STRUCTURE OF CELL – ITS COMPONENTS AND THEIR FUNCTIONS.pptx

STRUCTURE OF CELL – ITS
COMPONENTS AND THEIR FUNCTIONS
Unit2

Learning objective
At the end of this chapter, the students should be able to:
- Explain structure and characteristics of human cell
- Describe movements of particles across cell membrane
- Discuss organelles of human cell & their function
- Explain abnormal cellular function (cancer)

2.1 Cell
Cell is the basic living structural and functional unit of the body.
Cytology: - It is a branch of science concerned with a study of cells
Cell Theory explains about
a) All living organisms are composed of cell and cell products.
b) Cell is the basic unit of structure & function of all living organisms.
c) All cells come from the division of pre existing cell.
d) An organism as a whole can be understood through the collective activities & interactions of its cells.

To know more about cell, we can divide the cell in to four principal parts: -
Plasma (cell) membrane: it is the outer lining, limiting membrane separating the cell
internal parts from extra cellular materials & external environment.
Cytoplasm: cytoplasm is the substance that surrounds organelles and is located
between the nucleus and plasma membrane

Human Anatomy and Physiology
Organelles: these are permanent structures with characteristic morphology that are
highly specialized in specific cellular activity.
Inclusions: they are the secretions and storage products of cells.
Extra cellular materials are also referred to as the matrix, which are substances
external to the cell surface.

Figure: 2.1 structure of a cell (Source:
Carola, R., Harley,J.P., Noback R.C., (1992),
Human anatomy and physiology, Mc Graw hill
inc, New York, 2nd
Ed, pp 57)

2.1.1 Plasma Membrane

Plasma membrane is a thin outer membrane, which maintains the integrity of the cell. It keeps the cell and its
contents separate and distinct from the surrounding. It is a double layered measuring about 4.5 nm and made of
phospholipids, cholesterol, glyco-lipid, & carbohydrate (oligosaccharides). The bi-layer is self-sealing. If a needle
is injected and pulled out, it automatically seals.

Functions: -
1. Separate the cytoplasm inside a cell from extra cellular fluid.
2. Separate cell from one another
3. Provide an abundant surface on which chemical reaction can occur.
4. Regulate the passage of materials in to and out of cells. It
 also let some things in and keeps others out. The quality selective permeability

Movement across-cell membrane
Movements a cross membrane takes place in two ways.
These are passive and active movements. Passive movement uses energy whereas
active movement consumes energy in the form of ATP.

Passive movement: includes
a. Simple diffusion, the random movements of molecules
 from area of high concentration to the area of low concentration. Example air in alveoli of
lung
b. Facilitated diffusion, larger molecules, which are not soluble in lipid need protein channel to
pass through the plasma membrane. No direct energy needed. Example: - Amino acid
passes through the cell membrane.
c. Osmosis, a special type of diffusion referring to the passage of water through a selectively
permeable membrane from an area of high water concentration to lower water
concentration.
d. Filtration, small molecules pass through selectively permeable membrane in response to
force of pressure. Example: - filtration in the kidney in the process of urine formation.

Active movements across membranes
Substances move through a selectively permeable membrane from areas of low
concentration on side of a membrane to an
area of higher concentration on the other side. This is against concentration gradient.
Therefore, it requires energy.
 a) Active Transport: till equilibrium substances could more by passive movement. But
if equilibrium reached and still more molecules are needed, they must be pumped

 through the membrane against concentration gradient. This process requires the use of
ATP. One example of such processes is Sodium – potassium pump and calcium pump. In
this process all follows similar process. These are molecules bind to carrier protein,
molecule- carrier complex pass through the membrane, assisted by an enzyme & ATP
and carrier protein returns to its original shape & repeat the process.
b) Endocytosis, pocketing in by plasma membrane. It includes:
 Pinocytoss – cell drinking
 Receptor – mediated Endocytosis- Endocytosis with the help of receptor.
 Phagocytosis- cell eating.
c) Exocytosis, opposite to Endocytosis, to remove out undigested particles.

