Summary of "AQA GCSE Biology - Cell Biology for Combined Science | Whole topic"
1) Cell types and subcellular structures
Eukaryotic cells (animal and plant)
- Shared structures
- Nucleus — controls the cell; contains DNA needed for protein synthesis.
- Cytoplasm — site of many chemical reactions.
- Cell membrane — partially permeable; controls entry and exit of substances.
- Animal cell (specific)
- No cell wall.
- Organelles: ribosomes (make proteins) and mitochondria (release energy by respiration — do not say “produce energy”).
- Plant cell (specific)
- Cell wall made of cellulose — provides strength and support.
- Large central vacuole containing cell sap (sugary solution).
- Chloroplasts containing chlorophyll for photosynthesis.
- These structures are called subcellular structures.
Prokaryotic cells (bacteria)
- No nucleus and no membrane-bound organelles (no mitochondria).
- Contain: cytoplasm, ribosomes, cell membrane, cell wall (not made of cellulose), circular/main DNA strand, and plasmids (small extra DNA rings).
Relative sizes (approximate)
- Animal cell: ~10 µm
- Plant cell: ~50 µm
- Prokaryote (bacterium): ~5 µm
2) Cell differentiation and specialized cells
Differentiation
- Undifferentiated cells from the fertilized egg become specialized to carry out particular functions.
Animals
- Examples of specialized cells: nerve cells, muscle cells, sperm.
- Most animal cells cannot redifferentiate once differentiated (except for some repair divisions).
Plants
- Many plant cells retain the ability to differentiate.
- Meristem cells (in root and shoot tips) can form any plant cell — useful for cloning plants.
Examples of specialized cell adaptations
- Nerve cell: cell body, dendrites, long axon, insulating sheath, axon terminals — adapted for rapid impulse transmission.
- Sperm cell: acrosome with enzymes, nucleus with 23 chromosomes, mid-piece with many mitochondria, tail for swimming.
- Muscle cell: contractile fibers, many mitochondria for respiration, glycogen stores.
- Xylem: dead, hollow tube-like cells reinforced with lignin rings, no end plates or cytoplasm — for water transport and support.
- Phloem: living elongated cells with little cytoplasm; sieve end plates with pores; companion cells provide energy for sugar transport.
- Root hair cell: large surface area and many mitochondria — adapted for mineral and water uptake (active transport and diffusion).
3) Microscopy, scales and magnification
Microscopes
- Historical progression: simple/light microscopes → electron microscopes.
- Electron microscopes have much higher magnification and resolving power (can show ribosomes and internal structures of mitochondria).
Units and conversions
- 1 mm = 1000 µm
- 1 µm = 1000 nm
- Conversion rules:
- mm → µm: × 1000
- µm → nm: × 1000
- Reverse conversions: divide by 1000 per step
Magnification formula
- magnification = image size / real size
- Ensure image size and real size use the same units.
4) Cell cycle and mitosis
Cell cycle stages
- Stage 1 — Interphase: cell grows, increases organelles (e.g., ribosomes, mitochondria), and DNA replicates (two copies of each chromosome).
- Stage 2 — Mitosis: chromosomes line up and are pulled to opposite poles; nuclei divide.
- Stage 3 — Cytokinesis: cytoplasm and cell membranes divide to form two genetically identical daughter cells (identical to each other and to the parent).
Purposes of mitosis
- Growth, development, and repair.
5) Stem cells
Types and locations
- Embryonic stem cells: undifferentiated, can be cloned, and can become many cell types — potential for medical therapies (e.g., paralysis, diabetes).
- Adult stem cells: for example in bone marrow — mainly differentiate into blood cells.
- Plant meristem stem cells: found in root and shoot tips; can form whole plants and are used for cloning rare or commercially desirable plants.
Therapeutic cloning (process and pros/cons)
- Process:
- Remove nucleus from a patient somatic cell (e.g., skin cell).
- Remove nucleus from a donor egg (enucleated egg).
- Insert patient nucleus into the enucleated egg.
- Clone the egg to produce stem cells that can be differentiated into tissues for the patient.
- Advantage: lower risk of immune rejection.
- Risks/objections: potential viral transfer; moral and religious objections.
6) Diffusion and active transport
Diffusion — net movement of particles from a region of higher concentration to a region of lower concentration (in a solution or gas) until equilibrium is reached.
