1.1Cells, Organ Systems and Ecosystems
Cells and Movement Across Membranes — WJEC GCSE Biology Revision Notes & Topical Questions
5 min read 4 parts Lesson 1 of 6
Section 1 of 4Cell structure and magnification
| Structure | Animal | Plant | Bacteria | Function |
|---|---|---|---|---|
| Nucleus | Yes | Yes | No | Contains genetic material (DNA) controlling cell activities |
| Cytoplasm | Yes | Yes | Yes | Site of cellular chemical reactions |
| Cell Membrane | Yes | Yes | Yes | Controls entry and exit of substances; selectively permeable |
| Mitochondria | Yes | Yes | No | Site of aerobic respiration to release ATP energy |
| Ribosomes | Yes | Yes | Yes | Site of protein synthesis |
| Cell Wall | No | Yes (cellulose) | Yes | Strengthens and supports cell structure |
| Chloroplasts | No | Yes (leaves) | No | Contains chlorophyll to absorb light for photosynthesis |
| Permanent Vacuole | No | Yes | No | Contains cell sap; maintains turgidity |
| Plasmids | No | No | Yes | Small circular loops of non-chromosomal DNA |
- Magnification equation:
Image size = Actual size × Magnification
(Always convert measurements to the same units before calculating: 1 mm = 1000 µm).
Section 2 of 4Movement of substances across membranes
Diffusion
Net movement of solute particles from an area of higher concentration to lower concentration down a concentration gradient (passive process, requires no ATP).
Osmosis
Net movement of water molecules from a high water potential (dilute solution) to a lower water potential (concentrated solution) across a selectively permeable membrane:
- Plant cells in pure water become turgid (cell wall prevents bursting);
- in concentrated sugar/salt solutions they become flaccid/plasmolysed.
- Animal cells (red blood cells) burst (lyse) in pure water and shrivel (crenate) in concentrated salt solution.
Active Transport
Movement of substances from low to high concentration against a concentration gradient using carrier proteins and respiratory energy (ATP).
Section 3 of 4Key terms
| Term | Meaning |
|---|---|
| Selectively permeable | A membrane allowing certain small molecules to cross freely while barring larger ones. |
| Osmosis | The net diffusion of water molecules across a selectively permeable membrane down a water potential gradient. |
| Active transport | Energy-requiring movement of substances against a concentration gradient via membrane carrier proteins. |
| Turgor pressure | Internal pressure exerted by water inside a plant vacuole pressing firmly against the cell wall. |
Section 4 of 4Exam technique
Avoid these mistakes
Stating animal cells have a cell wall — animal cells never possess a cell wall.
Forgetting units when calculating magnification — always convert mm to µm by multiplying by 1000 before dividing.
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