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π¬ Think about it
A tent stays soft and moves in the wind. A brick house stands firm and stays in shape. Plants cannot run from wind or rain, so they need a firm wall. Let us see what gives plant cells their strength.
Which cells have an extra layer outside the membrane?
- Cells of plants, fungi and bacteria have an extra layer.
- This extra layer is called the cell wall.
- It lies around the cell membrane.
Why do plant cells need a cell wall?
- Plants cannot move, so they need a rigid support.
- The wall helps them withstand wind and rain.
- It keeps leaves and flowers firm and in shape.
- It helps plants stay upright.
Example:
A firm stem holds a sunflower upright even on a windy day.
Is the cell wall permeable?
- Yes, the cell wall is permeable.
- Water and some dissolved minerals can pass through.
- Along with the membrane, it helps roots absorb water.
In this Activity, we will compare onion (plant) cells and cheek (animal) cells under a microscope, and see how each reacts to a strong sugar solution.
Materials needed
Onion or Rhoeo (Cradle lily) leaf, safranin stain, cheek cells, methylene blue stain, glass slides, coverslips, a microscope, and 20 per cent sugar solution.
Procedure
1. Prepare a temporary slide of a thin peel of an onion or Rhoeo leaf, mount it with safranin using a cover slip, and observe plant cells.
2. Prepare a slide of cheek cells by gently scraping the inner cheek with a cotton swab or the blunt end of a toothpick.
3. Spread the cheek cells on a clean glass slide.
4. Add a drop of water and a few drops of methylene blue stain, then place a coverslip.
5. Observe both slides under a microscope.
6. Now prepare fresh slides and put 20 per cent sugar solution on them; observe again after half an hour.
2. Prepare a slide of cheek cells by gently scraping the inner cheek with a cotton swab or the blunt end of a toothpick.
3. Spread the cheek cells on a clean glass slide.
4. Add a drop of water and a few drops of methylene blue stain, then place a coverslip.
5. Observe both slides under a microscope.
6. Now prepare fresh slides and put 20 per cent sugar solution on them; observe again after half an hour.
Observation
Onion peel cells are box-shaped and regularly arranged; cheek cells are irregularly arranged. In sugar solution, the plant cell boundary stays the same but its inner content shrinks; cheek cells shrink considerably.
Explanation
In a strong sugar solution plant cells lose water by osmosis, but their rigid cell wall keeps the outer shape. Only the inner content pulls away from the wall. This shows the cell wall keeps plant cells firm and in shape.
β Summary
- Plant cells: regular boxes.
- Cheek cells: irregular shapes.
- Sugar: inner content shrinks.
- Wall keeps shape.
What is the cell wall made of?
- The plant cell wall is mainly made of cellulose.
- Cellulose is a carbohydrate of many glucose units linked together.
- Fungi and bacteria also have a cell wall for protection.
Example:
Cellulose in our food acts as roughage and helps in digestion.
How are plant cells different from animal cells here?
- Plant cells have a rigid cell wall, so they keep their shape.
- Animal cells have no cell wall.
- In strong sugar solution animal cells lose water and shrink.
- No wall means animal cells can change shape easily.
Plant cell
Animal cell
Has a rigid cell wall.
Has no cell wall.
Keeps its shape in sugar solution.
Shrinks in sugar solution.
Stays firm and upright.
Can change shape easily.
Important Points
- The cell wall is found in plants, fungi and bacteria.
- It is permeable, while the membrane is selectively permeable.
- The plant cell wall is mainly made of cellulose.
- Animal cells have no cell wall, so they can change shape.
βΈοΈ Pause and Ponder
-
What argument would you give for the necessity of a cell wall in plants usually fixed in one place versus in animals usually moving from one place to the other?
A fixed plant needs a rigid cell wall for support against wind and rain and to stay upright. A moving animal needs flexible cells with no wall, so it can change shape and move easily. -
What consequences would you predict for a plant cell if its cell wall were to become as flexible as a cell membrane?
The plant cell would lose its rigid support. The plant could not stay firm or upright and would droop, and it could not hold its shape in changing conditions. -
Why is it important to cut the two potato pieces in roughly equal size and measure their initial weight before placing them in different liquids?
Equal size and a recorded starting weight make it a fair test. Then any change in weight can be compared correctly to show how much water moved in or out.
β Test Yourself
-
Which three kinds of cells have a cell wall?
View Answer
Cells of plants, fungi and bacteria. -
Is the cell wall permeable or selectively permeable?
View Answer
Permeable — water and some minerals pass through freely. -
What is the plant cell wall mainly made of?
View Answer
Cellulose, a carbohydrate made of many glucose units. -
Why do plant cells keep their shape in sugar solution?
View Answer
Their rigid cell wall holds the outer shape, even as the inside shrinks. -
Why can animal cells change shape easily?
View Answer
They have no rigid cell wall.
Important Definitions
- Cell wall — an extra rigid layer outside the membrane in plant, fungal and bacterial cells.
- Cellulose — a carbohydrate of many linked glucose units that makes the plant cell wall.
- Roughage — cellulose in our diet that helps in digestion.
π NCERT Question 1 — Differentiate between the following pairs
Differentiate cell membrane and cell wall, RER and SER, chloroplasts and chromoplasts.
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π NCERT Question 8 — A researcher carried out an
A researcher placed two carrots in plain water and concentrated salt solution — explain her observations.
View Answer →
π NCERT Question 10 — Carry out the following experiment
Potato cups with sugar, salt, and a boiled potato — explain where water gathers and why.
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