BOT 103 MSc I Ch II. CYTOGENETICS & MOLECULAR BIOLOGY
Ch II. Cell and its components
Prof. Dr. Sadhana Nikam
Vice Principal RJ College
M.Sc , B.Ed, PGDDE, Ph.D
Defination
of Cell:
A basic
structural and functional unit of all living organism is called Cell.
Types of
Cell :
A.Prokaryotic
Cell
B.Eukaryotic
Cell
Q. Write a
note on Cell Structure in Prokaryotes (5 Marks)
Prokaryotic Cell
Structure :
Prokaryotic cells are not as complex as eukaryotic cells. They have no
true nucleus as the DNA is not contained within a membrane
or separated from the rest of the cell, but is coiled up in a region of
the cytoplasm called the
nucleoid. Prokaryotic organisms have varying cell shapes. The most common bacteria shapes are spherical,
rod-shaped, and spiral.
Using bacteria as our sample prokaryote, the
following structures and organelles can be found
in bacterial cells:
- Capsule -
Found in some bacterial cells, this additional outer covering protects the
cell when it is engulfed by other organisms, assists in retaining
moisture, and helps the cell adhere to surfaces and nutrients.
- Cell
Wall - The cell wall is an
outer covering that protects the bacterial cell and gives it shape.
- Cytoplasm -
Cytoplasm is a gel-like substance composed mainly of water that also
contains enzymes, salts, cell components, and various organic molecules.
- Cell
Membrane or Plasma Membrane - The cell membrane surrounds
the cell's cytoplasm and regulates the flow of substances in and out of
the cell.
- Pili (Pilus
singular)- Hair-like structures on the surface of the cell that
attach to other bacterial cells. Shorter pili called fimbriae help
bacteria attach to surfaces.
- Flagella - Flagella are
long, whip-like protrusion that aids in cellular locomotion.
- Ribosomes - Ribosomes are
cell structures responsible for protein production.
- Plasmids -
Plasmids are gene carrying,
circular DNA structures that are not involved in reproduction.
- Nucleiod
Region - Area of the cytoplasm that
contains the single bacterial DNA molecule.
Prokaryotic cells lack organelles found in
eukaryoitic cells such as mitochondria, endoplasmic reticuli, and Golgi complexes. According to
the Endosymbiotic Theory,
eukaryotic organelles are thought to have evolved from prokaryotic cells living
in endosymbiotic relationships with one another.
Like plant cells, bacteria have a cell
wall. Some bacteria also have a polysaccharide capsule layer surrounding the
cell wall. It is in this layer where bacteria produce biofilm,
a slimy substance that helps bacterial colonies adhere to surfaces and to each
other for protection against antibiotics, chemicals, and other hazardous
substances.
Similar to plants and algae, some prokaryotes also have photosynthetic pigments. These light-absorbing pigments enable photosynthetic bacteria to obtain nutrition from light.
Q. Write a note on Cell structure in Eukaryotes (5 Marks)
Eukaryotic cells are present in complex living
organisms like animals, humans, and plants. They formed as a result of
evolutionary changes that took lace in the prokaryotic cells. You can refer to the
following image for understanding the cell structure.
Cell Wall:
It is a distinguishing part of plant cells, and is absent in animals. It imparts rigidity. Its material is different for different plant species with cell shapes being elongated, oval, round, rectangular, or square-shaped.
Cell Membrane:
The outermost part of the cell is the cell membrane, which
encloses all the cell organelles. Protecting the cell, providing rigidity, and
controlling the flow of nutrients within the cells are important functions of
the cell membrane.
Cell Cytoplasm:
This liquid gel-like substance is called matrix,
within which the cell organelles float and/or are embedded. It provides the
right environment to carry out all the metabolic reactions.
Nucleus:
Eukaryotic cells are considered advanced and complex. The
nucleus is made up of genetic material, i.e., the DNA (Deoxyribonucleic acid)
and the chromosomes, owing to which it is considered as the brain of
the cell. It basically controls all the cell functions, and guides it properly.
Nucleolus:
The interior of the nucleus has a dark stained area called the
nucleolus, which is responsible for protein formation.
Nuclear Membrane:
Peculiar to the eukaryotic cells, the main function of this
membrane is to protect the nucleus by formation of a protective sheath around
it.
Nucleoplasm:
Nucleus is filled with this dense fluid that contains chromatin
fibers, chromosomes, and genes that carry the genetic information.
Mitochondria:
They are among the largest cell organelles present in the
eukaryotic cells. They are characterized by their own Mitochondrial DNA, RNA,
and ribosomes; and hence, can self-replicate. It is the key site for production
of energy in the form of ATP molecules, and thus aids photosynthesis and
respiration.
Plastids:
Another peculiar organelle present in eukaryotic plant cells are
the plastids. Photosynthesis is the unique process, by which plants prepare
their own food with the aid of these organelles. Plants generally contain
chloroplasts that are characterized by the presence of a green colored pigment
called chlorophyll.
