Important Points: The Fundamental Unit of Life | Science Class 9 PDF Download

Introduction to Cells

• Cells are the fundamental organizational units of living organisms.
• Enclosed by a plasma membrane composed of lipids and proteins.
• The plasma membrane regulates the movement of materials in and out of the cell.

Cell Components

Plasma Membrane:

• Active part of the cell.
• Regulates material movement between the cell interior and the external environment.

Cell Wall (in plant cells):

• Composed mainly of cellulose.
• Provides structural strength and prevents bursting in hypotonic media.

Cell Structures

Nucleus

• Separated from the cytoplasm by a double-layered membrane.
• Directs the life processes of the cell.

Nucleus

Endoplasmic Reticulum (ER)

• Network of membrane-bound tubes and sheets.
• Rough ER (RER) has ribosomes, while smooth ER (SER) synthesizes lipids.

Endoplasmic Reticulum

Golgi Apparatus

• Consists of stacks of membrane-bound vesicles.
• Modifies, packages, and stores substances manufactured in the cell.

Golgi Apparatus

Lysosomes

• Membrane-bound sacs with digestive enzymes.
• Involved in waste disposal and digestion of foreign materials.

Lysosomes

Mitochondria

• Powerhouses of the cell.
• Produce energy in the form of ATP through chemical reactions.

Mitochondria

Plastids (in plant cells)

• Chromoplasts contain chlorophyll for photosynthesis.
• Leucoplasts store substances like starch, oils, and proteins.

Plastids

Vacuoles

• Small in animal cells, and large in plant cells.
• Store cell sap and various substances.
• Provide turgidity and rigidity to plant cells.

Vacuole in Plant Cell

Cell Division

Mitosis:

• Process for growth and tissue repair.
• The mother cell divides into two identical daughter cells.

Meiosis:

• Process for forming gametes.
• Involves two consecutive divisions, resulting in four daughter cells with half the chromosome number.

Historical Perspective

• Cells were first discovered by Robert Hooke in 1665.
• Anton van Leeuwenhoek (1674) observed free-living cells in pond water.
• Robert Brown (1831) discovered the nucleus.
• Cell theory (Schleiden and Schwann, 1838-39) states that all plants and animals are composed of cells.
• Virchow (1855) expanded the cell theory by suggesting that all cells arise from pre-existing cells.
• Electron microscope (1940) allowed the observation of complex cell structures and organelles.

Conclusion

• Cells are the basic structural and functional units of life.
• Different organelles and structures contribute to the diverse functions of cells.
• Cell division is essential for growth, repair, and reproduction.