1.2.2
Different Cell Types
[A] Plant Tissues
(a)
Parenchyma :
made up of parenchyma cells
unspecialised cell with thin cell wall, possess only primary cell wall
Tissue
Living /
dead
Cell shape
Main function
Distribution
Parenchyma
Living
Usually
isodiametric
, sometimes
elongated
1. As ground tissue and packing tissue
of plant body
2. Important in the maintenance of
turgidity of the cell especially in
herbaceous plant
3. Sites for metabolic activities,
storage, deposition of waste
products
4. Site of meristematic function
5. Large intercellular air spaces among
cell for gaseous exchange
6. may be photosynthetic
Cortex, pith,
medullary rays
and packing
tissue in xylem
and phloem
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Modified parenchyma:
1.
Epidermis
- Usually surface covered with cuticle made of cutin
- Guard cells present in leaves or young stem, with unequal cell wall thickening
- At the root tip, root hair may extend from these cells.
How the structures of the epidermis are adapted to its functions
Structure
Functions
it is a continuous, compact layer of cells Give mechanical support and protection against certain fungi
on the surface of the plant body
and bacteria
it is one cell thick and transparent
Allow light to penetrate into the mesophyll for
photosynthesis
epidermal cells of the stem and leaves Reduce water loss through transpiration from the leaf surface
have a waxy cuticle on their outer walls
Guard cells surrounding stomata
Allow gaseous exchange between the leaf & atmosphere
Guard cells with thicker inner wall and For the formation of stomata by the uneven stretching of the
thinner outer wall
cell wall as the cell becomes turgid. It can control the
opening and closing of the stomata and thus control the gas
exchange and water loss.
Guards cells contain chloroplasts
It can regulate the water potential of the guard cells and thus
which allow the photosynthesis to carry control the stomatal opening.
out.
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2.
mesophyll
- in green leaves, with abundant chloroplasts for photosynthesis
Shape
Amount of chloroplasts
Intercellular air spaces
Palisade Mesophyll
Spongy Mesophyll
Regular, columnar / cylindrical Irregular
Abundant
few
Negligible and closely packed Large and very loosely arranged
3.
endodermis
- the innermost one-celled layer of cortex surrounding vascular tissue
- with Casparian strips (made of suberin, water-proof fatty substance, surrounding the
transverse & radial wall of the cell )
- At certain intervals, with passage cells (with NO Casparian strips)
4.
pericycle
- occurs only in root, one to several layers outside the central vascular tissue
- gives rise to lateral root and takes part in secondary growth
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5.
-
(b)
companion cell
- found adjacent to sieve tube
With dense cytoplasm & prominent nucleus, metabolically active
- may be used to supply energy for active movement of substances into the companion cells
and help the transport in sieve tube
Collenchyma :
Tissue
made up of collenchyma cells
Living /dead
Collenchyma Living
Cell shape
Main function
Distribution
Elongated and polygonal Support (a mechanical Outer region of cortex
e.g. angle of stems,
function)
with tapering ends
midrib of leaves
(Cell wall thickened at the
corners)
-
cytoplasm is scarce, small lumen and small nucleus
 support is given entirely by the cell wall
- its cell wall is usually thickened at the corners with
deposition of extra cellulose, hemicellulose and pectin.
- The cell walls are generally flexible and allow certain degree
of stretching.
- intercellular space absent or extremely scarce.
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(c)
Sclerenchyma :
Tissue
Sclerenchyma
made up of fibres and sclereids
Living /
dead
Cell shape
Dead
1.
Fibres
Elongated & polygonal
with tapering
interlocking ends
2.
Sclereids
Roughly isodiametric
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Main
function
Support &
mechanical
protection
Distribution
Outer region of cortex, pericycle
of stems, xylem & phloem
Cortex, pith, phloem, fruits &
seeds
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-
-
Support is provided by the cell wall which is evenly thickened. This extra-thickening is mainly due
to the deposition of lignin :
a hard substance with great tensile (i.e. not easily break on stretching) and compressional strength.
