B. Biology: Tissues, Organs, and Systems of Living Things · Tissues, Organs, and Systems of Living Things

Cells Up Close: Microscopes, Field of View and Plant and Animal Cells

B2.3 B2.1 A1.2 A1.4 A1.12 A1.13About 75 minutes

Learning goals

I can name the main organelles of plant and animal cells and describe what each one does.
I can prepare (or describe how to prepare) wet mounts of cheek cells and onion cells safely.
I can calculate the field of view at a higher magnification and use it to estimate the size of a cell.
I can draw a labelled scientific diagram that shows the differences between plant and animal cells.

Words to know

organelle
A tiny structure inside a cell that does a specific job, such as the nucleus or a mitochondrion.

field of view
The circle of light you see when you look through a microscope.

micrometre (µm)
One millionth of a metre; 1 mm = 1000 µm. Cells are usually measured in micrometres.

wet mount
A slide made by placing a thin specimen in a drop of water (or stain) under a cover slip.

stain
A dye, such as iodine or methylene blue, that makes cell parts easier to see.

magnification
How many times larger an image looks than the real object.

cell wall
A stiff layer outside the cell membrane of plant cells, made mostly of cellulose.

vacuole
A membrane sac that stores water, food or wastes; plant cells usually have one large central vacuole.

A city in a drop of spit

Scrape the inside of your cheek gently with a clean toothpick and you will collect thousands of living cells. Each one is too small to see with your eyes, yet each one is busy: making proteins, releasing energy, taking in food and pushing out wastes. This unit is about how those tiny cells divide, specialize and team up to build tissues, organs and whole organ systems, in you and in plants. We start where every biologist starts: looking at cells, carefully, and measuring them.

Remember from Grade 8

In Grade 8 Cells you learned the cell theory, the parts of a microscope, how to find total magnification (eyepiece × objective, such as 10 × 40 = 400×), how to make a wet mount, the rules of scientific drawing, the main organelles of plant and animal cells, and how to use the field of view to estimate the size of a cell. In Grade 10 you will use those skills with more precision: calculating the field of view at any magnification, working out the magnification of your own drawings, judging uncertainty in your estimates, and staining and comparing real plant and animal cells, the way a lab scientist does.

The organelles, one more time (with a few new ones)

Every cell is a little factory. These are the parts you need to know in Grade 10:

Cell membrane: a thin, flexible layer around the cell. It controls what enters and leaves (it is selectively permeable).
Cytoplasm: the jelly-like fluid that fills the cell and holds the organelles.
Nucleus: holds the cell's DNA, the instructions for building and running the cell. Inside it, the darker nucleolus helps make ribosomes.
Mitochondria: release energy from glucose by cellular respiration (you studied this in Grade 9).
Ribosomes: tiny structures that build proteins.
Endoplasmic reticulum (ER): a folded network of membranes that moves materials around the cell. Rough ER is covered in ribosomes.
Golgi apparatus: packages proteins and sends them where they need to go, sometimes out of the cell.
Vacuoles and vesicles: membrane sacs that store or carry materials.
Lysosomes: sacs of digestive enzymes that break down worn-out parts and wastes (found mainly in animal cells).

Plant cells also have:

a cell wall made of cellulose outside the membrane, which gives support and a box-like shape,
a large central vacuole that stores water and pushes outward to keep the plant firm,
chloroplasts, where photosynthesis happens (only in cells that get light, such as leaf cells).

Two cells you can prepare yourself. The cheek cell has an irregular shape and no cell wall. The onion cell has a rigid cell wall, a large central vacuole and a nucleus pushed to the side. Notice that onion bulb cells have no chloroplasts: they grow inside the bulb, away from light.

Feature	Animal cell (cheek)	Plant cell (onion)
Shape	Irregular, rounded	Regular, box-like
Cell wall	No	Yes (cellulose)
Vacuole	Small vacuoles or vesicles	One large central vacuole
Chloroplasts	No	Not in onion bulb cells; yes in leaf cells
Nucleus	Usually near the centre	Often pushed to the edge by the vacuole
Good stain	Methylene blue	Iodine solution

How the two cells you will observe compare.

