INTRODUCTION
The cell is the basic structural, functional, and
biological unit of all known organisms. A cell is the smallest unit
of life. Cells are often called the "building blocks of life".
The study of cells is called cell biology, cellular biology, or cytology.
General Characteristics of Cell
Each cell
in the body:
1. Needs
nutrition and oxygen.
2.
Produces its own energy necessary for its growth,repair and other activities.
3.
Eliminates carbon dioxide and other metabolic wastes.
4.
Maintains the medium, i.e. the environment for its survival.
5. Shows
immediate response to the entry of invaders like bacteria or toxic substances
into the body
6.
Reproduces by division. There are some exceptionslike neuron, which do not
reproduce.
TISSUE
Tissue is
defined as the group of cells having similar function. There are many types of
tissues in the body. All the tissues are classified into four major types which
are called the primary tissues. The primary tissues include:
1. Muscle
tissue (skeletal muscle, smooth muscle and cardiac
muscle)
2.
Nervous tissue (neurons and supporting cells)
3.
Epithelial tissue (squamous, columnar and cuboidal
epithelial cells)
4.
Connective tissue (connective tissue proper,
cartilage, bone and blood).
ORGAN
Organ, a group of tissues in a living
organism that have been adapted to perform a specific function. In higher
animals, organs are grouped into organ systems; e.g., the oesophagus, stomach,
and liver are organs of the digestive system.
SYSTEM
The organ
system is defined as group of organs that work together to carry out specific
functions of the body.Each system performs a specific function.
1. Digestive system: is concerned
with digestion of food particles.
2. Excretory system: eliminates
unwanted substances.
3. Cardiovascular system: is responsible
for transport ofsubstances between the organs.
4. Respiratory system: is concerned
with the supply of oxygen and removal of carbon dioxide.
5. Reproductive system: is involved in
the reproduction of species.
6. Endocrine system: is concerned with
growth of the body and regulation and maintenance of normal life.
7. Musculoskeletal system: is responsible
for stability and movements of the body.
8. Nervous system: controls the
locomotion and other activities including the intellectual functions.
CELL
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Cell types
Cells are
of two types:
1.Prokaryotic
cells
Prokaryotes include bacteria and archaea,
two of the three domains
of life. Prokaryotic cells were the first form of life on Earth, characterized by having vital biological
processes including cell signaling. They are simpler and smaller than
eukaryotic cells, and lack a nucleus, and
other membrane-bound organelles. The DNA of a prokaryotic cell
consists of a single circular chromosome that is in direct contact
with the cytoplasm. The nuclear region in the cytoplasm is called
the nucleoid. Most prokaryotes are the smallest of all organisms ranging
from 0.5 to 2.0 µm in diameter.
A prokaryotic
cell has three regions:
1.
Enclosing the cell is the cell envelope – generally
consisting of a plasma membrane covered by a cell
wall which, for some bacteria, may be further covered by a third layer
called a capsule. Though most prokaryotes have both a cell membrane and a
cell wall, there are exceptions such as Mycoplasma (bacteria)
and Thermoplasma (archaea)
which only possess the cell membrane layer. The envelope gives rigidity to the
cell and separates the interior of the cell from its environment, serving as a
protective filter. The cell wall consists of peptidoglycan in
bacteria, and acts as an additional barrier against exterior forces. It also
prevents the cell from expanding and bursting (cytolysis) from osmotic pressure due
to a hypotonic environment. Some eukaryotic cells (plant cells and fungal cells)
also have a cell wall.
2.
Inside the cell is the cytoplasmic region that
contains the genome (DNA),
ribosomes and various sorts of inclusions.[4] The genetic material
is freely found in the cytoplasm. Prokaryotes can carry extrachromosomal DNA elements
called plasmids, which
are usually circular. Linear bacterial plasmids have been identified in several
species of spirochete bacteria,
including members of the genus Borrelia notably Borrelia burgdorferi, which
causes Lyme disease.[14] Though
not forming a nucleus, the DNA is condensed in a nucleoid.
Plasmids encode additional genes, such as antibiotic resistance genes.
3.
On the outside, flagella and pili project from
the cell's surface. These are structures (not present in all prokaryotes) made
of proteins that facilitate movement and communication between cells.
