– 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.
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 organelles in prokaryotic cells
1. Cell Envelope – It is a chemically complex protective covering. – It is made of 3 tightly bound layers.
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.
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
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.
POLYSOME
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.
To be continue….next page….