Chapter – 4
CELL
It is the basic
structural and functional unit of life, which is able to carry out all the life
processes.
CELL THEORY
The cell theory was
collectively proposed by “Schleiden(1838), Schawnn(1839) and Virchow (1858).
IMPORTANT POSTULATES
The fundamental points of the cell theory are:
(a) The cell is the structural and functional unit of all living organism.
(b) All organisms are composed of one or more cells.
(c) New cells can arise only by division of pre-existing cells.
Thus cell theory established the concept that the function of an organism is
the result of activities and interaction of the cell units.
MICROSCOPE
DEFINITION
An instrument with the help of which we see small, tiny
and minute objects which can’t be observe by naked human eye.
TYPES OF MICROSCOPE
There are three
main types of microscope.
1. LIGHT MICRO SCOPE
In this microscope visible light is used as source of
illumination.
2. X-RAY MICROSCOPE
X-Rays are used as source of illumination.
3. ELECTRON MICROSCOPE
Electron beam is used as source of illumination.
There are further two sub-types of electron microscope which are:
(A)TRANSMISSION ELECTRON
MICROSCOPE
In this type resultant image is obtained on a fluorescent
screen or photographic film.
(B)SCANNING ELECTRON MICROSCOPE
In this type resultant image is obtained on a television
screen.
MAGNIFICATION OF MICROSCOPE
Ability of
microscope to increase the shape and size of the objects image. It can be
calculated by multiplying the power of its eye pieces with its magnifying power
of its objective.
RESOLUTION OF MICROSCOPE
The capacity of
microscope to separate adjacent forms or object. Also known as “Minimum
Resolved Distance”.
CONTRAST
It is important to
distinguishing one part of cell from another.
(Difference between
light and electron microscope – From Text page #57)
Prokaryotes and eukaryotes – From Text page #58)
CELL MEMBRANE
Each cell is
covered by an asymmetrical, porous, thin, semi permeable sheet called cell
membrane or plasmalemma.
CHARACTERISTICS OF CELL MEMBRANE
Living part of the
cell, consist of lipid + protein.
§ 1.5 micron in thickness.
§ Consist of two layers of lipid.
§ Lipid of plasma membrane are,
1. Phospho-lipids
2. Glycolipids
3. Sterol
4. Cholesterol.
STRUCTURE OF CELL MEMBRANE
Cell membrane made
up of phospho-lipids bilayer and each layer consists of ,
1. Head (hydrophilic end)
2. Tail (hydrophobic end)
1. HEAD (HYDROPHILIC/POLAR END)
Present towards the surface and formed of phosphates.
2. TAIL (HYDROPHOBIC/NON-POLAR
END)
Present towards the center and formed of fatty acids.
The non-polar ends of phospho lipids face each other, whereas their polar ends
are in association with protein or carbohydrates between every two phospo
lipids molecule lies a molecule of “Cholesterol”.
FLUID MOSAIC MODEL
INTRODUCTION
The fluid mosaic, bilayer model was proposed by “Singer
and Nicolson (1972).
POSTULATES OF FLUID MOSAIC MODEL
Important
postulates of this model are,
(a) The cell membrane consists of lipid bilayer, in which a variety of proteins
are present.
(b) These proteins float in the fluid matrix of lipid (as ice bergs in the sea)
(FIGURE 4.4 Page #61)
ARRANGEMENT OF PROTEINS
According to the fluid mosaic model proteins are:
1. INTRINSIC/INTEGRAL PROTEINS
These proteins peneterate the membrane surface and enter the lipid layers
(partially or wholly)
2. EXTRINSIC/PERIPHERAL PROTEINS
These are located adjacent to outer and inner surface of membrane and float
like ice-berg in the sea.
ARRANGEMENT OF LIPIDS
The non-polar end face each other while their polar ends
are towards the surface.
SIGNIFICANCE OF MODEL
§ Cell membrane is flexible.
§ Can change shape (because the protein and lipid of the membrane
can move).
FUNCTION OF MEMBRANE PROTEIN
§ Certain proteins themselves act as enzymes.
§ Some protein act as carrier for active transport.
§ Provide elasticity to membrane.
§ Pores are lined by the proteins.
FUNCTION OF LIPIDS PRESENT IN
MEMBRANE
§ The lipids give rigidity to cell membrane.
§ They lower the surface tension.
FUNCTIONS OF CELL MEMBRANE
§ It performs the two main function.
§ Protection of Protoplasm.
§ Regulation of material (In and Out of cell) through its
permeabality.
PERMEABILITY OF MEMBRANE
The permeability of
membrane is regulated by two processes.