Ribosomes:
They are essential for protein synthesis, which includes
transcription and translation. All the ribosomes are of 80S type, except the
one from mitochondria and plastids, which is of the 70S type.
Lysosomes:
They mainly help to undertake phagocytosis, and
promote intracellular digestion. They are also responsible for secretion of
enzymes, which are necessary for breaking down the cell debris.
Centrosomes:
Centrioles contained within the centrosomes are important for
the process of initiation of cell division, the result being either mitosis or
meiosis.
Endoplasmic Reticulum (ER):
These interconnecting flattened tubular tunnels are of two
types: Rough Endoplasmic Reticulum (RER) and Smooth Endoplasmic Reticulum
(SER). In combination with the ribosomes, they help in functions related to
protein transport. ER is regarded as one of the most important cell organelles
after mitochondria.
Golgi Apparatus:
Their function includes protein processing so that active
protein chains are released whenever required.
Vacuoles :
Alike in plants and animals, vacuoles are water-filled
organelles responsible for storage.
Various Cell Organelles :
Cell Wall: Nonliving
outermost rigid covering of plant cell is called Cell Wall.
Cell Wall consist of three layers as
A. Middle Pamela
B.Primary Wall
C.Secondary Wall
A. Middle Lamela : It is formed between adjacent
Cell Wall during Cell division. It is made up
of
Pectin, Cellulose, Calcium and Polymers.
B.Primary Wall : It is formed during early
stage of growth. It is 1 to 3 mm thick. It is made up of
Cellulose, hemicellulose and Pectic compound.
It is elastic layer. In many fleshy ROOTS fruits
and leaves only primary wall and middle lamela present.
C. Secondary Wall : When primary wall stop its
growth secondary wall form down to primary cell wall.
It is 5 to 10 mm thick. It is 5 – 10 mm thick.
It consist of three
layers as outer, middle and inner. Secondary Cell wall consist of many layers
of closely pack microfibrils made up of cellulose.
In some tissue tertiary cell wall is present on surface of
secondary cell wall. It is thin layer occur in xylem trachieds of Gymnosperm
composed of xylan.
A major role of the cell wall is to form a
framework for the cell to prevent over expansion. Cellulose fibers, structural
proteins, and other polysaccharides help to maintain the shape and form of the
cell.
Additional functions of the cell
wall include:
- Support: The
cell wall provides mechanical strength and support. It also controls the
direction of cell growth.
- Withstand
turgor pressure: Turgor
pressure is the force exerted against the cell wall as the contents of the
cell push the plasma membrane against the cell wall. This pressure helps a
plant to remain rigid and erect, but can also cause a cell to rupture.
- Regulate
growth: The cell wall sends signals
for the cell to enter the cell cyclein
order to divide and grow.
- Regulate
diffusion: The cell wall is porous
allowing some substances, including proteins,
to pass into the cell while keeping other substances out.
- Communication: Cells
communicate with one another via plasmodesmata (pores or channels between
plant cell walls that allow molecules and communication signals to pass
between individual plant cells).
- Protection: The
cell wall provides a barrier to protect against plant viruses and
other pathogens. It also helps to prevent water loss.
- Storage: The
cell wall stores carbohydrates for use in plant growth, especially in
seeds.
Endoplasmic Reticulum :
1.Endoplasmic
Reticulum is a cell organelle
found in all plant
and animal cell
and absent in
prokaryotes, in ova and
mature RBCs.
2. The term Endoplasmic
reticulum was first
used by Porter
and Kallman in
1952.
3. E.R. consist
of an interconnected system
of membrane bound
channels in the
cytoplasm.
4. E.R. consist
of three componants
as Cisternae, tubules and
vesicles.
5. The Cisternae are
broad flat membrane
bound stacks arranged
parallel to each
other to form
lamellae.
6. The tubules are
irregular branching having
diameter of 50 – 100 A.
7. The Vesicles are
spherical membrane bound
cavities present on
cisternae.
8. E. R. is a continuous
system connecting one
end with the
nuclear membrane and
other end with
the plasma membrane.
9. The lumen of
E.R. have diameter with
400 – 700 A diameter
filled with an
endoplamic matrix.
10. Each membrane of
cisternae, tubules and vesicles
of E.R. is about
50 – 60 A thick.
11. There are two
basic morphological types
of E.R.as Rough
E.R. or granular E.R. and
Smooth E.R. or agranular
E.R.
Functions of Endoplamic Reticulum :
1.Supporting
Framework :
E. R. act as intracellular
supporting framework.
2. Mechanical Support
: E.R. provide mechanical support
to the colloidal
structure of cytoplasm.
3. Protein Synthesis :
RER take
part in protein
synthesis.
4. Detoxification : SER
is useful for
detoxification in the
liver.
5. Glycogen Synthesis
and Storage :SER membrane
synthesize and store
Glycogen.
