paper viii unit i- 11-12 cell wall and plasma membrane
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D. D. Khedkar, Department of Botany, Shri Shivaji Science College, Amravati
CELL WALL
The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some
types of cells. It is located outside the cell membrane and provides these cells with structural
support and protection, and also acts as a filtering mechanism. A major function of the cell wall
is to act as a pressure vessel, preventing over-expansion when water enters the cell. They are
found in plants, bacteria, fungi, algae, and some archaea. Animals and protozoa do not have cell
walls.
Plant cell walls are thick walls that encase the cell, which can be numerous micrometers thick.
Cell walls are made of microfibrils of cellulose set in a base of proteins and other
polysaccharides. The wall itself consists of a primary cell wall, a secondary cell wall, and a
middle lamella. The plant cell also has many holes on its perimeter as well.
The material in the cell wall varies between species, and can also differ depending on cell type
and developmental stage. In bacteria, peptidoglycan forms the cell wall. Archaean cell walls
have various compositions, and may be formed of glycoprotein S-layers, pseudopeptidoglycan,
or polysaccharides. Fungi possess cell walls made of the glucosamine polymer chitin, and algae
typically possess walls made of glycoproteins and polysaccharides. Unusually, diatoms have a
cell wall composed of silicic acid. Often, other accessory molecules are found anchored to the
cell wall.
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D. D. Khedkar, Department of Botany, Shri Shivaji Science College, Amravati
PLANT WALL LAYERS
Many plant cells have walls that are strong enough to
withstand the osmotic pressure from the difference in solute
concentration between the cell interior and distilled water.
Up to three strata or layers may be found in plant cell walls:
1. The middle lamella, a layer rich in pectins. Thisoutermost layer forms the interface between adjacent
plant cells and glues them together.
2. The primary cell wall, generally a thin, flexible andextensible layer of cellulose formed while the cell is
growing.
3. The secondary cell wall, a thick layer formed insidethe primary cell wall after the cell is fully grown. It is not found in all cell types. In some
cells, such as found xylem, the secondary wall contains lignin, which strengthens and
waterproofs the wall.
The secondary cell wall consists mainly of cellulose, but also other polysaccharides, lignin, and
glycoproteins. It sometimes consists of three distinct layers - S1, S2 and S3 - where the direction
of the Cellulose microfibrils differs between the layers. Apparently there are no Structural
proteins or enzymes in the secondary wall.
The secondary cell wall has different ratios of wall constituents compared to the primary wall.
An example of this is that wood secondary walls contain xylans, whereas the primary wall
contains xyloglucans and the cellulose fraction is higher in the secondary wall. Pectins may also
be absent from the secondary wall and apparently it contain no Structural proteins or enzymes.
The Cellulose microfibrils give tensile strength, whereas lignification in addition to making thesecondary wall impermeable to water also give a "brittle" texture. Conceptually this give
lignified secondary wall properties resembling armored concrete, where the cellulose
microfibrils act as the armoring and the lignin as concrete.
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Lignification of the secondary wall confer resistance to pathogens by two mechanisms. As lignin
repel water, hydrolytic enzymes are less likely to attack and successfully penetrate the wall and it
lowers the nutritional value of the wall, providing less energy to pathogens.
Wood consists mostly of secondary cell wall, and holds the plant up against gravity.
Some secondary cell walls store nutrients, such as those in the cotyledons and the endosperm.
These contain little cellulose, and mostly other polysaccharides
COMPONENTS OF THE CELL WALL (CHEMISTRY OF CELL WALL)
Multiple layers of the cell wall possesses different components. Broadly one can classify them as
follows
1. Carbohydrates: Cellulose (23%), Hemicellulose (24%), Pectin (34%), etc.2. Proteins (19%)3. Lignin4. Lipids: Suberin, wax, cutin5. Water
The composition of the cell wall varies greatly amongst the plant species and even in the
different cell types of the same plant organ. Ex. Anatomical studies of the stem shows various
types of cells viz. parenchyma, collenchymas, sclerenchyma, xylem elements and phloem
elements; all these cells from the same location of plant organ has different chemical
composition.
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D. D. Khedkar, Department of Botany, Shri Shivaji Science College, Amravati
ULTRASTRUCTURE OF CELL WALL
Electron Microscopy has shown that the cell wall is constructed on the same architectural
principle which applied well in the construction of animal bones and such common building
materials as fibre glass (plastic + glass) or reinforced concrete (concrete + metal framework).
The strong fibres (cellulose microfibrils) resistance to tension embedded in an amorphous matrix
(comprising hemicelloulose, pectins and proteins). The plant cell wall is 0.2 micrometer thick
and completely coats the outside of the plant cells plasma membrane. This structure serves some
of the same functions as those of the extracellular matrix produced by animal cells, even though
the two structures are composed of entirely different macromolecules and have a different
organization. Like the extracellular matrix, the plant cell wall connects cells into tissues, signals
a plant cell to grow and divide, and controls the shape of plant organs. Just as the extracellular
matrix helps define the shapes of animal cells, the cell wall defines the shapes of plant cells.
When the cell wall is digested away from plant cells by hydrolytic enzymes, spherical cells
enclosed by a plasma membrane are left. In the past, the plant cell wall was viewed as an
inanimate rigid box, but it is now recognized as a dynamic structure that plays important roles in
controlling the differentiation of plant cells during embryogenesis and growth.
Because a major function of a plant cell wall is to withstand the osmotic turgor pressure of the
cell, the cell wall is built for lateral strength. It is arranged into layers of cellulose microfibrils
bundles of long, linear, extensively hydrogenbonded polymers of glucose in glycosidic
linkages. The cellulose microfibrils are embedded in a matrix composed of pectin, a polymer of
D-galacturonic acid and other monosaccharides, and hemicellulose, a short, highly branched
polymer of several five- and six-carbon monosaccharides.
The mechanical strength of the cell wall depends on crosslinking of the microfibrils by
hemicellulose chains. The layers of microfibrils prevent the cell wall fromstretching laterally.
Cellulose microfibrils are synthesized on the exoplasmic face of the plasma membrane from
UDPglucose and ADP-glucose formed in the cytosol. The polymerizing enzyme, called cellulose
synthase, moves within the plane of the plasma membrane as cellulose is formed, in directions
determined by the underlying microtubule cytoskeleton. Unlike cellulose, pectin and
hemicellulose are synthesized in the Golgi apparatus and transported to the cell surface where
they form an interlinked network that helps bind the walls of adjacent cells to one another and
cushions them. When purified, pectin binds water and forms a gel in the presence of Ca2+ and
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borate ionshence the use of pectins in many processed foods. As much as 15 percent of the cell
wall may be composed of extensin, a glycoprotein that contains abundant hydroxyproline and
serine. Most of the hydroxyproline residues are linked to short chains of arabinose (a five-carbon
monosaccharide), and the serine
residues are linked to galactose.
Carbohydrate accounts for about 65
percent of extensin by weight, and its
protein backbone forms an extended
rodlike helix with the hydroxyl or O-
linked carbohydrates protruding
outward. Lignina complex,
insoluble polymer of phenolic
residuesassociates with cellulose and is a strengthening material. Like cartilage proteoglycans,
lignin resists compression forces on the matrix.
