CELL N TISSUES
Tissue is a cellular organizational level intermediate between
cells and a complete
organism.
A tissue is an ensemble of cells, not necessarily identical, but from
the same
origin, that together carry out a specific
function. These are called tissues because of their identical
functioning.
Organs are then formed by the
functional grouping together of multiple tissues.
Animal tissues
Animal tissues can be grouped into four basic types:
connective,
muscle,
nervous, and
epithelial.
Multiple tissue types comprise 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 animals is derived from the
ectoderm
and
endoderm
with a small contribution from the
mesoderm
which forms the
endothelium. 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 organismal
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. Endothelium, which comprises the
vasculature, is a specialized type of
epithelium.
Connective tissue
Connective tissues are fibrous tissues. They are made up of cells
separated by non-living material, which is called
extracellular matrix. Connective tissue
gives shape to organs and holds them in place. Both blood and bone are
examples of connective tissue. As the name implies, connective tissue
serves a "connecting" function. It supports and binds other tissues.
Unlike epithelial tissue, connective tissue typically has cells
scattered throughout an extracellular matrix.
Muscle tissue
Muscle cells form the active contractile tissue of the body known as
muscle tissue. Muscle tissue functions to produce force and cause
motion, either locomotion or movement within internal organs. Muscle
tissue is separated into three distinct categories: visceral or
smooth muscle, which is found in the inner
linings of organs;
skeletal muscle, in which is found
attached to bone providing for gross movement; and
cardiac muscle which is found in the
heart,
allowing it to contract and pump blood throughout an organism.
Nervous tissue
Epithelial tissue
The epithelial tissues are formed by cells that cover organ surfaces
such as the surface of the
skin, the
airways, the
reproductive tract,
and the inner lining of the
digestive tract. The cells comprising an
epithelial layer are linked via semi-permeable,
tight junctions; hence, this tissue
provides a barrier between the external environment and the organ it
covers. In addition to this protective function, epithelial tissue may
also be specialized to function in
secretion
and
absorption.
Epithelial tissue helps to protect organisms from microorganisms,
injury, and fluid loss.
Plant tissues
Plant tissues can also be divided differently into two types:
- Meristematic tissues
- Permanent tissues
Meristematic tissues
Meristematic tissue consists of actively dividing cells, and leads to
increase in length and thickness of the plant. The primary growth of a
plant occurs only in certain, specific regions, such as in the tips of
stems or roots. It is in these regions that meristematic tissue is
present. Cells in these tissues are roughly spherical or polyhedral, to
rectangular in shape, and have thin cell walls. New cells produced by
meristem
are initially those of meristem itself, but as the new cells grow and
mature, their characteristics slowly change and they become
differentiated as components of the region of occurrence of meristimatic
tissues, they are classified as:
-
- a) Apical Meristem - It is present at the growing tips of
stems and roots and increases the length of the stem and root. They
form growing parts at the apices of roots and stems and are responsible
for increase in length,also called primary growth.This meristem
is responsible for the linear growth of an organ.
-
- b) Lateral Meristem - This meristem consist of cells which
mainly divide in one plane and cause the organ to increase in diameter
and growth. Lateral Meristem usually occurs beneath the bark of the
tree in the form of Cork Cambium and in vascular bundles of dicots in
the form of vascular cambium. The activity of this
cambium results in the formation of secondary growth.
-
- c) Intercalary Meristem - This meristem
is located in between permanent tissues. It is usually present at the
base of node, inter node and on leaf base. They are responsible for
growth in length of the plant.This adds growth in the girth of stem.
The cells of meristematic tissues are similar in structure and have
thin and elastic primary cell wall made up of
cellulose.
They are compactly arranged without inter-cellular spaces between them.
Each cell contains a dense
cytoplasm
and a prominent
nucleus. Dense
protoplasm
of meristematic cells contains very few vacuoles. Normally the
meristematic cells are oval,
polygonal or rectangular in shape.
Meristemetic tissue cells have a large nucleus with small or no
vacuoles, they have no inter cellular spaces.
Permanent tissues
The meristematic tissues that take up a specific role lose the
ability to divide. This process of taking up a permanent shape, size and
a function is called
cellular differentiation. Cells of
meristematic tissue differentiate to form different types of permanent
tissue. There are 2 types of permanent tissues:
1. simple permanent tissues
2. complex permanent tissues
Simple permanent
tissues
These tissues are called simple because they are composed of similar
types of cells which have common origin and function. They are further
classified into:
- Parenchyma
- Collenchyma
- Sclerenchyma
- Epidermis
Parenchyma
It consists of relatively unspecialised cells with thin cell walls.
They are live cells. They are usually loosely packed, so that large
spaces between cells(intercellular spaces)are found in this tissue. This
tissue provides support to plants and also stores food.In some
situations , it contains chlorophyll and performs photosynthesis, and
then it is called chlorenchyma. In aquatic plants,large air cavities are
present in parenchyma to give support to them to float on water. Such a
parenchyma type is called aerenchyma.