Eukaryotic
cells
Plants, animals, fungi, slime
moulds, protozoa, and algae are all eukaryotic. These cells
are about fifteen times wider than a typical prokaryote and can be as much as a
thousand times greater in volume. The main distinguishing feature of eukaryotes
as compared to prokaryotes is compartmentalization: the presence of
membrane-bound organelles (compartments) in which specific activities
take place. Most important among these is a cell nucleus, an organelle
that houses the cell's DNA. This
nucleus gives the eukaryote its name, which means "true kernel
(nucleus)".:
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Comparison of
features of prokaryotic and eukaryotic cells
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Prokaryotes
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Eukaryotes
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Typical organisms
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bacteria, archaea
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protists, fungi, plants, animals
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Typical size
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~
10–100 µm
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Type of nucleus
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nucleoid
region; no true nucleus
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true
nucleus with double membrane
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DNA
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circular (usually)
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linear
molecules (chromosomes) with histone proteins
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coupled
in the cytoplasm
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RNA
synthesis in the nucleus
protein synthesis in the cytoplasm |
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50S and 30S
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60S and 40S
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Cytoplasmic
structure
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very
few structures
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highly
structured by endomembrane and a cytoskeleton
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Flagella
and cilia containing microtubules;
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none
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one to
several thousand
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none
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Organization
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usually
single cells
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single
cells, colonies, higher multicellular organisms with specialized cells
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Cell division
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binary
fission (simple division)
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Chromosomes
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single
chromosome
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more
than one chromosome
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Membranes
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cell
membrane
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Cell
membrane and membrane-bound organelles
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STRUCTURE OF THE CELL
Each cell
is formed by three part-:
1. Cell
membrane
2.
Cytoplasm
3.
Nucleus.
1.CELL
MEMBRANE
The cell membrane, or plasma membrane, is a biological membrane that
surrounds the cytoplasm of a cell. In animals, the plasma membrane is the outer
boundary of the cell, while in plants and prokaryotes it is usually covered by
a cell
wall. This membrane serves to separate and protect a cell from its
surrounding environment .Cell membrane is a protective sheath, enveloping the cell
body. It is also known as plasma membrane or plasmalemma. Thickness of the cell membrane
varies from 75 to 111Å .
COMPOSITION
OF CELL MEMBRANE
Cell
membrane is composed of three types of substances:
1.
Proteins (55%)
2. Lipids
(40%)
3.
Carbohydrates (5%).
STRUCTURE
OF CELL MEMBRANE
On the basis of structure, cell membrane is called a unit membrane or a three-layered membrane.
The three layers of cell membrane, namely, one central electron-lucent
layer and two electron- dense
layers. The two electron-dense layers are placed one on either side of the
central layer. The central layer is a lipid layer formed by lipid substances. The other two layers are protein
layers formed by proteins.Cell membrane contains some carbohydrate molecules also.
Structural Model of the Cell Membrane
1. Danielli-Davson
model:-This model was basically a ‘sandwich of
lipids’ covered by proteins on
both
sides.
2. Unit
membrane model:- Unit membrane model’ on the
basis of electron microscopic studies.
3. Fluid
mosaic model:- According to them, the membrane is a fluid
with mosaic of proteins.
This
model is accepted by the scientists till now. In this model, the proteins are
found to float in the lipid layer instead of forming the layers of the
sandwich-type model.
Lipid Layers of the Cell Membrane
The
central lipid layer is a bilayered structure. This is formed by a thin film of lipids.
The characteristic feature of lipid layer is that, it is fluid in nature and
not a solid structure. So, the portions of the membrane move from one point to
another point along the surface of the cell. The materials dissolved in lipid
layer also move to all areas of the cell membrane.
Major
lipids are:
1.
Phospholipids
2.
Cholesterol.
1. Phospholipids:- Phospholipids
are the lipid substances containing phosphorus and fatty acids.
Phospholipid molecules are arranged in two layers.Each phospholipid
molecule resembles the headed pin in shape. The outer part of the phospholipid molecule
is called the head portion and the inner portion
is called the tail portion. Head portion is the polar end and it is soluble in water and has
strong affinity for water (hydrophilic). Tail portion is the non-polar end. It
is insoluble in water and repelled by water (hydrophobic).
