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On the basis of their function, animal tissues are
of four major types. They are epithelial tissue, connective tissue, muscle
tissue and nervous tissue.
Let us examine the structure the structure and
functions of these four kinds of tissues in animals ( including man).
Epithelial tissue: epithelial tissue forms a
covering over the surface of the body as well as on the outside and inside of
several internal organs example skin is an epithelial tissue on the outer side
of the body. Lining of the digestive tract is a covering of epithelial cells on
the internal organ.
Epithelial cells perform several functions- they
offer protection, produce structure like hair, nails, feather and horns, in the
form of gland cells, they secrete a variety of chemical and may involve in
receiving external stimuli.
Cells in the epithelial are very close to each other
and are joined together by a cementing substance. Epithelial cells occur in
various shapes, flat, cube like, long and cylindrical.
Epithelialcells lining the digestive and respiratory system of several animals have hair
like projections called cilia. They help in moving the food particles or mucus.
Epithelial cells may be present in a single layer or
may occur in multiple layer stratified epithelium.
There are simple and complex tissues in plants.
Parenchyma, collenchyma, sclerenchyma, are examples of simple tissues . PlantTissues fall into two large categories: Meristematic, or Permanent
(Non-Meristematic)
Xylem and phloem are example for complex tissues.
Simple tissues have only one type of cell while complex tissues have several
different types of cells.
Parenchyma:parenchyma is the fundamental and simple tissue in plants. Almost all the
primary structure like leaves, flowers and young stem made up of parenchyma.
Cells in parenchyma are living cells. They can be
either round, suare or polyhedral. There are different types of parenchymatous
cells.
Chlorechyma:
cells in this type of parenchyma have chloroplasts and are commonly seen in
leaves.
Aerenchyma:
air spaces are present between cells in this type of parenchyma.
This type of
parenchyma is seen in plants which float on water such plants are called
hydrophytes.
Storage tissue:
cells in this type of parenchyma store food materials. This type of parenchyma
is seen in tubers and rhizomes.
Water storage:
cells in this type of parenchyma store water – such cells are seen in desert
plants.
Collenchyma :
collenchymas is a simple living tissue.
This tissue is present in the stems of herbs and shrubs. It gives mechanical
strength and support. Cells may have chloroplasts. Cells in collenchymas are
short and square or long and fibre like with pointed ends. Cell wall is made up
of cellulose and pectin which gives. Collenchyma gives flexibility and tensile strength to the
plants.
Sclerenchyma :
sclerenchyma is a tissue with dead cells and is designed to give mechanical
strength to the plant. It also protects the parenchyma from damage due to
stretching, bending and pressure lignin is the major component in cell walls of
the cells present in sclerenchyma.
Xylem:xylem is a conductive tissue which has both
living and non-living cells. It conducts water and minerals from roots to other
parts of the plant. A part from this
xylem gives mechanical strength to plant. Xylem tissue is also helpful in
identifying the plant species.
There are three kinds of non-living cells in xylem-
they are fibers, tracheas and vessels. Xylem parenchyma is the living component
in xylem. Cell walls of the xylem cells are thick because of lignin deposition.
It is also commercially very useful.
Phloem:
phloem is also called bast or leptome. Unlike xylem, phloem is a living
tissues. Cell walls have lignin deposition. Phloem is composed of five types of
cells. They are sieve cells, sieve tubes, companion cells, phloem fibres and
phloem parenchyma.
Sieve cells and sieve tubes are long with tapering
ends. Sieve plate with one to many sieve pores is present at the ends of sieve
tubes. The conductuion of food material from leaves to other parts takes place
through phloem tissues. Besides, phloem has commercial value- bast fibres
present in phloem are used for making ropes.
Unlike plants, animals cannot prepare their own food. Instead, they procure food by feeding on other organisms.
Hence, they are called Heterotrophs. Heterotrophic nutrition is of two types. Saprozoic and Holozoic.
