Using the Microscope
Everything you have read about cells so far, you can now see for yourself. In this practical you will prepare a wet mount slide of onion epidermis, focus it under a light microscope, identify the cytoplasm, cell wall and nucleus, and then measure how big a cell actually is using a simple grid. Reading about a cell is one thing; calculating that it is 60 micrometres wide is another.
Get oriented
Make a prediction, read the goals and the key terms, then clear the trap before you start.
Before You Read
warm-upUnder a microscope a single cell can fill the whole field of view, yet you know cells are microscopic.
Before reading: if a cell looks 30 mm wide down the microscope at ×400 magnification, how could you work out its real size? Write your idea, even if you are not sure of the exact maths.
Learning Intentions
goalsKnow
- The steps to prepare a wet mount slide
- How to identify cytoplasm, cell wall and nucleus
- The magnification equation linking image, real size and magnification
Understand
- Why a stain such as iodine is used in a wet mount
- How a mini-grid on the stage gives a cell’s real size directly
- Why units (µm) and consistent scaling matter
Can Do
- Prepare a wet mount and locate the three named structures
- Calculate the real size of a cell from a mini-grid measurement
- Rearrange the magnification equation for any unknown
Scan these before reading
vocabMisconceptions To Fix
watch outBuild the first idea
The core of the dot point, then two checks you can answer straight from the card.
Core Content
Preparing a Wet Mount Slide
+5 XPMethod, safety and the structures you will see
A good wet mount keeps the specimen flat, hydrated and stained so its structures are clear and free of trapped air bubbles.
Method: place a drop of water on a clean slide; peel a thin layer of onion epidermis (or scrape inner cheek cells onto the slide); lay the specimen in the water; add a drop of iodine (onion) or methylene blue (cheek) to stain it; lower a coverslip at an angle to avoid bubbles; focus on low power first, then high power. Under the light microscope you can then identify the cytoplasm (the pale jelly filling the cell), the cell wall (the firm outer boundary, in onion) and the nucleus (a darker, stained, rounded structure).
Safety/validity: handle slides and coverslips carefully (sharp glass); iodine stains skin and clothes. Lowering the coverslip at an angle reduces bubbles that could be mistaken for cells.
Pause, copy the highlighted sentence on identifying the three structures into your book.
A stain such as iodine is added to a wet mount mainly to increase _____ so structures stand out.
Odd one out. Three of these steps belong in a correct wet-mount method. Which one does not?
Take it further
The next move, with the diagram to work from and two more checks.
Measuring Cell Size with a Mini-Grid
+5 XPThe magnification equation, worked through
We just observed the cell. That raises a question: how big is it, really? This card answers it → a known grid plus one equation converts the image into a real measurement.
A grid lying on the stage is magnified with the specimen, so reading a cell against it gives the real size straight away; the equation is what converts between a real size and a magnified image.
Text description
- Field of view: a stage grid of 100 µm squares lies under the specimen; the cell spans about 4 squares.
- Real size = 4 × 100 µm = 400 µm. The grid is magnified along with the cell, so there is no division.
- The equation, used forwards: magnification = image size ÷ real size. At ×100 that 400 µm cell appears 40 mm wide.
- Divide by the magnification only when you measured a magnified image.
The key equation is magnification = image size ÷ real size, which rearranges to real size = image size ÷ magnification. Which way you use it depends on what you measured. From the stage grid: the grid sits beside the specimen and is magnified by the same amount, so its squares are a ruler marked in real micrometres — a cell spanning 4 squares of 100 µm is 400 µm across, and there is nothing to divide. The equation then works forwards as a check: at ×100 that cell fills about 40 mm of the field. From a magnified image: only a length read off an image, such as a ruler laid on a photomicrograph, gets divided — a cell measured as 30 mm on a ×100 photomicrograph is 30 000 ÷ 100 = 300 µm across. Always keep units consistent (convert mm to µm: ×1000), and check the answer against the 10–100 µm range most cells fall in — a result below 1 µm usually means the magnification was applied twice.
Pause, copy the equation and the worked example into your book.
A cell measures 600 µm in the image at ×300 magnification. What is its real size?
Fill the gaps. A cell measures 900 µm in the image at ×300. Rearranged, real size image size ÷ magnification, so the real size is µm. To convert 0.5 mm to micrometres you multiply by , giving µm.
Work it yourself
Put the idea on paper in your own book before you read on.
Activity 1
Calculate Three Cell Sizes
Pattern C, Worked calculation set
Using real size = image size ÷ magnification, calculate each cell's real size and write it in µm. (a) image 1200 µm at ×400; (b) image 0.9 mm at ×100 (convert to µm first); (c) image 2400 µm at ×600. Show your working for each, including the unit conversion in (b).
Finish the picture
The last piece of the dot point, then a true or false check across all three cards.
Specialised Cells Vary in Size
+5 XPWhy measuring a variety of cells matters
We just calculated one cell's size. That raises a question: are all cells the same size? This card answers it → no, and that is exactly why the dot point asks for a variety of specialised cells.