Mature cells are dead with empty lumen at the centre.
Function : cannot carry out photosynthesis, not for food storage, solely for support
2 kinds:
1.
fibres
- elongated, with interlocking tapering ends
- closely packed with little or no air-space
- polygonal and closely packed leaving little or no air spaces between them.
- usually found in the outline of the cortex of stem and root and frequently associated with
vascular tissues.
e.g. sclerenchyma sheath around the vascular bundles
2.
(d)
Xylem :
sclereids or stone cells
- short and spherical in shape
- found in cortex, phloem and medulla of stem and in flesh of some fruits such as pear.
made up of xylem tracheids and xylem vessels
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-
1.
Tissue
Living /dead
Cell shape
Main function
Distribution
Tracheids &
vessels
dead when
mature
Elongated &
tubular
Transport of water &
mineral salts;
Support
Vascular
system
Xylem fibres and
parenchyma
(similar to that of sclerenchyma fibres and parenchyma)
Xylem consists of non-living cells with supporting function
includes tracheids and vessels.
- Walls of tracheids and vessels are thickened with cellulose and lignin
- they offer resistance to collapse especially during transpiration.
Xylem tracheid
2.
Elongated with fine tapering ends (L.S.), polygonal (X.S.)
Wall is heavily lignified & pitted (secondary wall) when mature
They have no perforation plates but they have pits connecting the tracheid to the adjacent one.
No protoplast (when mature)
Xylem vessel
- more important in angiosperms for water transport and support.
Structural adaptation of vessel :
- made up of dead hollow cells (xylem vessel elements) joined end to end with no end wall
minimum resistance
- thick lignified wall 
prevent wall collapsing and give mechanical support
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
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-
Secondary wall shows various patterns of thickening due to the deposition of lignin.
Bordered pits in the longitudinal wall with adjacent cell perforated end wall
Vessel has greater diameter and large lumen than tracheid
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Relationship between the featuresand functions of xylem vessels
(e)
Features
Functions
All the transverse walls are absent, the
vessels effectively become continuous
tubes.
Unlignified area (pits) occur in the side wall
of pitted vessels
The wall is thickened with lignified
cellulose
To enable the continuous conduction of water and mineral
salts through the plant.
To facilitate the lateral movement of water since the
lignified wall themselves are impervious.
For mechanical support – the strength of their lignified walls
enable xylem vessels to resist compression and tension :
Phloem :
made up of sieve (tube) elements and companion cells
- Cell wall mainly made of cellulose.
Tissue
Living
/dead
Cell shape
Main function
Sieve tubes
Living
Elongated &
tubular
Translocation of organic
solutes (food)
Companion cells
Living
Elongated &
narrow
Work in association with
sieve tubes
Phloem fibres
and parenchyma
(similar to that of fibres and parenchyma described before)
Distribution
Vascular system
1.
Sieve (tube) elements
- During development from meristematic cell, nucleus degenerates.
 sieve element is a living cell with no nucleus.
- Sieve elements are joined end to end forming a sieve tube, each element being separated from
the next by a sieve plate.
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-
When plasmodesmata of end walls enlarge greatly, sieve pores are formed on the sieve plate.
 Sieve tubes are tube-like structure with a wide lumen and a very narrow, indistinct,
peripheral layer of living cytoplasm bounded by a plasma membrane. Sieve plates are
regions of thick primary cell wall, with protoplasmic strands passing through.
-
Sieve areas are also present in the side wall for the lateral translocation of organic solutes to the
neighbouring cells.
-  sieve tubes are living.
 contain obstructions (sieve plate and cytoplasm) to the flow of solution.
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2. Companion cells
- are parenchyma cells derived from same parent cell as the neighbouring sieve element and
closely associated with sieve element.
