NCERT.UNIT 8.CELL:- THE UNIT OF LIFE

– A cell is the fundamental, structural, hereditary, and functional unit of all living organisms

Robert Hooke: Discovered cell (dead cell, from cork plant)

Anton Von Leeuwenhoek: First observed and described a live cell.

– The invention of the compound & electron microscopes revealed all the structural details of the cell.

                               CELL THEORY

Matthias Schleiden (1838) observed that all plants are composed of different kinds of cells.

Theodore Schwann (1839) found that cells have a thin outer layer (plasma membrane). He also found that plant cells have cell wall.

-He proposed the hypothesis that animals and plants are composed of cells and products of cells.

Schleiden & Schwann formulated the cell theory.

Rudolf Virchow (1855) first explained that cells divide and new cells are formed from pre-existing cells (Omnis cellula-e cellular).

-He modified the cell theory. – Cell theory states that:

(i) All living organisms are composed of cells and products of cells.

(ii) Cells arise from pre-existing cells.


                                                               AN OVERVIEW OF CELL

– All cells contains

o Cytoplasm: A semi-fluid matrix where cellular activities and chemical reactions occur. This keeps the cell in ‘living state’.

o Ribosomes: Non-membrane bound organelles seen in cytoplasm, chloroplasts, mitochondria & on rough ER.

# Cells differ in size, shape, and activities.

o Smallest cells: Mycoplasmas (0.3 µm in length).

o Largest isolated single cell: Egg of ostrich.

o Longest cells Eg Nerve cell.

o Size of bacteria: 3 to 5 µm.

o Human RBCs are about 7.0 µm in diameter.

– Based on the functions, the shape of cells may be disc-like, polygonal, columnar, cuboid, threadlike, or irregular.

source: ncert biology

Cells are 2 types: Prokaryotic & Eukaryotic cells.

                                                                         PROKARYOTIC CELLS

They have no membrane bound nucleus and organelles.

– They include bacteria, blue-green algae, mycoplasma & PPLO (Pleuro Pneumonia Like Organisms) and are generally smaller and multiply more rapidly than the eukaryotic cells.


– They vary in shape & size. Bacteria have 4 basic shapes :-Bacillus, Coccus, Vibrio, and Spirillum.      

cell-biology-notes-bacteria
SHAPES OF BACTERIA
source:- NCERT BIOLOGY

                           

      # Cell organelles in prokaryotic cells

1.  Cell Envelope – It is a chemically complex protective covering. – It is made of 3 tightly bound layers.

CELL ENVELOPE

o Glycocalyx: Outer layer. Its composition and thickness vary in different bacteria. It may be a slime layer (loose sheath) or capsule (thick & tough)

o Cell wall: Middle layer. Seen in all prokaryotes except mycoplasma. It gives shape to the cell and provides structural support to prevent the bacterium from bursting or collapsing.

o Plasma membrane: Inner layer. It is semi-permeable in nature and interacts with the outside. This is structurally similar to that of the eukaryotes.

– Based on the types of cell envelopes and response to Gram staining (developed by Gram), bacteria are 2 types:

o Gram-positive: They take up and retain the gram stain.

o Gram-negative: They do not retain the gram stain.

GRAM POSITIVE AND NEGATIVE BACTERIA STAIN


2. Mesosomes & Chromatophores (Membranous structures)


– Mesosome is formed by the infolding of plasma membrane. It includes vesicles, tubules & lamellae.

Functions : Mesosomes helps in 

o In cell wall formation.

o In DNA (chromosome) replication.

o In distribution of chromosomes to daughter cells.

o In respiration and secretion processes.

o To increase the surface area of the plasma membrane and enzymatic content.


– Chromatophores are membranous infoldings in some prokaryotes (e.g. cyanobacteria). They contain pigments.


3. Nucleoid – It is formed of non-membranous (naked) circular genomic DNA (single chromosome/ Genetic material) & protein.

– Many bacteria have small circular DNA (plasmid) outside the genomic DNA. It gives some unique phenotypic characters (e.g. resistance to antibiotics) to bacteria.

4. Flagella – These are thin filamentous extensions from the cell wall of motile bacteria.

Their number and arrangement are varied in different bacteria. – The bacterial flagellum has 3 parts

filament, hook, and basal body. The filament is the longest portion and extends from the cell surface to the outside.

5. Pili and Fimbriae – These are surface structures that have no role in motility.

– Pili (sing. Pilus) are elongated tubular structures made of a special protein (pilin).

– Fimbriae are small bristle-like fibers sprouting out of the cell. In some bacteria, they help to attach the bacteria to rocks in streams and to the host tissue

RIBOSOME

6. Ribosomes – They are associated with the plasma membrane of prokaryotes.

– They are about 15 nm by 20 nm in size. – They are made of 2 subunits – 50S & 30S (Svedberg’s unit). – They together form 70S prokaryotic ribosomes. (S= sedimentation coefficient; a measure of density & size)

– Function: Ribosomes are the site of translation (protein synthesis). Several ribosomes may attach to a single mRNA to form a chain called polyribosomes (polysome).

Ribosomes translate the mRNA into proteins.

7. Inclusion Bodies – These are non-membranous, stored reserve material seen freely in the cytoplasm of prokaryotic cells. – E.g. phosphate granules, cyanophycean granules, and glycogen granules, gas vacuoles, etc.

Gas vacuoles are found in blue-green and purple and green photosynthetic bacteria.

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