Preparing the slides (lab procedure)

Onion epidermis: peel the thin, see-through skin from the inside of one onion layer with tweezers. Lay it flat in a drop of water on a slide. Lower a cover slip at an angle to avoid bubbles. Add a drop of iodine at one edge of the cover slip and touch a piece of paper towel to the opposite edge to pull the stain under.

Cheek cells: gently rub the inside of your cheek with the flat end of a clean toothpick. Stir the toothpick in a drop of water on a slide, then add a cover slip and a drop of methylene blue stain the same way. Cheek cells are thin and nearly clear, so without stain they are very hard to see.

Safety and WHMIS

Stains are lab chemicals. In school they come with a WHMIS label and a Safety Data Sheet (SDS). Wear goggles and gloves if your teacher asks; iodine and methylene blue stain skin and clothes and must never be swallowed.
Cheek cells are biological material. Use only your own cells, put used toothpicks straight into the container your teacher provides (often one with disinfectant), and wash your hands afterwards.
Glass slides and cover slips break. Report broken glass; never pick it up with bare hands.

Measuring with a microscope: field of view (a Grade 8 skill, sharpened)

You can't put a ruler on a cell. Instead, you measure the field of view (the circle you see) and compare the cell to it.

At low power, put a clear plastic ruler on the stage and count the millimetres across the circle. On many school microscopes the low-power (40×) field of view is about 4.5 mm across, but measure your own.
At higher power, the field of view is too small to measure with a ruler, so you calculate it. When magnification goes up, the field of view goes down by the same factor:

high-power field diameter = low-power field diameter × (low magnification ÷ high magnification)

Estimate cell size: count how many cells would fit across the field, then

size of one cell ≈ field diameter ÷ number of cells across

Cells are measured in micrometres (µm): 1 mm = 1000 µm. Always show units, and don't report more precision than your counting allows (a cell that is "about 56 µm" should not be written as 56.25 µm).

Going from 40× to 400× multiplies the magnification by 10, so the field of view shrinks by a factor of 10: 4.5 mm becomes 0.45 mm. If 8 cells fit across 450 µm, each cell is about 450 ÷ 8 ≈ 56 µm wide.

Worked example

Let's estimate the size of an onion cell

At 40×, a student measures the field of view as 4.5 mm. At 100× she sees about 4 onion cells end to end across the field. About how long is one onion cell, in micrometres?

Change the low-power diameter to micrometres: 4.5 mm × 1000 = 4500 µm.
Find the 100× field of view: 4500 µm × (40 ÷ 100) = 4500 × 0.4 = 1800 µm.
Divide by the number of cells across: 1800 µm ÷ 4 = 450 µm.
Check that it is sensible: onion epidermis cells are long and can be several hundred micrometres, so this is reasonable. Our count of "about 4" is an estimate, so we report the answer as about 450 µm (or 0.45 mm).

Each onion cell is about 450 µm (0.45 mm) long.

Scientific drawings, Grade 10 style

Your drawing is data. A good biological drawing:

is done in sharp pencil, large (at least a third of a page), with no shading or colouring (use stippling, small dots, for darker areas),
shows only what you actually see, not what the textbook shows,
has labels on straight, horizontal, ruled lines that don't cross, lined up to one side,
has a title, the specimen, the magnification you used, and an estimated size or a scale bar (a short line labelled with the real length it represents, such as 50 µm).

You can also calculate the drawing magnification: drawing size ÷ actual size. If you draw a 56 µm cheek cell 56 mm wide, your drawing is 56 000 µm ÷ 56 µm = 1000× larger than life.

Worked example

Let's work out a drawing magnification

Your drawing of a single onion cell is 90 mm long. The real cell is about 450 µm long. What is the magnification of your drawing?

Use the same units: 90 mm = 90 × 1000 = 90 000 µm.
Magnification = drawing size ÷ actual size = 90 000 µm ÷ 450 µm = 200.
Write it under your drawing as ×200 (or 200×). This is the drawing's magnification, which is not the same as the microscope's magnification.

