AQA GCSE (9-1) Biology: Cell Structure

Key Takeaways: Cell Structure

1. Prokaryotic vs. Eukaryotic Cells

• Prokaryotic Cells (e.g., bacteria):
  • No nucleus (DNA in a single loop/plasmids).
  • Smaller (1–10 µm).
  • Cell wall (not cellulose), cytoplasm, ribosomes.
• Eukaryotic Cells (plants, animals, fungi):
  • Have a nucleus and membrane-bound organelles.
  • Larger (10–100 µm).
  • Plant cells: Cellulose cell wall, chloroplasts, vacuole.

2. Animal vs. Plant Cells

Shared Features: Nucleus, mitochondria, ribosomes, cytoplasm, cell membrane.

3. Specialized Cells & Adaptations

• Sperm Cell: Tail (movement), mitochondria (energy), streamlined.
• Nerve Cell: Long axon, myelin sheath (fast impulses), dendrites (signal reception).
• Red Blood Cell: Biconcave shape (maximizes oxygen), no nucleus.
• Root Hair Cell: Long extension (increases surface area for absorption).
• Xylem: Hollow, lignin-strengthened tubes (water transport).
• Phloem: Sieve plates (translocate sugars), companion cells.

Cell Differentiation:

• Cells specialize during development (e.g., muscle cells develop more mitochondria).
• Plants retain differentiation ability throughout life.

4. Microscopy

• Light Microscope:
  • Magnification: ~×1,500.
  • Resolution: ~200 nm.
  • Uses light; can view live cells.
• Electron Microscope:
  • Magnification: Up to ×2,000,000.
  • Resolution: 0.1 nm.
  • Types: TEM (internal detail) and SEM (3D surface images).

Magnification Formula:

• Units: 1 mm = 1,000 µm; 1 µm = 1,000 nm.

5. Key Processes & Terms

• Photosynthesis: Occurs in chloroplasts (chlorophyll absorbs light).
• Respiration: Occurs in mitochondria (releases energy).
• Turgidity: Vacuole presses cytoplasm against cell wall (keeps plant rigid).
• Transpiration: Water loss from leaves (drives water uptake via xylem).

Quick Revision Tips:

• Use flashcards for organelle functions.
• Practice labeling cell diagrams.
• Memorize magnification formula and unit conversions.
• Compare adaptations of specialized cells using tables.

50 GCSE Biology Questions on Cell Structure

Answers provided at the end.

Section 1: Prokaryotic vs. Eukaryotic Cells

1. State three structural differences between prokaryotic and eukaryotic cells.
2. Name the genetic material found in prokaryotic cells.
3. What are plasmids, and in which type of cell are they found?
4. Explain why prokaryotic cells are generally smaller than eukaryotic cells.
5. Why do scientists believe prokaryotes evolved before eukaryotes?

Section 2: Animal and Plant Cells

6. List three structures present in plant cells but absent in animal cells.
7. What is the function of the vacuole in plant cells?
8. Why are chloroplasts absent in root hair cells?
9. Describe the role of the cellulose cell wall in plant cells.
10. How does the shape of animal cells differ from plant cells?

Section 3: Specialised Cells

11. Explain how a sperm cell is adapted for its function.
12. Why do muscle cells contain many mitochondria?
13. Describe two adaptations of red blood cells.
14. How does the structure of a nerve cell support its role in transmitting impulses?
15. What is the function of root hair cells, and how are they adapted?
16. Why are xylem cells hollow, and how does this aid their function?
17. What is the role of phloem cells, and how do sieve plates assist this process?
18. Why do root hair cells lack chloroplasts?
19. Explain how the biconcave shape of red blood cells increases efficiency.
20. Compare the structure of xylem and phloem cells.

Section 4: Microscopy

21. Calculate the total magnification if the eyepiece lens is ×10 and the objective lens is ×40.
22. State two advantages of electron microscopes over light microscopes.
23. Why are electron microscope images black and white?
24. What is resolution, and why is it important in microscopy?
25. Describe how to prepare a slide of onion epidermal cells.
26. Explain why iodine solution is used when observing plant cells.
27. What is the maximum resolution of a light microscope?
28. How does a transmission electron microscope (TEM) differ from a scanning electron microscope (SEM)?
29. Why are specimens for electron microscopes often dead?
30. Name two parts of a light microscope and their functions.

Section 5: Cell Processes and Organelles

31. Where in the cell does respiration occur, and what is its purpose?
32. What is the function of ribosomes?
33. Explain the role of chlorophyll in photosynthesis.
34. Define turgidity and its importance in plants.
35. How does the nucleus control cell activities?
36. What is cell differentiation, and why is it important?
37. Why can plant cells differentiate throughout their life, unlike animal cells?
38. Describe the function of the cell membrane.
39. What is transpiration, and how does it relate to xylem function?
40. How do mitochondria structure (e.g., folded membranes) relate to their function?