The cell wall is a selective filter whose permeability is controlled largely by pectins in the wall
matrix. Whereas water and ions diffuse freely across cell walls, the diffusion of large molecules,
including proteins larger than 20
kDa, is limited. This limitation may
account for why many plant
hormones are small, water-solublemolecules, which can diffuse across
the cell wall and interact with
receptors in the plasma membrane of
plant cells. The cell wall microfibrils
are linked with plasma membrane in its lipid bilayer.
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D. D. Khedkar, Department of Botany, Shri Shivaji Science College, Amravati
FUNCTIONS OF THE CELL WALL:
The cell wall serves a variety of purposes including:
1. Maintaining/determining cell shape (analogous to an external skeleton for every cell). Sinceprotoplasts are invariably round, this is good evidence that the wall ultimately determines the
shape of plant cells.
2. Support and mechanical strength (allows plants to get tall, hold out thin leaves to obtainlight)
3. It prevents the cell membrane from bursting in a hypotonic medium (i.e., resists waterpressure)
4. It controls the rate and direction of cell growth and regulates cell volume5.
Cell wall is ultimately responsible for the plant architectural design and controlling plantmorphogenesis since the wall dictates that plants develop by cell addition (not cell migration)
6. Cell wall components has a metabolic role (i.e., some of the proteins in the wall are enzymesfor transport, secretion)
7. It is a main physical barrier to: (a) pathogens; and (b) water in suberized cells8. Cell wall is a carbohydrate storage - the components of the wall can be reused in other
metabolic processes (especially in seeds). Thus, in one sense the wall serves as a storage
repository for carbohydrates. The cell wall carbohydrates reserve can be used dire/starvation
situations.
9. Signaling - fragments of wall, called oligosaccharins, act as hormones. Oligosaccharins,which can result from normal development or pathogen attack, serve a variety of functions
including: (a) stimulate ethylene synthesis; (b) induce phytoalexin (defense chemicals
produced in response to a fungal/bacterial infection) synthesis; etc.
10.Economic products - cell walls are important for products such as paper, wood, fiber, energy,shelter, and even roughage in our diet.
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PLASMA MEMBRANE
The plasma membrane is the outermost boundary of the prokaryotic and eukaryotic cell. It
separates cytoplasm from its surrounding. Plasma membranes are used to compartmentalize the
cells. It is a ultrathin, elastic, living, dynamic and selective transport barrier. It is fluid mosaic
assembly of molecules of lipid (phospholipid and cholesterol), proteins and carbohydrates.
Plasma membrane controls the entry of nutrients and exit of waste products, and generates
difference in ion concentration between interior and exterior of the cell. It also acts as a sensor of
external signals and allows the cells to react or change in response to the environmental signals.
All membranes including plasma membrane and internal membranes of eukaryotic cells like
bounding membranes of Nucleus, Mitochondrion, Chloroplasts, Endoplasmic reticulum, Golgi
bodies, etc. are same in structure and selective permeability but differing in other functions and
compositions.
The plasma membrane is also called as Cytoplasmic Membrane, Cell Membrane, Cell Membrane
or Plasmalemma. The term Cell Membrane was firstly used by Nageli and Cramer (1955) and
Plasmalemma by Plowe (1931)
The plasma membrane and othercellular membranes are composed primarily of two layers of
phospholipid molecules. These bipartite
molecules have a water-loving
(hydrophilic) end and a water-hating
(hydrophobic) end. The two phospholipid
layers of a membraneare oriented with all
the hydrophilic ends directed toward the
inner and outer surfaces and the
hydrophobic ends buried within the
interior. Smaller amounts of other lipids,
such as cholesterol, and many kinds of proteins are inserted into the phospholipid framework.
The lipid molecules and some proteins can float sidewise in the plane of the membrane, giving
membranes a fluid character. This fluidity allows cells to change shape and even move.
However, the attachment of some membrane proteins to other molecules inside or outside the
cell restricts their lateral movement.
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CHEMICAL COMPOSITION
Overall plasma membrane composed of
20 % Water with 80 % Organic
Contents. The entire amount of organic
contents share following amount (%) of
chief constituents -
CONSTITU
ENTS
CE
LL
(R.
B.
C.)
CELL
ORGANELLES
(MITOCHOND
RION)
Proteins 18 76
Lipids 79 24
Carbohydrate
s03 00
PROTEINS
Membrane proteins are defined by their location within or at the surface of a phospholipid
bilayer. Although every biological membrane has the same basic bilayer structure, the proteins
associated with a particular membrane are responsible for its distinctive activities. The density
and complement of proteins associated with biomembranes vary, depending on cell type and
subcellular location.
Membrane proteins can be classified into three categories integral, lipid-anchored, and
peripheralon the basis of the nature of the membraneprotein interactions.
I. Integral membrane proteins, also called transmembrane proteins, span a phospholipidbilayer and are built of three segments.
II. Lipid-anchored membrane proteins are bound covalently to one or more lipid molecules.The hydrophobic carbon chain of the attached lipid is embedded in one leaflet of the
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membrane and anchors the protein to the membrane. The polypeptide chain itself does not
enter the phospholipids bilayer.
III. Peripheral membrane proteins do not interact with the hydrophobic core of thephospholipid bilayer. Instead theyare usually bound to the membrane indirectly by
interactions with integral membrane proteins or directly by interactions with lipid head
groups. Peripheral proteins are localized to either the cytosolic or the exoplasmic face of the
plasma membrane. Following are some examples of the proteins -
Peripheral proteins (Cytoskeleton formation) :
Spectrin, Ankyrins, Actin, etc.
Integral Proteins (Surface Reacting Transport, Reception, Recognition)
Glycophorin A, Glycophorin B, Glycophorin C, etc.
In addition to these proteins, which are closely associated with the bilayer, cytoskeletal filaments
are more loosely associated with the cytosolic face, usually through one or more
LIPIDS
Phospholipids of the composition present in cells spontaneously form sheetlike phospholipid
bilayers, which are two molecules thick. The hydrocarbon chains of the phospholipids in each
layer, or leaflet, form a hydrophobic core that is 34 nm thick in most biomembranes. Electron
microscopy of thin membrane sections stained with osmium tetroxide, which binds strongly to
the polar head groups of phospholipids, reveals the bilayer structure (Figure).
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D. D. Khedkar, Department of Botany, Shri Shivaji Science College, Amravati
PLASMA MEMBRANE ULTRASTRUCTURE
The plasma membrane does the major function of regulating transportation of substances from
inside the cell to the outside and vice versa. The specificity of plasma membrane structure plays
a crucial role in the overall functioning of the cell. In simple terms, it acts in a similar manner to
the skin of animals. Various scientific hypotheses have been proposed to explain the structure of
the plasma membrane, out of which the most popularly accepted theory is the fluid mosaic
model.
I. THE PHOSPHOLIPID BILAYERThe fundamental part of the plasma membrane structure is the lipid bilayer. Types of lipids
present in the plasma membrane are phospholipids, cholesterol and glycolipids. However, as
majority of the molecules are of phospholipid type (containing a phosphate group), the two lipid
layers are better known as phospholipid layers.