The drawing is 200× life size.

Just for fun

Why did the onion cell refuse to share its feelings?

It had built a wall around itself. (Luckily for biologists, iodine still gets through.)

Hands-on activity

Kitchen-table cell lab

You need: an onion, tweezers (or your fingernails), a clear plastic ruler, a magnifying glass or a phone camera zoomed all the way in, and a microscope if you have one (a toy or student microscope is fine). Ask a grown-up to cut the onion; knives are sharp.

Peel the thin, see-through skin from the inside of one onion layer. Hold it up to a window (never look at the Sun).
Look at it with the magnifier or phone zoom. Can you see a pattern of tiny boxes? Those are cell walls. With a microscope, make a wet mount; if you have iodine from a first-aid kit, a grown-up can add one drop (it stains, and it is not safe to swallow).
With a microscope: measure your low-power field of view with the ruler, then estimate the length of one cell at a higher power using the method from this lesson.
Make a proper scientific drawing of 3 or 4 cells with a title, labels, magnification and estimated size.

What to look for: box-shaped cells in a brick-like pattern, thick lines (walls) between them, and, if stained, a dark dot (nucleus) in each cell. Without a microscope, you will see the grid pattern but not the nuclei, which is a good reminder of why cell biology needed the microscope. Throw the onion skin in the compost and wash your hands.

Your turn

Name three structures found in plant cells but not in animal cells.

Why can you see the nucleus of an onion cell clearly after adding iodine, but hardly at all before?

Onion cells are plant cells. Why don't they have chloroplasts?

Convert 0.08 mm to micrometres.

The low-power (40×) field of view is 4.5 mm. What is the field of view at 100×?

At 400× the field of view is 450 µm. About 9 cells fit across it. Estimate the size of one cell.

Give two safety rules for the cheek cell lab.

More practice: warm-up

Which organelle releases energy from glucose?

Which organelle builds proteins?

What is the cell wall of a plant made of?

How many micrometres are in 1 mm?

A microscope has a 10× eyepiece and a 40× objective. What is the total magnification?

More practice: on your own

Low power is 40× with a field of view of 5.0 mm. Find the field of view at 400× in µm.

A student says, "At high power I can see more cells." Is she right? Explain.

Your drawing of a cheek cell is 60 mm wide. The real cell is about 60 µm wide. What is the magnification of your drawing?

Explain how the large central vacuole helps a plant stand up.

Why are cheek cells stained with methylene blue rather than looked at unstained?

Challenge

At 100×, a field of view of 1.8 mm holds about 3 cells across. At 400×, how many of the same cells would fit across the field?

A red blood cell is about 7.5 µm across. At 400× with a 450 µm field, about how many would fit across the field side by side? Why might it be hard to count them accurately?

Two students estimate the length of the same onion cell as 420 µm and 480 µm. Suggest two sources of error, and one way to get a more reliable value.

Check your understanding

Quiz

Which structure is found in an onion epidermis cell but not in a human cheek cell?

The field of view at 40× is 4.0 mm. What is it at 400×?

About 5 cells fit across a 300 µm field of view. About how wide is each cell?

Which organelle packages proteins and ships them to where they are needed?

Which stain is commonly used to see cheek cells?

Which is a correct rule for a scientific drawing?

For parents and teachers

Big idea: this lesson reviews cells from Grade 8 and adds the measurement skills (field of view, micrometres, drawing magnification) used throughout Grade 10 biology. Expectation B2.3 asks students to examine and draw plant and animal cells and show their differences.

How to help: You don't need a microscope. Peeling onion skin and looking at it with a phone camera or magnifier shows the grid of cell walls. If you have a student microscope, let your teen make a wet mount; supervise any iodine and the knife used to cut the onion. Ask them to explain the field-of-view calculation out loud: "magnification up 10 times, field down 10 times."

Common difficulties:

Forgetting to convert mm to µm (× 1000), or dividing when they should multiply.
Thinking higher power shows more cells (it shows fewer, larger ones).
Saying onion cells have chloroplasts (bulb cells don't).
Drawing from memory instead of from what they see, and adding colour or shading.