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1.1.51 · Cell Structure

Calculating Actual Size — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 7 parts Lesson 6 of 13 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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1.1.5 Calculating Actual Size — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Calculate the actual size of cells and organelles from images, scale bars and graticule measurements, using suitable units.Equation — actual size = image size ÷ magnification.It is a rearr…

  • Magnification given
  • Scale bar given
  • Graticule used
  • Unit relationships
  • Plausibility check
  • Significant figures

Part 1 of 5The actual size equation

Equation
  • Equation actual size = image size ÷ magnification.
  • It is a rearrangement of the magnification equation triangle.
Image size
  • Image size measured with a ruler, so it is usually in mm.
Magnification
  • Magnification may be given, calculated from a scale bar, or found from eyepiece × objective magnification.
Unit
  • Unit the answer is a real length and needs a unit, usually µm for cells and nm for organelles like ribosomes.

Part 2 of 5Three routes to actual size

Magnification given
  • Magnification given measure the image in mm, convert to µm, then divide by the magnification.
Scale bar given
  • Scale bar given measure the bar, convert to µm, calculate magnification, then measure the structure.
    • (bar ÷ labelled length)
  • Alternatively use a proportion: structure length ÷ bar length × labelled length.
Graticule used
  • Graticule used number of graticule divisions × calibration value of one division.

Part 3 of 5Choosing sensible units

StructureTypical actual size
Plant or animal cell10–100 µm
Nucleus5–10 µm
Mitochondrion1–10 µm long
Bacterium1–5 µm
Ribosome20–25 nm
Virus20–300 nm
Unit relationships
  • 1 mm = 1000 µm and 1 µm = 1000 nm.
Plausibility check
  • Plausibility check an organelle larger than the nucleus, or a cell smaller than a ribosome, signals a conversion error.

Part 4 of 5Accuracy of results

Significant figures
  • Significant figures give the answer to the same number of significant figures as the least precise measurement, usually 2 or 3.
Rounding
  • Rounding keep full values during the calculation and round only at the end.
Repeats
  • Repeats measure several cells and calculate a mean, because cells of one type vary in size.
Consistent measurement
  • Consistent measurement measure the longest length or widest diameter each time.
Sections
  • Sections structures cut at an angle can look different in size or shape.

Part 5 of 5More from the specification

1
Actual Size Calculation
  • The real dimensions of a biological specimen observed under a light microscope are determined using the magnification equation: Actual Size = Image Size / Magnification.
2
Stage Micrometer Calibration
  • An eyepiece graticule is calibrated against a stage micrometer slide of known division spacing on the microscope stage to assign precise micrometer lengths to graticule units.
3
Resolution of Light Microscopes
  • Resolution defines the minimum distance between two points at which they can still be distinguished as separate entities, limited to ~200 nm by the wavelength of visible light.
4
Biological Drawing Guidelines
  • Microscope drawings of specimen slides must show clear continuous outlines, accurate cellular proportions, no cross-hatching,
  • and include a calibrated scale bar.
5
Starch
  • An insoluble polysaccharide of glucose used by plants for energy storage.
6
Biological drawing
  • A labelled, clear line drawing recording what is observed.
7
Stage micrometer
  • A slide with an accurately known scale used to calibrate the graticule.
8
Light microscope
  • Microscope using light, magnifying up to about x1500.
9
Magnification factor
  • The µm per graticule division at a given objective lens.
10
Starch grain
  • Store of starch in the stroma.
11
Specimen
  • The sample being observed.
12
Resolution
  • The ability to distinguish between two objects that are close together.
13
Micrometer
  • A unit of length; 1 mm = 1,000 micrometers
14
Coverslip
  • A thin glass or plastic square placed over the specimen to flatten it and protect the lens.

Key terms

TermMeaning
Actual sizeThe true size of the specimen, image size ÷ magnification.
Scale barA bar on an image labelled with the real length it represents.
Significant figuresThe meaningful digits in a quoted value.
Calibration valueThe real length of one eyepiece graticule division.
MicrographA photograph taken through a microscope.
MeanThe sum of a set of values divided by the number of values.

Avoid these mistakes

1
Leaving the answer in mm or without a unit — state a unit that is sensible for the structure.
2
Swapping image size and actual size in the equation — the image should be larger than the real object.
3
Rounding too early — keep the full values until the final answer.
4
Measuring the scale bar label instead of the line — measure the line on the page and use the label as its real length.

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