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Cell : The Unit of Life
Prokaryotic Cell
What is a Cell?


A fundamental, structural and functional unit of all living organisms



Anton von Leeuwenhoek first described the live cells
...




All cells arise from pre-existing cells
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3 μm)



Largest cell: Ostrich egg



Longest cell: Nerve cell
Prokaryotic Cells



Represented by bacteria, blue-green algae, PPLO and Mycoplasma



Smaller and rapidly multiplying



Vary greatly in shape and size



Characteristic features:



Have cell wall surrounding the cell membrane



Absence of a well-defined nucleus



May have plasmids − small, circular, extra-chromosomal DNAs present in addition to the
genomic DNAs; this confers characteristics like antibiotic resistance to bacteria, and help in
bacterial transformation with foreign DNA
...




Functions: Cell wall formation; DNA replication and distribution; respiration and secretion
processes; increase surface area of plasma membrane and enzymatic content



Prokaryotic Cell may be − Motile or Non-Motile



Motile: Have flagella



Non-motile: Lack flagella



Flagella has three parts: Filament, Hook, Basal body



Pili (tubular structures made of proteins) and fimbriae (bristle-like fibres) are also present
along with flagella, but their function is attachment (to the substratum or the host cell)
...




Suspended freely in the cytoplasm, e
...
, phosphate granules or glycogen granules



Gas vacuoles: found in blue-green, purple and green photosynthetic bacteria

Let us find out some more differences between gram positive and gram negative
bacteria
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stain during gram staining
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They possess the outer membrane
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and single layered
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nm thick
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content
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nature
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Pseudomonas, Salmonella, etc
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

Animal cells possess centrioles, lysosomes, numerous small-sized vacuoles, which are
absent from plant cells
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So, the non-polar, hydrophobic tail is protected from aqueous
environment
...




Membrane proteins can be integral (lying buried in the membrane) or peripheral (lying on
the surface of the membrane)
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

Fluidity is the measure of the ability of the membrane to allow movement within itself
...




Transport across plasma membrane:



Passive transport − does not require energy; neutral solute moves by diffusion, along the
concentration gradient;
Water moves through osmosis; polar molecules that cannot diffuse move through
facilitated diffusion, carried out by carrier proteins



Active transport − requires energy for transport against the concentration gradient
Cell Wall



Present exterior to the cell membrane in fungi and plants



Gives shape and protection to cell



Helps in cell to cell interactions; provides a barrier against undesirable macromolecules



Algae cell wall: Has cellulose, galactans, mannans, minerals (CaCO3)



Plant cell wall: Has cellulose, hemicelluloses, pectins and proteins



In a young plant cell, the primary cell wall is present; it diminishes as the cell matures and
is replaced by the secondary cell wall, towards the inner side of the cell
...




Plasmodesmata traverses the cell wall and the middle lamella, and connects cytoplasm of
the neighbouring cells
...




Endomembrane system includes the endoplasmic reticulum (ER), Golgi complex,
lysosomes and vacuole
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

SER is the major site for lipid synthesis
...




Function − packaging of materials, either to be delivered to the intracellular targets or to be
secreted outside the cell



Materials to be packaged in the form of vesicles from ER fuse with the cis face of the golgi
apparatus and move towards the trans face
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

Lysosomes:



Lysosomes are vesicles formed as a result of the packaging in the Golgi apparatus
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

Vacuole:



Membrane-bound space in cells; containing water, sap, excretory products and other waste
materials



Vacuole membrane − Tonoplast



Tonoplast has the ability to pump ions into the vacuole against the concentration gradient
...




E
...
, Contractile vacuole is found in Amoeba for excretion; food vacuole is found in protists
formed by the engulfment of food particles
Ribosomes



Granular structures, first observed by George Palade under an electron microscope



Composition: RNA and proteins



Not surrounded by any membrane



Prokaryotic cells have ribosomes 70S
...
S stands for
Svedberg's Unit, and is indicative of density and size
...




Importance: Mitochondria are called the ‘power houses of cells’ since they are the site for
aerobic respiration, and form ATP (source of cellular energy)
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

Mitochondria divide by fission
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e
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Small Organelles of Eukaryotic Cell
Cytoskeleton


Filamentous, proteinaceous structures present in cytoplasm



Functions:



Mechanical support



Motility



Maintenance of the shape of a cell
Cilia and Flagella



These are hair-like outgrowths of the plasma membrane
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

Axoneme − core of the internal portions of the cilia and the flagella



Axoneme contains microtubules
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

This arrangement is called 9 + 2 array
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

Both centrioles in a centrosome lie perpendicular to each other
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

A centriole is made up of nine peripheral fibrils of tubulin protein
...




Hub: Central part of the proximal region of the centriole



Radial spokes: Connect the hub with the tubules of the peripheral triplets



Importance of centriole: Forms the basal body of the cilia and the flagella, and the spindle
fibre
Microbodies



Membrane-bound minute vesicles



Present in both plant and animal cells



Contain enzymes
Nucleus and Chromosomes
Nucleus



Every cell has a nucleus, except some such as the RBCs of mammals and the sieve tube cells
in vascular plants
...




Nucleus is bound by a nuclear envelope which consists of two membranes with perinuclear
space (10 − 50 nm) between them



Perinuclear space acts as a barrier for the flow of materials between the inside of the
nucleus and the cytoplasm
...
Nuclear pores are formed at places where the two membranes fuse
...
)



Nucleolus (pl
...




Contains DNA, histone proteins, non-histone proteins, and also RNA



DNA is distributed among 23 pairs (46) of chromosomes



A chromosome has a primary constriction called centromere
...

Classification of Chromosomes



Based upon the position of the centromere, chromosomes are of four types:



Metacentric − centromere located in the middle, forming two equal arms of the
chromosome



Sub-metacentric − centromere located slightly away from the middle, resulting in one arm
being longer than the other



Acrocentric − centromere located close to the end, resulting in one arm being extremely
longer than the other



Telocentric − centromere located at the terminal point



Satellites: Small fragments that appear due to the non-staining secondary constrictions
present at a constant location on the chromosomes
DNA and Its Structure
Deoxyribonucleic acid or DNA is a macromolecule found inside the nucleus
...
A DNA molecule is made
up of repeating units of nucleotides
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These are :






Adenine (A)
Guanine (G)
Cytosine (C)
Thymine (T)
Adenine pairs with Thymine with the help of two hydrogen bonds, while Guanine pairs
with Cytosine with the help of three hydrogen bonds
...
This replication process
takes place during mitosis, in which the helical structure of DNA gets open at one end and
the free strands give rise to new, complementary strands
...
It is located on a chromosome and controls the
development of one or more traits through proteins encoded by it
...
Every
person has two copies of each gene, one inherited from each parent
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