(1) Passive Transport (Osmosis and Diffusion)
(2) Active Transport (Endocytosis, Exocytosis)
1. PASSIVE TRANSPORT
Such type of molecules transport which does not require
energy. It is further divided into,
DIFFUSION
Spreading and free movement of molecules (or ions) from
the region of higher concentration to the region of lower concentration (till
equilibrium state)
SIGNIFICANCE
§ Movement of oxygen and digested food (glucose, amino acids,
fatty acids) into the cell.
§ Movement of excretory waste out of cell.
OSMOSIS
Diffusion of water by semipermeable membrane or the
movement of solvent molecules from higher to lower concentration across semi
permeable membrane.
SIGNIFICANCE
§ Liquids, primarily water molecules enter and leave the cell by
Osmosis.
§ It helps to maintain a balance (osmotic pressure) in and out of
cell.
2. ACTIVE TRANSPORT
Such type of molecule transport which require energy. Or
Movement of molecules against the concentration by the expenditure of energy
through a carrier (i.e. movement of molecules from the region of lower concentration
to higher concentration by protein using ATP as energy.
SIGNIFICANCE
Absorption of excess food (glucose), ions (K+ and Na+)
takes place by Active transport.
CONDITIONS
§ It is unidirectional.
§ ATP provides energy.
§ Protein act as carrier.
Active transport is
further subdivided into,
(1) Phagocytosis and Pinocytosis (Endocytosis).
(2) Exocytosis.
PHAGOCYTOSIS
Process of picking and ingestion of large solid particle
by plasma membrane (which can not enter by diffusion, osmosis or active
transport).
SIGNIFICANCE
Ingestion of solid food particles.
WBCs pick foreign particles (certain bacteria)
PINOCYTOSIS
Process of fluid intake, for absorbing fluid by forming
pinocytic vesicle (the fluid which cannot be absorbed by osmosis, enters
through it)
SIGNIFICANCE
Helps in absorption of harmones, lipids etc.
CELL WALL
The cell wall is
the outer most covering of a plant cell. It is composed of cellulose (a
carbohydrate) and some other chemical substances.
This hard covering gives form, firmness and strength to the plant cell.
In a young cell it is thin and delicate but in a mature cell it becomes thick
due to the deposition of various chemical substances on its inner surface.
There are three layer of cell wall.
1. MIDDLE (LAMELLA)
§ First formed cell plate.
§ Cementing layer between two daughter cells.
§ Composed of Ca++ and Mg++ pectate.
§ Cells are separated when this layer is dissolved.
2. PRIMARY WALL
§ First product of cell synthesized by protoplast.
§ In young cells it is thin and elastic while it becomes thick and
rigid on maturity.
§ Made up of Hemicellulose (50%), cellulose (25%) and pectate
substances.
3. SECONDARY WALL
§ Composed of cellulose.
§ Present inside the primary wall.
§ Can be modified through the deposition of lignin and other
substances.
NUCLEUS
It control all the
activities of the cell and was discovered by Robert Brown in 1831.
It consist of the following parts,
(1) Nuclear Membrane.
(2) Nucleoplasm or Karyoplasm.
(3) Nucleolus.
(4) Chromatin Network.
1. NUCLEAR MEMBRANE
The nucleus is
bounded by a double layered membrane which bears pores and is known as “Nuclear
Membrane”
2. NUCLEOPLASM
Inside the nuclear
membrane is a structure less fluid called “Nucleoplasm” and highly rich with
proteins.
3. NUCLEOLUS
It is a patch work
of granules rich in R.N.A formed in the nucleus. They may be more than one in a
single nucleus. It contains mRNA formed from DNA, later mRNA comes out of
nucleus to control protein formation.
4. CHROMATIN NETWORK
There is a network
of threads dispersed in the karyoplasm called (Chromatin network)
Each individual thread is called (Chromosomes).
These are made up of DNA and are carrier of genes.
NOTE:(Types of Chromosomes from Book Page# 66)
MEMBRANE BOUND ORGANELLES
(1) ENDOPLASMIC RETUCULUM
It is a complex
series of tubules in the cytoplasm. Endoplasmic reticulum are of two types,
(1) Agranular or
Smooth EPR.
(2) Granular or Rough EPR.
SMOOTH EPR
§ It has no attached ribosome’s.
§ Function is to synthesis lipid.
ROUGH EPR
§ It has ribosome’s attached to its outer surface.
§ Synthesize protein and also transport material within the cell.
