Observing and Comparing Cells

Cells are the basic structural and functional units of living organisms. Although plant and animal cells are both eukaryotic cells, they are not identical.

Using a light microscope allows us to observe real cells and compare their visible structures. Common classroom specimens include:

• onion epidermal cells
• Elodea or other aquatic plant cells
• human cheek cells
• prepared animal tissue slides

When observing cells, scientists carefully record what they can actually see rather than simply drawing what they know should be present.

Observing Plant Cells

Plant cells can be observed using a thin piece of plant tissue.

Onion epidermis is commonly used because the tissue is thin and the cells are relatively large and easy to observe.

Under a light microscope, onion epidermal cells often appear:

• rectangular or brick-like
• arranged closely together
• surrounded by clearly visible cell walls
• relatively regular in size and shape

Depending on the preparation and staining, the nucleus may also be visible.

Structures Visible in Plant Cells

Several structures may be visible using a school light microscope.

Cell wall – A rigid outer layer that supports and protects the cell.

Cell membrane – A thin membrane controlling movement of substances into and out of the cell. It can be difficult to distinguish clearly from the cell wall in some specimens.

Cytoplasm – The material inside the cell where many chemical reactions occur.

Nucleus – Contains the cell's genetic material and controls many cell activities.

Vacuole – Plant cells usually contain a large permanent vacuole filled with cell sap.

Chloroplasts – Contain chlorophyll and are the site of photosynthesis. They are visible in some green plant tissues.

Not All Plant Cells Have Chloroplasts

It is easy to assume that every plant cell contains chloroplasts.

This is not true.

Chloroplasts are mainly found in cells that carry out photosynthesis.

For example, leaf cells may contain many chloroplasts because they receive light and perform photosynthesis.

Onion bulb cells normally grow underground and do not perform much photosynthesis. Therefore, onion epidermal cells usually do not contain visible chloroplasts.

This demonstrates an important idea:

Different cells can have different structures depending on their functions.

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Observing Animal Cells

Human cheek cells are commonly used to observe animal cells.

Cells can be collected gently from the inside of the cheek, placed on a microscope slide, and stained to make structures easier to see.

Under a light microscope, cheek cells often appear:

• irregular or rounded
• relatively flat
• separate or loosely grouped
• without a rigid cell wall

The nucleus may appear as a darker region when the cells are stained.

Structures Visible in Animal Cells

A typical animal cell contains:

Cell membrane – Controls movement of substances into and out of the cell.

Cytoplasm – Contains many of the cell's chemical reactions.

Nucleus – Contains genetic material and helps control cell activities.

Animal cells do not have:

• cell walls
• chloroplasts
• large permanent central vacuoles

This gives animal cells more flexibility in shape than many plant cells.

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Comparing Plant and Animal Cells

Plant and animal cells share several important structures because both are eukaryotic cells.

Some structures, such as mitochondria, are normally too small to be clearly resolved with a standard school light microscope.

This means there is an important difference between:

structures we know are present

and

structures we can actually observe

When recording microscope observations, focus on what is genuinely visible.

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Similarities Between Plant and Animal Cells

Both plant and animal cells usually contain:

• a cell membrane
• cytoplasm
• a nucleus
• mitochondria
• ribosomes

These similarities exist because both types of cells carry out many of the same basic life processes.

For example, both need to:

• release energy
• make proteins
• control chemical reactions
• transport substances
• contain and use genetic information

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Differences Between Plant and Animal Cells

Plant cells have several structures that animal cells do not.

Cell Wall

Plant cells have a rigid cell wall made mainly from cellulose.

The cell wall:

• provides support
• protects the cell
• helps maintain cell shape

This helps explain why many plant cells have a more regular shape.

Animal cells do not have cell walls.

Chloroplasts

Many plant cells contain chloroplasts.

Chloroplasts contain chlorophyll, which absorbs light energy for photosynthesis.

Animal cells do not contain chloroplasts.

Large Permanent Vacuole

Many mature plant cells contain a large central vacuole.

The vacuole contains cell sap and helps support the cell.

Animal cells may contain small vesicles or vacuoles, but they do not normally have the large permanent central vacuole typical of plant cells.

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Comparing Cell Shape

One of the easiest differences to observe under a microscope is cell shape.

Plant cells often have:

• straight edges
• regular shapes
• clearly defined boundaries
• close arrangement

Animal cells often have:

• curved edges
• irregular shapes
• less rigid boundaries
• more variable arrangements

The difference is largely related to the presence of the rigid cell wall in plant cells.

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Using Stains

Many cells are almost transparent.

A stain increases contrast, making certain structures easier to observe.

For example, stains may make the nucleus appear darker than the surrounding cytoplasm.

