$\displaystyle \small \bullet$ A cell is a structural and functional unit in living organisms.
$\displaystyle \small \bullet$ Robert Hooke (1665) was the first person to observe cells. He studied a thin slice of bottle cork under self-made microscope and observed honeycomb like structure and called them as cells.
$\displaystyle \small \bullet$ Anton Von Leeuwenhoek first saw and described a live cell.
$\displaystyle \small \bullet$ Robert Brown later discovered the nucleus.
$\displaystyle \small \bullet$ The invention of the microscope and its improvement leading to the electron microscope revealed all the structural details of the cell.
$\displaystyle \small \bullet$ All organisms are composed of cells. Some are composed of a single cell and are called unicellular organisms while others, composed of many cells are called multicellular organisms.
$\displaystyle \small \bullet$ Study of structure, composition and form of cells is called cytology.
$\displaystyle \small \bullet$ The shape and size of cells varies greatly according to function and their position.
$\displaystyle \small \bullet$ Ostrich egg is the largest and mycoplasma is the smallest isolated cell.
CELL THEORY
$\displaystyle \small \bullet$ Matthias Schleiden, a German botanist and Theodore Schwann, a British zoologist, together formulated the cell theory.
$\displaystyle \small \bullet$ Cell theory failed to explain how and from where the new cells were formed.
$\displaystyle \small \bullet$ In 1855 Rudolf Virchow modified the hypothesis of Schleiden and Schwann to give the modern cell theory which states that
$\displaystyle \small \circ$ All living organisms are composed of cells and products of cells.
$\displaystyle \small \circ$ All cells arise from pre-existing cells.
$\displaystyle \small \circ$ All cells are basically alike in chemical composition and metabolic activities.
$\displaystyle \small \circ$ The function of an organism as a whole is the outcome of activities and interactions of the constituent cells.
On the basis of organization, complexity and variety, all cells can be grouped into two types i.e. prokaryotic cells and eukaryotic cells
PROKARYOTIC CELLS
$\displaystyle \small \bullet$ The prokaryotic cells do not have a true nucleus.
$\displaystyle \small \bullet$ Their genetic material is not enclosed by a membrane and lies free in the cytoplasm.
$\displaystyle \small \bullet$ The genetic material is in the form of a single circular chromosome present in the cytoplasm in a nuclear region called nucleoid.
$\displaystyle \small \bullet$ They lack all membrane bounded organelles and it indicates they are primitive.
$\displaystyle \small \bullet$ They seem to have appeared on earth about 3500 million years ago.
They are represented by bacteria, cyanobacteria (blue-green algae), mycoplasm or PPLO (pleuropneumonia-like organisms), spirochaete and rickettsia.
$\displaystyle \small \bullet$ They may vary greatly in shape and size. The four basic shapes of bacteria are bacillus (rod like), coccus (spherical), vibrio (comma shaped) and spirillum (spiral).
$\displaystyle \small \bullet$ Bacteria can be seen in all kinds of environment like soil, water, air, hot springs, glaciers etc,.
$\displaystyle \small \bullet$ In addition to the genomic DNA (the single chromosome/circular DNA), many bacteria have small circular DNA outside the genomic DNA. These smaller DNA are called plasmids.
$\displaystyle \small \bullet$ The plasmid DNA confers certain unique phenotypic characters such as antibiotic resistance.
$\displaystyle \small \bullet$ A specialized differentiated form of cell membrane called mesosome is the characteristic of prokaryotes. They are essentially infoldings of cell membrane.
Cell Envelope and its Modifications/ Ultrastructure of a Prokaryotic Cell
Cell Wall
$\displaystyle \small \bullet$ Most prokaryotic cells, particularly the bacterial cells have a chemically complex cell envelope.
$\displaystyle \small \bullet$ The cell wall determines the shape of the cell and provides a strong structural support to prevent the bacterium from bursting or collapsing.
$\displaystyle \small \bullet$ Bacteria can be classified into two groups on the basis of the differences in the cell envelopes and the manner in which they respond to the staining procedure developed by Gram viz.,
$\displaystyle \small \bullet$ Those that take up the gram stain are Gram positive and the others that do not are called Gram negative bacteria.
Capsule and Slime Layer
$\displaystyle \small \bullet$ Glycocalyx differs in composition and thickness among different bacteria. It could be a loose sheath called the slime layer in some, while in others it may be thick and tough, called the capsule.
Plasma Membrane
$\displaystyle \small \bullet$ In a plasma membrane each layer of the envelope performs distinct function; they act together as a single protective unit.
$\displaystyle \small \bullet$ It is semi-permeable in nature and interacts with the outside world.
$\displaystyle \small \bullet$ A special membranous structure is the mesosome which is formed by the extensions of plasma membrane into the cell.