6. Lipid synthesis :
SER synthesize Triglyceride and
RER synthsize Phospholopids.
7.Synthesis of Cholesterol
and Steroid hormones :SER
synthesize Cholesterol and
Steroid hormones.
8. Circulation and
Exchange : ER provide internal
surface for exchange
of material between
the matrix of
the cytoplasm and
lumen of cisternae.
9. Membrane Flow :
E.R. is essential for
transport of ions, molecules and particles
into and out
of the cells through
membrane flow.
10.Production : RER
give rise to
SER.
Mitochondria
Mitochondria (singular: mitochondrion) are organelles within eukaryotic cells that produce adenosine triphosphate (ATP), the main energy molecule used by the cell. For this reason, the mitochondrion is sometimes referred to as “the powerhouse of the cell”. Mitochondria are found in all eukaryotes.
Structure of
Mitochondria:
1.Mitochondria have two membranes, an outer membrane and an inner membrane. 2.These membranes are made of phospholipid layers, just like the cell’s outer membrane. 3.The outer membrane covers the surface of the mitochondrion, while the inner membrane is located within and has many folds called cristae. 4.The folds increase surface area of the membrane, which is important because the inner membrane holds the proteins involved in the electron transport chain. 5.It is also where many other chemical reactions take place to carry out the mitochondria’s many functions. 6.An increased surface area creates more space for more reactions to occur, and increases the mitochondria’s output. 7.The space between the outer and inner membranes is called the intermembrane space, and the space inside the inner membrane is called the matrix.
Functions of Mitochondria :
1.Mitochondria act as power house of the cell.
2.Mitochondria
produce ATP through process of cellular respiration—specifically, aerobic respiration, which
requires oxygen.
3..Mitochondria supply biological energy.
4. Mitochondria convert pyruvic acid
to carbon di oxide and water.
5. Mitochondria act as respiratory centres of the cell.
6. Kreb cycle occur in matrix of mitochondria.
7.oxidative phosphorylation, which
also takes place in the mitochondria.
8. Mitochondria is essential for transport of ATP.
9. Mitochondria is useful
for synthesis of
Lipids.
10.They can store calcium, which maintains homeostasis of calcium levels in the cell. 11.They also regulate the cell’s metabolism and have roles in apoptosis (controlled cell death), cell signaling, and thermogenesis (heat production). Lysosomes : 1.Lysosome is a cell organelle present in all animal cells but absent in plant cell. 2. de Duve in 1955 termed as lysosome.
3. Lysosomes originate from Golgi associated endoplasmic reticulum.(GERL).
4. Lysosomes are
irregular shape having
diameter of 0.2
to 0.8 micrometer.
5.Lysosomes are
made up of
single unit membrane
which is composed
of Lipoprotein.
6. Lysosomes inside
contain various hydrolytic
enzymes.
7. Lysosomes are
polymorphic in nature
having four types
as
a. Primary Lysosomes
b. Secondary Lysosomes
c. Residual bodies
d. Autophagic vacuole.
Functions of Lysosome :
1.Extracellular digestion :
Lysosomes by
secreting hydrolytic enzymes
digest the dead
cell.
2. Intracellular digestion :
Lusosomes digest
intracellular substances by
autophagy or heterophagy.
3.Cellular digestion :
Lysosomes eliminate
excess cell organelles
by autophagy.
4. Suicide bags :
Lysosomes are
called Suicide bags
of the cell
as they digest
themselves.
5.Fertilization :
Lysosomes help in fertilization as sperm releases hyaluronidase enzyme through acrosome Which help the sperm to penetrate the ovum.
6. Removal :
Lysosomes help
in removal of
carcinogens.
7. Developmental processes :
Lysosomes are
important for remodeling
of tadpole tail
and insect larval
tissue.
8. Digestion :
Lysosomes help
in digestion of
food and foreign
substances by phagocytosis.
Microbody :
Structure :
1.A microbody is usually a vesicle with a spherical shape, ranging
from 0.2-1.5 micrometres in diameter.
2.The microbodies are found in the cytoplasm of a cell, but they
are only visible with the help of an electron microscope.
3.They are surrounded by a single phospholipid bilayer membrane
and they contain a matrix of intracellular material including enzymes and other
proteins, but they do not seem to contain any genetic material to allow them to
self-replicate.
Functions:
1.Microbodies contain enzymes that participate in the preparatory or
intermediate stages of biochemical reactions within the cell.
2.This facilitates the breakdown of fats, alcohols and amino
acids.
3. Generally, microbodies are involved in detoxification of
peroxides and in photorespiration in plants.
4.Different types of microbodies have different functions:
A peroxisome is a type of microbody that functions to help the
body break down large molecules and detoxify hazardous substances.
5.Glyoxysomes are specialized peroxisomes found in plants and
mold, which help to convert stored lipids into carbohydrates so that they can
be used for plant growth.
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