Lesson 3 of 5 / Cell structure and organisation
Reading a light microscope view
Why does a real cell look less tidy than a textbook diagram?
I can use visible evidence to identify structures and explain how slide preparation improves a view.
I can identify the six named cell structures and functions from diagrams and light microscopy.Syllabus
Syllabus K224 / K225, Topic 1(a). Identify the six named cell structures and functions from diagrams and light microscopy.
About 5 min
Make a guess. You are not marked.
You add iodine to an onion cell and a dark round body appears that you could not see before. What happened?
Show the answer
The stain made an existing nucleus easier to see.
A stain adds contrast. The nucleus was there all along but was nearly transparent before staining.
The key idea
A stain improves contrast; it does not add a new organelle. Identify what the image actually resolves and distinguish an observation from an inference.
A micrograph is a photograph made through a microscope. Compare real stained onion cells, cheek cells and photosynthetic leaf cells below. First describe what you can see, then check the explanation.
Explore the idea
What can you actually see?
Look at real cells
Find a cell wall and a nucleus. Can you distinguish the membrane separately from the wall?
Image size changes with your screen. Use a stated scale bar for measurements; do not calculate from the display size alone. Practise slide preparation and focusing in your laboratory.
Explanation
A drawing in a book shows each part clearly. A real microscope view is messier. Cells overlap, the stain is uneven and parts are at different depths. First find the repeated cell edges. Then look for a nucleus and the cytoplasm around it. Only label what you can really see.
To look at onion cells, place a thin layer of onion skin flat in a drop of water on a slide. Add a stain such as iodine solution, following your teacher's instructions. The stain makes parts easier to see. Lower the coverslip slowly at an angle so that fewer air bubbles are trapped.
Start with the low-power lens to find the cells and move them to the centre. Adjust the light and focus. Then change to a higher power if you need to, and use the fine focus. A higher power makes the image bigger, but it cannot show details that are too small for the microscope.
In onion skin, the cell walls make a repeated pattern. The nucleus may sit near the edge of the cell, because the large vacuole takes up most of the space. The membrane is pressed against the wall, so it is hard to see on its own. Onion bulb cells have no chloroplasts, so you should not expect to see any.
On a prepared slide of animal cells, look for a membrane, cytoplasm and a stained nucleus, but no cell wall. Compare what you see with a diagram, but record what is really there. Ribosomes are too small to see without an electron microscope.
Step by step
- 1
Prepare for a clear view
Use a thin, flat specimen, an appropriate stain and a carefully lowered coverslip.
- 2
Observe before naming
Record visible boundaries and darker bodies. A large round air bubble is not automatically a nucleus.
- 3
Recognise the limit
Do not claim that a structure is absent simply because it cannot be resolved or stained clearly.
Worked example
Before and after staining
Before staining, an onion cell has a clear outline but no obvious nucleus. After staining, a dark body becomes visible near its edge. Explain the change.
One way to explain it
The stain increased contrast between the nucleus and its surroundings, making an existing structure easier to see. It did not create a nucleus. The nucleus can be near the edge because the large vacuole occupies much of the cell.
Why this answer works
- Separate visibility from presence.
- Explain the purpose of staining.
- Use plant cell structure to interpret the position.
Watch out for this
A student says: "If I cannot see a cell membrane, the cell has no membrane." What is wrong with this?
Show the answer
A cell membrane may be too thin or too close to the wall to distinguish separately in this view. Limited resolution or contrast is not proof of absence.
Check your understanding
Which conclusion is justified when a nucleus becomes visible after staining?
Show the answer
An existing nucleus is now easier to distinguish.
Contrast increased. Visibility before staining was insufficient to show the nucleus clearly.
Exam-style questions
Check your understanding
A drawing of a cell is 60 mm long. The real cell is 0.12 mm long. What is the magnification of the drawing?
Show the answer
x500
Magnification = drawing size / real size = 60 / 0.12 = 500.
Quick recall
Why does a real cell look less tidy than a textbook diagram?