2.1.2 Cytoplasm
Cytoplasm is a matrix or ground substance in which various cellular components are
found. It is thick semi transparent, elastic fluid containing suspended particles and a
series of minute tubules and filaments that form cytoskeleton. Water constitutes 75-90%
of the cytoplasm. It also contains solid components, proteins, carbohydrates, lipids and
inorganic substances. The inorganic components exist as solutions
because they are soluble in water. The majority of organic substances however are
found as colloids. Colloids are particles that remain suspended in the surrounding
medium

2.1.3 Organelles
Organelles are specialized portion of the cell with a characteristic shape that assume specific role in growth,
maintenance, repair and control.
a) Nucleus, Oval in shape and is the largest structure in the cell. Contain the hereditary factor in the cell. Hence it
controls cell activity & structure. Most cell contain single nucleus but some like matured Red Blood cell do not
contain. However Muscle cell contain several nucleuses. The nucleus separated from other cell structure by double
membrane called nuclear membrane. Pores over the nuclear membrane allow the nucleus to communicate with
the cytoplasm. In the nucleus a jelly like fluid that fill the nucleus is karylymph (neucleoplasm), which contain the
genetic material called chromosome. Nucleus also contain dark, somewhat spherical, non-membrane bound mass
called nucleolus. It contains DNA, RNA and protein, which assist in the construction of ribosome.
b) Ribosome, tiny granules, composed of Ribosomal RNA (rRNA). They are site of protein synthesis

c) Endoplasmic reticulum is a double membrane channel. It is continuous with the nuclear membrane. It
involved in intracellular exchange of material with the cytoplasm. Various products are transported from
one portion of the cell to another via the endoplasmic reticulum. So it is considered as intracellular
transportation. It is also storage for synthesized molecules. Together with the Golgi complex it serves as
synthesis & packaging center.
 Endoplasmic reticulum (ER) is divided in to two. These are, granular E.R. Containing granule and involving
in synthesis of protein and agranular E.R. that synthesize lipid & involves in detoxification.

d)Golgi Complex, near to the nucleus. It consist 4-8
 membranous sacs. It process, sort, pack & deliver protein to various parts of the
cell.
e)Mitochondria, a small, spherical, rod shaped or filamentous structure. It generates
energy. Each mitochondria posses two membrane, one is smooth (upper)
membrane and the other is arranged with series of folds called cristae. The central
cavity of a mitochondrion enclosed by the inner membrane is the matrix.
f) Lysosomes appear as membrane enclosed spheres. They are formed from Golgi
complexes & have single membrane. They contain powerful digestive (hydrolytic

 enzyme capable of breaking down many kinds of molecules. The lysosomal enzyme believed to
be synthesized in the granular endoplasmic reticulum and Golgi complex.
g) The cyto-skeleton, the cytoplasm has a complex internal structure consisting of a series of
exceedingly small microfilaments, microtubule & intermediate filaments together referred to as
the cyto-skeleton.
h) Centrosme, a dense area of cytoplasm generally spherical and located near the nucleus it
contain centrioles. It also contains DNA that controls their replication. Centrosmes are made of
microtubules, which seam drinking straws. They are Involved in the movement of chromosome
during cell division.
i) Cilia/flagella, thread like appendages, which are made of microtubules. When they are beating
forms rhythmic movement. They are found in female reproductive organ and upper respiratory
tube.

1. TISSUE
Cells are highly organized units. But in multicultural organisms, they do not function in
isolation. They work together in-group of similar cells called tissue. Tissue is a group of
similar cell and their intercellular substance that have a similar embryological origin and
function together to perform a specialized activity. A science that deals with the study of a
tissue is Histology.
The various tissues of the body are classified in to four principal parts according to their
function & structure. These are epithelial, connective, muscular, and Nervous tissue.

1. Epithelial tissue
Epithelial tissues covers body surface, lines body cavity & ducts and form glands. They are
subdivided in to:
 Covering & lining epithelium
 Glandular epithelium
Covering and lining epithelium: it forms the outer covering of external body surface and
outer covering of some internal organs. It lines body cavity, interior of respiratory & gastro
intestinal tracts, blood vessels & ducts and make up along with the nervous tissue (the parts of
sense organs for smell,

hearing, vision and touch). It is a tissue from which gametes (egg & sperm) develops.
Covering and lining epithelium are classified based on the arrangement of layers and cell shape.
According to the arrangement of layers covering and lining epithelium is grouped in to:
a) Simple epithelium: it is specialized for absorption, and filtration with minimal wear & tear. It is a single
layered
b) Stratified epithelium, it is many layered and found in an area with high degree of wear & tear.
c) Pseudo-stratified, is a single layered but seam to have many layer.

