What you need to know
- Unit 3Comment on a record or technique you are shown, including a drawing or a microscope measurement, and on a realistic improvement.
Cell structure and tissue identity are taught in Topics 3 and 4. This lesson is the commentary: labels, scale, plan diagrams, and magnification. It does not add organelles to the specification.
By the end, you should be able to…
- List what a comment on a biological drawing should notice.
- Calculate magnificationHow many times larger an image is than the real object: magnification = image size ÷ actual size. only after the image and the object are in the same unit.
- Distinguish a high-power cell drawing from a low-power plan.
Build your understanding
What the comment is looking for
A useful biological drawing has clear outlines, no shading, a title, and label lines that touch the structure they name. The label lines do not cross. A scale bar or a magnificationHow many times larger an image is than the real object: magnification = image size ÷ actual size. is part of the record if a size is being claimed.
When you comment, name the fault you can see. Shading, sketchy overlapping lines, a label that stops in empty space, or a missing scale are comments. “The drawing is untidy” is not specific enough.
Plan or cells: read the instruction
A low-power plan of a root, stem or leaf shows the outlines of tissues, not every cell. A high-power drawing shows a few cells with their walls and any structures the question names. Drawing every cell on a plan, or only a tissue outline when cells were required, misses the instruction.
Do not invent tissues that are not in the specification list for that practical. If you cannot see a layer, say that it is not distinct rather than guessing a name. A comment can also say that a photograph was traced without a scale, so the image cannot be converted to a real size.
Magnification is a calculation, then a unit check
Magnification = size of the image ÷ size of the real object, with both sizes in the same unit. An illustrative eyepiece graticuleA scale in the microscope eyepiece. It has no units until it is calibrated with a stage micrometer. must be calibrated against a stage micrometerA microscope slide with a scale of known divisions, used to calibrate the eyepiece graticule. before its divisions are converted to micrometres. Quoting “12 eyepiece units” is not yet a length.
Worked illustrative example: an image of a cell is 18 mm long on the drawing and the real cell is 20 µm. Convert 18 mm to 18 000 µm. Magnification = 18 000 ÷ 20 = ×900. Leaving the lengths in mixed units, or multiplying them, is the usual error. A magnification has no unit, but the working must show the conversion.
NOT: “untidy” is not a comment. Name the missing line, label or scale.
- No shading on a biological drawing
- A plan is tissue outlines, not every cell
- Convert millimetres and micrometres before dividing
- Drawing magnification is not the microscope’s magnification
Two traps this diagram blocks
- Do not invent organelles that the practical did not ask you to draw.
- Do not use the microscope magnification as the magnification of the drawing.
From the diagram alone
Question
State three features of a biological drawing that make it useful as a record. [3]
Show a model answer
It has a clear unshaded outline, label lines that touch the structures, and a scale bar or magnification so size can be interpreted. A plan and a cell drawing are not the same kind of record.
Make your reasoning visible
Comment on what is visible
Use the drawing in the question. Do not deliver a memorised essay about the microscope.
Convert, then divide
Write the unit conversion on its own line. Then divide image by object. Check that the answer is a magnificationHow many times larger an image is than the real object: magnification = image size ÷ actual size. and not a length.
Key explanations
- Magnification is image size divided by actual size, with both lengths in the same unit.
Avoid this misconception
The claim: A plan diagram should show as many individual cells as possible.
The correction: A plan shows tissue boundaries. Individual cells belong on a high-power drawing when that is what was asked.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
A low-power stem drawing shades the vascular tissue and has no scale. Give two comments.
Show suggested answer
Shading should not be used; outlines should show the tissue. A scale bar or magnification is missing, so the drawing does not record size.
Practice question
Retrieval and explanation · suggested answer
An illustrative drawing of a cell is 24 mm across. The real cell is 15 µm across. Calculate the magnification.
Show suggested answer
24 mm = 24 000 µm. Magnification = 24 000 ÷ 15 = ×1600.
Practice question
Retrieval and explanation · suggested answer
A student records a chloroplast as “6 eyepiece graticule units” and calls that the actual length. What is missing?
Show suggested answer
The graticule has not been calibrated. The units must be converted to a length, such as micrometres, using the calibration for that objective lens.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
You are shown an unfamiliar photograph of a cell with a scale bar labelled 5 µm. On the printed photograph the bar is 20 mm long and the nucleus is 12 mm long. Calculate the actual nucleus length. State one assumption.
Show a hint
The scale bar converts the printed millimetres. Do not use ×400 unless it is printed and you are told to.
Explore the reasoning
20 mm on the image represents 5 µm, so 1 mm represents 5 ÷ 20 = 0.25 µm. The nucleus is 12 × 0.25 = 3 µm. The assumption is that the photograph was not resized after the scale bar was applied, and that you are measuring the printed lengths you were given.
Recall this lesson
Lesson recall questions
Question 1 of 5: Compare the range of a 0–110 °C thermometer with its resolution in an enzyme-temperature investigation.
Unit 3 implementation and measurements
Practice question
Retrieval and explanation · suggested answer
Compare the range of a 0–110 °C thermometer with its resolution in an enzyme-temperature investigation.
Lesson: I1: Readings, range and significant figures
Try this from memory first.
Beyond the classroomA micrograph keeps detail a drawing leaves outEvidence · images · Enrichment, not required recall
The linked electron micrograph is a public-domain record of a leukocyte, used in Topic 3 to show a Golgi apparatus. A drawing of the same cell would keep only the structures the biologist chose to outline. That selection is why a title, labels and a scale have to say what was kept and how big it was.
Back to the spec: Unit 3 implementation: comment on whether a drawing or measurement is an honest record. The micrograph itself is enrichment, not extra Unit 3 recall.
Wikimedia Commons: leukocyte Golgi transmission electron micrograph
Question
Why can a carefully labelled drawing still be a weaker size record than the same view with a scale bar?
Explore a possible response
Labels name structures. Without a scale bar or a magnification calculated from known lengths, the drawing does not show actual size. A viewer cannot tell whether the cell was micrometres or millimetres across.