CHAPTER
1Cell: The Unit of Life
Cell
Cell is the fundamental structural and functional unit of all
living organisms.
Anton Von Leeuwenhoek first saw and described a live cell.
Cell Theory
Cell theory was given by two scientists Schleiden and Schwann.
It states that all plants or animals are composed of cells and
their products.
Rudolf Virchow explained that new cells arise from pre-
existing cells (Omnis cellula-e cellula) and finally modified
the cell theory as:
(i) All living organisms are composed of cells and products
of cells.
(ii) All cells arise from pre-existing cells.
An Overview of Cell
Cells differ greatly in size, shape and activities for example,
Mycoplasma is smallest cell (0.3µm), Egg of an ostrich is the
largest isolated single cell.
Cytoplasm is main arena of cellular activities in both plant
and animal cells.
Prokaryotic Cells
Lack membrane bound nucleus and cell organelles.
Represented by bacteria, blue-green algae, mycoplasma and
PPLO (0.1µm).
In addition to genomic DNA, many bacteria have small
circular DNA outside the genomic DNA called plasmids.
All prokaryotes have a cell wall surrounding the cell membrane
(except mycoplasma).
Most prokaryotic cells have cell envelope which is tightly
bound three layered structure i.e., the outermost glycocalyx
followed by the cell wall and then the plasma membrane.
Mesosomes are extensions of plasma membrane into the cell.
It helps in cell wall formation, DNA replication, distribution
of daughter cells, respiration, secretion process and increase
the surface area of plasma membrane.
In cyanobacteria, chromatophores contain pigments.
Prokaryotic cells have surface structures - Flagella, pili and
fimbriae.
Bacteria may be motile or non-motile. If motile they have
flagella, composed of three parts: Filament (longest portion),
hook and basal body.
Bacteria, on the basis of the staining, can be Gram positive
or Gram negative.
Ribosomes are non-membrane bound organelles.
Ribosomes are 70S, has subunits 50S and 30S. Several
ribosomes may attach to a single mRNA and form a chain
called polyribosome or polysome.
Reserve material is stored in the form of inclusion bodies in
prokaryotic cytoplasm. Eg. phosphate granules, cyanophycean
granules and glycogen granules.
Gas vacuoles are found in blue-green, purple and green
photosynthetic bacteria.
Eukaryotic Cells
The eukaryotes include all the protists, plants, animals and
fungi.
Besides the nucleus, eukaryotic cells have other membrane
bound structure called organelles like ER, Golgi complex
etc.
Cell membrane is mainly composed of proteins and lipids
(mainly phospholipids).
Membrane proteins can be integral or peripheral.
Most accepted model for structure of cell membrane is fluid
mosaic model given by Singer and Nicolson (1972).
Membrane is selectively permeable.
The quasi-fluid nature of lipid enables lateral movement
of proteins within the overall bilayer. This ability to move
within the membrane is measured as its fluidity.
The fluid nature of membrane is important for cell growth,
formation of intercellular junctions, secretion, endocytosis,
cell division etc.
Cell wall is a non-living rigid structure forms an outer
covering of the plasma membrane in fungi and plants.
Primary wall: Cell wall of a young plant cell is capable of
growth which gradually diminishes as the cell matures.
Secondary wall: As cell matures, it is formed on inner side
(towards membrane) of the cell.
Middle lamella: It is a layer mainly composed of calcium
pectate that holds the different neighbouring cells together.