(2) MITOCHONDRIA
An oval body
bounded by a double membrane. The inner membrane is folded to form
shelves/incomplete partitions. Which are known as “Crista”, here oxidative
enzyme are present. They are sites for aerobic cellular respiration and the
energy is produced. Therefore also known as “Power house of cell”
(3) GOLGI APPARATUS(DICTYOSOMES)
These are thin,
plate like structures and are usually located near the nucleus. These are the
site of formation of lysosomes and also conjugate protein, modify structure of
substances, synthesized by EPR to form lysosomes and secretary vesides. Golgi
bodies of plants and lower animals (mostly invertebrates) are known as
“Dictyosomes”.
(4) LYSOSOMES
They are large,
some what irregular structure formed in the cytoplasm formed by golgi-bodies.
They contain hydrolytic enzymes which destroys foreign particles. They are also
known as “Suicide Sacs” because after secreting the enzymes they digese their
own proteins (Autophagy).
NOTE:(Lysosomal Storage Diseases From Text Page # 71)
(5) PLASTIDS
They are
specialized organelles of plant cell that contain pigment or they synthesize
reserve substances.
They are of three kinds,
(A) LEUKOPLAST
leuco = white
Leukoplast are colourless and store nutrient material.
(B)CHLOROPLAST
Chloroplast are green having chlorophyll that performs
photosynthesis.
(C) CHROMOPLAST
Chromo = Colour
Chromoplast contain different coloured (red, yellow, orange or other than
green) pigments. They are found in the cells of different coloured flowers and
fruits.
(6) MICRO BODIES
It includes
peroxisome and glyoxysome.
(A) PEROXISOME
These are the single membrane bounded microbodies contain
enzymes for transferring hydrogen atom to oxygen i.e. forming hydrogen
peroxide.
Hydrogen peroxide is very toxic to the cell therefore it is immediately break
down to water by enzyme catalyst.
These microbodies help in detoxyfication of alcohal and mostly present in liver
cells.
(B) GLYOXYSOME
It is a single layered membrane bound structure containing
enzymes which metabolize some molecules in photosynthesis and respiration.
They also cause oxidation of fatty acids.
CYTOSKELETON
Cytoskeleton means
skeleton of the cell, which is mostly composed of microtubules, microfilaments
and intermediate filaments.
(A) MICRO TUBULES
§ Microtubules are hollow cylinders with an outerdiameter of 25nm.
§ They are made up of a special type of globular protein tubulin.
§ In single microtubule consist of hundredth of thousands of
tubulin sub units, which are usually arranged in 13 columns called
Protofilaments.
§ Microtubules are arranged in assemble and disassemble manner.
§ In animal cells and lower plants they also form centriole, cilia
and flagella.
(B) MICROFILAMENTS
§ Microfilaments are solid structures, thread like with a diameter
of 7nm.
§ They are also composed of globular proteins.
§ Each microfilament consist of two actin (Protein) chains that
inter wing in a helical fashion.
(C) INTERMEDIATE FILAMENTS
§ They are intermediate in size having a diameter of 8nm to 11nm.
§ They are rope like polymers of Fibrous protein.
§ In skin and hair these filaments are made up of protein keratin.
§ They provide mechanical strength to the cell and support the
nuclear envelope.
NON MEMBRANE BOUND CYTOPLASMIC ORGENELLE
(1)RIBOSOMES
§ These are small structures concerned with protein synthesis in
all type of the cells i.e. Prokaryotic as well as Eukaryote.
§ They are freely dispersed in cytoplasm of Prokaryotic cell but
in Eukaryotic cells they may be free or attached with endoplasmic reticulum.
§ More than 50 type of proteins are present in ribosome structure
and they contain high quantity of RNA.
§ Under the direction of Nucleus ribosome produce the protein made
it by the cell.
§ Each Ribosome consist of two unequal parts.
§ These are the smallest and most vital cellular components,
manufactured in the nucleolus.
(2) CENTRIOLE
§ They are only present in animal cells and certain lower plants.
§ Mostly near the nucleus.
§ Each centriole consist of two cylinders lying perpendicular to
one another.
§ Each cylinder consist of nine parallel triplets of hollow
cylindrical microtubules.
§ During the cell division they replicate and move towards
opposite poles of the cell.
§ In mitosis and meiosis they form thread like fibers which
rediate from each centriole are known as mitotic apparatus.
(3)VACUOLES
§ These are non-protoplasmic fluid filled cavities in the
cytoplasm.
§ Their membrane is known as Tonoplast.
§ They are more prominent in mature cells.
§ In plant cells vacuoles are filled with cell sap and act as
store, house.
§ They also play an important role in plant defence.
§ In animal cells vacuole contain hydrolytic enzymes (i.e.
lysosomes)
(Note: Techniques
to isolate components of the cell Text Page # 58).
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