Cell structure

Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 16
Cell structure
Syllabus : The variety of cell structure and function as exemplified by the following : leaf epidermis, parenchyma,
collenchyma, sclerenchyma, phloem, xylem, epithelia (squamous, ciliated and stratified), blood cells
and neurones.
The ultrastructures and their functions in plant and animal cells : nucleus, cell wall, cell membrane,
vacuole, chloroplast, mitochondrion, lysosome, ribosome, endoplasmic reticulum and Golgi
The fluid mosaic model of membranes.
The structure of prokaryotic cells and eukaryotic cells.
His t o l o g y
it is the study of the structure and arrangement of tissues.
During the course of evolution, many plants and animals have attained a specialisation and
division of labour of their cells.
The cells that make up the body of an organism are not all alike, those of the same shape
grow together and combine into a group for the discharge of a common function. This
specialisation allows the cells to function more efficiently. Each group of mature cells gives
rise to a tissue.
A tissue is a group of cells and their intercellular substance which are linked together and
perform a particular function. There may be more than one cell type in one type of tissue,
e.g. in xylem.
Plant Histology
Simple Tissues
- They are made up of one type of cells only, forming a homogeneous or uniform mass.
1. Parenchyma :
- consists of a collection of cells which are more or less isodiametric (equally expanded on
all sides)
- typically oval, spherical or polygonal in shape
- their walls are thin and made of cellulose
- usually living
BS 3 rd ed. P169 fig 6.2
Fig. 17 Structure of parenchyma cells. (a) Drawing of TS of parenchyma cells; (b) TS Helianthus stem pith
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 17
- Functions :
(1) mainly store food materials such as starch grains
(2) form the bulk of packing tissue within the plant, when tightly packed and turgid they
provide support for herbaceous plants
(3) may be modified in certain parts of the plant e.g. epidermis, palisade and spongy
(4) some contain chloroplasts, often called chlorenchyma, manufactures sugar and starch
(5) some are star- like and with radiating arms, leaving a lot of air cavities, is called
arenchyma, as in many aquatic plants for buoyancy
2. Collenchyma :
- consists of elongated cells with corners or intercellular spaces much thickened with
additional deposit of cellulose, hemicellulose and pectin (never lignified)
- in a transverse section (T.S.) of the stem the cells look circular or oval
- they are living and may contain chloroplasts
- often found under the epidermis of herbaceous dicotyledons e.g. sunflower, it is absent
from the root and the monocotyledon except in special cases
BSI 3 rd ed. P173 fig 6.5
Fig. 18 Structure of collenchyma cells. (a) Drawing of TS of collenchyma cells; (b) TS Helianthus stem
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 18
- Functions :
(1) it is a mechanical tissue, providing mechanical support for those organ in which it is
found, since it is living, so it can grow and stretch without imposing limitations on the
growth of other cells (more elastic than sclerenchyma)
(2) contain chloroplast, so can manufacture sugar and starch
(3) for food storage
3. Sclerenchyma :
- cell wall is evenly thickened and is elongated with interlocking tapering ends
- have large deposits of lignin ( which is a hard organic substance with great tensile and
compressional strength) on the primary cell wall and the cell contents are lost with the result
that the mature cells are dead with empty lumen
- they are incapable of elongation when the cells are mature, so they are inelastic
- its sole function is to provide mechanical support and strength for the plant
UB p64 fig 5.17 + photo
Fig. 19 Structure of sclerenchyma cells. (a) Drawing ; (b) Photo
- their walls are provided with pits here and there
Pits : In plants, lignin is not deposited due to the presence of plasmodesmata (strand of
cytoplasm that connect neighbouring cells through the minute pores in the adjacent
cell walls) in the primary cell wall. Such region are called pits
Each group of plasmodesmata forms one pit. Their function is for transport of liquid
or materials between adjacent cells.
UBI p223 fig 8.7
Fig. 20 Development of simple pits in sclerenchyma fibres and sclereids
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 19
- two basic types of sclerenchyma : sclereids and fibres but the differences between them are
not always clear- cut
Fibres : They are very long, narrow, thick- walled and lignified cells with interlocking
tapering ends (fibre like in appearance). They often have simple pits on their
walls. They are dead cells and serve solely for support.
Sclereids or stone cells :
These cells are very thick- walled
and strongly lignified and are
BSI 3 rd ed. P176 fig 6.8
mostly spherical or irregular in
shape or slightly elongated.