$\displaystyle \small \bullet$ These extensions are in the form of vesicles, tubules and lamellae. They help in cell wall formation, DNA replication and distribution to daughter cells.
$\displaystyle \small \bullet$ They also help in respiration, secretion processes, to increase the surface area of the plasma membrane and enzymatic content.
$\displaystyle \small \bullet$ In some prokaryotes like cyanobacteria, there are other membranous extensions into the cytoplasm called chromatophores which contain pigments.
Cytoplasm
$\displaystyle \small \bullet$ The cytoplasm appears granular due to presence of ribosomes and inclusion bodies.
Flagella
$\displaystyle \small \bullet$ Bacterial cells may be motile or non-motile. If motile, they have thin filamentous extensions from their cell wall called flagella.
$\displaystyle \small \bullet$ Bacterial flagellum is composed of three parts – filament, hook and basal body.
Pili and Fimbriae
$\displaystyle \small \bullet$ Pili and Fimbriae are also surface structures of the bacteria but do not play a role in motility.
$\displaystyle \small \bullet$ The pili are elongated tubular structures made of a special protein. The fimbriae are small bristle like fibres sprouting out of the cell.
$\displaystyle \small \bullet$ These help in the attachment of bacteria to other cells or surfaces.
Ribosomes
$\displaystyle \small \bullet$ In prokaryotes ribosomes are associated with the plasma membrane of the cell.
$\displaystyle \small \bullet$ They are about 15 nm ✕ 20 nm in size .
$\displaystyle \small \bullet$ A complete prokaryotic ribosome is of 70S (S-Svedberg unit). It is made up of two subunits 50S and 30S.
$\displaystyle \small \bullet$ Ribosomes are the site of protein synthesis. Several ribosomes may attach to a single mRNA and form a chain called polyribosomes or polysome.
$\displaystyle \small \bullet$ The ribosomes of a polysome translate the mRNA into proteins.
Inclusion Bodies
$\displaystyle \small \bullet$ Reserve material in prokaryotic cells are stored in the cytoplasm in the form of inclusion bodies.
$\displaystyle \small \bullet$ These are not bounded by any membrane system and lie free in the cytoplasm, e.g., phosphate granules, cyanophycean granules and glycogen granules.
$\displaystyle \small \bullet$ Gas vacuoles are found in blue green, purple and green photosynthetic bacteria.
$\displaystyle \small \bullet$ Robert Hooke (1665) was the first person to observe cells. He studied a thin slice of bottle cork under self-made microscope and observed honeycomb like structure and called them as cells.
$\displaystyle \small \bullet$ Anton Von Leeuwenhoek first saw and described a live cell.
$\displaystyle \small \bullet$ Robert Brown later discovered the nucleus.
$\displaystyle \small \bullet$ The invention of the microscope and its improvement leading to the electron microscope revealed all the structural details of the cell.
$\displaystyle \small \bullet$ All organisms are composed of cells. Some are composed of a single cell and are called unicellular organisms while others, composed of many cells are called multicellular organisms.
$\displaystyle \small \bullet$ Study of structure, composition and form of cells is called cytology.
$\displaystyle \small \bullet$ The shape and size of cells varies greatly according to function and their position.
$\displaystyle \small \bullet$ Ostrich egg is the largest and mycoplasma is the smallest isolated cell.
CELL THEORY
$\displaystyle \small \bullet$ Matthias Schleiden, a German botanist and Theodore Schwann, a British zoologist, together formulated the cell theory.
$\displaystyle \small \bullet$ Cell theory failed to explain how and from where the new cells were formed.
$\displaystyle \small \bullet$ In 1855 Rudolf Virchow modified the hypothesis of Schleiden and Schwann to give the modern cell theory which states that
$\displaystyle \small \circ$ All living organisms are composed of cells and products of cells.
$\displaystyle \small \circ$ All cells arise from pre-existing cells.
$\displaystyle \small \circ$ All cells are basically alike in chemical composition and metabolic activities.
$\displaystyle \small \circ$ The function of an organism as a whole is the outcome of activities and interactions of the constituent cells.
On the basis of organization, complexity and variety, all cells can be grouped into two types i.e. prokaryotic cells and eukaryotic cells
PROKARYOTIC CELLS
$\displaystyle \small \bullet$ The prokaryotic cells do not have a true nucleus.
$\displaystyle \small \bullet$ Their genetic material is not enclosed by a membrane and lies free in the cytoplasm.
$\displaystyle \small \bullet$ The genetic material is in the form of a single circular chromosome present in the cytoplasm in a nuclear region called nucleoid.
$\displaystyle \small \bullet$ They lack all membrane bounded organelles and it indicates they are primitive.
