krit.club logo

The Fundamental Unit of Life - Plasma Membrane and Cell Wall

Grade 9CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

•

The Plasma Membrane is the outermost covering of the cell that separates the contents of the cell from its external environment. It is composed of organic molecules called lipids and proteins.

•

The Plasma Membrane is called a selectively permeable membrane because it permits the entry and exit of some materials in and out of the cell while preventing others.

•

Diffusion is the spontaneous movement of a substance from a region of high concentration to a region of low concentration. For example, gaseous exchange of CO2CO_2 and O2O_2 across the cell membrane.

•

Osmosis is the passage of water from a region of high water concentration through a selectively permeable membrane to a region of low water concentration until equilibrium is reached.

•

Tonicity Effects: In a Hypotonic solution, the cell gains water and swells. In an Isotonic solution, there is no net movement of H2OH_2O. In a Hypertonic solution, the cell loses water and shrinks.

•

The Cell Wall is a rigid outer covering found only in plant cells, fungi, and bacteria, located outside the plasma membrane. In plants, it is mainly composed of cellulose, providing structural strength.

•

Plasmolysis occurs when a living plant cell loses water through osmosis, causing the contents of the cell to shrink away from the cell wall.

•

Endocytosis is the process by which the flexibility of the cell membrane enables the cell to engulf food and other material from its external environment (e.g., Amoeba).

📐Formulae

Net Movement=High Concentration→Low Concentration\text{Net Movement} = \text{High Concentration} \rightarrow \text{Low Concentration}

Rate of Diffusion∝ΔConcentration\text{Rate of Diffusion} \propto \Delta \text{Concentration}

Hypotonic Solution: [H2O]ext>[H2O]int⇒Cell Swells\text{Hypotonic Solution: } [H_2O]_{ext} > [H_2O]_{int} \Rightarrow \text{Cell Swells}

Hypertonic Solution: [H2O]ext<[H2O]int⇒Cell Shrinks\text{Hypertonic Solution: } [H_2O]_{ext} < [H_2O]_{int} \Rightarrow \text{Cell Shrinks}

Isotonic Solution: [H2O]ext=[H2O]int⇒No Change\text{Isotonic Solution: } [H_2O]_{ext} = [H_2O]_{int} \Rightarrow \text{No Change}

💡Examples

Problem 1:

Explain what happens when a plant cell is placed in a concentrated salt solution.

Solution:

The process of Plasmolysis occurs.

Explanation:

Because the concentration of H2OH_2O is higher inside the cell than in the external salt solution (Hypertonic), water moves out of the cell via osmosis. This causes the cytoplasm and protoplast to shrink away from the Cell WallCell \ Wall.

Problem 2:

How do CO2CO_2 and O2O_2 move across the cell membrane?

Solution:

Through the process of simple diffusion.

Explanation:

When CO2CO_2 accumulates in high concentration inside a cell and is low outside, it moves out. Conversely, when [O2][O_2] decreases inside the cell, O2O_2 enters from the outside where its concentration is higher. The movement follows the gradient: High C→Low C\text{High } C \rightarrow \text{Low } C.

Problem 3:

Why can plant cells withstand much greater changes in the surrounding medium than animal cells?

Solution:

Due to the presence of the Cell Wall.

Explanation:

When a plant cell swells due to osmosis in a hypotonic medium, it builds up pressure against the Cell WallCell \ Wall. The wall exerts an equal pressure against the swollen cell, preventing it from bursting, unlike animal cells which lack this rigid structure.