Revision notes · Cell biology
Cell structure
Eukaryotes and prokaryotes4.1.1.1
Definition: Eukaryotic cells have their genetic material enclosed in a nucleus; prokaryotic cells do not — their DNA lies free in the cytoplasm.
All living organisms are built from one of two basic types of cell.
- •Animal and plant cells are eukaryotic; bacteria are prokaryotic.
- •Eukaryotic cells are generally much larger than prokaryotic cells.
- •A prokaryotic cell's DNA is a single circular strand, not enclosed in a nucleus; it may also carry one or more small rings of extra DNA called plasmids.
- •Prokaryotic cells lack membrane-bound organelles such as mitochondria and chloroplasts.
⚠️ Common mistake: Thinking a bacterial cell has a nucleus, just a 'loose' one. It has no nucleus at all — the DNA floats free in the cytoplasm.
🧠 Remember: Pro = no nucleus; Eu = has one (‘Eu’ sounds like ‘you’ — you’ve got a nucleus).
Animal and plant cells4.1.1.2
Definition: A sub-cellular structure (organelle) is a distinct part inside a cell that carries out a specific function.
Animal and plant cells share several structures, but plant cells have three extra ones.
| Structure | Function | Animal | Plant |
|---|---|---|---|
| Nucleus | Contains DNA; controls the cell | ✓ | ✓ |
| Cytoplasm | Site of most chemical reactions | ✓ | ✓ |
| Cell membrane | Controls what enters/leaves | ✓ | ✓ |
| Mitochondria | Site of aerobic respiration | ✓ | ✓ |
| Ribosomes | Site of protein synthesis | ✓ | ✓ |
| Cell wall (cellulose) | Strengthens and supports the cell | ✗ | ✓ |
| Chloroplasts | Absorb light for photosynthesis | ✗ | ✓ |
| Permanent vacuole | Contains cell sap; keeps the cell rigid | ✗ | ✓ |
⚠️ Common mistake: Forgetting plant cells still have mitochondria and ribosomes as well as chloroplasts — the extra structures are additional, not a replacement.
🧠 Remember: Plant cells get three extras: cell Wall, Vacuole, Chloroplasts — WVC.
Cell specialisation4.1.1.3
Definition: A specialised cell has changed in structure to be well-adapted for a particular function.
Different cells are adapted with different structures depending on their job in the organism.
- •Sperm cells have a long tail and many mitochondria to swim to the egg.
- •Nerve cells are long and branched, to carry electrical impulses quickly across the body.
- •Muscle cells contain protein fibres that can contract, and many mitochondria for energy.
- •Root hair cells have a large surface area to absorb water and mineral ions efficiently.
- •Xylem cells are hollow, joined end to end, and strengthened with lignin, to transport water.
- •Phloem cells form tubes with perforated end walls (sieve plates) to transport dissolved sugars.
⚠️ Common mistake: Assuming root hair cells contain chloroplasts — they don't; they're underground, so photosynthesis is impossible there.
🧠 Remember: Structure fits function — always link a specialised cell's shape to the job it does.
Cell differentiation4.1.1.4
Definition: Differentiation is the process by which a cell changes to become specialised for its function.
As an organism develops, unspecialised cells differentiate into the many types of specialised cell it needs.
- •Most animal cells differentiate early in development; once specialised, they mostly lose the ability to differentiate again.
- •Many plant cells retain the ability to differentiate throughout the plant's life.
- •In mature animals, cell division is mainly used for growth, and to repair or replace damaged cells.
- •As a cell differentiates, it develops the sub-cellular structures it needs for its specific function.
⚠️ Common mistake: Muddling differentiation (becoming specialised) with mitosis (making identical copies) — a cell can divide without differentiating, and vice versa.
🧠 Remember: Differentiation = becoming different (specialised); mitosis = making identical copies.
Microscopy4.1.1.5
Definition: Magnification is how many times bigger an image is than the real object; resolution is the ability to distinguish two close points as separate.
Microscopes let us see cells and structures too small for the naked eye.
| Light microscope | Electron microscope | |
|---|---|---|
| Magnification | Up to ×2,000 | Up to ×2,000,000 |
| Resolution | Lower — cannot show detail inside organelles | Much higher — shows internal organelle structure |
| Sample | Living or dead; can be coloured | Must be dead, in a vacuum; black and white |
| Practicality | Portable and cheap | Large, expensive, specialist |
- •Magnification = image size ÷ actual size.
- •Convert mm to μm by ×1000 (and μm to mm by ÷1000) before calculating.
⚠️ Common mistake: Forgetting to convert image size and actual size to the same units before dividing — the single most common lost mark in magnification questions.
🧠 Remember: I = A × M (Image size = Actual size × Magnification) — rearrange to find whichever value is missing.
Culturing microorganisms4.1.1.6
Definition: Aseptic technique is the set of sterile procedures used to stop unwanted microorganisms contaminating a culture.
Bacteria reproduce rapidly by binary fission, so cultures must be grown and handled carefully to stay uncontaminated and safe.
- •Bacteria reproduce by binary fission (splitting in two) as often as every 20 minutes in ideal conditions.
- •The lid is taped in place but not fully sealed, to still allow some air exchange.
- 1Sterilise the Petri dish, agar and equipment (e.g. in an autoclave) to kill unwanted microorganisms.
- 2Sterilise the inoculating loop by passing it through a Bunsen burner flame until red hot, then let it cool.
- 3Use the loop to transfer microorganisms onto the agar plate.
- 4Briefly lift the lid to inoculate the agar, minimising the time it is open.
- 5Secure the lid with tape around the edge (not fully sealed) and store the dish upside down.
- 6Incubate at a maximum of 25°C in school labs, to reduce the risk of growing pathogens harmful to humans.
⚠️ Common mistake: Thinking the lid should be sealed completely airtight — this actually encourages growth of dangerous anaerobic bacteria; it should be taped, not fully sealed.
🧠 Remember: School cultures: incubate at 25°C max — cooler than body temperature, to avoid growing pathogens that infect humans.
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