
The cell (from Latin cella, meaning "small room") is the basic
structural,
functional,
and
biological
unit
of
all
known living organisms. Cells are the smallest unit of life that
can replicate independently, and are often called the "building
blocks of life". The study cells is called biology

Cells consist of a protoplasm enclosed within a membrane, which
contains many bio molecules such as proteins and nucleic
acids. Organisms can be classified as unicellular (consisting of a
single
cell;
including
most
bacteria)
or
multicellular (including plants and animals). While the number of
cells in plants and animals varies from species to species, humans
contain about 100 trillion(1014) cells. Most plant and animal cells
are visible only under the microscope, with dimensions between 1
and 100 micrometres.

The cell was discovered by Robert Hooke in 1665, who named the
biological unit for its resemblance to cells inhabited by Christian
monks in a monastery. The cell theory, first developed in 1839
by Matthias Jakob Schneider and Theodor Schwann, states that
all organisms are composed of one or more cells, that all cells
come from preexisting cells, that vital functions of an organism
occur within cells, and that all cells contain the hereditary
information necessary for regulating cell functions and for
transmitting information to the next generation of cells. Cells
emerged on Earth at least 3.5 billion years ago.

There are two types of cells, eukaryotes, which contain a nucleus, and prokaryotes, which do
not. Prokaryotic cells are usually single-celled organisms, while eukaryotic cells can be either
single-celled or part of multicellular organisms.

Prokaryotic cells were the first form of life on Earth, as they have signaling and
self-sustaining processes. They are simpler and smaller than eukaryotic cells, and
lack membrane-bound organelles such as the nucleus. Prokaryotes include two of
the domains of life, bacteriaand archaea. The DNA of a prokaryotic cell consists
of a single chromosome that is in direct contact with the cytoplasm. The nuclear
region in the cytoplasm is called the nucleoid. Most of the prokaryotes are
smallest of all organisms. Most prokaryotes range from 0.5 to 2.0 µm in diameter.

Inside the cell is the cytoplasm region that contains the genome (DNA),
ribosomes and various sorts of inclusions. The prokaryotic chromosome is
usually a circular molecule (an exception is that of the bacterium Borrelia
burgdorferi, which causes Lyme disease).

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 membranebound compartments in which specific metabolic
activities take place. Most important among these is
a cell nucleus, a membrane-delineated compartment
that houses the eukaryotic cell's DNA. This nucleus
gives the eukaryote its name, which means "true
nucleus".

The plasma membrane resembles that of
prokaryotes in function, with minor differences in
the setup. Cell walls may or may not be present.

Eukaryotes can move using motile cilia or flagella.
Eukaryotic flagella are less complex than those of
prokaryotes.

Animal cells are eukarytoic. Animal cells are have
outer boundary known as the plasma membrane. The
nucleus and the organelles of the cell are bound by a
membrane. The genetic material (DNA) in animal
cells is within the nucleus that is bound by a
double membrane. The cell organelles have a
vast range of functions to perform like hormone
and enzyme production to providing energy for the
cells.

All animal cells are multicellular. They are eukaryotic
cells. Animal cells are surrounded by plasma
membrane and it contains the nucleus and organelles
that are membrane bound.

Animal cells are of various sizes and have
irregular shapes. Most of the cells size range between
1 and 100 micrometers and are visible only with help
of microscope. Trillions of cells are found in the
human body. There are many different types of cells,
approximately 210 distinct cell types in adult human
body.

The components of animal cells are centrioles, cilia
and flagella, endoplasmic reticulum, golgi apparatus,
lysosomes, microfilaments, microtubules,
mitochondria, nucleus, peroxisomes, plasma
membrane and ribosomes.

In biology, tissue is a cellular organizational level intermediate
between cells and a complete organ. A tissue is an ensemble of
similar cells from the same origin that together carry out a specific
function. Organs are then formed by the functional grouping
together of multiple tissues.

The study of tissue is known as histology or, in connection with
disease, histopathology. The classical tools for studying tissues are
the paraffin block in which tissue is embedded and then sectioned,
the histological stain, and the optical microscope. In the last couple
of
decades,
developments
in
electron
microscopy, immunofluorescence, and the use of frozen tissue
sections have enhanced the detail that can be observed in tissues.

With these tools, the classical appearances of tissues can be
examined in health and disease, enabling considerable refinement
of medical diagnosis and prognosis.

Grouped in to four basic types: connective, muscle, nervous, and epithelial. Multiple tissue types compose organs and body structures.
While all animals can generally be considered to contain the four tissue types, the manifestation of these tissues can differ depending on
the type of organism.

For example, the origin of the cells comprising a particular tissue type may differ developmentally for different classifications of
animals.

The epithelium in all birds and animals is derived from the ectoderm and endoderm with a small contribution from the mesoderm,
forming the endothelium, a specialized type of epithelium that composes the vasculature. By contrast, a true epithelial tissue is present
only in a single layer of cells held together via occluding junctions called tight junctions, to create a selectively permeable barrier.

This tissue covers all organism surfaces that come in contact with the external environment such as the skin, the airways, and the
digestive tract. It serves functions of protection, secretion, and absorption, and is separated from other tissues below by a basal lamina.

Plant tissues are categorized broadly into three tissue
systems: the epidermis, the ground tissue, and
the vascular tissue.

Epidermis - Cells forming the outer surface of
the leaves and of the young plant body.

Vascular tissue - The primary components of
vascular tissue are the xylem and phloem. These
transport fluid and nutrients internally.

Ground
tissue
Ground
tissue
is
less differentiated than other tissues. Ground
tissue
manufactures
nutrients
by photosynthesis and stores reserve nutrients.

Plant tissues can also be divided differently into
two types:

Meristematic tissues

Permanent tissues.

In biology, an organ (or viscus) is a collection
of tissues joined in a structural unit to serve a
common function. In anatomy, a viscus is an
internal organ, and viscera is the plural form.

Organs
are
composed
of
main
tissue,
parenchyma,
and
"sporadic"
tissues, stroma. The main tissue is that which is
unique for the specific organ, such as the
myocardium, the main tissue of the heart, while
sporadic tissues include the nerves, blood
vessels, and connective tissues. Functionally
related organs often cooperate to form
whole organ systems. Organs exist in all higher
biological organisms, in particular they are not
restricted to animals, but can also be identified
in plants. In single-cell organisms like bacteria,
the functional analogue of an organ is called

A hollow organ is a visceral organ that forms a
hollow
tube
or
pouch,
such
as
the stomach or intestine, or that includes a
cavity, like the heart or urinary bladder.

A biological system is a complex network of biologically
relevant entities. As biological organization spans several scales,
examples of biological systems are populations of organisms, or
on the organ- and tissue scale in mammals and other animals,
the circulatory system, the respiratory system, the nervous
system, etc.

On the micro- to the nanoscopic scale, examples of biological
systems are cells, organelles, macromolecular complexes and
regulatory pathways.

A biological system is not to be confused with a living system,
which is commonly referred to as life.
Group Members:
1. M. Salman Hassan
2. Abdullah Maqsood
3. Mustafa Kashif
4. Umar Siddiqui
Class: 7
Section: N