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Chapter 6: TISSUES
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In unicellular organisms, a single cell performs all basic functions. E.g., in Amoeba, a single cell carries out movement, food intake, gas exchange and excretion.
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But in multicellular organisms, cells are specialised to carry out specific functions (division of labour).
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Each specialized function is performed by a different group of cells, which increases efficiency.
- A group of cells that are similar in structure and/or work together for a particular function is called a tissue. E.g., blood, phloem and muscle.
ARE PLANTS & ANIMALS MADE OF SAME TYPES OF TISSUES?
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Plants and animals differ in structure and function.
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Plants are stationary, with more supportive tissue (often made of dead
cells) to stay upright.
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Animals move and have mostly living tissues, consuming more energy.
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Pattern
of growth: Plant growth is limited to certain
regions. Based on the dividing capacity, plant tissues are 2 types: growing
or meristematic & permanent.
Cell growth in animals is more uniform. So, there is no
dividing and nondividing regions in animals.
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The
structural organisation of organs and organ systems is more specialised and localised
in complex animals compared to
plants. This reflects the different lifestyles and feeding methods in plants and animals.
- These differences are also due to sedentary existence of plants and active locomotion of animals, leading to variations in organ system design.
PLANTS TISSUES
MERISTEMATIC
(DIVIDING) TISSUE
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It is the
tissue responsible for growth and cell division.
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Plant
growth occurs only in specific regions, such as the tips of roots and shoots,
because meristematic tissue is located at these points. This can be proved by
following experiment:
•
Place
onion bulbs in two glass jars filled with water.
•
Observe and measure the root growth in the bulbs daily for three days.
• On day 4, cut 1 cm from the root tips of the onion bulb in jar 2. Continue observing and measuring root growth in both jars for five more days. Record the observations.
|
Length |
Day 1 |
Day 2 |
Day 3 |
Day 4 |
Day 5 |
|
Jar 1 |
2 cm |
2.5cm |
3cm |
3.5cm |
4.5cm |
|
Jar 2 |
2 cm |
2.5cm |
3cm |
Nil |
Nil |
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First 3 days, roots in both bulbs grow similarly but after cutting the root tips, growth stopped in jar B.
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Based on the location, meristematic tissues are 3 types:
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Apical meristem: Seen at the growing tips of stems and roots. It increases the length of stem and root.
·
Lateral meristem (cambium): It increases the girth of the stem or root.
·
Intercalary meristem: Seen near the node.
|
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Meristematic cells are very active, they have dense cytoplasm, thin cellulose walls and prominent nuclei. They lack vacuoles because they are rapidly dividing and no storage food is required. |
|
|
Location of meristematic tissue in
plant body |
PERMANENT
TISSUE
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Cells formed
by meristematic tissue take up a specific role and lose the ability to divide.
As a result, they form a permanent tissue.
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This
process of taking up a permanent shape, size, and a function is called differentiation.
It leads to the development of various types of permanent tissues.
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On
observing a stained thin section of stem under microscope, the following
tissues can be seen.
(i) SIMPLE PERMANENT TISSUE
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A few
layers of cells beneath the epidermis.
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They are
made of one type of cells.
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3 types: Parenchyma,
Collenchyma & Sclerenchyma.
Parenchyma:
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Most common
simple permanent tissue.
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It
consists of relatively unspecialised living cells with thin cell walls. Cells
are usually loosely arranged, creating large intercellular spaces.
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This
tissue generally stores food.
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Chlorenchyma: Parenchyma containing chlorophyll. It performs photosynthesis.
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Aerenchyma: Parenchyma with large air cavities. Found in aquatic plants to help
them float.
Collenchyma:
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A permanent
tissue that gives flexibility in plants.
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It allows bending
of parts like tendrils and stems of climbers without breaking.
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It also
provides mechanical support.
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This tissue
is found in leaf stalks below the epidermis.
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Cells are
living, elongated and irregularly thickened at the corners. There is very
little intercellular space.
Sclerenchyma:
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It makes
the plant hard and stiff. E.g., Husk of a coconut.
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Cells
are dead, long and narrow as the walls are thickened due to lignin.