Without staining, some cells may appear almost colourless under a light microscope.

A stain does not increase magnification.

Instead, it improves contrast.

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Preparing Cells for Observation

A typical procedure for observing plant cells is:

1. Obtain a very thin piece of plant tissue.
2. Place it onto a clean microscope slide.
3. Add a drop of water or appropriate stain.
4. Carefully lower a coverslip over the specimen.
5. Place the slide onto the microscope stage.
6. Begin with the lowest-power objective.
7. Focus using the coarse focus knob.
8. Sharpen the image using the fine focus knob.
9. Centre the cells.
10. Increase magnification if greater detail is required.

Starting with low magnification makes the specimen easier to locate.

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Comparing Two Specimens

Suppose a student observes two unknown cell samples.

Specimen A

The cells:

• have straight edges
• are arranged in rows
• have clearly visible cell walls
• contain several green structures

Specimen B

The cells:

• have irregular shapes
• do not have cell walls
• contain dark-stained nuclei
• do not contain green structures

The student could conclude that:

Specimen A is plant tissue.

The green structures are likely chloroplasts.

Specimen B is animal tissue.

The absence of cell walls and chloroplasts supports this conclusion.

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Recording Scientific Observations

Good scientific observations describe what can actually be seen.

Instead of writing:

"The cells look weird."

A more scientific description would be:

"The cells have irregular shapes and contain dark-stained nuclei."

Instead of:

"The plant cells look like bricks."

A more precise description would be:

"The cells are rectangular and arranged closely together in regular rows."

Scientific observations should use appropriate biological terminology.

Useful terms include:

• nucleus
• cytoplasm
• cell membrane
• cell wall
• chloroplast
• vacuole
• regular
• irregular
• rectangular
• circular
• stained
• transparent
• magnification
• field of view

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Observation vs Explanation

Scientists distinguish between what they observe and how they explain the observation.

Observation

"The plant cells have straight, clearly defined boundaries."

This describes what was seen.

Explanation

"The regular shape is caused by the rigid cell wall."

This uses biological knowledge to explain the observation.

Both are valuable, but they serve different purposes.

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Recording Observations in a Table

A comparison table is an effective way to record microscope observations.

For example:

This makes similarities and differences easier to identify.

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Scientific Drawings

A scientific drawing is a careful representation of what was actually observed.

Scientific drawings are not artistic drawings.

The goal is to communicate biological information clearly and accurately.

Rules for Scientific Drawings

A good scientific drawing should:

• be large enough to show important details
• use a sharp pencil
• use clear, single lines
• show correct proportions
• show only structures that were actually observed
• include appropriate labels
• use straight label lines
• avoid unnecessary shading
• include a title
• include magnification or a scale where appropriate

Avoid:

• sketchy lines
• colouring
• decorative shading
• tiny drawings
• overlapping label lines
• structures that were not actually visible

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Drawing What You See

Suppose you observe onion epidermal cells.

You know from biology that the cells contain mitochondria and ribosomes.

However, you cannot see these structures using the school light microscope.

Therefore, you should not draw them.

Your drawing should represent the structures actually visible in the specimen.

This is an important principle in scientific work:

Record the evidence you observe, not the result you expect.

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Labelling Scientific Drawings

Labels should identify visible structures accurately.

For a plant cell drawing, possible labels include:

• cell wall
• nucleus
• cytoplasm
• vacuole
• chloroplasts, if visible

For an animal cell drawing:

• cell membrane
• cytoplasm
• nucleus

Label lines should be straight and should point clearly to the structure being identified.

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Adding Magnification

The magnification used should normally be recorded with the observation.

For example:

Onion epidermal cells at 100× magnification

or:

Human cheek cells at 400× magnification

Remember that total microscope magnification is:

Total magnification = eyepiece magnification × objective magnification

For example:

10× eyepiece × 40× objective = 400×

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Comparing Different Plant Cells

Not all plant cells look the same.

Different plant cells are specialized for different functions.

For example:

Onion Epidermal Cells

Usually:

• rectangular
• transparent
• tightly packed
• no visible chloroplasts

Leaf Cells

May contain:

• many chloroplasts
• visible cell walls
• large vacuoles

Guard Cells

Have a specialized curved shape and contain chloroplasts.

This means that even cells from the same organism can have different structures.

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Comparing Different Animal Cells

Animal cells also show considerable variation.

For example:

Cheek Cells

• flat
• irregular or rounded
• visible nucleus when stained

Muscle Cells

• elongated
• specialized for contraction

Nerve Cells

• long extensions
• specialized for transmitting electrical signals

Red Blood Cells

• small
• disc-shaped
• specialized for carrying oxygen
• mature human red blood cells do not contain nuclei

These differences demonstrate an important biological principle:

Cell structure is related to cell function.

