Cell: The Building Block of Life · Lesson 3 of 9
Cell wall — The outer covering of cells
“Plant cells bring their own walls because standing upright all day is serious work.”
• Compare the visible organisation of plant and animal cells. • Explain how a concentrated solution affects plant and animal cells. • Describe the position, permeability and functions of the cell wall. • Relate cellulose to cell-wall strength and dietary roughage. • Explain why a cell wall benefits organisms that cannot move away from environmental stress.
Introduction
When a thin piece of onion peel is observed under a microscope, its cells look like small rectangular boxes arranged neatly next to one another, much like the bricks in a wall. Human cheek cells look very different. They are usually rounded or irregular in shape and do not have a fixed, box-like outline.
Both onion cells and human cheek cells are eukaryotic cells, meaning they contain a well-defined nucleus and other membrane-bound structures. So why do they have such different shapes? The main reason is the cell wall. An onion cell has a strong, rigid cell wall surrounding its cell membrane. This wall provides support, protects the cell and helps it maintain a regular shape. A human cheek cell has only a flexible cell membrane and no cell wall. Therefore, it can bend and take on a more irregular shape.
Activity: Let us investigate
A temporary mount makes living or recently collected cells thin enough for light to pass through them. Stains increase contrast: safranin helps the onion or Rhoeo leaf peel become easier to observe, while methylene blue improves the visibility of cheek cells.
| Preparation | Important steps | Likely observation |
|---|---|---|
| Onion or Rhoeo leaf peel | Take a thin peel, add safranin, place a coverslip and observe. | Cells are box-shaped and regularly arranged. |
| Human cheek cells | Collect gently with a clean swab or blunt toothpick, add water and methylene blue, then place a coverslip. | Cells are more irregularly arranged and have flexible outlines. |
Use a clean cotton swab or the blunt end of a toothpick for cheek cells. Handle stains and glass slides carefully, and do not share collection tools.
When both preparations are exposed to a concentrated sugar solution, water moves out of the cells by osmosis. The cheek cells shrink as a whole. In the plant cells, the inner contents shrink and the cell membrane moves away from the cell wall, but the strong outer wall largely retains its shape.
Problem
Slide A shows rectangular cells whose outer boundaries remain fixed after concentrated sugar solution is added. Slide B shows cells that shrink completely. Identify the likely cell types.
- 1.A fixed outer boundary indicates the presence of a rigid cell wall.
- 2.Slide A therefore contains plant cells, such as onion or Rhoeo peel cells.
- 3.Complete shrinking indicates that no rigid wall is holding the original outline.
- 4.Slide B therefore contains animal cells, such as cheek cells.
- 5.Both lose water by osmosis; their different coverings produce different visible results.
Cell Wall — The Outer Covering of Cells
A rigid and permeable covering outside the cell membrane in plant cells and in the cells of fungi and bacteria. It provides protection, support and a stable shape.
A plant cannot usually move away from wind, rain or changes in water availability. Its cell walls help tissues resist these stresses, keep leaves and flowers firm and support the upright body. Although rigid, the wall is permeable: water and some dissolved minerals can pass through it. The cell membrane beneath it remains selectively permeable and controls exchange with the living interior.
| Feature | Cell membrane | Cell wall |
|---|---|---|
| Position in a plant cell | Immediately around the living contents | Outside the cell membrane |
| Nature | Thin and flexible | Rigid |
| Permeability | Selectively permeable | Permeable to water and some dissolved substances |
| Presence | All cells | Plants, fungi and bacteria; absent in animal cells |
| Main role | Controlled exchange and separation | Protection, shape and structural support |
The plant cell wall is mainly made of cellulose, a carbohydrate built from many linked glucose units. Its arrangement gives the wall strength. Humans do not digest cellulose as a source of glucose, but cellulose in food acts as roughage and helps material move through the digestive system.
Problem
A plant wilts during a dry afternoon but becomes firm after watering. Explain the contribution of its cell coverings.
- 1.During water shortage, plant cells lose water and their inner pressure falls.
- 2.The cell walls retain the general outlines of the cells but cannot alone keep the tissue fully firm.
- 3.After watering, water enters the cells by osmosis.
- 4.The enlarged cell contents press outward against the rigid walls.
- 5.The supported cells make the tissue firm again.
Pause and Ponder
A cell wall is especially useful to a plant because the plant usually remains fixed in one place and must withstand environmental forces. An animal gains movement from more flexible cells and supports its body through tissues and other structures rather than a wall around every cell.
Quiz
Which description best matches Cell wall?
Which term matches this description: A rigid and permeable covering outside the cell membrane in plant cells and in the cells of fungi and bacteria.
Which statement is a key takeaway from this lesson?
Which additional statement is also a key takeaway from this lesson?
Which further statement is also a key takeaway from this lesson?
Practice Problems
- Why do onion-peel cells look more regularly arranged than cheek cells?
- Describe what happens to the cell membrane and cell wall of a plant cell in concentrated sugar solution.
- Explain why the cell wall can be permeable while the cell membrane is selectively permeable.
- Predict one consequence if a plant cell wall became as flexible as its cell membrane.
- Why must the initial masses of potato pieces be measured before comparing their final masses?
Key Takeaways
• Plant cells have a rigid cell wall outside the cell membrane. • A concentrated solution removes water from both plant and animal cells by osmosis. • The plant cell wall retains the outer shape even when the inner contents shrink. • Cell walls are permeable and provide protection, support and firmness. • Plant cell walls are mainly made of cellulose.