The plasma membrane is the most thoroughly studied of all cell membranes, and it is largely
through investigations of the plasma membrane
that our current concepts of membrane structure
have evolved. In 1925, two Dutch scientists (E.
Gorter and R. Grendel) extracted the membrane
lipids from a known number of red blood cells,
corresponding to a known surface area of plasmamembrane. They then determined the surface area
occupied by a monolayer of the extracted lipid
spread out at an air-water interface. The surface
area of the lipid monolayer turned out to be twice that occupied by the erythrocyte plasma
membranes, leading to the conclusion that the membranes consisted of lipid bilayers rather than
monolayers.
The bilayer structure of the erythrocyte plasma membrane is clearly evident in high-
magnification electron micrographs. The plasma membrane appears as two dense lines separated
by an intervening spacea morphology frequently referred to as a railroad track appearance.
This image results from the binding of the electron-dense heavy metals used as stains in
transmission electron microscopy to the polar head groups of the phospholipids, which therefore
appear as dark lines. These dense lines are separated by the lightly stained interior portion of the
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membrane, which contains the hydrophobic fatty acid chains. The lipid tails are water repelling
(hydrophobic), while phosphate heads are water-attracted (hydrophilic). The phospholipid
bilayer is arranged in a specific fashion, with the hydrophobic tails orienting towards the inside
(facing each other) and the hydrophilic head aligning to the outside. Thus, both sides of the
plasma membrane, one that faces the cytosol and the other facing the outside environment, are
hydrophilic in nature.
II. PROTEIN LIPID BILAYER MODELIn 1935, Hugh Davson and James Danielli proposed a model of the cell membrane in which the
phospholipid bilayer lay between two layers of globular protein. It is also called as Davson-
Danielli sandwich model. The phosopholipid bilayer had already been proposed by Gorter and
Grendel in 1925, but the DavsonDanielli model's flanking proteinaceous layers were novel and
intended to explain Danielli's observations on the surface tension of lipid bilayers. (It is now
known that the phospholipid
head groups are sufficient to
explain the measured surface
tension.)
The DavsonDanielli model
predominated until Singer and
Nicolson advanced the fluid
mosaic model in 1972. The
fluid mosaic model expanded on the DavsonDanielli model by including transmembrane
proteins, and eliminated the previously-proposed flanking protein layers that were not well-
supported by experimental evidence.
Limitation: The model was considering stable nature of plasma membrane and hence dynamic
nature was not explained.
III. FLUID MOSAIC MODEL OF THE PLASMA MEMBRANEDissecting "Fluid Mosaic" revealed that -
"fluid" = soluble, constantly changing movement."mosaic" = composed of a plethora of different macromolecules (ie proteins, phospholipids, and
fats).
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FUNCTIONS OF THE PLASMA MEMBRANE
Although the lipid composition of a membrane largely determines its physical characteristics, its
complement of proteins is primarily responsible for a membranes functional properties. We
have alluded to many functions of the plasma membrane in the preceding discussion and briefly
consider its major functions here.
1. In all cells, the plasma membrane acts as a permeability barrier that prevents the entryof unwanted materials from the extracellular milieu and the exit of needed metabolites.
2. Specific membrane transport proteins in the plasma membrane permit the passage ofnutrients into the cell and metabolic wastes out of it; others function to maintain the
proper ionic composition and pH (7.2) of the cytosol.
a. Some of the transport process happens "passively" without the cell needing to expendany energy to make them happen. These processes are called "passive transport
processes".
b. Other transport processes require energy from the cell's reserves to "power" them.These processes are called "active transport processes".
3. The plasma membrane is highly permeable to water but poorly permeable to salts and smallmolecules such as sugars and amino acids. Owing to osmosis, water moves across such a
semi permeable membrane from a solution of low solute (high water) concentration to one
of high solute (low water) concentration until the total solute concentrations and thus the
water concentrations on both sides are equal.
4. Plasma membrane protects and separate the interior part of cell (protoplasm) fromexternal environment.
5. Plasma membrane help to adhere with adjacent cells to form tissue and maintainsconnection with adjacent cells via pores on membrane known as plasmodesmata (in plants)
and desmosome (in animals).
6. Unlike animal cells, bacterial, fungal, and plant cells are surrounded by a rigid cell wall andlack the extracellular matrix found in animal tissues. The plasma membrane is intimately
engaged in the assembly of cell walls, which in plants are built primarily of cellulose. The
cell wall prevents the swelling or shrinking of a cell that would otherwise occur when it is
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placed in a hypotonic or hypertonic medium, respectively. For this reason, cells surrounded
by a wall can grow in media having an osmotic strength much less than that of the cytosol.
7. In addition to these universal functions, the plasma membrane has other crucial roles inmulticellular organisms. Specialized areas of the plasma membrane contain proteins and
glycolipids that form specific junctions between cells to strengthen tissues and to allow the
exchange of metabolites between cells.
8. Still other proteins in the plasma membrane act as anchoring points for many of thecytoskeletal fibers that permeate the cytosol, imparting shape and strength to cells.
9. The plasma membranes of many types of eukaryotic cells also contain receptor proteinsthat bind specific signaling molecules (e.g., hormones, growth factors, neurotransmitters),
leading to various cellular responses. These proteins, which are critical for cell development
and functioning.
10.Finally, peripheral cytosolic proteins that are recruited to the membrane surface function asenzymes, intracellular signal transducers, and structural proteins for stabilizing the
membrane.
11.Like the plasma membrane, the membrane surrounding each organelle in eukaryotic cellscontains a unique set of proteins essential for its proper functioning.
12.Plasma membrane also carry out exocytosis (excretion of waste outside the cell),endocytosis (intake of large particles inside the cell) and pinocytosis (a mechanism by
which cells ingest extracellular fluid and its contents- drinking)
13.Function of plasma membrane of some cells (phagocytes) include important role inimmunity
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GOLGI COMPLEX
Discovery
Due to its fairly large size, the golgi apparatus was one of the first
organelles to be discovered and observed in detail. The apparatus was
discovered in 1897 by Italian physician Camillo Golgi during an
investigation of the nervous system. After first observing it under his
microscope he termed the structure the internal reticular apparatus. The
structure was then renamed after Golgi not long after the announcement
of his discovery in 1898. However, some doubted the discovery at first,
arguing that the appearance of the structure was merely an optical illusion created by the
observation technique used by Golgi. With the development of modern microscopes in the 20th
century, the discovery was confirmed.
The synonymous terms are Golgi Apparatus, Lipochondria, Chondriome, Lipoidal
Mitohondria and Dictyosome.
GC occurs in all cells except in prokaryotes and certain fungi, bryophytes and pteridophytes. The
number of GC varies from cell to cell even from the same organism.