Saprozoic nutrition in very primitive animals, especially some protozoans, and parasites exhibit saprozoic mode of nutrition. They absorb food materials dissolved in water through their body surfaces.
Some animals secrete enzymes into the external medium. These enzymes breakdown the complex organic molecutes into simple organic molecules. These are then absorbed by the animal through its body surface. As the food is already digested and in the form of small molecules, some of these animals may not have special digestive organs. Even if present, the digestive system in these animals is very simple.
Holozoic nutrition Majority of animals take solid particulate food or liquid food through special feeding mechanisms and digest them with the help of digestive enzymes. This mode of nutrition is know as holozoic nutrition.
The size of food particles varies considerably ranging from microscopic organisms to large animals and plants. Depending on the size of the food, different mechanisms to capture the food.
Some animals use pseudopodia (Amoeba), tentacles (Hydra) for capture the food. These animals do not cut the food into small pieces and swallow the entire food at one time. Other animals which feed on solid food have mechanisms to cut the food into small pieces before they consume it.
For example, snails have snarp teeth like structures on their tongue called Radula with which they scrape the surface of food and swallow the food particles.
Some animals, like lions and tigers, are equipped with organs to chase, seize (capture) kill, cut and cat the prey.
Some animals feed only on liquid food. For example scorpions and spiders kill the prey and inject digestive enzymes into the body of the dead animal. These enzymes digest the food and convert it into liquid form which the animal will feed.
Houseflies, butterflies, mosquitoes, aphids, leeches etc.,also feed on liquid food. Butterflies have long tube like proboscis which is inserted into the flowers and nectar is sucked.
Mosquitoes have special organs to pierce through the skin and suck the blood. Blood sucking animals such as mosquitoes and leeches secrete substances into the blood to prevent blood from clothing while they are feeding.
Animals like earth worms feed on soil containing decomposed organic material.
Thus,a variety of feeding mechanisms are seen in animal kingdom and the type of feeding mechanisms in the animal depends on the type and nature of the food.
Some animals feed on only one type of food material and these are called monophagous animals.
A typical example for this is the caterpillar larva of silkworm which feeds only on mulberry leaves.
Several animals feed on plant material are called herbivores while those feeding on other animals are called carnivores. Animals which eat both plants and animals are called omnivores.
Diet of an animal may be different in different sexes or at different stages of its life. For example, male mosquito feeds on plant juices white female mosquito feeds on blood. Similarly, caterpillar larva feeds on leaves while the butterfly feeds on the nectar.
Thus, there is a great variation in the type of food consumed and the method by which it is consumed in holozoic animals.
The third type of nutrition is called mixotrophic nutrition. In this type of nutrition, there is an association of two different living organisms.
Nutrients may be obtained by both the organisms or
only by one organism. In the former condition, both the organisms are
benefited while in the latter case only once of the organisms is benefited of
the association.
When two organisms live together, exchange nutrients
and are benefited mutually. This nutrition is called symbiotic nutrition.
One of the organisms provide nutrients required for
the other organism which the other provides shelter or nutrients or both.
Symbiotic mode of nutrition is seen in both plants
and animals. Nitrogen fixing bacteria living in the root nodules of leguminous
plants is a typical example for symbiotic nutrition.
Plant provides shelter and nutrients to the bacteria
while bacteria provide nitrogenous compounds to the plant.
Similarly, symbiotic association is seen in lichens
where algae supplies food to fungi, fungi provides protection to algae and live
together.
In animal kingdom, the association of certain crabs with sea anemones is an example of symbiotic nutrition.
Sea anemones give protection to the animal while the small pieces of food particles are provided to sea anemones by the crab.
Parasitism is the second type of mixotrophic
nutrition – parasite is an organism which lives inside (called endoparasite) or
outside (called ectoparasite) of another organism (called host ) and obtain the
nutrients form the host organism.
In this mode of nutrition, only parasite is benefited. Body functions of the
host are usually affected badly due to the invasion of the parasite and the
host may even die.
