Lesson 6 of 6 / Cells and microscopy
Draw, measure and compare specimens
What separates a drawing that scores from a sketch that does not?
I can make a plan diagram and a cell drawing to the marked rules, calculate actual size from an image to 2 significant figures, and compare two specimens in an observable-differences table.
I can recognise the specified cell structures in drawings and micrographs; use a microscope and calibrated graticule.Syllabus
Syllabus 9477, Core 1(b). Recognise the specified cell structures in drawings and micrographs; use a microscope and calibrated graticule.
About 8 min
Make a guess. You are not marked.
You are asked for a plan diagram of a leaf section. Should you draw the individual cells?
Show the answer
Show tissue outlines, in correct proportion.
A plan diagram maps tissue layers. Drawing individual cells turns it into a cell drawing and loses marks.
The key idea
Draw only what you see with clear, continuous single lines and no shading; label with ruled lines outside the drawing; calculate actual size = image size / magnification and give it to 2 significant figures with a unit.
A plan diagram shows only the outlines of tissues and their relative positions, with no individual cells. A cell drawing shows a few cells in detail, including cell walls and the structures you can see inside.
Explore the idea
Would this drawing score?
Five faults lose marks here: broken, overlapping strokes; shading; a chloroplast drawn that was not visible; label lines with arrowheads that cross; and labels written on the drawing.
Original schematic drawings, not traced from a specimen. Actual size = image size / magnification, to 2 significant figures with a unit.
Original schematic. Features, cell sizes and distances are not to scale. Use several visible clues when interpreting an unfamiliar section.
Explanation
Practical papers mark drawings against fixed rules. Use a sharp pencil and draw clear, continuous single lines that join up, with no sketchy or overlapping strokes and no shading or colouring. Draw only what you can see in the specimen, not what a textbook shows. Keep the proportions of the parts the same as in the specimen, and make the drawing large, filling most of the space provided.
A plan diagram is a low-power drawing. It shows the outlines of the tissues, such as the epidermis, the cortex and the vascular bundles, in their correct positions and proportions. It never shows individual cells. A cell drawing is a high-power drawing of a small number of cells, usually two to four adjacent cells. Draw plant cell walls as two lines to show their thickness, and show shared walls between neighbouring cells. Include only the organelles you can actually see at that magnification, such as the nucleus.
Label with ruled lines drawn with a ruler. Each line starts outside the drawing and ends exactly on the structure it names, with no arrowheads. Label lines should not cross each other, and labels sit outside the drawing, ideally lined up at one side. Give the drawing a title and, if asked, its magnification.
To find actual size, first find the magnification of the image. If a scale bar 20 mm long on the image represents 50 micrometres, the magnification is 20,000 micrometres / 50 micrometres = x400. A cell 57 mm long on the same image has an actual length of 57,000 micrometres / 400 = 142.5 micrometres, which is 140 micrometres to 2 significant figures. Convert mm to micrometres first (1 mm = 1,000 micrometres), round only at the end, and always write the unit.
An observable-differences table compares two specimens. Use three columns: the feature, specimen A and specimen B. Each row names one feature you can see in both, such as the shape of the cells or the number of layers, and describes it for each specimen in matching terms. Include only differences you can observe, not reasons or functions, and give a description in both columns of every row.
Step by step
- 1
Choose the drawing type
Plan diagram for tissue layout at low power; cell drawing for a few cells at high power.
- 2
Draw what is there
Clean single lines, correct proportions, no shading, nothing that is not visible.
- 3
Label and title
Ruled label lines ending on each structure, labels outside the drawing.
- 4
Calculate size
Magnification from the scale bar, then actual size = image size / magnification, to 2 significant figures.
Worked example
Work through the evidence
On a photomicrograph, a scale bar 20 mm long represents 50 micrometres. A cell measures 57 mm. Calculate the magnification and the actual length of the cell to 2 significant figures.
One way to explain it
Magnification = 20,000 / 50 = x400. Actual length = 57,000 / 400 = 142.5 micrometres = 140 micrometres to 2 significant figures.
Why this answer works
- Convert both image measurements to micrometres first.
- Round only the final answer.
- Give the unit with the answer.
Watch out for this
A student says: "Shading and a few extra organelles make a drawing more accurate." What is wrong with this?
Show the answer
Shading is not credited and blurs outlines. Adding structures that are not visible in the specimen loses the mark for drawing what you see.
Check your understanding
A scale bar 15 mm long represents 20 micrometres. A guard cell measures 28 mm on the same image. What is its actual length to 2 significant figures?
Show the answer
37 micrometres
Magnification = 15,000 / 20 = x750, so actual length = 28,000 / 750 = 37.3, which is 37 micrometres to 2 significant figures.
Quick recall
What separates a drawing that scores from a sketch that does not?