Stone cells occur in hard seeds,
nuts and stony hardness of the part
concerned. The flesh of pear is
gritty because of the presence of
stone cells in it.
Fig. 21 Structure of sclerenchyma sclereids
Exercise : (93 I 3)
Compare and contrast the structures and functions of collenchyma and sclerenchyma tissues.
[7 marks]
Complex Tissues
- They consist more than one type of cell, some living and some dead.
Fig. 22 Drawing showing the T.S structure of a photosynthetic leaf.
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 20
1. Leaf Epidermis
- it is the epithelium of leaves
- its cells are usually flattened, and often irregular when looked at in surface view
- they fit together like a jigsaw, forming a protective layer covering the more delicate tissues
- with the exception of the stomatal guard cells, plant epidermal cells lack chloroplasts
- their outer walls are frequently thickened and covered with a layer of waxy material which
constituents the cuticle
- the cuticle is impermeable to water and prevents excessive evaporation in dry conditions
2. Xylem
- it is mainly a conducting tissue of transporting water and minerals upwards from the root
to the leaf and is composed of tracheids, xylem vessels, xylem fibres and xylem parenchyma
- except xylem parenchyma, all others are lignified, thick- walled and dead
(1) Tracheids
- elongated, tube- like dead cells with hard, thick and lignified walls and a large cell cavity
- the walls possess many pits
- occur alone in the wood of ferns and gymnosperms, whereas in the wood of angiosperms
they occur associated with the vessels
- Functions
a) conduction of water and mineral salts from the root to the leaf - - - water can pass
through the empty lumens without being obstructed by living contents, water can also
pass from tracheid to tracheid through the pits
b) being lignified and hard, they give strength to the plant body
(2) Xylem vessel
- rows of elongated tube- like dead cells, place end to end, with their transverse or
end- walls broken down, resulting in long tubes very much like a series of water pipes
forming a pipe line
- their walls are thickened in various ways, and according to the mode of thickening
vessels have received their names such as annular, spiral, scalariform, reticulate
(netted) and pitted
- they have large cell cavities which serve for conduction of water and mineral salts from
roots to the leaves
- they are dead, thick- walled and lignified so that they also serve the mechanical function
of strengthening the plant body
(3) Xylem fibres (wood fibres)
- sclerenchyma cells associated with xylem are known as xylem fibres
- occur abundantly in woody dicotyledons
- since they do not conduct water, they can have much thicker walls and narrower lumens
than vessels and are therefore stronger and confer additional mechanical strength to
the xylem
(4) Xylem parenchyma
- parenchyma cells associated with xylem together form the xylem parenchyma
- they are alive and thin- walled
- assists directly or indirectly in the conduction of water upwards through the vessels and
the tracheids, and also serves for food storage
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 21
- they may form the radial sheets of tissue called medullary rays which maintain a living
link through the wood between the pith and cortex
[Note] the first xylem to appear in the growing plant is called primary xylem while the later xylem formed after
secondary growth due to the activity of vascular cambium is known as secondary xylem
BSI 3 rd ed. P177 fig 6.9
Fig. 23 Structure of Primary xylem. (a) TS; (b) LS primary xylem from Helianthus stem
3. Phloem
- another conduction tissues (transport food materials from leaf to storage organs and
growing organs, and vice versa)
- composed of living cells (sieve tubes, companion cells and phloem parenchyma) with
cytoplasm and have no mechanical function
(1) Sieve tubes
- slender, tube- like structures composed of elongated cells placed end to end
- their walls are thin and made of cellulose and pectic substances
- the transverse partition walls are perforated by a number of pores which is known as
the sieve- plate
Sieve- plate : Derived from the two adjoining end walls of neighbouring sieve
elements. Originally plasmodesmata run through the walls but the canals
enlarge to form pores, allowing a flow of solution from one element to the
next by means of cytoplasmic streaming. Thus sieve tubes are spanned at
intervals by sieve plated that mark successive sieve elements.