Two layers of phospholipids
are arranged in such a way that the hydrophobic tail portions meet in the
center of the membrane. Hydrophilic head portions of outerlayer face the ECF
and those of the inner layer face ICF (cytoplasm).
2. Cholesterol:-Cholesterol
molecules are arranged in between the phospholipid molecules.
Phospholipids are soft and oily
structures and cholesterol helps to ‘pack’ the phospholipids in the membrane.
So, cholesterol is responsible for the structural integrity of lipid layer of
the cell membrane.
Functions of Lipid Layer in Cell Membrane
Lipid layer of the cell membrane is a semipermeable membrane and
allows only the fat-soluble substances to pass through it. Thus, the
fat-soluble substances like oxygen, carbon dioxide and alcohol can pass through
this lipid layer. The water-soluble substances such as glucose, urea and
electrolytes cannot pass through this
layer.
Protein Layers of the Cell Membrane
Protein
layers of the cell membrane are electron-dense layers. These layers cover the
two surfaces of the
central
lipid layer. Protein layers give protection to the central lipid layer. The
protein substances present in these layers are mostly glycoproteins.
Protein
molecules are classified into two categories:
1.
Integral proteins or transmembrane proteins.
2.
Peripheral proteins or peripheral membrane proteins.
1.
Integral proteins or transmembrane proteins:-.The
proteins that pass through entire thickness of cell membrane from one side to
the other side. These proteins are tightly bound with the cell membrane.
Examples
of integral protein:
i. Cell
adhesion proteins
ii. Cell junction proteins
iii. Some
carrier (transport) proteins
iv.
Channel proteins
v. Some
hormone receptors
vi.
Antigens
vii. Some
enzymes.
2. Peripheral proteins or peripheral membrane proteins:-Peripheral proteins or peripheral membrane proteins are the
proteins which are partially embedded in the outer and inner surfaces of the
cell membrane and do not penetrate the cell membrane. Peripheral proteins are
loosely bound with integral proteins or lipid layer of cell membrane. So, these
protein molecules dissociate readily from the cell membrane.
Examples
of peripheral proteins:
i.
Proteins of cytoskeleton
ii. Some
carrier (transport) proteins
iii. Some
enzymes.
Functions of Proteins in Cell Membrane
1. Integral proteins provide the structural integrity of the cell
membrane.
2. Channel proteins help in the diffusion of watersoluble substances
like glucose and electrolytes.
3. Carrier or transport proteins help in the transport of substances
across the cell membrane by means of
active or passive transport.
4. Pump: Some carrier proteins act as pumps, by which ions are
transported actively across the cell
membrane
5. Receptor proteins serve as the receptor sites for hormones and
neurotransmitters.
6. Enzymes: Some of the protein molecules form the enzymes and
control chemical (metabolic) reactions
within the cell membrane
7. Antigens: Some proteins act as antigens and inducethe process
of antibody formation
8. Cell adhesion molecules or the integral proteins are responsible
for attachment of cells to their neighborsor to basal lamina.
Carbohydrates of the Cell Membrane
Some of the carbohydrate molecules present in cell membrane are
attached to proteins and form
glycoproteins (proteoglycans). Some
carbohydrate molecules are attached to lipids and form glycolipids.
Carbohydrate molecules form a thin and loose covering over the
entire surface of the cell membrane
called glycocalyx.
Functions of Carbohydrates in Cell Membrance
1.
Carbohydrate molecules are negatively charged and do not permit the negatively
charged substances to move in and out of the cell
2.
Glycocalyx from the neighboring cells helps in the tight fixation of cells with
one another.
3. Some
carbohydrate molecules function as the receptors for some hormones.
FUNCTIONS
OF CELL MEMBRANE
1. Protective
function: Cell membrane protects the cytoplasm and the organelles present
in the cytoplasm
2. Selective
permeability: Cell membrane acts as a semipermeable membrane, which allows
only some
substances
to pass through it and acts as a barrierfor other substances
3. Absorptive
function: Nutrients are absorbed into thecell through the cell membrane
4. Excretory
function: Metabolites and other waste products from the cell are excreted
out through the
cell
membrane
5. Exchange
of gases: Oxygen enters the cell from the blood and carbon dioxide leaves
the cell and enters the blood through the cell membrane
6. Maintenance
of shape and size of the cell: Cell membrane is responsible for the
maintenance of shape
and size
of the cell.
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