CELL-THE UNIT OF LIFE
• Robert Hooke: He first discovered cells which were dead cells from the
bark of oak tree. He observed compartments in beehive / honeycomb like
structure and called them 'cella'.
• Leeuvenhoek: He discovered first living cells from mud water and
observed bacteria and protozoans.
• Robert Brown: He discovered nucleus.
• Purkinje: He give the term protoplasm.
• Huxley: He called protoplasm as physical basis of life.
CELL THEORY
Cell theory is given by Schleiden ( botanist) and Schwann ( zoologist).
Postulates of cell theory are:
1. All living organisms are made up of cells. So cell is the structural and
functional unit of life.
2. All cells contains a cell membrane, nucleus and cell organelles. Cell wall
may or may not be present.
3. An activity of an organism is because of interaction between different cells.
The theory failed to explain that how the new cell arises. So Rudolf Virchow give
the statement ' omnis cellula-e-cellula', that is the cells arises from pre-existing
cells. So he modified the cell theory. Schwann gives hypothesis that all plants and
animals are made up of cells and their products hence cell is the structural and
functional unit of life.
Exceptions to cell theory:
Exceptions to cell theory are prokaryotes, viruses, unicellular eukaryotes.
PROKARYOTIC CELL
, • Bacteria are very simple in structure and complex in behaviour.
• Bacteria is a single envelope system consisting of three layers:
1. Glycocalyx
2. Cell wall
3. Cell membrane
Glycocalyx
• Glycocalyx is the outermost layer of the prokaryotic cells. It can either be loosely
arranged which is called slime layer or compactly arranged which is called
capsule. • Loosely arranged slime layer is made up of three types of
polysaccharides:
1) Dextrin
2) Dextran
3) Lavan
• Compactly arranged capsule is made up of polysaccharides and D-Glutamic acid
(amino acids).
• Presence of capsule protect the bacteria from host immune system and so such
bacteria are pathogenic.
Cell wall
Cell wall of bacteria are made up of peptidoglycan or murein or mucopeptide
which is furthur made up of amino acids and sugars. Amino acids are L-lysine, D-
Alanine, L-Alanine, D-Glutamic acid. Sugars present are NAG- N Acetyl
Glucosamine and NAM- N Acetyl Muramic acid.
On the basis of cell wall, bacteria are of two types: gram positive bacteria and
gram negative bacteria. Christian Gram differentiated them on the basis of gram
staining in which bacteria are stained with safranin stain which is counter stain.
One kind of bacteria appeared purple in colour these are gram positive and the
other kind of bacteria appear pink in colour these are gram negative bacteria.
➢ Gram positive bacteria: Cell wall of gram positive bacteria are made
up of peptidoglycan. Peptidoglycan is a thick layer. Porins are absent. They
appear purple after gram staining. These bacteria contain all the four amino
2
• Robert Hooke: He first discovered cells which were dead cells from the
bark of oak tree. He observed compartments in beehive / honeycomb like
structure and called them 'cella'.
• Leeuvenhoek: He discovered first living cells from mud water and
observed bacteria and protozoans.
• Robert Brown: He discovered nucleus.
• Purkinje: He give the term protoplasm.
• Huxley: He called protoplasm as physical basis of life.
CELL THEORY
Cell theory is given by Schleiden ( botanist) and Schwann ( zoologist).
Postulates of cell theory are:
1. All living organisms are made up of cells. So cell is the structural and
functional unit of life.
2. All cells contains a cell membrane, nucleus and cell organelles. Cell wall
may or may not be present.
3. An activity of an organism is because of interaction between different cells.
The theory failed to explain that how the new cell arises. So Rudolf Virchow give
the statement ' omnis cellula-e-cellula', that is the cells arises from pre-existing
cells. So he modified the cell theory. Schwann gives hypothesis that all plants and
animals are made up of cells and their products hence cell is the structural and
functional unit of life.
Exceptions to cell theory:
Exceptions to cell theory are prokaryotes, viruses, unicellular eukaryotes.
PROKARYOTIC CELL
, • Bacteria are very simple in structure and complex in behaviour.
• Bacteria is a single envelope system consisting of three layers:
1. Glycocalyx
2. Cell wall
3. Cell membrane
Glycocalyx
• Glycocalyx is the outermost layer of the prokaryotic cells. It can either be loosely
arranged which is called slime layer or compactly arranged which is called
capsule. • Loosely arranged slime layer is made up of three types of
polysaccharides:
1) Dextrin
2) Dextran
3) Lavan
• Compactly arranged capsule is made up of polysaccharides and D-Glutamic acid
(amino acids).
• Presence of capsule protect the bacteria from host immune system and so such
bacteria are pathogenic.
Cell wall
Cell wall of bacteria are made up of peptidoglycan or murein or mucopeptide
which is furthur made up of amino acids and sugars. Amino acids are L-lysine, D-
Alanine, L-Alanine, D-Glutamic acid. Sugars present are NAG- N Acetyl
Glucosamine and NAM- N Acetyl Muramic acid.
On the basis of cell wall, bacteria are of two types: gram positive bacteria and
gram negative bacteria. Christian Gram differentiated them on the basis of gram
staining in which bacteria are stained with safranin stain which is counter stain.
One kind of bacteria appeared purple in colour these are gram positive and the
other kind of bacteria appear pink in colour these are gram negative bacteria.
➢ Gram positive bacteria: Cell wall of gram positive bacteria are made
up of peptidoglycan. Peptidoglycan is a thick layer. Porins are absent. They
appear purple after gram staining. These bacteria contain all the four amino
2