- Biological examples: oxygen diffusing from blood to cells; carbon dioxide diffusing from cells to blood; urea diffusing out of cells.
- Factors affecting rate of diffusion:
- Concentration/diffusion gradient — larger difference → faster diffusion.
- Temperature — higher temperature → faster diffusion.
- Surface area — larger surface area → faster diffusion.
Active transport — movement of particles from a region of lower concentration to a region of higher concentration that requires energy from respiration.
- Examples: uptake of sugars from the small intestine into blood against a concentration gradient; uptake of mineral ions by root hair cells from dilute soil.
7) Exchange surfaces and surface area : volume ratio
Concept
- Small organisms and cells have large surface area : volume (SA:V) ratios, allowing sufficient diffusion. Larger organisms have smaller SA:V and therefore need specialized exchange and transport systems.
Calculations
- Calculate surface area and volume for simple shapes (e.g., cubes), then compute SA:V as surface area ÷ volume. As size increases (e.g., larger cubes), SA:V decreases.
Adaptations in exchange systems
- Small intestine: villi and microvilli increase surface area; thin walls; many mitochondria in microvilli; rich blood supply to maintain a concentration gradient.
- Lungs: many alveoli → large surface area; thin (one-cell thick) walls; rich capillary network; ventilation maintains gradients.
- Leaves: flat and thin with stomata and internal air spaces to increase diffusion.
- Fish gills: many filaments and lamellae for large surface area; thin capillary walls; counter-current flow and constant water flow maintain gradients.
8) Osmosis
Osmosis — diffusion of water from a dilute solution (high water potential) to a concentrated solution (low water potential) through a partially permeable membrane.
- In plant cells:
- Water entering the cell makes it turgid (supported by the cell wall; plant cells do not burst).
- Water leaving the cell makes it flaccid or plasmolysed depending on conditions.
- Example: root hair cells absorb water from dilute soil by osmosis.
9) Practical investigation — effect of sugar/salt solutions on mass of plant tissue (potato experiment)
Purpose
- Test how varying external solution concentrations affect mass via osmosis and find the isotonic point (no net mass change).
Materials
- Potato (or other plant tissue such as carrot), cork borer, cutting tool, paper towel, balance, boiling tubes, solutions of different concentrations (e.g., 0, 0.2, 0.4, 0.6, 0.8 mol/dm³), water.
Method (step-by-step)
- Prepare equal volumes (e.g., 30 cm³) of each solution concentration in separate boiling tubes (include pure water = 0).
- Use a cork borer to cut equal-sized cylinders of potato; trim to equal lengths so pieces have equal shape and, as far as possible, equal mass.
- Dry the surface of each cylinder with a paper towel to remove surface water.
- Weigh each potato piece (initial mass) and place one piece per tube.
- Leave for a set time (24 hours preferred; at least a few hours).
- Remove pieces, dry surface again with a paper towel, and re-weigh (final mass).
- Record initial and final masses, calculate change in mass and percentage change (percentage change accounts for starting mass differences).
- Plot concentration (x-axis) vs percentage change in mass (y-axis) and draw a best-fit line. The concentration where the line crosses the x-axis is the isotonic point (no net gain or loss); example value given ~0.24 mol/dm³.
Controls to keep constant
- Temperature, type/age of potato (or plant tissue), volume of solution, sample size.
Interpretation
- If the potato loses mass: the external solution is more concentrated; water moved out by osmosis.
- If the potato gains mass: the potato’s internal solution is more concentrated; water moved in by osmosis.
10) Final notes and examination relevance
- Key exam points:
- Organelle functions and differences between prokaryotes and eukaryotes.
- Definitions of diffusion, osmosis and active transport, and factors affecting diffusion.
- Stages of mitosis and the cell cycle.
- Functions and adaptations of specialized cells and exchange surfaces.
- Stem cell types and therapeutic cloning pros/cons.
- Magnification calculations and unit conversions.
- Practical method variables and data interpretation (finding the isotonic point).
- Reminder: use correct phrasing in answers (for example, say “release energy by respiration” rather than “produce energy”), and always match units when using the magnification formula.
Speakers / sources featured
- Video presenter / teacher (unnamed) — single narrator throughout the subtitles.
- Content aligned with the AQA GCSE Biology specification (topic: Cell Biology).
Category
Educational
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