Cells are specialised for different jobs, and their sizes and shapes vary widely, measuring several reveals this range.
A red blood cell is about 8 µm across; a human cheek cell about 60 µm; an onion epidermal cell can be 200–300 µm long; a nerve cell may be over a metre long. Measuring a variety of specialised cells with the mini-grid method shows that cell size relates to function, and that repeating the measurement on several cells improves the reliability of an estimate. Always state results to an appropriate number of significant figures and include the unit.
Add the idea that measuring several cells improves reliability to your notes before the check.
Real size is found by multiplying the image size by the magnification.
Lowering the coverslip at an angle helps avoid trapping air bubbles.
Measuring several cells rather than one improves the reliability of a size estimate.
See where it goes
Where this shows up outside the classroom, and why the dot point is worth knowing.
Cell size in the pathology lab
Measuring cells is not just a school exercise. In a pathology lab, a haematologist measures red blood cell size to diagnose anaemias, abnormally small cells (microcytic) or large cells (macrocytic) point to different causes. The same magnification equation you use on onion cells underlies those clinical measurements. Reliable measurement, repeated across many cells, is exactly what makes a diagnosis trustworthy.
Describe how you would prepare a wet mount slide of onion epidermis and identify the cytoplasm, cell wall and nucleus under a light microscope.
A practical describe is marked on whether another student could follow it. Two things earn marks that students routinely leave out: a reason attached to each step, and what you would actually see at the end.
6 marks. Roughly four for a method another person could repeat, and two for identifying the three named structures by their appearance.
Place a single drop of water in the centre of a clean glass slide. Using forceps, peel a thin layer of epidermis from the inner surface of an onion scale and lay it flat in the drop without folding it, since a folded or thick specimen gives overlapping layers that cannot be focused together.
Add one drop of iodine solution to the specimen to stain it, which increases contrast so the nucleus and cell wall become visible against the cytoplasm. Lower a coverslip onto the specimen slowly from one edge at about 45 degrees using a mounting needle, so that air is pushed out ahead of the liquid and no bubbles are trapped, and blot any excess stain with filter paper.
Place the slide on the stage and secure it with the clips. Select the lowest-power objective first, use the coarse focus while watching from the side to bring the objective close to the slide, then focus upwards and away from the slide using the coarse and then the fine adjustment. Only once the specimen is in focus and centred should you rotate to a higher-power objective and refocus with the fine adjustment alone.
Under the microscope the cell wall appears as a firm, straight, darkly stained line forming a rectangular boundary shared between adjacent cells. The cytoplasm is the pale, evenly stained region filling the space inside the wall. The nucleus appears as a small, distinctly darker ovoid body within the cytoplasm, usually pressed against one side of the cell because a large central vacuole occupies most of the interior.
Place a single drop of water in the centre of a clean slide, then use forceps to peel a thin layer of epidermis from the inner surface of an onion scale and lay it flat in the drop without folding, since light must pass through the specimen and overlapping layers cannot be brought into focus together. Add one drop of iodine solution to stain the specimen and raise the contrast so the nucleus and cell wall stand out against the cytoplasm. Lower a coverslip slowly from one edge at roughly 45 degrees using a mounting needle so air is pushed ahead of the liquid and no bubbles are trapped, and blot excess stain with filter paper. Secure the slide on the stage, select the lowest-power objective, bring the objective close to the slide using the coarse focus while watching from the side, then focus upwards and away from the slide, first with the coarse and then the fine adjustment; once the specimen is centred and sharp, rotate to a higher-power objective and refocus using the fine adjustment only. The cell wall is then visible as a firm, straight, darkly stained line forming a rectangular boundary shared with the neighbouring cell; the cytoplasm is the pale, evenly stained region within it; and the nucleus is a small, distinctly darker ovoid body inside the cytoplasm, usually displaced to one side of the cell by the large central vacuole.
Outline how you would determine the real size of a specialised cell viewed under a light microscope using a mini-grid, and state your answer in micrometres.
Outline means sketch the essential features in sequence. It is shorter than describe, but a sizing question still has a non-negotiable core: a measured image size, a stated magnification, one division, and a unit.
5 marks. Roughly two for calibrating and reading the size off the grid, one for the relationship, one for using it in a direction the method allows, and one for units and a sensible final value.
Place the mini-grid on the stage and focus on it at the magnification you intend to use. The side length of one square is known — a 1 mm grid divided into ten gives squares of 100 micrometres. Because the grid sits on the stage alongside the specimen, it is magnified by exactly the same amount, so the grid is a ruler marked in REAL micrometres, not in magnified ones.
Replace the grid with the prepared slide without changing the objective, and count how many squares the cell spans — for example, a cell covering two-fifths of a 100 micrometre square has a real width of 0.4 × 100, that is 40 micrometres. Measure the same dimension on several cells and use the mean to reduce random error.
The equation relates the three quantities: magnification equals image size divided by real size. Use it to check the result or to work in the other direction. At a total magnification of 400 times — a 40 times objective with a 10 times eyepiece — a real width of 40 micrometres appears as an image 40 × 400 = 16 000 micrometres, that is 16 mm across the field. Equally, measuring that cell as 16 mm on a photomicrograph and dividing by its real 40 micrometres returns the magnification of 400 times.