-  with prominent nucleus, dense cytoplasm, small vacuole, numerous mitochondria and ribosomes
 companion cells are very active metabolically.
found in angiosperm only
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How the structural features of phloem related with their functions
Features
The sieve plate of sieve tubes with protoplasmic
strands passing through
Companion cell
- has pitted wall for communication with the
cytoplasm of the sieve tube by plasmodesmata
- with dense cytoplasm, nucleus & is metabolic
active.
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Functions
Enable the transport of solute through the tube.
-
It is functionally associated with sieve (tube)
elements
may be used to supply energy for active
movement of substances into the
companion cells and help the transport in
sieve tube
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[B] Animal Tissues
(a)
Epithelium : made up of epithelial cells
- in single (simple epithelium) or multilayered sheets (compound epithelium)
- covers the internal and external surfaces of the body of an organism for protection.
- The bottom layer of cells rests on a basement membrane.
- The free surface of the epithelium is often highly differentiated for various purposes:
squamous epithelium
lining of capillaries (endothelium)
Alveoli in lungs
Bowman’s capsule of kidney
ciliated epithelium
lining the oviducts,
ventricles of brain, spinal cord,
respiratory passages
stratified epithelium
skin
oesophagus
1.
squamous epithelium : with smooth lining,
to provide a relatively friction-free passage of fluids through hollow structures such as blood
vessels, heart chambers (endothelium) and the inside of the body cavity (peritoneum).
The thinness of squamous epithelium facilitates diffusion across it.
The smoothness facilitates the passage of fluid and lubricates the movement between adjacent
surface.
2.
Epithelium bears numerous cilia and always associated with mucus-secretory goblet cells (ciliated
epithelium lining the oviducts, ventricles of brain, spinal cord, respiratory passages)
serve to move materials along the passage.
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3.
stratified epithelium : consists of several layers of cells making up a tough, thick and impervious
lining of skin, oesophagus, vagina etc.
protect underlying structures from injury through
abrasion or pressure, and from infection.
The cells of the outermost layers are dead and are
continuously replaced from below.
The innermost layer of cells, called generative
layer, is in an active state of mitosis. Above the
generative layer, the cells become progressively
isolated from the oxygen and nutrients supplied by
the blood. The cells die and eventually slough off.
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Special feature of stratified epithelium related to the function
• The multilayered structure provides protection from abrasion.
• e.g. A stratified epithelium occurs in the buccal cavity and in the vagina
• e.g. in the skin (epidermis) additional protection is afforded by the dying cells, the proteins of
which become modified into a hard waterproof layer. The protein of this layer (keratin)
provides a tough surface referred to as “cornified”•The greater the friction that a given
area of skin experiences, the thicker the cornified layer becomes.
(b)
Blood Cells
1.
Red Blood Cell (Erythrocytes)
Shape & structure
biconcave discs
 Have a larger SA/V ratio for gaseous exchange.
Thin
 permit efficient diffusion of gases from its surface inwards.
Non-nucleated in human RBC
 Passive and no activity; only acting as a carrier; no nucleus to occupy any space or volume in
cell.
 Permit more Hb to be packed in the cell to carry more O 2 (Whole interior filled with Hb :
surrounded by a thin membrane.
No nucleus (in human RBC) and cytoplasm containing haemoglobin (takes about 90% of the
dry mass)
– enable the RBC to carry O2 as oxyhaemoglobin and CO2 as carbamino haemoglobin
formed in bone marrow
Function : as O2 and CO2 (some) carriers
2.
White Blood Cell (Leucocytes)
Shape & structure
All are nucleated  very active.
(may be) amoeboid shape
formed in bone marrow & lymphatic system
fewer number than RBC (1 WBC : 500 RBC)
Function : for body defence.
Types :
1.
Granulocytes : granular cytoplasm and lobed nucleus (polymorphonuclear)
- capable of amoeboid movement.
- Types:
(1)
Neutrophils (phagocytes)
- able to squeeze between the cells of capillary wall and enter intercellular
spaces.
- They can be stained by neutral dye.
- Function : actively phagocytic : engulf and digest disease-causing bacteria in
the intercellular spaces.