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Often the
walls are so thick that there is no internal space inside the cell.
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Found in
stems, around vascular bundles, in leaf veins and hard covering of seeds and
nuts.
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It provides
strength to the plant parts.
Activity
•
Stretch
and break a freshly plucked Rhoeo leaf by applying pressure. Some peel
or skin projects out from the cut.
•
Put the peel
in a petri dish filled with water.
•
Add a few
drops of safranin.
•
After few
minutes, transfer it onto a slide. Gently place a cover slip over it.
•
Observe
under microscope.
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The outermost
layer of cells is called epidermis. It is usually made of a single layer
of cells.
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In plants
in dry habitats, the epidermis may be thicker to prevent water loss.
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The
epidermis covers the entire surface of the plant. It protects all the parts of
the plant.
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Epidermal
cells on the aerial parts of the plant often secrete a waxy, water-resistant layer.
It protects against loss of water, mechanical injury and parasitic fungi. Due
to the protective role, epidermal cells form a continuous layer without
intercellular spaces.
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Most
epidermal cells are relatively flat. Outer and side walls are thicker than the
inner wall.
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The epidermis
of leaf has small pores called stomata.
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Stomata
are enclosed by two kidney-shaped cells called guard cells.
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Stomata help
for gas exchange and Transpiration (loss of water vapour).
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In roots,
epidermal cells help in water absorption. Their long hairlike parts increase
absorptive surface area.
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Desert plants
have a thick coating of cutin (waxy chemical with waterproof quality) on
outer surface of the epidermis. It reduces
water loss through transpiration.
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The outer
layer of a tree branch is different from the outer layer of a young stem. As
plants grow older, the outer protective tissue changes, forming cork (layers
of cells from secondary meristem cells in the cortex).
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Cells of
cork are dead and compactly arranged without intercellular spaces. They have suberin
in their walls that makes them impervious to gases and water.
|
Protective tissue |
|
(ii) COMPLEX PERMANENT TISSUE
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These are
made of more than one type of cells working together for a common function.
E.g., Xylem & phloem.
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Xylem
& phloem are conducting tissues and constitute a vascular bundle.
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Vascular
tissue is a distinctive feature of the complex plants. It enables their
survival on the land.
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Xylem
consists of tracheids, vessels, xylem parenchyma and xylem fibres.
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Tracheids
and vessels have thick walls, and many are dead cells when mature. Their tubular
shape allows them to transport water and minerals vertically.
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Xylem parenchyma
stores food.
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Xylem fibres
are mainly supportive in function.
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Phloem consists of 5 types of cells: sieve
cells, sieve tubes, companion cells, phloem
fibres & phloem parenchyma.
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Sieve
tubes are tubular cells with perforated walls.
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Phloem
transports food from leaves to other parts.
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Phloem cells
are living except phloem fibres.
Types
of complex tissue: (a) Tracheid (b) Vessel
(c) Xylem parenchyma (d) Section of phloem
ANIMALS TISSUES
Based on the functions, animal tissues are 4 types: epithelial,
connective, muscular & nervous.
EPITHELIAL
TISSUE (EPITHELIUM)
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These are
the covering or protective tissues.
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It covers most
organs and cavities in the body. It also forms a barrier to separate different
body systems.
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Location: Skin, lining of mouth, blood vessels, lung alveoli and kidney
tubules.
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Epithelial
cells are tightly packed forming a continuous sheet. They have very little
cementing material between them and almost no intercellular spaces.
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The permeability
of the epithelial cells regulates the exchange of materials between the body
and external environment and also between different body parts.
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Epithelium
is separated from the underlying tissue by an extracellular fibrous basement
membrane.
Types of epithelial
tissues
·
Simple squamous
epithelium: They have single layer of very thin and
flat cells and form a delicate lining.
Found in lining of blood vessels or lung alveoli, where transport of
substances occurs through a selectively permeable surface.
·
Stratified
squamous epithelium: In this, cells are arranged in
many layers to prevent wear and tear.
Found in skin, lining of the mouth and oesophagus.
·
Columnar
epithelium: Tall, pillar-like cells found in areas
of absorption and secretion. E.g., inner lining of the intestine. This
facilitates movement across the epithelial barrier.
