Cell: The Building Block of Life | Class 9 Science Notes

2.2 Structure of a Cell

Cells are organized into specialized tissues and organs and perform specific functions. For cells to function effectively, they must interact with each other and their environment. This interaction occurs at the cell boundary, where substances move between the cell and its surroundings. Even unicellular organisms exchange materials and respond to their environment through the cell membrane. The cell membrane, also called the plasma membrane, is a thin boundary surrounding the cell that protects its contents and defines the cell's individuality. It is selectively permeable, allowing some substances to pass while blocking others. For example, oxygen and carbon dioxide move across the membranes of alveoli in lungs. The cell membrane is about 7 to 10 nanometres thick and is made up of lipids and proteins arranged in the fluid-mosaic model. This model describes a lipid bilayer with water-attracting heads outward and water-repelling tails inward, with proteins embedded that act as gatekeepers. The molecules can move sideways, flip, and rotate, making the membrane fluid. Cells of plants, fungi, and bacteria have an additional layer outside the cell membrane called the cell wall, which provides rigidity and protection. The chapter includes an activity demonstrating osmosis using potato pieces placed in pure water and concentrated salt or sugar solutions. The potato piece in pure water swells due to water intake, while the piece in concentrated solution shrinks due to water loss. This experiment illustrates osmosis, the diffusion of water across a selectively permeable membrane from a dilute to a concentrated solution. The cell wall in plant cells helps maintain shape and firmness even when water is lost, unlike animal cells which lack a cell wall and shrink when losing water.

📊 Diagram: Fig. 2.4: Electron micrograph of lower surface of a Colocasia leaf showing stomata; Fig. 2.5: Experimental set-up and initial and final states of potato pieces in plain water and 20% salt solution; Fig. 2.6: Effect of solutions of different concentrations on a cell; Fig. 2.7: Structure of a cell membrane; Fig. 2.8: (a) The onion peel cells, and (b) human cheek cells.

🧪 Activity: Activity 2.2: Experiment with potato pieces in water and concentrated salt/sugar solution to observe osmosis; Activity 2.3: Preparing slides of onion peel and cheek cells to observe cell structure and response to sugar solution.

🔗 Connection: Leads to detailed study of cell interior and organelles.