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 22
- sieve tube elements have a very distinctive feature that they have their nuclei
degenerated when mature, the cytoplasm becomes confined to a thin layer around the
periphery of the cell but it remain alive and is metabolically dependent on the adjacent
companion cells
- used for the longitudinal transmission of prepared food materials, proteins and
carbohydrates, from the leaves to the storage organs in the downward direction, and
later from the storage organs to the growing regions in the upward direction
(2) Companion cells
- they are thin- walled, elongated cells
- associated with each sieve tube and connected with it by simple pits
- living, containing protoplasm and a large elongated nucleus
- present only in angiosperms
- they are thought to regulate the
metabolic functions of the sieve tubes
(3) Phloem parenchyma
- mostly found in dicotyledons
- elongated and thin walled cells
- living and serve as food storage
(similar to xylem parenchyma)
BSI 3 rd ed. P182 fig 6.13
Fig. 24 Structure of phloem. (a) TS; (b) LS primary phloem from Cucurbita stem
Exercise :
(91 II 2d)
How are the structures of xylem vessels and sieve tubes related to their functions ?
[7 marks]
(92 II 2ai, iii)
For each of the following, explain how its structure is related to its function(s).
(a) a sieve tube
(b) a xylem vessel
[4 marks]
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 23
Animal Histology
- It is a cellular layer forming a bounding surface either externally or internally
- They are derived from all three germ layers:
Ectoderm : Epithelial lining of skin, nervous system, parts of the digestive tract such as mouth
and anus
Mesoderm : Endothelium of blood vessels
Endoderm : The lining of respiratory tract and digestive tract and the associated glands of
digestive system such as liver and pancreases
- Characteristics :
a) can be arranged as single or multi- layer
b) to cover the internal and external surface of the body for protection of the underlying
structures from mechanical injury and from infection
c) the cells are closely packed with no or very little intercellular space, adhesion between cells
is strong
d)do not contain any blood and lymph vessel, but contain sensory cells and nerve endings may
penetrate into the epithelial tissue for stimulus reception
e) the bottom layer of cells rest on a collagenous fibres called basement membrane
f) the free surface of the epithelium is often highly differentiated and may be absorptive,
secretory or excretory in function
They can be classified, according to the number of cell layers and the morphology of the cells
in the surface layer, into two main groups :
I Simple epithelium :
- there are only one layer of cell over a
basement membrane
1. Squamous epithelium
- composed of thin, flat cells
- the cells contain little cytoplasm enclosing a
centrally placed disc- shaped nucleus
- since they are highly permeable, so they form
UB p58 fig 5.1+ 5.2
permeable surface for rapid diffusion of materials
- examples : lining of mouth, Bowman’s capsule, the
alveolar of the lung, blood capillary wall and
peritoneum (lining of body cavity)
Fig. 25 Squamous epithelium
2. Columnar epithelium
- composed of tall and narrow cells with
nucleus near the base
- the free surface of the cells possess
microvilli to increase the surface area
for absorption and secretion
- these cells usually associated with
mucus secreting goblets cells which
protect them from digestion of
Fig. 26 Columnar epithelium (LS)
- examples : the lining of alimentary canal except buccal cavity and oesophagus
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 24
3. Ciliated epithelium
- the cells are usually columnar in shape
and bear cilia at their free surface
- they are often associated with goblet
cells which produce fluids for the cilia
to set up currents (materials can then
be transported by cilial movement)
- examples : oviducts, trachea
UB p58 fig 5.4
Fig. 27 Ciliated epithelium (LS)
II Compound epithelia
- more than one cell layer thick
e.g. Stratified epithelium
- made up of a number of layers of cells
- those cells attached to the basement
membrane form the germinative layer
and they undergo mitosis, they are
cubical in shape and nucleus stains darkly
- as multiplication of germinative cells
continue, they are pushed outward and
move towards the free surface, there they
UB p59 fig 5.5
become flattened and eventually flake off
Fig.28 Stratified epithelium (LS)
and then replaced by new ones just
- in the area of external skin surface, such as buccal cavity, vagina and anal canal,
the cells are transformed into dead cornified layers, because keratin is continuously
deposited onto them, this increases the protection against abrasion and infection
Exercise : (92 I 3)
Tabulate the types of epithelial tissues forming the inner linings of the structures listed below,
and indicate how their structures are related to the function of the following :
(a) the capillary
(b) the small intestine
(c) the oviduct
(d) the trachea
[6 marks]
Blood cells
- Blood is a connective tissue with a large amount of fluid intercellular substance but has no
fibres and the matrix is not secreted by the blood cells.