Collenchyma
Cross section of collenchyma cells
Collenchyma is Greek word
where "Collen" means gum and "enchyma" means infusion. It is a living
tissue of primary body like
Parenchyma.
Cells are thin-walled but possess thickening of
cellulose
and
pectin
substances at the corners where number of cells join together. This
tissue gives a tensile strength to the plant and the cells are compactly
arranged and do not have inter-cellular spaces. It occurs chiefly in
hypodermis of stems and leaves. It is absent in
monocots and in roots.
Collenchymatous tissue acts as a supporting tissue in stems of young
plants. It provides mechanical support, elasticity, and tensile strength
to the plant body. It helps in manufacturing sugar and storing it as
starch. It is present in margin of leaves and resist tearing effect of
the wind.
Sclerenchyma
Sclerenchyma is Greek word
where "Sclrenes" means hard and "enchyma" means infusion. This tissue
consists of thick-walled, dead cells. These cells have hard and
extremely thick secondary walls due to uniform distribution of
lignin.
Lignin
deposition is so thick that the cell walls become strong, rigid and
impermeable to water. Sclerenchymatous cells are closely packed without
inter-cellular spaces between them. Thus, they appear as hexagonal net
in
transverse section.
The cells are cemented with the help of
lamella. The
middle lamella is a wall that lies between adjacent cells.
Sclerenchymatous cells mainly occur in
hypodermis,
pericycle,
secondary xylem and phloem. They also occur in endocorp of almond and
coconut. It is made of
pectin,
lignin,
protein.
The cells of sclerenchymatous cells can be classified as :
- Fibres- Fibres are long, elongated sclerenchymatous cells with
pointed ends.
- Sclerides- Sclerenchymatous cells which are short and possess
extremely thick, lamellated, lignified walls with long singular piths. They are called sclerides.
The main function of Sclerenchymatous tissues is to give support to
the plant.
Epidermis
The entire surface of the plant consists of a single layer of cells
called epidermis or surface tissue. The entire surface of the plant has
this outer layer of epidermis. Hence it is also called surface tissue.
Most of the epidermal cells are relatively flat. the outer and lateral
walls of the cell are often thicker than the inner walls. The cells
forms a continuous sheet without inter cellular spaces. It protects all
parts of the plant.
Complex permanent
tissue
A complex permanent tissue may be classified as a group of more than
one type of tissue having a common origin and working together as a unit
to perform a function. These tissues are concerned with
transportation of water,
mineral, nutrients and organic substances. The important complex tissues
in vascular plants are
xylem,
phloem.
Xylem
Xylem
is a chief, conducting tissue of
vascular plants. It is responsible for
conduction of water and mineral ions.
Xylem is a very important plant tissue as it is part of the
‘plumbing’ of a plant. Think of bundles of pipes running along the main
axis of stems and roots. It carries water and dissolved substances
throughout and consists of a combination of parenchyma cells, fibers,
vessels, tracheids and ray cells. Long tubes made up of individual cells
are the vessels, while vessel members are open at each end. Internally,
there may be bars of wall material extending across the open space.
These cells are joined end to end to form long tubes. Vessel members and
tracheids are dead at maturity. Tracheids have thick secondary cell
walls and are tapered at the ends. They do not have end openings such as
the vessels. The tracheids ends overlap with each other, with pairs of
pits present. The pit pairs allow water to pass from cell to cell. While
most conduction in the xylem is up and down, there is some side-to-side
or lateral conduction via rays. Rays are horizontal rows of long-living
parenchyma cells that arise out of the vascular cambium. In trees, and
other woody plants, ray will radiate out from the center of stems and
roots and in cross-section will look like the spokes of a wheel.
Phloem
Phloem is an equally important plant tissue as it also is part of the
‘plumbing’ of a plant. Primarily, phloem carries dissolved food
substances throughout the plant. This conduction system is composed of
sieve-tube member and companion cells, that are without secondary walls.
The parent cells of the vascular cambium produce both xylem and phloem.
This usually also includes fibers, parenchyma and ray cells. Sieve
tubes are formed from sieve-tube members laid end to end. The end walls,
unlike vessel members in xylem, do not have openings. The end walls,
however, are full of small pores where cytoplasm extends from cell to
cell. These porous connections are called sieve plates. In spite of the
fact that their cytoplasm is actively involved in the conduction of food
materials, sieve-tube members do not have nuclei at maturity. It is the
companion cells that are nestled between sieve-tube members that
function in some manner bringing about the conduction of food.
Sieve-tube members that are alive contain a polymer called callose.
Callose stays in solution as long at the cell contents are under
pressure. As a repair mechanism, if an insect injures a cell and the
pressure drops, the callose will precipitate. However, the callose and a
phloem protein will be moved through the nearest sieve plate where they
will form a plug. This prevents further leakage of sieve tube contents
and the injury is not necessarily fatal to overall plant turgor
pressure. Phloem transports food and materials in plants in upwards and
downwards as required