·
Ciliated
columnar epithelium: Columnar cells with cilia
(hair-like projections). Found in respiratory tract where the cilia move mucus forward
to clear it.
·
Cuboidal
epithelium: Cube-shaped cells. They form the lining
of kidney tubules and ducts of salivary glands. It provides mechanical support.
·
Glandular
epithelium: Specialized epithelial cells (gland
cells) that secrete substances. Sometimes, the epithelial tissue folds inward
to form multicellular glands.
CONNECTIVE
TISSUE
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The tissue
in which the cells are loosely spaced and embedded in an intercellular matrix.
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The matrix
may be jelly like, fluid, dense or rigid based on the function of the specific
connective tissue.
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E.g., Bone,
cartilage, blood etc.
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Bone: It forms the framework that supports the body, anchors the muscles
and protects vital organs.
It is a strong and nonflexible tissue. So bone can provide structural
support and protection for the body.
Bone cells are embedded in a hard matrix composed of calcium
and phosphorus compounds.
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Cartilage: Widely spaced cells. The solid matrix is composed of proteins and
sugars.
Cartilage smoothens bone surfaces at joints.
It is also present in the nose, ear, trachea and larynx.
The cartilage of the ears can be folded, but the arm bones cannot be
bent.
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Blood: It has a fluid matrix called plasma, in which red blood
corpuscles (RBCs), white blood corpuscles (WBCs) and platelets are
suspended.
The plasma contains proteins, salts and hormones.
Blood transports gases, digested food, hormones and waste materials
to different parts of the body.
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Areolar
tissue: Found between the skin and muscles, around
blood vessels and nerves and in bone marrow. It fills the space inside the
organs, supports internal organs and helps in tissue repair.
|
Areolar tissue |
|
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Adipose
tissue: It stores fat. Found below the skin and
between internal organs. The cells are filled with fat globules which allows it
to act as an insulator.
|
Adipose tissue |
|
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Ligament: It connects two bones each other. Very elastic and has considerable
strength with very little matrix.
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Tendon: It connects muscles to bones. It is a fibrous tissue with great
strength but limited flexibility.
MUSCULAR
TISSUE
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This is responsible
for movement in the body.
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It consists
of elongated cells (muscle fibres).
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Muscles
contain special proteins called contractile proteins. They contract and
relax to cause movement.
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Voluntary
(Skeletal/ Striated) muscles: Seen attached to
skeleton and can move by conscious will. E.g., limb muscles. Under the
microscope, these muscles show alternate light and dark bands (striations) when
stained. Cells are long, cylindrical, unbranched and multinucleate.
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Smooth
muscles (unstriated/ Involuntary muscles): They
cannot be moved by conscious will. Striations are absent. They are involved in involuntary
movements, such as moving food through the alimentary canal and contraction of
blood vessels. They are also found in the iris of the eye, ureters and bronchi.
The cells are long spindle-shaped and uninucleate.
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Cardiac
muscles: The involuntary muscles found in the heart.
They show rhythmic contraction and relaxation throughout life. The cells are cylindrical,
branched and uninucleate.
|
Features |
Striated |
Smooth |
Cardiac |
|
Shape |
Cylindrical |
Spindle-shape |
Cylindrical, branched |
|
Number of nuclei |
Multinucleate |
Uninucleate |
Uninucleate |
|
Position of nuclei |
Peripheral |
Central |
Central |
NERVOUS
TISSUE
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The tissue
which contains specialised cells (neurons or nerve cells) which are
highly stimulated and rapidly transmit the stimuli within the body.
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Found in brain,
spinal cord and nerves.
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A neuron consists
of a cell body (a nucleus + cytoplasm), from which long thin hair-like parts
arise.
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A neuron has
a single long process called the axon, and many short, branched processes
called dendrites.
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An
individual nerve cell may be up to a metre long.
- Many nerve fibres bound together by connective tissue
to form a nerve.
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The signal
that passes along the nerve fibre is called a nerve impulse. It allows muscle
movement.
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The
combination of nerve and muscle tissue is essential for rapid movement in
response to stimuli.