STRUCTURE
GC has following three important constituents
1. Cisternae2. Tubules3. Vescicles
1. Cisternae or Flattened sacsThe Golgi is composed of stacks of
membrane-bound structures known as
cisternae (1m diameter). An individualstack is sometimes called a dictyosome
(from Greek dictyon, net + soma, body),
especially in plant cells. A mammalian
cell typically contains 40 to 100 stacks.
http://en.wikipedia.org/wiki/Cisternaehttp://en.wikipedia.org/wiki/Cisternaehttp://en.wikipedia.org/wiki/Cisternae -
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Between four and eight cisternae are usually present in a stack; however, in some protists as
many as sixty have been observed. Each cisterna comprises a flattened membrane disk, and
carries Golgi enzymes to help or to modify cargo proteins that travel through them. They are
found in both plant and animal cells.
The cisternae stack has four functional regions: the cis-Golgi network, medial-Golgi, endo-
Golgi, and trans-Golgi network. The margins of each cisternae are gently curved so that it looks
like bow.
Vesicles from the endoplasmic reticulum fuse with the network and subsequently progress
through the stack to the trans Golgi network, where they are packaged and sent to the required
destination. Each region contains different enzymes which selectively modify the contents
depending on where they reside. The cisternae also carry structural proteins important for their
maintenance as flattened membranes which stack upon each other.
FUNCTION
The Golgi apparatus is working in modifying, sorting, and packaging the macromolecules for
cell secretion (exocytosis) or use within the cell. In this respect it can be thought of as similar to
a post office; it packages and labels items which it then sends to different parts of the cell.
Following are the functions of GC:
1. It primarily modifies proteins delivered from the rough endoplasmic reticulum.2. Also involved in the transport of lipids around the cell3. It is useful in creation of lysosomes for disposal of the waste and used materials in the cell.4. Enzymes within the cisternae are able to modify substances by the addition of
carbohydrates (glycosylation) and phosphates (phosphorylation).
5. The Golgi plays an important role in the synthesis of proteoglycans, which are moleculespresent in the extracellular matrix of animals.
6. It is also a major site of carbohydrate synthesis.7. Another task of the Golgi involves the sulfation of certain molecules passing through its
lumen via sulphotranferases that gain their sulphur molecule from a donor called PAPs.
This process occurs on the GAGs of proteoglycans as well as on the core protein. The level
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of sulfation is very important to the proteoglycans' signalling abilities as well as giving the
proteoglycan its overall negative charge.
8. The Golgi has a important role in apoptosis (Programmed Cell Death) 9. Transportation of Vesicles is conducted by GC. The vesicles that leave the rough
endoplasmic reticulum are transported to the cis face of the Golgi apparatus, where they
fuse with the Golgi membrane and empty their contents into the lumen. Once inside they
are modified, sorted and shipped towards their final destination.
10.As such, the Golgi apparatus tends to be more prominent and numerous in cellssynthesizing and secreting many substances: plasma B cells, the antibody-secreting cells of
the immune system, have prominent Golgi complexes.
11.Those proteins destined for areas of the cell other than either the endoplasmic reticulum orGolgi apparatus are moved towards the trans face, to a complex network of membranes
and associated vesicles known as the trans-Golgi network(TGN).
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ENDOPLASMIC RETICULUM
The endoplasmic reticulum (ER) is a eukaryotic organelle that forms an interconnected network
of tubules, vesicles, and cisternae within cells. Rough endoplasmic reticulums synthesize
proteins, while smooth endoplasmic reticulums synthesize lipids and steroids, metabolize
carbohydrates and steroids, and regulate calcium concentration, drug detoxification, andattachment of receptors on cell membrane proteins. Sarcoplasmic reticulums solely regulate
calcium levels.
The Lacey membranes of the endoplasmic reticulum were first seen by Keith R. Porter, Albert
Claude, and Ernest F. Fullam in 1945.
Structure
The general structure of the endoplasmic reticulum is an extensive membrane network ofcisternae (sac-like structures) held together by the
cytoskeleton. The phospholipid membrane encloses a
space, the cisternal space (or lumen), from the cytosol,
which is continuous with the perinuclear space. The
functions of the endoplasmic reticulum vary greatly
depending on the exact type of endoplasmic reticulum and
the type of cell in which it resides. The three varieties are
called rough endoplasmic reticulum, smooth endoplasmic reticulum and sarcoplasmic reticulum.
The quantity of RER and SER in a cell can quickly interchange from one type to the other,
depending on changing metabolic needs: one type will undergo numerous changes including new
proteins embedded in the membranes in order to transform. Also, massive changes in the protein
content can occur without any noticeable structural changes, depending on the enzymatic needs
of the cell.
Rough endoplasmic reticulum
The surface of the rough endoplasmic reticulum (RER) is studded with protein-manufacturing
ribosomes giving it a "rough" appearance (hence its name).[2]
However, the ribosomes bound to
the RER at any one time are not a stable part of this organelle's structure as ribosomes are
constantly being bound and released from the membrane. A ribosome only binds to the ER once
http://en.wikipedia.org/wiki/Ribosomehttp://en.wikipedia.org/wiki/Endoplasmic_reticulum#cite_note-campbell-1http://en.wikipedia.org/wiki/Endoplasmic_reticulum#cite_note-campbell-1http://en.wikipedia.org/wiki/Endoplasmic_reticulum#cite_note-campbell-1http://en.wikipedia.org/wiki/Endoplasmic_reticulum#cite_note-campbell-1http://en.wikipedia.org/wiki/Ribosome -
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it begins to synthesize a protein destined for the secretory pathway. Here, a ribosome in the
cytosol begins synthesizing a protein until a signal recognition particle recognizes the pre-piece
of 5-15 hydrophobic amino acids preceded by a positively charged amino acid. This signal
sequence allows the recognition particle to bind to the ribosome, causing the ribosome to bind to
the RER and pass the new protein through the ER membrane. The pre-piece is then cleaved off
within the lumen of the ER and the ribosome released back into the cytosol.
The membrane of the RER is continuous with the outer layer of the nuclear envelope. Although
there is no continuous membrane between the RER and the Golgi apparatus, membrane-bound
vesicles shuttle proteins between these two compartments.[4]Vesicles are surrounded by coating
proteins called COPI and COPII. COPII targets vesicles to the golgi and COPI marks them to be
brought back to the RER. The RER works in concert with the Golgi complex to target new
proteins to their proper destinations. A second method of transport out of the ER are areas called
membrane contact sites, where the membranes of the ER and other organelles are held closely
together, allowing the transfer of lipids and other small molecules.
The RER is key in multiple functions:
1. Lysosomal enzymes synthesis required in the cis-Golgi network2. Secreted proteins, either secreted constitutively with no tag, or regulated secretion
involving clathrin and paired basic amino acids in the signal peptide.
3. Integral membrane proteins that stay imbedded in the membrane as vesicles exit and bindto new membranes.
Smooth endoplasmic reticulum
The smooth endoplasmic reticulum (SER) has functions in several metabolic processes,
including synthesis of lipids and steroids, metabolism of carbohydrates, regulation of calcium
concentration, drug detoxification, attachment of receptors on cell membrane proteins, and
steroid metabolism. It is connected to the nuclear envelope. Smooth endoplasmic reticulum is
found in a variety of cell types (both animal and plant) and it serves different functions in each.
