Give two differences between prokaryotic and eukaryotic cells.
- What is the role of cholesterol in membranes?
- Cell A has ψ = −400 kPa and cell B −700 kPa. In which direction does water move?
Show answers
- Any two: no nucleus vs nucleus; 70S vs 80S ribosomes; no membrane-bound organelles; murein wall; plasmids.
- It regulates membrane fluidity and stability.
- From A to B (from higher to lower water potential).
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