CB1 — Key Concepts in Biology
Cell structure: eukaryotes and prokaryotes
All living organisms are made of cells. Edexcel 1BI0 requires you to distinguish between eukaryotic and prokaryotic cells with precision.
| Feature | Eukaryotic (animal/plant) | Prokaryotic (bacteria) |
|---|---|---|
| Nucleus | Present (membrane-bound) | Absent — DNA free in cytoplasm |
| Size | Typically 10–100 µm | Typically 1–10 µm |
| Mitochondria | Present (animal + plant) | Absent |
| Chloroplasts | Plant cells only | Absent |
| Cell wall | Plant: cellulose; Animal: none | Murein (peptidoglycan) |
| Ribosomes | 80S (larger) | 70S (smaller) |
| Plasmids | Rare | Often present |
Animal cell organelles: nucleus, cytoplasm, cell membrane, mitochondria, ribosomes. Plant cell additional structures: cell wall (cellulose), large central vacuole (cell sap), chloroplasts (contain chlorophyll). Bacterial cell structures: cell wall (murein), cell membrane, cytoplasm, 70S ribosomes, circular DNA (chromosome), plasmids, flagellum (some).
Microscopy — CP1 (Core Practical 1)
The light (optical) microscope uses visible light and glass lenses. Maximum useful magnification ~×1500; resolution ~200 nm. You can see cells, nuclei, chloroplasts, cell walls.
Electron microscopes use beams of electrons:
- Transmission electron microscope (TEM): passes electrons through a thin specimen; produces 2D high-resolution images of internal structure. Resolution ~0.5 nm.
- Scanning electron microscope (SEM): scans surface; gives 3D images of surfaces. Resolution ~1 nm.
Electron microscopes can resolve mitochondria inner membranes (cristae), ribosomes, endoplasmic reticulum — structures invisible in light microscopy.
Magnification formula (must know for Paper 1): $$\text{magnification} = \frac{\text{image size}}{\text{actual size}}$$
Rearranged: actual size = image size ÷ magnification; image size = actual size × magnification. Use the same units (convert µm ↔ mm: 1 mm = 1000 µm).
CP1 — Making and observing a slide: cut thin section → place on slide → add stain (iodine for starch/nucleus in plant cells; methylene blue for animal cells) → add coverslip at 45° to avoid bubbles → observe under low then high power. Draw a labelled biological diagram (no shading, single clear lines, ruler for label lines, scale bar or magnification stated).
Stem cells
Stem cells are undifferentiated cells that can divide (by mitosis) and differentiate into specialised cell types.
- Embryonic stem cells (from blastocyst): pluripotent — can become almost any cell type.
- Adult stem cells (e.g., in bone marrow): multipotent — limited range (e.g., blood cell types).
- Induced pluripotent stem cells (iPSCs): adult cells reprogrammed to pluripotent state.
Medical uses: treating leukaemia (bone-marrow transplant), potential treatments for Parkinson's, diabetes, spinal cord injury.
Ethical issues: embryonic stem cells require destruction of embryos → ethical objections; adult stem cells avoid this but are less versatile.
Diffusion, osmosis and active transport
Diffusion: net movement of particles from high → low concentration (down the concentration gradient). Passive (no ATP). Rate increased by: steeper gradient, higher temperature, larger surface area, shorter diffusion distance, smaller particles.
Osmosis: diffusion of water molecules across a partially permeable membrane from a region of high water potential (dilute solution) to low water potential (concentrated solution). Passive.
Core Practical CP3 — Osmosis in potato cylinders: cut identical potato cylinders, measure mass/length, place in solutions of different sucrose concentrations (0, 0.2, 0.4, 0.6, 0.8, 1.0 mol/dm³), leave 30 min, remove, dry gently, re-measure. Plot % change in mass vs concentration. The concentration where mass does not change = water potential of potato cells. Cylinders in dilute solutions gain mass (turgid); in concentrated solutions lose mass (plasmolysed/flaccid).
Active transport: movement of substances from low → high concentration (against the gradient) using carrier proteins and ATP (energy from respiration). Example: uptake of mineral ions by plant root hair cells; glucose uptake from gut into blood.
Comparison table:
| Feature | Diffusion | Osmosis | Active transport |
|---|---|---|---|
| Direction | High → low conc. | High → low ψ (water) | Low → high conc. |
| Membrane needed | No | Yes (partially permeable) | Yes |
| Energy (ATP) | No | No | Yes |
| Example | O₂ into red blood cells | Water into root cells | Glucose into ileum cells |
Enzyme activity and CP2
Enzymes are biological catalysts — proteins that speed up specific reactions without being used up. The active site matches the substrate (lock-and-key model); the induced fit model shows the active site changes shape slightly on substrate binding.
Factors affecting enzyme activity:
- Temperature: rate increases up to optimum (~37°C for human enzymes). Above optimum: enzyme denatures — active site shape changes permanently, no substrate binding.
- pH: each enzyme has an optimum pH (e.g., pepsin pH 2, amylase pH 7, trypsin pH 8). Extreme pH denatures.
- Substrate concentration: rate increases until enzymes are saturated.
CP2 — Effect of pH on enzyme activity: mix amylase + starch in buffer solutions at different pH. Spot on iodine-spotted tile every 30 s. Time until colour no longer turns blue-black (starch fully digested). pH with fastest digestion = optimum.
Common exam mistakes (Edexcel 1BI0 Paper 1)
- Saying bacteria have a "nucleus" — they do NOT. DNA is in the cytoplasm (nucleoid region).
- Confusing magnification formula — dividing actual by image instead of image by actual.
- In osmosis questions, direction — water moves to the MORE concentrated solution (lower water potential).
- Confusing diffusion and active transport — active transport needs ATP, goes against the gradient.
- Stating enzymes are "destroyed" in reactions — they are not; they are reused (unless denatured).
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