Finally, check the result against what you know: most plant and animal cells are between about 10 and 100 micrometres across, so 40 micrometres is plausible. An answer in millimetres, or one below 1 micrometre, signals either a misplaced factor of 1000 or the magnification applied twice.
Place the mini-grid on the stage and focus on it at the magnification to be used; a 1 mm grid divided into ten gives squares of 100 micrometres, and because the grid lies on the stage beside the specimen it is magnified by exactly the same factor, so it reads in real micrometres rather than magnified ones. Without changing the objective, replace the grid with the prepared slide and count the squares the cell spans: a cell covering two-fifths of a 100 micrometre square has a real width of 0.4 × 100, that is 40 micrometres. Repeat on several cells and take the mean to reduce random error. The magnification equation relates the three quantities, magnification being image size divided by real size, and is used here to work in the other direction or to check: at a total magnification of 400 times, from a 40 times objective and a 10 times eyepiece, a real width of 40 micrometres appears as an image of 40 × 400 = 16 000 micrometres, or 16 mm across the field, and measuring that same cell as 16 mm on a photomicrograph and dividing by its real 40 micrometres returns the magnification of 400 times. The grid reading must not itself be divided by the magnification: it is already a real length, and dividing again applies the magnification twice and would report 0.1 micrometres for an ordinary cell. Divide by magnification only when the measured length came from a magnified image. Finally check for plausibility: most plant and animal cells fall between roughly 10 and 100 micrometres, so 40 micrometres is reasonable.
Practise independently
Answer in your own words first. The model answers are for checking, not for copying.
Multiple Choice
+5 XPA fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
Short Answer, 12 marks
+5 XPApplyBand 4(5 marks) 1. Outline how you would prepare a wet mount slide of onion epidermis and identify the cytoplasm, cell wall and nucleus. Then calculate the real size of a cell whose image is 1500 µm at ×500 magnification, showing your working.
2 marks: correct wet-mount method (water, specimen, stain, coverslip at an angle) · 1 mark: the three structures correctly identified · 2 marks: real size = 1500 ÷ 500 = 3 µm with working and unit
ApplyBand 4(3 marks) 2. Under a ×100 objective with a ×10 eyepiece, a specialised cell spans 12 squares of a mini-grid whose squares are 100 µm across in the image. Calculate the real size of the cell, showing your working.
1 mark: total magnification calculated as 1000 · 1 mark: image size calculated as 1200 µm · 1 mark: correct real size with unit
EvaluateBand 5–6(4 marks) 3. A student measures one onion cell once and reports its length as 250 µm. Assess the reliability and validity of this result and describe two changes that would improve it.
1 mark: identifies a single measurement as unreliable · 1 mark: identifies a specific source of error · 2 marks: two improvements described with the reason each helps
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Short Answer Model Responses
Q1 (5 marks): Method: place a drop of water on a clean slide, lay a thin peel of onion epidermis in the water, add a drop of iodine to stain it, then lower a coverslip at an angle to avoid trapping air bubbles, and focus under low then high power. Identification: the cytoplasm is the pale jelly filling the cell, the cell wall is the firm outer boundary of each cell, and the nucleus appears as a darker, stained rounded structure. Calculation: real size = image size ÷ magnification = 1500 µm ÷ 500 = 3 µm.
Q2 (3 marks): Total magnification eyepiece × objective 10 × 100 ×1000 [1]. Image size 12 squares × 100 µm 1200 µm [1]. Real size image size ÷ magnification 1200 ÷ 1000 1.2 µm [1].
Q3 (4 marks): A single measurement gives no way to tell whether the result is typical or an outlier, so it is not reliable [1]. Cells of one type vary in size, and reading how many grid squares a cell spans involves judgement at each edge, so one reading can easily be several squares out [1]. Measuring a number of cells and reporting a mean would improve reliability, because random reading errors partly cancel and an unrepresentative cell no longer decides the result [1]. Measuring the same cell at a higher total magnification, and reporting the answer to an appropriate number of significant figures with the unit, would improve accuracy, because each grid square then covers a smaller real distance and the edges of the cell can be located more precisely [1].
Retrieve what stuck
Work the spaced-review set first. It mixes this lesson with earlier ones on purpose.
Check what actually stuck
A short retrieval set drawn from this lesson and the ones before it, so the earlier work keeps coming back rather than fading.
Lock it in
bossYou can now prepare a wet mount, identify the three named structures, and calculate cell size with the magnification equation. Next lesson steps back to ask how improving microscope technology changed what scientists could know about cells.
↩ Back to focus-area hubCompare it with where you started
Read your opening prediction again, then mark the lesson complete.
How did your thinking change?
At the start you were asked how to find a cell's real size from a 30 mm image at ×400. The answer is real size = image size ÷ magnification = 30 mm ÷ 400 = 0.075 mm = 75 µm. If your instinct was to multiply, revisit the misconception box, the image is the enlarged view, so you divide to get back to the smaller real cell.