Based on the cell shape covering and lining epithelium is grouped in to:
a)Squamous: - flattened & scale like
b)Cuboidal: - cube shaped
c) Columnar: - tall & cylindrical
d)Transitional: - combination of cell shape found where there is a great degree of
distention or expansion, these may be cuboidal to columnar, cuboidal to polyhydral
and cuboidal to Squamous

 Therefore considering the number of layers and cell shape we can classify covering and lining
epithelium in to the following groups
Simple epithelium
a)Simple – Squamous epithelium, contain single layer of
 flat, scale like resemble tiled floor. It is highly adapted to diffusion, osmosis & filtration. Thus, it
lines the air sacs of lung, in kidneys, blood vessels and lymph vessels.
b)Simple – cuboidal epithelium, Flat polygon that covers the
 surface of ovary, lines the anterior surface of lens of the eye, retina & tubules of kidney
c) Simple – columnar epithelium, Similar to simple cuboidal. It is modified in several ways
depending on location & function. It lines the gastro-intestinal tract gall bladder, excretory ducts
of many glands. It functions in secretions, absorption, protection & lubrication.

Stratified epithelium
It is more durable, protects underlying tissues form external
environment and from wear & tear.
a) Stratified Squamous epithelium: In this type of epithelium, the
outer cells are flat. Stratified squamous epithelium is subdivided in
to two based on presence of keratin. These are Non-Keratnized
and Keratinized stratified squamous

epithelium. Non-Keratnized stratified squamous epithelium is found in wet surface that
are subjected to considerable wear and tear. Example: - Mouth, tongue and vagina. In
Keratinized, stratified squamous epithelium the surface cell of this type forms a tough
layer of material containing keratin. Example: skin. Keratin, is a waterproof protein, resists
friction and bacterial invasion.
b)Stratified cuboidal epithelium, rare type of epithelium. It is found in seat glands duct,
conjunctiva of eye, and cavernous urethra of the male urogenital system, pharynx &
epiglottis. Its main function is secretion.

b)Stratified columnar epithelium, uncommon to the body. Stratified columnar epithelium is
found in milk duct of mammary gland & anus layers. It functions in protection and secretion.
Transitional epithelium
The distinction is that cells of the outer layer in transitional
epithelium tend to be large and rounded rather than flat. The feature allows the tissue to be
stretched with out breakage. It is found in Urinary bladder, part of Ureters & urethra.
Pseudo stratified epithelium
Lines the larger excretory ducts of many glands, epididymis,
parts of male urethra and auditory tubes. Its main function is protection & secretion

3.1.2 Glandular Epithelium
Their main function is secretion. A gland may consist of one cell or a group of highly specialized epithelial
cell. Glands can be classified into exocrine and endocrine according to where they release their secretion.
Exocrine: Those glands that empties their secretion in to ducts/tubes that empty at the surface of covering.
Their main products are mucous, oil, wax, perspiration and digestive enzyme. Sweat & salivary glands are
exocrine glands.
Endocrine: They ultimately secret their products into the blood system. The secretions of endocrine glands
are always hormones. Hormones are chemicals that regulate various physiological activities. Pituitary,
thyroid & adrenal glands are endocrine.

Classification of exocrine glands
They are classified by their structure and shape of the secretary portion. According to structural classification
they
are grouped into:
a)Unicellular gland: Single celled. The best examples are goblet cell in Respiratory, Gastrointestinal &
Genitourinary system.
b)Multicultural gland: Found in several different forms
 By looking in to the secretary portion exocrine glands are grouped into
a)Tubular gland: If the secretary portion of a gland is tubular.
b) Acinar gland: If the secretary portion is flask like.
C) Tubulo-acinar: if it contains both tubular & flask shaped secretary portion.

Further more if the duct does not branch it is referred as a simple gland and if it branch's it is compound gland. By
combining the shape of the secretary portion with the degree of branching of the duct of exocrine glands are classified
in to
 Unicellular
 Multi-cellular
 Simple tubular Branched tubular Coiled tubular Acinar
 Branched Acinar
 - Compound
 Tubular
 Acinar
 Tubulo-acinar

3.1.2 Connective tissue
Connective tissues of the body are classified into embryonic connective tissue and adult connective tissue.
Embryonic connective tissue
Embrayonic connective tissue contains mesenchyme &
mucous connective tissue. Mesenchyme is the tissue from which all other connective tissue eventually arises. It is
located beneath the skin and along the developing bone of the embryo. Mucous (Wharton’s Jelly) connective tissue is
found primarily in the fetus and located in the umbilical cord of the fetus where it supports the cord.
Adult connective tissue
It is differentiated from mesenchyme and does not change after birth. Adult connective tissue composes connective
tissue proper, cartilage, osseous (bone) & vascular (blood)
tissue

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