$\displaystyle \small \bullet$ They seem to have appeared on earth about 3500 million years ago.
They are represented by bacteria, cyanobacteria (blue-green algae), mycoplasm or PPLO (pleuropneumonia-like organisms), spirochaete and rickettsia.
$\displaystyle \small \bullet$ They may vary greatly in shape and size. The four basic shapes of bacteria are bacillus (rod like), coccus (spherical), vibrio (comma shaped) and spirillum (spiral).
$\displaystyle \small \bullet$ Bacteria can be seen in all kinds of environment like soil, water, air, hot springs, glaciers etc,.
$\displaystyle \small \bullet$ In addition to the genomic DNA (the single chromosome/circular DNA), many bacteria have small circular DNA outside the genomic DNA. These smaller DNA are called plasmids.
$\displaystyle \small \bullet$ The plasmid DNA confers certain unique phenotypic characters such as antibiotic resistance.
$\displaystyle \small \bullet$ A specialized differentiated form of cell membrane called mesosome is the characteristic of prokaryotes. They are essentially infoldings of cell membrane.
Cell Envelope and its Modifications/ Ultrastructure of a Prokaryotic Cell
$\displaystyle \small \bullet$ Most prokaryotic cells, particularly the bacterial cells have a chemically complex cell envelope.
$\displaystyle \small \bullet$ The cell wall determines the shape of the cell and provides a strong structural support to prevent the bacterium from bursting or collapsing.
$\displaystyle \small \bullet$ Bacteria can be classified into two groups on the basis of the differences in the cell envelopes and the manner in which they respond to the staining procedure developed by Gram viz.,
$\displaystyle \small \bullet$ Those that take up the gram stain are Gram positive and the others that do not are called Gram negative bacteria.
Capsule and Slime Layer
$\displaystyle \small \bullet$ Glycocalyx differs in composition and thickness among different bacteria. It could be a loose sheath called the slime layer in some, while in others it may be thick and tough, called the capsule.
Plasma Membrane
$\displaystyle \small \bullet$ In a plasma membrane each layer of the envelope performs distinct function; they act together as a single protective unit.
$\displaystyle \small \bullet$ It is semi-permeable in nature and interacts with the outside world.
$\displaystyle \small \bullet$ A special membranous structure is the mesosome which is formed by the extensions of plasma membrane into the cell.
$\displaystyle \small \bullet$ These extensions are in the form of vesicles, tubules and lamellae. They help in cell wall formation, DNA replication and distribution to daughter cells.
$\displaystyle \small \bullet$ They also help in respiration, secretion processes, to increase the surface area of the plasma membrane and enzymatic content.
$\displaystyle \small \bullet$ In some prokaryotes like cyanobacteria, there are other membranous extensions into the cytoplasm called chromatophores which contain pigments.
Cytoplasm
$\displaystyle \small \bullet$ The cytoplasm appears granular due to presence of ribosomes and inclusion bodies.
Flagella
$\displaystyle \small \bullet$ Bacterial cells may be motile or non-motile. If motile, they have thin filamentous extensions from their cell wall called flagella.
$\displaystyle \small \bullet$ Bacterial flagellum is composed of three parts – filament, hook and basal body.
Pili and Fimbriae
$\displaystyle \small \bullet$ Pili and Fimbriae are also surface structures of the bacteria but do not play a role in motility.
$\displaystyle \small \bullet$ The pili are elongated tubular structures made of a special protein. The fimbriae are small bristle like fibres sprouting out of the cell.
$\displaystyle \small \bullet$ These help in the attachment of bacteria to other cells or surfaces.
Ribosomes
$\displaystyle \small \bullet$ In prokaryotes ribosomes are associated with the plasma membrane of the cell.
$\displaystyle \small \bullet$ They are about 15 nm ✕ 20 nm in size .
$\displaystyle \small \bullet$ A complete prokaryotic ribosome is of 70S (S-Svedberg unit). It is made up of two subunits 50S and 30S.
$\displaystyle \small \bullet$ Ribosomes are the site of protein synthesis. Several ribosomes may attach to a single mRNA and form a chain called polyribosomes or polysome.
$\displaystyle \small \bullet$ The ribosomes of a polysome translate the mRNA into proteins.
Inclusion Bodies
$\displaystyle \small \bullet$ Reserve material in prokaryotic cells are stored in the cytoplasm in the form of inclusion bodies.
$\displaystyle \small \bullet$ These are not bounded by any membrane system and lie free in the cytoplasm, e.g., phosphate granules, cyanophycean granules and glycogen granules.
$\displaystyle \small \bullet$ Gas vacuoles are found in blue green, purple and green photosynthetic bacteria.
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