The Invisible Living World: Beyond Our Naked Eye · Lesson 2 of 9
Exploring Plant and Animal Cells
“Observe two familiar tissues to discover the parts shared by cells and the features that differ.”
• Describe what an onion peel and a cheek-cell preparation reveal. • Explain the functions of the membrane, cytoplasm, nucleus and plant cell wall. • Compare the additional structures shown in plant and animal cell diagrams. • Relate chloroplasts and vacuoles to their functions.
Looking at cells directly
A thin onion peel looks like one transparent sheet until you place it under a microscope. In the chapter’s teacher demonstration, the peel is stained with safranin, rinsed, placed in glycerin on a slide and gently covered with a coverslip. Stain makes parts easier to distinguish; glycerin limits drying, and lowering the coverslip carefully avoids trapped air. The microscope reveals many adjoining, roughly rectangular cells, rather like bricks placed close together.
A gentle scrape from the inside of the cheek provides another preparation. Methylene blue increases contrast; a drop of glycerin and a coverslip help keep the sample visible. The cells appear flattened and less regular, often polygonal. Looking at both preparations allows us to compare real observations rather than assuming that every cell is box-shaped.
Problem
You see a close-packed pattern of rectangular outlines. What is a reasonable interpretation?
- 1.Identify one repeated compartment rather than calling the entire pattern one cell.
- 2.Compare its boundary and interior with the labeled onion-peel drawing; the pattern shows many adjacent cells.
The shared parts of a typical cell
Both observed cell types have a cell membrane around the cell, cytoplasm within it and a nucleus. These parts do different jobs. The membrane separates the inside from the surroundings and regulates the movement of needed materials in and wastes out. Cytoplasm holds cell components and is where many life processes occur. The nucleus helps regulate the cell’s activities and growth.
The thin boundary enclosing a cell that controls exchange of substances with its surroundings.
The material inside the cell membrane in which many cell processes take place.
A cell structure that regulates many activities of the cell and its growth.
These labels describe a typical plant or animal cell in this chapter. Later we will meet bacteria, whose genetic material is not enclosed in a well-defined nucleus. Do not assume the same labeled drawing fits every kind of cell.
What makes the plant drawing different?
An onion cell has a cell wall outside its membrane. This firm outer layer provides support and helps the plant cell retain a stable outline. Many plant cells also have a large vacuole, a space that stores substances, helps handle wastes and contributes to cell shape. Some plastids are chloroplasts containing green chlorophyll, which helps a plant use sunlight in photosynthesis. Plastids in non-green parts can store substances. An onion peel need not appear green simply because it is a plant tissue.
A firm outer covering outside the membrane of a plant cell that provides strength and support.
| Feature | Onion-peel/plant cell | Cheek/animal cell |
|---|---|---|
| Cell membrane, cytoplasm, nucleus | Present in the typical cells shown | Present |
| Cell wall | Present outside the membrane | Absent |
| Large vacuole | Common in plant cells | Usually absent; any vacuoles are generally small |
| Chloroplasts | Present in many green plant cells, not necessarily onion peel | Absent |
The source diagram also labels mitochondria, but does not develop their role here. The important comparison is the position of the wall, the shared basic parts, and the extra plant structures. The drawing represents typical features; individual cells can vary.
Problem
Why are onion-peel cells regularly box-like while cheek cells have less regular edges?
- 1.Both have a membrane, so a membrane alone cannot explain the firm rectangular outline.
- 2.Plant cells also have a rigid wall that supports a more fixed shape; cheek cells lack that wall.
Problem
A green leaf cell has chloroplasts. What do they help it do?
- 1.Chlorophyll in chloroplasts captures light.
- 2.The cell can use that light in photosynthesis to make food.
Chloroplasts are important in green parts, but an onion peel need not contain visible green chloroplasts. A plant cell is identified by more than its colour.
Check your understanding
Use the two microscope observations as evidence: identify what both cells share before explaining why their boundaries and shapes differ.
Quiz
Why is safranin used in the onion-peel preparation?
Which layer lies outside the membrane in the onion cell?
What is a main function of the cell membrane?
Which structure contains chlorophyll in many green plant cells?
Why may cheek cells lack a fixed box-like outline?
Practice Problems
- Write the order of structures you would cross when moving from outside a plant cell into its cytoplasm.
- Explain why a stained, carefully covered slide can be easier to interpret than a dry, unstained peel.
- Name three parts visible in the typical onion and cheek cells and explain one function of each.
- A student says every plant cell must look green. Use the onion peel and plastids to respond.
- Compare a large plant vacuole with the smaller or absent vacuoles shown in animal cells.
- Use the plant–animal diagram to explain why a cell wall is not the same thing as a cell membrane.
Key Takeaways
• Onion peel and cheek tissue both reveal many cells under a microscope. • Typical cells shown share membrane, cytoplasm and nucleus. • A plant cell wall supports a firmer outline; animal cells lack it. • Chloroplasts support photosynthesis in green plant parts, and a large plant vacuole aids storage and shape.