CELL
All the living things are composed of
cells. A single cell is the smallest unit that has all the characteristics of
life. Cell is defined as the structural and functional unit of the living body.
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 exceptions like 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
An organ is defined as
the structure that is formed by two or more primary types of tissues, which
execute the functions
of the organ. Some organs are composed of all the four types of
primary tissues. The organs are of two types, namely tubular
or hollow
organs and
compact or parenchymal
organs. Some
of the organs in the body are brain, heart, lungs, stomach, intestine,
liver, gallbladder, pancreas, kidneys, endocrine glands, etc.
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. Digestive system is concerned
with digestion of food particles. Excretory system
eliminates unwanted substances. Cardiovascular system is responsible for
transport of substances between the organs. Respiratory system is concerned with the
supply of oxygen and removal of carbon dioxide. Reproductive system is
involved in the reproduction of species. Endocrine system is concerned with growth of the
body and regulation and maintenance of normal life.Musculoskeletal
system is responsible for stability and movements of the body. Nervous system
controls the locomotion and other
activities including the intellectual functions.
STRUCTURE OF THE CELL
Each cell is formed by a cell body and a membrane covering
the cell body called the cell membrane. Cell
body has two parts, namely nucleus and
cytoplasm surrounding the nucleus. Thus, the structure
of the cell is studied under three
headings:
1. Cell membrane
2. Cytoplasm
3. Nucleus.
CELL MEMBRANE
Cell membrane is a protective sheath,
enveloping the cell body. It is also known as plasma membrane or
plasmalemma.
This membrane separates the fluid outside the cell called extracellular fluid
(ECF) and the fluid inside the cell called intracellular fluid (ICF). The cell membrane is a
semipermeable membrane. So, there is free exchange of
certain substances between ECF and ICF. 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 electron microscopic study reveals 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
‘DanielliDavson model’
was the first proposed basic model of membrane structure. It was proposed by
James F Danielli and Hugh
Davson in 1935. And it was accepted by scientists for many years. This model was basically a ‘sandwich of lipids’ covered by proteins
on both
sides.
2. Unit membrane model
In 1957, JD Robertson
replaced ‘DanielliDavson model’ by ‘Unit membrane model’ on the basis of
electron microscopic
studies.
3. Fluid mosaic model
Later in 1972, SJ
Singer and GL Nicholson proposed ‘The fluid mosaic model’. According to them,
the membrane is a fluid with mosaic of proteins (mosaic means pattern formed by arrangement
of different colored pieces of stone, tile, glass or other such materials).
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.
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