The Smooth ER also contains the enzyme glucose-6-phosphatase which converts glucose-6-
phosphate to glucose, a step in gluconeogenesis. The SER consists of tubules and vesicles that
http://en.wikipedia.org/wiki/Secretory_pathwayhttp://en.wikipedia.org/wiki/Signal_recognition_particlehttp://en.wikipedia.org/w/index.php?title=Pre-piece&action=edit&redlink=1http://en.wikipedia.org/wiki/Hydrophobichttp://en.wikipedia.org/wiki/Amino_acidhttp://en.wikipedia.org/wiki/Nuclear_envelopehttp://en.wikipedia.org/wiki/Golgi_apparatushttp://en.wikipedia.org/wiki/Endoplasmic_reticulum#cite_note-3http://en.wikipedia.org/wiki/Endoplasmic_reticulum#cite_note-3http://en.wikipedia.org/wiki/Endoplasmic_reticulum#cite_note-3http://en.wikipedia.org/wiki/COPIIhttp://en.wikipedia.org/wiki/COPIhttp://en.wikipedia.org/wiki/Golgi_complexhttp://en.wikipedia.org/wiki/Protein_targetinghttp://en.wikipedia.org/wiki/Protein_targetinghttp://en.wikipedia.org/wiki/Membrane_contact_sitehttp://en.wikipedia.org/wiki/Lysosomehttp://en.wikipedia.org/wiki/Secretionhttp://en.wikipedia.org/wiki/Clathrinhttp://en.wikipedia.org/wiki/Signal_peptidehttp://en.wikipedia.org/wiki/Integral_membrane_proteinshttp://en.wikipedia.org/wiki/Steroid_metabolismhttp://en.wikipedia.org/wiki/Glucose-6-phosphatasehttp://en.wikipedia.org/wiki/Glucose-6-phosphatehttp://en.wikipedia.org/wiki/Glucose-6-phosphatehttp://en.wikipedia.org/wiki/Gluconeogenesishttp://en.wikipedia.org/wiki/Gluconeogenesishttp://en.wikipedia.org/wiki/Glucose-6-phosphatehttp://en.wikipedia.org/wiki/Glucose-6-phosphatehttp://en.wikipedia.org/wiki/Glucose-6-phosphatasehttp://en.wikipedia.org/wiki/Steroid_metabolismhttp://en.wikipedia.org/wiki/Integral_membrane_proteinshttp://en.wikipedia.org/wiki/Signal_peptidehttp://en.wikipedia.org/wiki/Clathrinhttp://en.wikipedia.org/wiki/Secretionhttp://en.wikipedia.org/wiki/Lysosomehttp://en.wikipedia.org/wiki/Membrane_contact_sitehttp://en.wikipedia.org/wiki/Protein_targetinghttp://en.wikipedia.org/wiki/Protein_targetinghttp://en.wikipedia.org/wiki/Golgi_complexhttp://en.wikipedia.org/wiki/COPIhttp://en.wikipedia.org/wiki/COPIIhttp://en.wikipedia.org/wiki/Endoplasmic_reticulum#cite_note-3http://en.wikipedia.org/wiki/Golgi_apparatushttp://en.wikipedia.org/wiki/Nuclear_envelopehttp://en.wikipedia.org/wiki/Amino_acidhttp://en.wikipedia.org/wiki/Hydrophobichttp://en.wikipedia.org/w/index.php?title=Pre-piece&action=edit&redlink=1http://en.wikipedia.org/wiki/Signal_recognition_particlehttp://en.wikipedia.org/wiki/Secretory_pathway -
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branch forming a network. In some cells there are dilated areas like the sacs of RER. The
network of SER allows increased surface area for the action or storage of key enzymes and the
products of these enzymes.
Functions
The endoplasmic reticulum serves many general functions, including:
1. Facilitation of protein folding and the transport of synthesized proteins in sacs calledcisternae.
2. Correct folding of newly-made proteins is made possible by several endoplasmicreticulum chaperone proteins. Only properly-folded proteins are transported from the
rough ER to the Golgi complex.
3. Proteins that are transported by the endoplasmic reticulum and from there throughout thecell are marked with an address tag called a signal sequence. The N-terminus (one end) of
a polypeptide chain (i.e., a protein) contains a few amino acids that work as an address
tag, which are removed when the polypeptide reaches its destination.
4. Proteins that are destined for places outside the endoplasmic reticulum are packed intotransport vesicles and moved along the cytoskeleton toward their destination.
5. The endoplasmic reticulum is also part of a protein sorting pathway.It is, in essence, the transportation system of the eukaryotic cell. The majority of endoplasmic
reticulum resident proteins are retained in the endoplasmic reticulum through a retention
motif. This motif is composed of four amino acids at the end of the protein sequence.
Other functions
Insertion of proteins into the endoplasmic reticulum membrane Glycosylation: Glycosylation involves the attachment ofoligosaccharides. Disulfide bond formation and rearrangement: Disulfide bonds stabilize the tertiary
and quaternary structure of many proteins.
Drug Metabolism: The smooth ER is the site at which some drugs are modified bymicrosomal enzymes which include the cytochrome P450 enzymes.
http://en.wikipedia.org/wiki/Cisternaehttp://en.wikipedia.org/wiki/Chaperone_%28protein%29http://en.wikipedia.org/wiki/Golgi_complexhttp://en.wikipedia.org/wiki/Signal_peptidehttp://en.wikipedia.org/wiki/Polypeptidehttp://en.wikipedia.org/wiki/Amino_acidhttp://en.wikipedia.org/wiki/Vesicle_%28biology%29http://en.wikipedia.org/wiki/Cytoskeletonhttp://en.wikipedia.org/wiki/Glycosylationhttp://en.wikipedia.org/wiki/Oligosaccharidehttp://en.wikipedia.org/wiki/Cytochrome_P450http://en.wikipedia.org/wiki/Cytochrome_P450http://en.wikipedia.org/wiki/Oligosaccharidehttp://en.wikipedia.org/wiki/Glycosylationhttp://en.wikipedia.org/wiki/Cytoskeletonhttp://en.wikipedia.org/wiki/Vesicle_%28biology%29http://en.wikipedia.org/wiki/Amino_acidhttp://en.wikipedia.org/wiki/Polypeptidehttp://en.wikipedia.org/wiki/Signal_peptidehttp://en.wikipedia.org/wiki/Golgi_complexhttp://en.wikipedia.org/wiki/Chaperone_%28protein%29http://en.wikipedia.org/wiki/Cisternae -
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PEROXISOMES
Peroxysomes are organelles present in almost all eukaryotic cells. They participate in the
metabolism of fatty acids and many other metabolites. Peroxisomes harbour enzymes that rid the
cell of toxic peroxides. Peroxisomes are bound by a single membrane that separates their
contents from the cytosol (the internal fluid of the cell) and contain membrane proteins critical
for various functions, such as importing proteins into the organelles and aiding in proliferation.
Peroxisomes are formed from the endoplasmic reticulum. Peroxisomes were identified as
organelles by the Belgian cytologist Christian de Duve in
1967.