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Identifying an Unknown Cell

Imagine you observe an unknown cell under a microscope.

You can use visible evidence to identify it.

Evidence 1: A cell wall is visible.

This strongly suggests a plant cell rather than an animal cell.

Evidence 2: Chloroplasts are visible.

This provides even stronger evidence that the specimen is from photosynthetic plant tissue.

Evidence 3: No cell wall or chloroplasts are visible.

The specimen may be animal tissue.

Scientists therefore identify cells by examining several pieces of evidence rather than relying on only one characteristic.

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Worked Comparison

A student observes Cell X and Cell Y.

Cell X:

• has a regular rectangular shape
• has a clearly visible outer wall
• contains many small green structures

Cell Y:

• has an irregular shape
• has no visible cell wall
• contains a dark-stained nucleus

Identification

Cell X is most likely a plant cell.

Evidence:

• cell wall
• regular shape
• chloroplasts

Cell Y is most likely an animal cell.

Evidence:

• no cell wall
• irregular shape
• visible stained nucleus

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Similarities Are Important Too

When comparing cells, students sometimes focus only on differences.

However, similarities provide important biological information.

Plant and animal cells both:

• have cell membranes
• contain cytoplasm
• contain genetic material
• carry out cellular respiration
• produce proteins
• perform many similar chemical reactions

The presence of common structures reflects the fact that all eukaryotic cells share many fundamental biological processes.

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From Observation to Conclusion

A strong scientific comparison usually follows this pattern:

Observation → Evidence → Conclusion

For example:

Observation: The cells contain many green structures.

Evidence: Chloroplasts contain green chlorophyll.

Conclusion: The cells are probably from photosynthetic plant tissue.

Another example:

Observation: The cells have no rigid cell wall and have irregular shapes.

Evidence: Animal cells lack cell walls.

Conclusion: The specimen is probably animal tissue.

This approach is much stronger than simply saying, "It looks like an animal cell."

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Common Misconceptions

All plant cells contain chloroplasts.

Incorrect. Chloroplasts are mainly found in plant cells that carry out photosynthesis.

Animal cells do not have cell membranes.

Incorrect. Both plant and animal cells have cell membranes.

The cell wall and cell membrane are the same structure.

They are different. Plant cells have both a cell membrane and a cell wall.

Everything shown in a textbook cell diagram should be visible through a school microscope.

No. Many structures are too small to resolve clearly with a standard light microscope.

Scientific drawings should be artistic and highly detailed.

Scientific drawings should be clear, accurate, simple, and based on actual observations.

Plant cells are always rectangular.

Many plant cells have regular shapes because of their cell walls, but plant cells can have many different shapes depending on their function.

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Did You Know?

One of the first people to describe cells was Robert Hooke in 1665. He examined thin slices of cork using an early microscope and observed many small compartments.

He called these structures cells because they reminded him of small rooms.

Modern microscopes allow us to see far more detail, but the basic scientific process remains similar: observe carefully, record evidence, compare structures, and use those observations to make conclusions.

Key Terms

Plant cell – A eukaryotic cell containing a cell membrane, cytoplasm and nucleus, as well as a cell wall and usually a large permanent vacuole.

Animal cell – A eukaryotic cell containing a cell membrane, cytoplasm and nucleus but no cell wall or chloroplasts.

Cell wall – A rigid layer surrounding plant cells that provides support.

Cell membrane – A thin boundary controlling movement of substances into and out of a cell.

Cytoplasm – Material inside a cell where many chemical reactions occur.

Nucleus – A structure containing genetic material.

Chloroplast – An organelle containing chlorophyll where photosynthesis occurs.

Vacuole – A fluid-filled structure; plant cells commonly have a large permanent central vacuole.

Stain – A substance used to increase contrast and make cell structures easier to observe.

Scientific drawing – An accurate representation of a biological specimen based on direct observation.

Observation – Information obtained by examining or measuring something.

Key Takeaways

• Plant and animal cells can be observed and compared using a light microscope.
• Both plant and animal cells contain a cell membrane, cytoplasm and nucleus.
• Plant cells also have a cell wall and usually a large permanent vacuole.
• Photosynthetic plant cells may contain chloroplasts.
• Animal cells do not have cell walls or chloroplasts.
• Plant cells often appear more regular because of their rigid cell walls.
• Animal cells often have more irregular shapes.
• Not every structure known to exist in a cell can be seen using a school light microscope.
• Scientific observations should use accurate biological terminology.
• Scientific drawings should be large, clear, labelled and based only on what was actually observed.
• Different cell types can have different structures because cell structure is related to function.
• Strong comparisons identify both similarities and differences between cells.