Class 9, Science, Chapter-5, Lecture-1, Cell Theory (Notes)
Cell:
The structural and functional unit of biological activity surrounded by a selectively permeable membrane, and which is capable of self-reproduction is called a cell.
Cell Biology:
The branch of science that studies the structure and functions of cells is called the cell biology.
Cell Theory:
The Cell Theory proposed by M. J. Schleiden & T. Schwan in 1839 and modified by Rudolf Virchow in 1855 states that
- All the plants and animals are composed of cells
- The cell is the basic unit of life
- all cells arise from pre-existing cells.
Microscope:
A high resolution optical device used for observing fine details of very small objects is called a microscope.
- Compound Microscope (Light Microscope):
A high resolution optical device used for observing fine details of very small objects using glass lens for magnification and light for illumination is called a compound microscope or a light microscope.
Magnification - 100 X to 1500 X - Electron Microscope:
A high resolution optical device used for observing fine details of very small objects using electromagnets for magnification and electron for illumination is called an electron microscope.
Magnification - 100000 X to 500000 X
Differences Between
Unicellular Organisms |
Multi-cellular Organisms |
|
---|---|---|
Definition |
The organisms which consist of a single cell are called unicellular organisms. |
The organisms which consist of more than one cell are called multi-cellular organisms. |
Example |
Amoeba, Chlamydomonas, paramecium, Bacteria, Acetabularia (marine alga) |
Fungi, Plants, Animals |
Reproduction |
The parent cell loses identity after reproduction |
The parents maintain their identity after reproduction. |
Life-span |
Short |
Long |
Activities |
All activities are performed by a single cell |
Specific activities are performed by specific cells. |
Shapes of Cells:
Shape |
Example |
---|---|
Polygonal |
Liver cells |
Spherical |
Ovum (Egg) |
Elliptical |
Fat cells |
Spindle Shaped |
Smooth Muscle Cells |
Cuboidal |
Germ cells |
Stellate |
Bone cells |
Knobbed thread |
Sperm |
Discoidal (Biconcave disc) |
Human RBC |
Elongated & Branched |
Nerve cells (Neurons) |
Branched |
Pigment cells of skin |
Factors determining the shape of a cell:
- Function performed by the cell.
- Surface tension
- Viscosity of the protoplasm
- Mechanical action between adjoining cells
- Rigidity of the cell covering.
Size of Cells:
Example |
Size |
|
---|---|---|
Mycoplasma/PPLO |
0.1 µm to 0.5 µm |
Smallest cell |
Human RBC |
7 µm (diameter) |
|
Human Liver cells |
20 µm to 30 µm |
|
Neuron |
90 cm to 100 cm |
Longest animal cell |
Ostrich egg |
18 cm (diameter) |
Largest cell |
Acetabularia |
10 cm |
Largest unicellular organism |
Amoeba proteus |
60 µm |
Smaller units of length:
micron (micro metre) |
µm |
10–6 m |
nano metre |
Nm |
10–9 m |
Angstrom |
Å |
10–10 m |
Important Historical Years:
Year |
Event |
---|---|
1665 |
Robert Hooke discovered cell. |
1674 |
A. Van Leeuwenhoek designed Compound Microscope and observed the first LIVING cell. |
1831 |
Robert Brown discovered nucleus in a cell. |
1839 |
· J. E. Purkinjee coined the term ‘PROTOPLASM’ · M. J. Schleiden & T. Schwan proposed the ‘Cell Theory’ |
1855 |
Rudolf Virchow modified the Cell Theory (“omnis cellula-e-cellula”) |
1932 |
Knoll & Ruska designed Electron Microscope. |
PPLO = Pleuro Pneumonia Like Organisms