Proteins are selectively imported into peroxisomes. Since the
organelles contain no DNA or ribosomes and, thus, have nomeans of producing proteins, all of their proteins must be
imported across the membrane. It is believed that necessary
proteins enter through the endoplasmic reticulum during
biogenesis as well as through membrane proteins.
A specific protein signal (PTS or peroxisomal targeting signal) of three amino acids at theC-
terminusof many peroxisomal proteins signals the membrane of the peroxisome to import them
into the organelle. Other peroxisomal proteins contain a signal at the N-terminus. There are atleast 32 known peroxisomal proteins, called peroxins,
[17] which participate in the process of
importing proteins by means ofATP hydrolysis. Proteins do not have to unfold to be imported
into the peroxisome. The protein receptors, the peroxins PEX5 and PEX7, accompany their
cargoes (containing a PTS1 or a PTS2, respectively) all the way into the peroxisome where they
release the cargo and then return to the cytosol - a step named recycling. Overall, the import
cycle is referred to as the extended shuttle mechanism
Function
1. Peroxisomes contain oxidative enzymes, such as catalase, D-amino acid oxidase, and uricacid oxidase. However the last enzyme is absent in humans, explaining the disease known as
gout, caused by the accumulation of uric acid. Certain enzymes within the peroxisome, by
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using molecular oxygen, remove hydrogen atoms from specific organic substrates (labeled as
R), in an oxidative reaction, producing hydrogen peroxide (H2O2, itself toxic).
2. A major function of the peroxisome is the breakdown of fatty acid molecules, in a processcalled beta-oxidation. In this process, the fatty acids are broken down two carbons at a time,
converted to Acetyl-CoA, which is then transported back to the cytosol for further use. In
animal cells, beta-oxidation can also occur in the mitochondria. In yeast and plant cells, this
process is exclusive for the peroxisome.
3. The first reactions in the formation ofplasmalogen in animal cells also occur in peroxisomes.Plasmalogen is the most abundant phospholipid in myelin. Deficiency of plasmalogens
causes profound abnormalities in the myelination ofnerve cells, which is one of the reasons
that many peroxisomal disorders lead to neurological disease (adrenoleukodystrophy).
4. Peroxisomes also play a role in the production ofbile acids and proteins.5. In higher plants, peroxisomes contain also a complex battery of antioxidative enzymes such
as superoxide dismutase, the components of the ascorbate-glutathione cycle, and the NADP-
dehydrogenases of the pentose-phosphate pathway. It has been demonstrated the generation
of superoxide (O2-) and nitric oxide (
NO) radicals
6. The peroxisome of plant cells is polarised when fighting fungal penetration. Infection causesa glucosinolate molecule to play an antifungal role to be made and delivered to the outside of
the cell through the action of the peroxisomal proteins (PEN2 and PEN3).
7. Peroxisomal disorders are a class of conditions that lead to disorders of lipid metabolism anddiseases of the nervous system. Well-known examples are X-linked adrenoleukodystrophy,
the most frequent, and Zellweger syndrome [18][19]. Peroxisomes matrix proteins are
synthesized on free ribosomes in the cytosol and are imported post-translationally in pre-
existing vesicles.
http://en.wikipedia.org/wiki/Hydrogen_peroxidehttp://en.wikipedia.org/wiki/Fatty_acidhttp://en.wikipedia.org/wiki/Beta-oxidationhttp://en.wikipedia.org/wiki/Acetyl-CoAhttp://en.wikipedia.org/wiki/Cytosolhttp://en.wikipedia.org/wiki/Plasmalogenhttp://en.wikipedia.org/wiki/Myelinhttp://en.wikipedia.org/wiki/Neuronhttp://en.wikipedia.org/wiki/Adrenoleukodystrophyhttp://en.wikipedia.org/wiki/Bilehttp://en.wikipedia.org/wiki/Ascorbate-glutathione_cyclehttp://en.wikipedia.org/wiki/Nitric_oxidehttp://en.wikipedia.org/wiki/Glucosinolatehttp://en.wikipedia.org/wiki/Peroxisomal_disordershttp://en.wikipedia.org/wiki/Lipid_metabolismhttp://en.wikipedia.org/wiki/X-linkedhttp://en.wikipedia.org/wiki/Adrenoleukodystrophyhttp://en.wikipedia.org/wiki/Zellweger_syndromehttp://en.wikipedia.org/wiki/Peroxisome#cite_note-17http://en.wikipedia.org/wiki/Peroxisome#cite_note-17http://en.wikipedia.org/wiki/Peroxisome#cite_note-17http://en.wikipedia.org/wiki/Peroxisome#cite_note-17http://en.wikipedia.org/wiki/Zellweger_syndromehttp://en.wikipedia.org/wiki/Adrenoleukodystrophyhttp://en.wikipedia.org/wiki/X-linkedhttp://en.wikipedia.org/wiki/Lipid_metabolismhttp://en.wikipedia.org/wiki/Peroxisomal_disordershttp://en.wikipedia.org/wiki/Glucosinolatehttp://en.wikipedia.org/wiki/Nitric_oxidehttp://en.wikipedia.org/wiki/Ascorbate-glutathione_cyclehttp://en.wikipedia.org/wiki/Bilehttp://en.wikipedia.org/wiki/Adrenoleukodystrophyhttp://en.wikipedia.org/wiki/Neuronhttp://en.wikipedia.org/wiki/Myelinhttp://en.wikipedia.org/wiki/Plasmalogenhttp://en.wikipedia.org/wiki/Cytosolhttp://en.wikipedia.org/wiki/Acetyl-CoAhttp://en.wikipedia.org/wiki/Beta-oxidationhttp://en.wikipedia.org/wiki/Fatty_acidhttp://en.wikipedia.org/wiki/Hydrogen_peroxide -
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VACUOLE
A vacuole is a membrane-bound organelle which is present in all plant and fungal cells and some
protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are
filled with water containing inorganic and organic molecules including enzymes in solution,
though in certain cases they may contain solids
which have been engulfed. Vacuoles are formed
by the fusion of multiple membrane vesicles and
are effectively just larger forms of these. The
organelle has no basic shape or size; its structure
varies according to the needs of the cell. The
membrane of the vacuole is called as Tonoplast
The function and importance of vacuoles varies
greatly according to the type of cell in which they are present, having much greater prominence
in the cells of plants, fungi and certain protists than those of animals and bacteria. In general, the
functions of the vacuole include:
1. Isolating materials that might be harmful or a threat to the cell2. Containing waste products3. Containing water in plant cells4. Maintaining internal hydrostatic pressure or turgor within the cell5. Maintaining an acidic internal pH6. Containing small molecules7. Exporting unwanted substances from the cell8. Allows plants to support structures such as leaves and flowers due to the pressure of the
central vacuole
9. In seeds, stored proteins needed for germination are kept in 'protein bodies', which aremodified vacuoles.
10.Vacuoles also play a major role in autophagy, maintaining a balance between biogenesis(production) and degradation (or turnover), of many substances and cell structures in certain
organisms.11.They also aid in the lysis and recycling of misfolded proteins that have begun to build up
within the cell.