Section 6: Calculations and Units

41. Convert 5 mm to micrometres (µm).
42. An image of a cell measures 45 mm. If the actual cell is 15 µm, calculate the magnification.
43. A pinhead is 2 mm long. If magnified ×500, what is the image size in cm?
44. If a mitochondrion is 0.5 µm in real life, how long would it appear under a ×10,000 magnification?
45. Define nanometre (nm) and its relation to micrometres (µm).

Section 7: Application and Analysis

46. Orchids have green roots. Suggest why this adaptation might be beneficial.
47. Explain why muscle cells in the heart (cardiac muscle) have many mitochondria.
48. A student observes a cell with a cell wall and chloroplasts. Is it an animal, plant, or bacterial cell? Justify.
49. Why might a leaf cell have more chloroplasts than a root cell?
50. A scientist claims prokaryotic cells are less complex than eukaryotic cells. Evaluate this statement.

Detailed Answers

Section 1: Prokaryotic vs. Eukaryotic Cells

1. Differences: Prokaryotes lack a nucleus (DNA in a loop), are smaller (1–10 µm), and lack membrane-bound organelles.
2. Genetic material: Single circular DNA loop and plasmids.
3. Plasmids: Small rings of DNA; found in prokaryotes (e.g., bacteria).
4. Size: Prokaryotes lack complex organelles, reducing internal space needs.
5. Evolution: Fossil evidence shows prokaryotes existed ~3.5 billion years ago, earlier than eukaryotes.

Section 2: Animal and Plant Cells

6. Plant-only structures: Cell wall, chloroplasts, permanent vacuole.
7. Vacuole function: Stores cell sap, maintains turgidity.
8. Chloroplasts in roots: Roots are underground; no light for photosynthesis.
9. Cell wall role: Provides structural support and prevents bursting.
10. Shape: Animal cells are irregular; plant cells are regular (due to rigid wall).

Section 3: Specialised Cells

11. Sperm adaptations: Tail (movement), mitochondria (energy), streamlined shape.
12. Muscle mitochondria: High energy demand for contraction.
13. Red blood cells: Biconcave shape (increases surface area), no nucleus (more hemoglobin).
14. Nerve cell: Long axon (transmits impulses), myelin sheath (insulates axon).
15. Root hair cells: Increase surface area for water/mineral absorption; thin extension.
16. Xylem: Hollow tubes (dead cells) transport water; lignin strengthens walls.
17. Phloem: Transports sucrose via sieve plates; companion cells support function.
18. No chloroplasts in roots: No light for photosynthesis.
19. Biconcave shape: Maximizes oxygen absorption and flexibility.
20. Xylem vs. phloem: Xylem is dead/hollow; phloem is alive with sieve plates.

Section 4: Microscopy

21. Total magnification: ×10 × ×40 = ×400.
22. Electron advantages: Higher resolution (~0.1 nm), greater magnification (×2,000,000).
23. B&W images: Electrons lack color; color is added artificially.
24. Resolution: Minimum distance between distinguishable points; critical for detail clarity.
25. Onion slide: Peel epidermis, stain with iodine, add coverslip to avoid bubbles.
26. Iodine use: Stains starch in plant cells for visibility.
27. Light resolution: ~200 nm.
28. TEM vs. SEM: TEM shows internal structures; SEM gives 3D surface images.
29. Dead specimens: Electron beams require vacuums, killing live cells.
30. Microscope parts: Eyepiece (viewing), objective (magnification), stage (holds slide).

Section 5: Cell Processes and Organelles

31. Respiration: In mitochondria; releases energy (ATP) from glucose.
32. Ribosomes: Site of protein synthesis.
33. Chlorophyll: Absorbs light for photosynthesis.
34. Turgidity: Vacuole presses cytoplasm against wall, keeping plant rigid.
35. Nucleus: Contains DNA, which codes for proteins controlling cell activities.
36. Differentiation: Cells specialize for specific functions (e.g., nerve cells transmit signals).
37. Plant differentiation: Retain stem cells in meristems for growth/repair.
38. Cell membrane: Controls substance entry/exit; selectively permeable.
39. Transpiration: Water evaporates from leaves, pulling water up xylem.
40. Mitochondria folds: Increase surface area for respiration enzymes.

Section 6: Calculations and Units

41. 5 mm to µm: 5 × 1000 = 5000 µm.
42. Magnification: 45 mm = 45,000 µm; 45,000 ÷ 15 = ×3000.
43. Pinhead image: 2 mm × 500 = 1000 mm = 100 cm.
44. Mitochondrion image: 0.5 µm × 10,000 = 5000 µm (5 mm).
45. Nanometre: 1 nm = 0.001 µm; 1 µm = 1000 nm.

Section 7: Application and Analysis

46. Green roots: Chlorophyll allows photosynthesis in light-exposed roots.
47. Cardiac mitochondria: Continuous heartbeat requires constant energy.
48. Cell type: Plant (bacteria lack chloroplasts; animals lack cell walls).
49. Leaf chloroplasts: Leaves are photosynthetic; roots are not.
50. Prokaryote complexity: Simpler due to no nucleus/organelles, but still highly efficient.

Tip: Use diagrams, flashcards, and practice calculations to reinforce these concepts!