Cell Structures
Eukaryotic and Prokaryotic Cells
All living things are made of cells. There are two fundamental types:
Eukaryotic cells (animals, plants, fungi): Have a nucleus containing DNA; generally larger (10–100 μm); more complex internal structure with membrane-bound organelles.
Prokaryotic cells (bacteria): No nucleus — DNA is a circular strand free in the cytoplasm; much smaller (1–10 μm); no membrane-bound organelles.
Animal Cell Structures
| Structure | Function |
|---|---|
| Nucleus | Contains DNA (as chromosomes); controls cell activities |
| Cell membrane | Controls what enters and leaves the cell (partially permeable) |
| Cytoplasm | Site of chemical reactions; contains enzymes |
| Mitochondria | Site of aerobic respiration; produce ATP (energy) |
| Ribosomes | Site of protein synthesis |
Plant Cell Structures (additional)
| Structure | Function |
|---|---|
| Cell wall | Made of cellulose; provides support and strength |
| Chloroplasts | Contain chlorophyll; site of photosynthesis |
| Permanent vacuole | Contains cell sap; maintains cell turgor |
Plants also have all animal cell structures.
Bacterial Cell Structures
| Structure | Function |
|---|---|
| Cell wall | Made of peptidoglycan (not cellulose); provides support |
| Cell membrane | Controls transport in/out |
| Cytoplasm | Site of reactions |
| Ribosomes | Protein synthesis (smaller than eukaryotic ribosomes) |
| Plasmids | Small circular DNA molecules (not the main chromosome) |
| Flagellum | Tail-like structure for movement (not all bacteria) |
| Chromosomal DNA | Single circular loop; not enclosed in a nucleus |
Note: Bacteria have NO mitochondria, NO chloroplasts, NO nucleus, NO membrane-bound organelles.
Microscopy
Cells are studied using microscopes:
- Light microscope: Uses light and glass lenses; can see up to ~200 nm; can show live cells and colour; used for whole cells, tissues
- Electron microscope: Uses electron beams; resolution up to 0.1 nm; shows sub-cellular detail (organelles clearly); cells must be dead and specially prepared
Magnification formula: Magnification = Image size ÷ Actual size
Example: An image of a cell is 5 cm = 50,000 μm. The actual cell is 50 μm. Magnification = 50,000 ÷ 50 = ×1000.
Conversion: 1 mm = 1000 μm; 1 μm = 1000 nm.
Scale of Cells
- Bacterial cell: ~1–10 μm
- Animal cell: ~10–40 μm
- Plant cell: ~10–100 μm
Stem Cells
Embryonic stem cells: Can differentiate into ANY type of cell (totipotent/pluripotent). Used in research; potential for treating diseases. Ethical issues — use of embryos.
Adult stem cells: Found in bone marrow and a few other tissues. Can differentiate into a limited range of cell types (multipotent). Used in bone marrow transplants for leukaemia.
Therapeutic cloning: Creating an embryo genetically identical to the patient to harvest stem cells — avoids rejection but raises ethical issues.
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