12.The vacuole participates in the destruction of invading.
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RIBOSOME
Ribosomes are the components of cells that make proteins from all amino acids. One of the
central tenets of biology, often referred to as the "central dogma," is that DNA is used to make
RNA, which, in turn, is used to make protein. The DNA sequence in genes is copied into a
messenger RNA (mRNA). Ribosomes then read the information in this RNA and use it to create
proteins. This process is known as translation; i.e., the ribosome "translates" the geneticinformation from RNA into proteins. Ribosomes do this by binding to an mRNA and using it as
a template for the correct sequence of amino acids in a particular protein. The amino acids are
attached to transfer RNA (tRNA) molecules, which enter one part of the ribosome and bind to
the messenger RNA sequence. The attached amino acids are then joined together by another part
of the ribosome. The ribosome moves along the mRNA, "reading" its sequence and producing a
chain of amino acids.
Ribosomes are made from complexes of RNAs and proteins. Ribosomes are divided into two
subunits, one larger than the other. The smaller subunit binds to the mRNA, while the larger
subunit binds to the tRNA and the amino acids. When a ribosome finishes reading a mRNA,these two subunits split apart. Ribosomes have been classified as ribozymes, since the ribosomal
RNA seems to be most important for the peptidyl transferase activity that links amino acids
together.
Ribosomes from bacteria, archaea and eukaryotes (the three domains of life on Earth), have
significantly different structures and RNA sequences. These differences in structure allow some
antibiotics to kill bacteria by inhibiting their ribosomes, while leaving human ribosomes
unaffected. The ribosomes in the mitochondria of eukaryotic cells resemble those in bacteria,
reflecting the likely evolutionary origin of this organelle. The word ribosome comes from
ribonucleic acid and the Greek: soma (meaning body).
STRUCTURE
The ribosomal subunits of prokaryotes and eukaryotes are quite similar.
The unit of measurement is the Svedberg unit, a measure of the rate of sedimentation in
centrifugation rather than size and accounts for why fragment names do not add up (70S is made
of 50S and 30S).
Prokaryotes have 70S ribosomes, each consisting of a small (30S) and a large (50S) subunit.
Their large subunit is composed of a 5S RNA subunit (consisting of 120 nucleotides), a 23S
RNA subunit (2900 nucleotides) and 34 proteins. The 30S subunit has a 1540 nucleotide RNA
subunit (16S) bound to 21 proteins.
Eukaryotes have 80S ribosomes, each consisting of a small (40S) and large (60S) subunit. Their
large subunit is composed of a 5S RNA (120 nucleotides), a 28S RNA (4700 nucleotides), a 5.8S
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subunit (160 nucleotides) and ~49 proteins. The 40S subunit has a 1900 nucleotide (18S) RNA
and ~33 proteins.
The ribosomes found in chloroplasts and mitochondria of eukaryotes also consist of large and
small subunits bound together with proteins into one 70S particle. These organelles are believed
to be descendants of bacteria and as such their ribosomes are similar to those of bacteria.
The various ribosomes share a core structure, which is quite similar despite the large differences
in size. Much of the RNA is highly organized into various tertiary structural motifs, for example
pseudoknots that exhibit coaxial stacking. The extra RNA in the larger ribosomes is in several
long continuous insertions, such that they form loops out of the core structure without disrupting
or changing it. All of the catalytic activity of the ribosome is carried out by the RNA; the
proteins reside on the surface and seem to stabilize the structure.
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The differences between the bacterial and eukaryotic ribosomes are exploited by pharmaceutical
chemists to create antibiotics that can destroy a bacterial infection without harming the cells of
the infected person. Due to the differences in their structures, the bacterial 70S ribosomes are
vulnerable to these antibiotics while the eukaryotic 80S ribosomes are not. Even though
mitochondria possess ribosomes similar to the bacterial ones, mitochondria are not affected by
these antibiotics because they are surrounded by a double membrane that does not easily admit
these antibiotics into the organelle.
FUNCTION
Ribosomes are the workhorses of protein biosynthesis, the process of translating mRNA into
protein. The mRNA comprises a series of codons that dictate to the ribosome the sequence of the
amino acids needed to make the protein. Using the mRNA as a template, the ribosome traverses
each codon (3 nucleotides) of the mRNA, pairing it with the appropriate amino acid provided bya tRNA. Molecules of transfer RNA (tRNA) contain a complementary anticodon on one end and
the appropriate amino acid on the other. The small ribosomal subunit, typically bound to a tRNA
containing the amino acid methionine, binds to an AUG codon on the mRNA and recruits the
large ribosomal subunit. The ribosome then contains three RNA binding sites, designated A, P
and E. The A site binds an aminoacyl-tRNA (a tRNA bound to an amino acid); the P site binds a
peptidyl-tRNA (a tRNA bound to the peptide being synthesized); and the E site binds a free
tRNA before it exits the ribosome. Protein synthesis begins at a start codon AUG near the 5' end
of the mRNA. mRNA binds to the P site of the ribosome first. The ribosome is able to identify
the start codon by use of the Shine-Dalgarno sequence of the mRNA in prokaryotes and Kozak
box in eukaryotes.
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MITOCHONDRIA
Mitochondria were first observed by Altman (1894) as filamentous structures and he called them
as bioblasts, but Brenda (1897) called them as mitochondria. Unlike chloroplasts they are found
in almost all eukaryotic organisms. They are very important energy rich ATP producing
organelles and makes it available for other biological process in the form of energy rich
compounds like ATP and NADH2 and NADPH2. It also directs fatty acid oxidation products for
ATP synthesis.
Shape sand Size:
Using specific stains or fluorescent dyes, mitochondia can be observed cytologically. In the
presence of janus green they look like small rod shaped, greenish blue structures. The shape of
mitochondria is never constant; at one moment they look like small bacterial shaped or tubular
shaped structures of 0-.5 mm to 7 mm and in few moments later, they appear as long filaments orvesicles. A movie on mitochondria, pictured to show its variability in structure with time, it is
amazing to observe how the mitochondria divide and fuse with one another and divide - a
process continuum, perhaps in all organisms. Thus they exhibit polymorphism in their shape and
size.
Number and Distribution:
The number of mitochondria found in cells varies from cell type to cell type, from species to
species. Even physiological state of the cell determines the number. Trypanosomes contain only
one mitochondria. Yeast cells under glucose repression posses just one or two mitochondria per
cell. But a liver or meristems in roots contain 500-1600 mitochondria per cell. In some
amphibian oocytes which is very active, there can be 10,0000 or more mitochondria. The cell
that requires more energy contains more number of mitochondria, ex: flight muscle cells in
insects, heart cells have more mitochondria than their intestinal cells.
Even the intracellular distribution and orientation of mitochondria depends upon the structures
involved in a particulars function. For example, in flight muscle cells, in the vicinity of
centrosomes, mitochondria are arranged radially and mitochondria are packed longitudinally. At
the basal granules of flagella they are aggregated at the base; otherwise they are randomly
distributed in the cytoplasm.
