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1.4.9Inquiry Process

Cell Types and Structures: Skills — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 10 of 160

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Part 1 of 5Identifying Cells

1.4.9 Cell Types and Structures: Skills

Work through a micrograph by asking the same short list of questions in the same order each time, so nothing is missed.

Presence of nucleus
  • A clear dark nucleus indicates a eukaryote; no nucleus with scattered DNA suggests a prokaryote.
Cell wall and shape
  • Rectangular cells with thick walls point to plants, while rounded cells with no wall suggest animal cells.
Chloroplasts
  • Green oval bodies filled with stacked thylakoids confirm a photosynthetic plant or algal cell.
Size clues
  • A bacterium is usually about ten times smaller in length than a typical animal cell.
Mitochondria
  • Oval profiles with an inner folded membrane appear in nearly all eukaryotic cells under the electron microscope.
ObservationLikely conclusion
No nucleus, cell about 2 μm, wall visibleBacterium (prokaryote)
Nucleus, cellulose wall, chloroplasts, large vacuolePlant cell
Nucleus, no wall, no chloroplasts, irregular shapeAnimal cell
Nucleus, chitin wall, no chloroplasts, thread-like hyphaeFungus

Part 2 of 5Magnification Method

Always keep the units the same on both sides before dividing.

1
Measure the length of the image using a ruler, working in millimetres for convenience.
2
Convert the image length to micrometres by multiplying by 1000 so units match the actual size.
3
Use the formula magnification equals image size divided by actual size.
4
Rearrange to actual size equals image size divided by magnification when a scale factor is given.
5
Where a scale bar is shown, divide the measured bar length by the stated length it represents.

Part 3 of 5Microscope Types

Microscope Types
Light microscope
  • Shows living or stained cells at up to about 1500 times, resolving structures larger than roughly 200 nm.
Transmission EM
  • Passes electrons through thin slices to reveal internal ultrastructure such as ribosomes and membranes in great detail.
Scanning EM
  • Scans a coated surface to give three-dimensional images of the outer surface of cells and tissues.

Part 4 of 5Worked Example

Question

  • A drawing of a cell is 60 mm long.
  • The actual cell is 25 μm long.
  • Calculate the magnification.

  1. Convert the drawing to micrometres60 mm × 1000 = 60 000 μm.
  2. Divide image size by actual size60 000 ÷ 25.
  3. 60 000 ÷ 25 = 2400.

Answer

  • Magnification = ×2400.
  • As a check, 2400 × 25 μm = 60 000 μm = 60 mm, which matches the drawing.

Part 5 of 5Exam Checklist & Tips

1.4.9 – Key Points for the Exam

  • Magnification How many times larger an image is than the real object.
  • Scale bar A line on an image representing a known actual length.
  • Resolution The ability to distinguish two close points as separate.
  • Micrograph A photograph taken through a microscope.

Common Mistakes

  • Forgetting to convert units, such as dividing millimetres by micrometres.
  • Confusing magnification with resolution; a larger image need not be clearer.
  • Measuring the scale bar from the wrong edge or including the thickness of the line.
  • Calling any organelle with a double membrane a nucleus.

Last updated:

Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A  ⇔  A = I / M  ⇔  I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.

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