Discovery of Cell:
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In \(1665\), Robert Hooke observed a thin slice of cork under a self-designed microscope.
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He saw a honeycomb-like structure with many small compartments.
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He named these compartments cells (Latin: little rooms).
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This was the first time it was observed that living things are made of separate units.
Robert Hooke's microscope
History of Cell Discovery:
| Scientist | Year | Discovery |
| Robert Hooke | \(1665\) | Discovered cells in cork |
| Antonie van Leeuwenhoek | \(1674\) | Observed living cells |
| Robert Brown | \(1831\) | Discovered nucleus |
| Jan Evangelista Purkyne | \(1839\) | Coined protoplasm |
| Matthias Schleiden | \(1838\) | Proposed cell theory |
| Theodor Schwann | \(1839\) | Proposed cell theory |
| Rudolf Virchow | \(1855\) | Cells arise from pre-existing cells |
Cell Theory:
1. All organisms are made up of cells.
2. Cells are the basic units of life.
3. Cells comes from pre-existing cells that have multiplied.
Onion Peel Activity:
| Aspect | Description |
| Aim | To observe onion cells |
| Materials | Onion peel, slide, water, safranin, microscope |
| Procedure | A thin onion peel is placed on a slide with water, stained with safranin, covered with a cover slip, and observed under a microscope |
| Observation | Small box-like cells seen |
| Conclusion | An onion is made of cells |
An onion cell under a microscope
Based on the number of cells, organisms are classified into two: unicellular and multicellular.
| S.No. | Unicellular Organisms | Multicellular Organisms |
| 1 | Made up of a single cell. | Made up of many cells. |
| 2 | There is no division of labour. A single cell performs all the activities. | There is division of labour. Cells are specialized to perform various functions. |
| 3 | Can live independently. | Cells work together to form body parts. |
|
4
|
Example - Bacteria, amoeba, plankton, paramecium, euglena and chlamydomonas.
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Example - Fungi, plants, birds, insects, humans and animals. |
 Unicellular organisms
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 Left to right: a mushroom (fungi), plants, bird and insects
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Cell size and shape:
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Cells differ in shape and size.
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The shape and size of cells are related to the specific function they perform.
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Some cells, like Amoeba, have changing shapes.
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In some cases, the cell shape is fixed and peculiar for a particular type of cell (e.g, nerve cells).
Neuron
Egg of the ostrich
Human female ovum
Structure of Sperm
Cells are divided into prokaryotic and eukaryotic based on the presence or absence of a true nucleus.
|
S.No
|
Prokaryotic cell
|
Eukaryotic cell
|
|
1
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Size of a prokaryotic cell ranges from
\(1 µm\) to \(10 µm\).
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Size of a eukaryotic cell ranges from
\(5 µm\) to \(100 µm\).
|
|
2
|
Membrane-bound nucleus absent (not well defined).
|
Membrane-bound nucleus present (True nucleus).
|
|
3
|
It is unicellular.
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It is multicellular.
|
|
4
|
\(70S\) Ribosomes are present.
|
\(80S\) Ribosomes are present.
|
|
5
|
A single chromosome is present.
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More than one chromosome is present.
|
|
6
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Example: Bacteria, Archaea.
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Examples: Plants, Animals, Fungi, Protists.
|
 Unicellular-Bacterial cell
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 Multicllular-Animal cell
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Cells are classified into plant and animal cells based on differences in their structure and cell organelles.
|
S.No
|
Plant cell
|
Animal cell
|
|
1
|
Rectangular or cubic in shape.
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Round or irregular shape.
|
|
2
|
Larger in size.
|
Smaller in size.
|
|
3
|
Cell wall is present.
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Cell wall is absent.
|
|
4
|
Plastids are present.
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Plastids are absent.
|
|
5
|
A large central vacuole is present.
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Small vacuoles are present.
|
|
6
|
Nucleus present at the peripheral side of the cytoplasm.
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Nucleus lies down at the centre of the cell.
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|
7
|
Chloroplast is present.
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Chloroplast is absent.
|
|
8
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Centrosomes and centrioles are absent.
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Centrosomes and centrioles are present.
|
 Plant cell
|
 Animal Cell
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