Bihar Board - Class 11 biology - Chapter 8: Cell – The Unit of Life Handwritten Notes

The cell is the basic structural and functional unit of life. All living organisms, from unicellular bacteria to multicellular humans, are composed of cells. This chapter explores the discovery of the cell, its structure, types, and the functions of its organelles, highlighting its role as the fundamental building block of life.

Key Points

1. Discovery of the Cell

   • Robert Hooke (1665) discovered cells while observing cork slices under a microscope.
   • Anton van Leeuwenhoek observed living cells like bacteria and protozoa.
   • The Cell Theory, proposed by Schleiden and Schwann, states:
     • All living organisms are composed of cells.
     • The cell is the basic unit of life.
     • All cells arise from pre-existing cells (added by Rudolf Virchow).

2. Types of Cells

   • Prokaryotic Cells:
     • Found in organisms like bacteria.
     • Lack a true nucleus and membrane-bound organelles.
     • Genetic material is present in the nucleoid region.
     • Example: Bacteria, archaea.
   • Eukaryotic Cells:
     • Found in plants, animals, fungi, and protists.
     • Have a true nucleus enclosed by a nuclear membrane.
     • Contain membrane-bound organelles.
     • Example: Human cells, plant cells.

3. Cell Structure

   • Plasma Membrane:
     • A selectively permeable membrane made of a phospholipid bilayer and proteins.
     • Regulates the movement of substances in and out of the cell.
   • Cell Wall:
     • Found in plant cells, fungi, and prokaryotes.
     • Provides structural support and protection.
     • Composed of cellulose in plants and chitin in fungi.
   • Cytoplasm:
     • A gel-like substance that fills the cell and houses organelles.
     • Contains enzymes for metabolic reactions.
   • Nucleus:
     • Controls cell activities and contains genetic material (DNA).
     • Surrounded by the nuclear envelope with pores for material exchange.
     • Contains the nucleolus, responsible for ribosome synthesis.

4. Cell Organelles

   • Endoplasmic Reticulum (ER):
     • Rough ER: Studded with ribosomes; involved in protein synthesis.
     • Smooth ER: Involved in lipid synthesis and detoxification.
   • Golgi Apparatus:
     • Modifies, packages, and transports proteins and lipids.
   • Mitochondria:
     • The powerhouse of the cell; generates energy in the form of ATP.
     • Has a double membrane and its own DNA.
   • Chloroplasts (in plant cells):
     • Site of photosynthesis; contains chlorophyll.
     • Also has a double membrane and its own DNA.
   • Lysosomes:
     • Contain hydrolytic enzymes for breaking down waste and cellular debris.
   • Ribosomes:
     • Sites of protein synthesis; may be free or attached to the rough ER.
   • Centrioles (in animal cells):
     • Involved in cell division and formation of the spindle apparatus.
   • Vacuoles:
     • Store nutrients, waste products, and water.
     • Large central vacuole in plant cells maintains turgor pressure.

5. Cell Cycle and Division

   • Cell Cycle: The life cycle of a cell, divided into interphase and mitotic phase.
   • Mitosis: Division of a cell into two identical daughter cells for growth and repair.
   • Meiosis: Division to produce gametes (sex cells) with half the chromosome number.

6. Specialized Cells

   • Cells in multicellular organisms are specialized for specific functions. Examples:
     • Nerve cells for transmitting signals.
     • Muscle cells for contraction.
     • Red blood cells for oxygen transport.

Conclusion

The cell is the cornerstone of biology, being the smallest unit capable of performing all life processes. Understanding cell structure and function is essential for exploring the complexities of life. From prokaryotic simplicity to eukaryotic sophistication, cells form the foundation of all living organisms, highlighting their significance in biological studies.

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