Skip to main content

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

  1. All the plants and animals are composed of cells
  2. The cell is the basic unit of life
  3. 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.

  1. 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
  2. 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:

  1. Function performed by the cell.
  2. Surface tension
  3. Viscosity of the protoplasm
  4. Mechanical action between adjoining cells
  5. 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