Detailed Notes -2 – chapter-5 – class 9

What is a Cell Made Up of? What is the Structural Organisation of a Cell?

1. Components of a Cell:
  - A cell consists of three main features: plasma membrane, nucleus, and cytoplasm.
  - These components regulate the cell’s activities and interactions with its environment.
2. Plasma Membrane:
  - The outermost covering of the cell, separating its contents from the external environment.
  - Acts as a selectively permeable membrane, controlling the movement of substances into and out of the cell.
3. Diffusion and Osmosis:
  - Diffusion: Spontaneous movement of substances from areas of high concentration to areas of low concentration.
  - Osmosis: Movement of water molecules across a selectively permeable membrane toward areas of higher solute concentration.
4. Effects of Solutions on Cells:
  - Hypotonic Solution: Higher water concentration outside the cell, causing the cell to gain water and potentially swell.
  - Isotonic Solution: Same water concentration as the cell, resulting in no net movement of water.
  - Hypertonic Solution: Lower water concentration outside the cell, leading to water loss and potential cell shrinkage.

Activity 5.3:

1. 1. Objective:
    - To observe the effects of osmosis using a de-shelled egg.
  2. Materials:
    - Eggs
    - Dilute hydrochloric acid
    - Pure water
    - Concentrated salt solution
  3. Procedure: (a) Removing the eggshell:
    - Dissolve the eggshell in dilute hydrochloric acid, which mainly consists of calcium carbonate.
    - Once the shell is dissolved, a thin outer skin will enclose the egg.
    (b) Osmosis with pure water:
    - Place the de-shelled egg in pure water and observe it after 5 minutes.
    - Note the changes observed.
    (c) Osmosis with concentrated salt solution:
    - Place a similar de-shelled egg in a concentrated salt solution and observe it for 5 minutes.
    - Note the changes observed.
  4. Observations:
    - In the pure water, the egg swells because water passes into it by osmosis.
    - In the concentrated salt solution, the egg shrinks because water passes out of the egg solution into the salt solution, as the salt solution is more concentrated.
  5. Explanation:
    - In the pure water, osmosis occurs from the region of lower solute concentration (outside the egg) to the region of higher solute concentration (inside the egg), causing the egg to swell.
    - In the concentrated salt solution, osmosis occurs from the region of higher solute concentration (inside the egg) to the region of lower solute concentration (outside the egg), causing the egg to shrink.
  6. Conclusion:
    - The experiment demonstrates the principle of osmosis, where water moves across a selectively permeable membrane from an area of low solute concentration to an area of high solute concentration.

5.2.2 CELL WALL
Cell Wall in Plant Cells:
- Plant cells have an additional outer covering called the cell wall, which lies outside the plasma membrane.
- The cell wall provides structural support and rigidity to plant cells.
Composition of Cell Wall:
- The plant cell wall is mainly composed of cellulose, a complex substance.
- Cellulose fibers form a network that reinforces the cell wall, giving it strength.
Function of Cellulose:
- Cellulose provides structural strength to plant cells, allowing them to maintain their shape and resist mechanical stress.
Plasmolysis:
- Plasmolysis occurs when a living plant cell loses water through osmosis.
- As water exits the cell, the contents of the cell shrink or contract away from the cell wall.
Definition of Plasmolysis:
- Plasmolysis is the phenomenon where the protoplasm of a plant cell shrinks and detaches from the cell wall due to the loss of water.
Causes of Plasmolysis:
- Plasmolysis is caused by an imbalance in water concentration between the cell’s interior and its external environment.
- When the external solution has a lower water concentration (hypertonic solution), water moves out of the cell, leading to plasmolysis.
Importance of Cell Wall:
- The cell wall plays a crucial role in maintaining the structural integrity of plant cells.
- It provides support and protection, especially in non-lignified cells.
Effects of Plasmolysis:
- Plasmolysis disrupts normal cellular functions and can lead to wilting in plants.
- Severe plasmolysis can cause irreversible damage to the cell.
Relation to Osmosis:
- Plasmolysis is directly related to the process of osmosis, which involves the movement of water across a selectively permeable membrane.
- In hypertonic conditions, water moves out of the cell, leading to plasmolysis.

5.2.3 NUCLEUS

Iodine Solution and Onion Peel Observation:
- Iodine solution is applied to the onion peel before observation under the microscope to stain the cells, making them more visible.
- Without iodine solution, the cells may not be clearly visible, and it may be difficult to distinguish their structures.
Alternative Staining Solutions:
- Other staining solutions such as safranin solution or methylene blue solution can also be used to observe onion peel cells.
- These solutions provide contrast and coloration to different cell structures, enhancing visibility under the microscope.
Observing Cheek Cells:
- Cheek cells can be observed by gently scraping the inside surface of the cheek with an ice-cream spoon.
- The collected material is then transferred to a glass slide and spread evenly.
- Methylene blue solution is used to stain the cheek cell material for observation under the microscope.
Structure of the Nucleus:
- The darkly colored, spherical or oval structure near the center of each cell is called the nucleus.
- This structure is present in both cheek cells and onion peel cells.
Composition and Function of the Nucleus:
- The nucleus is surrounded by a double-layered covering called the nuclear membrane, which contains pores allowing the transfer of materials between the nucleus and cytoplasm.
- Chromosomes, composed of DNA and protein, are present in the nucleus and contain genetic information in the form of DNA molecules.
- Functional segments of DNA called genes are responsible for inheritance of traits from parents to offspring.
- The nucleus plays a central role in cellular reproduction and in determining cell development and maturity by directing the chemical activities of the cell.
Prokaryotic and Eukaryotic Cells:
- Prokaryotic cells lack a defined nucleus and nuclear membrane, and their nuclear region is called a nucleoid.
- Eukaryotic cells, like those in plants and animals, have a well-defined nucleus enclosed within a nuclear membrane.
5.2.4 CYTOPLASM
Cytoplasm:
- The cytoplasm is a large region within each cell enclosed by the cell membrane.
- It contains fluid and various specialized cell organelles.
- The cytoplasm takes up very little stain compared to other regions of the cell.
Cell Organelles:
- Cell organelles are specialized structures within the cytoplasm that perform specific functions for the cell.
- These organelles are enclosed by membranes, which help segregate their functions from the rest of the cell.
Prokaryotic vs. Eukaryotic Cells:
- Prokaryotic cells lack a defined nuclear region and membrane-bound organelles.
- Eukaryotic cells, on the other hand, have both a nuclear membrane and membrane-enclosed organelles.
Significance of Membranes:
- Membranes play a crucial role in cell structure and function.
- They separate the cell’s internal environment from its external surroundings and help compartmentalize different cellular processes.
- Membranes also facilitate the selective transport of molecules into and out of the cell, regulating cellular activities.
Example of Viruses:
- Viruses lack membranes and do not exhibit characteristics of life on their own.
- They rely on infecting living cells and hijacking their cellular machinery to replicate and multiply.