I Components of blood
A. Plasma
- it is a complex mixture of water, proteins, amino acids, carbohydrates, lipids, salts,
hormones and enzymes, antibodies, dissolved gases and urea
- it is slightly alkaline, with pH = 7.4 in nature
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 25
US p60 fig 5.9
Fig.29 Blood cells. (a) Blood cells ,x400 ; (b) Drawing of different types of blood cells
B. Cells
1. Red blood cells (erythrocytes)
- small, biconcave, disc- shaped cells,
[Note] biconcave disc shape is to increase the surface area and decrease the path for diffusion
of oxygen to occur
they lose their nuclei when mature, so as to increase space for carrying more O 2
contain haemoglobin (red pigment) for carrying O2
the number of RBC in human blood is 5 million/mm3
in foetus, RBC are manufactured chiefly at the liver, after birth they are produced at
the red bone marrow
- the average life span is around 127 days
- they are destroyed mainly in liver and spleen, where they are engulfed by large
phagocytic cells
2. White blood cells (leucocytes)
- their number varied from 5000 to 10000/ mm 3 ,in an inflammatory disease, the WBC
count may go up very high
- normally, the cells circulate freely and randomly, but the site of infection attracts
them, they fight against or remove bacteria and other infective agents
- there are two main types of white blood cells, all having nuclei
(1) Agranulocytes
- two types : lymphocytes (produce antibodies) and monocytes
- have, relatively, little cytoplasm which stained lightly, and a large nucleus
- without granules in the cytoplasm
- formed in lymphoid tissues such as the lymph nodes, tonsil, spleen and the thymus
(2) Granulocytes
- three types : basophils, eosinophils and neutrophils, they are stained by basic dyes,
eosin and neutral dyes respectively
- have granules in their cytoplasm and may possess more than one nucleus
- formed in red bone marrow
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 26
C. Blood platelets (thrombocytes)
- tiny, spherical and oval bodies, lacking nucleus
- formed by fragmentation of some cells in the red bone marrow
- to initiate blood clotting
II Functions
(a) transportation of gases : oxygen and carbon dioxide
(b) transport of nutrients, wastes, hormones and other metabolite
(c) distribution of heat and regulate of body temperature (by vaso- dilation and
vaso- constriction)
(d) defence against infection (function of WBC)
(e) blood clotting: this is to prevent blood lose and invasion of bacteria through wound
Nervous Tissues
- The cells which constitute nervous tissue always show branching protoplasmic processes,
some of which may be very long. Such processes are called nerve fibres.
- A nerve consists of bundle of fibres bound up with a connective tissue sheath.
Nerve cell (ne urones)
- each neurone consists of a cell body, which contains the nucleus and several thin fibres
extending from it
- one of theses fibres is the output fibre of the cell and it is called the axon which transmit
nerve impulse away from the cell body
- the other fibre are the dendrons and carry impulse towards the cell body
- the end of a dendron may branch into dendrites.
- functionally, neurones can be divided into three types :
sensory neurones : carry impulse from the receptors to central nervous system (CNS)
motor neurones : carry impulse from CNS to the effectors
association neurones : link up the sensory neurones with the motor neurones
- Functions :
(a) to receive stimuli from environment
(b) to transmit impulse rapidly from one part of the body to another
(c) to interpret nerve impulses
(d) to excite effectors and store information
(e) to give neurosecretion (hormones) for nerve impulse transmission
- Structure of the neurone
1. Cell body
- found in or near the CNS or ganglion
- they are round, oval, stellate in shape
- the nucleus is large with prominent nucleolus
- there are neurofibrils and rod- like Nissel granules except where the fibres arise (the
exit of the fibres)
2. Nerve fibres
- two types :
(1) Myelinated fibres (white fibres)
- found in cranial nerves, spinal nerves and white matter
- the myelin gives the white colour of the fibres
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 27
- the fibre is surrounded by a delicate membrane, the neurilemma, outside the
neurilemma is a white myelin sheath
- the myelin sheath is formed from Schwann cells which go spirally around the
fibre forming many turns, it produces myelin and later loses its cytoplasm
leaving layers
- the myelin sheath is interrupted in the peripheral fibres by constrictions, the
node of Ranvier, the neurilemma is continuous over the nodes of Ranvier
- the myelin sheath act as an insulator to speed up the flow of nervous impulses
through the nerve fibre and to protect the nerve fibre from injury, as well as
to supply nutrient to the fibre
(2) Non- myelinated fibres (grey fibres)
- absent of myelin sheath but the fibre enclosed by the neurilemma and
connective tissue only
- the nuclei of the neurilemma cause some swellings along the fibre
UB p62 fig 5.13b
Fig.30 Motor neurone
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