(2)
Eosinophils
- Stained by eosin (an acid dye)
- Function : have anti-histamine properties.
- It increases in number with allergic conditions such as asthma, hayfever,
parasitism etc.
(3)
Basophils
- rare in blood
- Granules stained by basic dye e.g. methylene blue
- Polymorphic nuclei;
- Function : produce heparin and histamine
- They can leave the blood and form‘mast cells’. Mast cells cause blood vessel
dilation at sites of inflammation such as those caused by insect bites.
2.
Agranulocytes : non-granular cytoplasm; oval or bean-shaped nucleus
- Types:
(1) Monocytes
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-
horseshoe-shaped / bean-shaped nucleus
Function : actively phagocytic and ingest bacteria and other large particles.
They are able to migrate from the bloodstream to inflamed area of the body
and carry out phagocytosis (in the same manner as neutrophils)
(2) Lymphocytes
rounded nucleus, only a small quantity of cytoplasm
2 types: T cell, B cell
Functions: To cause immune reactions
e.g. T cell : cell-mediated response - phagocytic
B cell : produce antibodies acting against antigens
3.
Platelets
(Thrombocytes)
Shape & structure
- Irregularly shaped membrane-bound cell fragments.
- They are irregularly shaped membrane-bound cell fragments produced by large cells in the
bone marrow.
Frequently (Number is between 150 000 and 400 000 platelets in each cubic millimeter of
blood.with no nucleus.)
Function : for blood clotting.
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(c)
Nervous tissue (Neurone)
Structure
1.
2.
3.
-
Basic unit is neurone (about 1010 in the brain)
All neurones have the following structures:
Cell body containing a nucleus.
Dendron – a fine cytoplasmic fibre which brings impulses in
towards the cell body.
Axon – a single fibre taking impulse away from the cell body.
many fine cytoplasmic fibres connecting to the cell body are
called dendrites) which transmit nerve impulses to the cell body.
Impulses leave via the axon.
Features
The cytoplasm of a neurone cell body is
densely packed with ribosomes, Golgi
apparatus, rough endoplasmic recticulum and
mitochondria
Microtubules run from the cell body cut along
the axons and tendons
Functions
Ribosomes, Golgi apparatus, rough ER are
responsible for synthesizing neurotransmitters.
Mitochnondria are responsible for the
production of energy for the process to take
place.
Function of a neurone
- responsible for initiating an electrical impulse and coordinating the activities of other tissues of the
body.
Neurone
Receptors (receive stimuli, e.g. sensory cells)
Effectors (response, e.g. muscle or glands)
Some axons are covered by a fatty myelin sheath formed by Schwann cells.
Different type of nerve fibres :
(a)
Myelinated nerve fibres
Fibres surrounded by fatty myelin sheath. A tough inelastic membrane, neurilemma
surround the myelin sheath.
Myelin sheath is absent at nodes of Ranvier leaving the fibre unprotected.
e.g. In cranial nerves and spinal nerves
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Functions of myelin sheath :
for protection
act as an electrically insulating sheath
help in the increase in the speed of impulse transmission.
(b)
Non-myelinated nerve fibre
no myelin sheath around nerve fibres
Functional classification of neurones
- Based on the direction in which they transmit impulse and on the positions they occupy in the
nervous system.
- Different types of neurone :
(a)
Afferent (sensory) neurone
It conducts impulses along a dendron from a receptor organ to the central nervous
system.
Cell bodies of sensory neurones occur in clusters called ganglia next to the spinal cord.
The axon carries the impulse into the central nervous tissues.
(b)
Efferent (motor) neurone
They transmit impulses along axon from the central nervous system to the muscles and
gland.
(c)
Intermediate (relay, association) neurone
They occur within the central nervous system (brain or spinal cord) or in sympathetic
ganaglia.
They receive impulses from sensory neurones from other intermediary neurone to motor
neurones or to other intermediary neurones.
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Nerve : a bundle of nerve fibres.
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