Thursday, July 30, 2020

Cell The Unit of Life - Notes | Class 11 | Part 1: Cell theory, An Overview of Cell

8. CELL: THE UNIT OF LIFE

-    A cell is the fundamental, structural and functional unit of all living organisms.

-    Robert Hooke: Discovered cell.

-    Anton Von Leeuwenhoek: First observed and described a live cell.

-    The invention of the compound & electron microscopes revealed all the structural details of the cell.


CELL THEORY


-    Matthias Schleiden (1838) observed that all plants are composed of different kinds of cells.

-    Theodore Schwann (1839) found that cells have a thin outer layer (plasma membrane). He also found that plant cells have cell wall. He proposed a hypothesis that animals and plants are composed of cells and products of cells.

-    Schleiden & Schwann formulated the cell theory.

-    Rudolf Virchow (1855) first explained that cells divide and new cells are formed from pre-existing cells (Omnis cellula-e cellula). He modified the cell theory.

-    Cell theory states that:

(i)      All living organisms are composed of cells and products of cells.

(ii)    All cells arise from pre-existing cells.


AN OVERVIEW OF CELL


-    All cells contain

o Cytoplasm: A semi-fluid matrix where cellular activities and chemical reactions occur. This keeps the cell in ‘living state’.

o Ribosomes: Non-membrane bound organelles seen in cytoplasm, chloroplasts, mitochondria & on rough ER.

-    Cells differ in size, shape and activities.

o Smallest cells: Mycoplasmas (0.3 µm in length).

o Largest isolated single cell: Egg of ostrich.

o Longest cells: E.g. Nerve cell.

o Size of bacteria: 3 to 5 µm (Typical: 1 to 2 µm).

o Human RBCs are about 7.0 µm in diameter.

-    Based on the functions, shape of cells may be disc-like, polygonal, columnar, cuboid, thread like, or irregular.

-    Cells are 2 types: Prokaryotic cells & Eukaryotic cells.

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