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STRUCTURE
Mitochondria are called the 'powerhouse of the cell'. Mitochondria contain a number of enzymes
and proteins that help in processing carbohydrates and fats obtained from food we eat to release
energy. Read on to know about the structure and functions of the organelle.
Be it the beating of the heart or moving of our hands, every action requires energy. This energy
is stored in ATP (adenosine triphosphate) molecules that are produced in the mitochondria by the
process ofoxidative phosphorylation. Although mitochondria are present in every cell, they are
found in high concentrations in the muscle cells that require more energy. Though the primary
function of mitochondria is to produce energy, they also play an important role in the metabolism
and synthesis of certain other substances in the body.
Structure
Mitochondria are rod-shaped structures that are enclosed within two membranes - the outermembrane and the inner membrane. The membranes are made up of phospholipids and proteins.
The space in between the two membranes is called the inter-membrane space which has the same
composition as the cytoplasm of the cell. However, the protein content in this space differs from
that in the cytoplasm. The structure of the various components of mitochondria are as follows:
Outer Membrane
The outer membrane is smooth unlike the inner membrane and has almost the same amount of
phospholipids as proteins. It has a large number of special proteins called porins, that allow
molecules of 5000 daltons or less in weight to pass through it. The outer membrane is completely
permeable to nutrient molecules, ions, ATP and ADP molecules.
Inner Membrane
The inner membrane is more complex in structure than the outer membrane as it contains the
complexes of the electron transport chain and the ATP synthetase complex. It is permeable only
to oxygen, carbon dioxide and water. It is made up of a large number of proteins that play an
important role in producing ATP, and also helps in regulating transfer of metabolites across the
membrane. The inner membrane has infoldings called the cristae that increase the surface area
for the complexes and proteins that aid in the production of ATP, the energy rich molecules.
Matrix
The matrix is a complex mixture of enzymes that are important for the synthesis of ATP
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molecules, special mitochondrial ribosomes, tRNAs and the mitochondrial DNA. Besides these,
it has oxygen, carbon dioxide and other recyclable intermediates.
F1 Particles:
F1 particles are also called as ATP synthase, which is an important enzyme that provides energy
for the cell to use through the synthesis of adenosine triphosphate (ATP). ATP is the most
commonly used "energy currency" of cells from most organisms. It is formed from adenosine
diphosphate (ADP) and inorganic phosphate (Pi), which releases energy.
The overall reaction sequence is: ATP synthase + ADP + Pi ATP Synthase + ATP
This energy is often in the form of protium or H+, moving down an electrochemical gradient,
such as from the lumen into the stroma of chloroplasts or from the inter-membrane space into the
matrix in mitochondria.
These F 1 Particles are located within the mitochondrial inner membrane and consists of 2
regions:
The FO portion is within the membrane. The F1 portion of the ATP synthase is above the membrane, inside the matrix of the
mitochondria.
Mitochondria structure: (1) inner membrane, (2) outer membrane, (3) cristae, (4) matrix
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The nomenclature of the enzyme suffers from a long history. The F1 fraction derives its name
from the term "Fraction 1" and FO (written as a subscript letter "o", not "zero") derives its name
from being the oligomycin binding fraction. Oligomycin, an antibiotic, is able to inhibit the FO
unit of ATP synthase.
F1- ATP Synthase structure
The F1 particle is large and can be seen in the transmission electron microscope by negative
staining. These are particles of 9 nm diameter that pepper the inner mitochondrial membrane.
They were originally called elementary particles and were thought to contain the entire
respiratory apparatus of the mitochondrion, but, through a long series of experiments, Ephraim
Racker and his colleagues (who first isolated the F1 particle in 1961) were able to show that this
particle is correlated with ATPase activity in uncoupled mitochondria and with the ATPase
activity in submitochondrial particles created by exposing mitochondria to ultrasound. This
ATPase activity was further associated with the creation of ATP by a long series of experiments
in many laboratories.
FO - ATP Synthase Structure
The FO region of ATP synthase is a proton pore that is emmbeded into the mitochondrial
membrane. It consists of three main subunits A, B, and C, and (in humans) six additional
subunits, d, e, f, g, F6, and 8 (or A6L).
Physiological role
Like other enzymes, the activity of F1FO ATP synthase is reversible. Large-enough quantities of
ATP cause it to create a transmembrane proton gradient, this is used by fermenting bacteria that
do not have an electron transport chain, and hydrolyze ATP to make a proton gradient, which
they use for flagella and transport of nutrients into the cell.
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Functions
1. Energy production: The main function of the mitochondrion is the production of energy, inthe form of adenosine triphosphate (ATP). The cell uses this energy to perform the specific
work necessary for cell survival and function.
2. Programmed cell death: Cell death can occur either by injury due to toxic exposure, bymechanical damage, or by an orderly process called programmed cell death or apoptosis.
Programmed cell death occurs during development as the organism is pruning away
unwanted, excess cells. It also occurs during infections with viruses, cancer therapy, or in the
immune response to illness. The process of programmed cell death is another function of
mitochondria.
3. Detoxification: Normally, ATP production is coupled to oxygen consumption. Duringabnormal states such as fever, cancer, or stroke, or when dysfunction occurs within the
mitochondria, more oxygen is consumed or required than is actually used to make ATP. The
mitochondria become partially uncoupled and produce highly reactive oxygen species
called free radicals. When the production of free radicals overwhelms the mitochondrias
ability to detoxify them, the excess free radicals damage mitochondrial function by
changing the mitochondrial DNA, proteins, and membranes. As this process continues, it can
induce the cell to undergo apoptosis. Abnormal cell death due to mitochondrial dysfunction
can interfere with organ function.
4. Cell-specific functions: Other functions of mitochondria are related to the cell type in whichthey are found.
a. Mitochondria are involved in building, breaking down, and recycling products neededfor proper cell functioning.
b. Also involved in making parts of blood and hormones such as estrogen andtestosterone. They are required for cholesterol metabolism, neurotransmitter
metabolism, and detoxification of ammonia in the urea cycle.
Thus, if mitochondria do not function properly, not only energy production but also cell-specific
products needed for normal cell functioning will be affected.
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CHLOROPLAST
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct
photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and
reduce NADP to NADPH through a complex set of processes called photosynthesis.
Chloroplasts are green because they contain the chlorophyll pigment. The term Plastid derived
from Greek word plastikas = formed or moulded and was used in 1885 by A. F. W. Schimperto include organelles involved in the formation and storage of carbohydrates. A. Meyer explored
green plastids which are now Chloroplasts. They vary in shape, size and number from cell to
cell. Shape: spheroid, ovoid, lens shaped or discoid chloroplast. Size: 4 8 micron in diameter
and 2 micron in thickness. Number: 20 40 per eukayotic cell. These are discrete double
membrane bound structure
Chloroplasts are observable as flat discs usually 2 to 10 micrometers in diameter and 1
micrometer thick. In land plants, they are, in general, 5 m in diameter and 2.3 m thick. The
chloroplast is contained by an envelope that consists of an inner and an outer phospholipid
membrane. Between these two layers is the intermembrane space. A typical parenchyma cellcontains about 10 to 100 chloroplasts.
The material within the chloroplast is called the stroma, corresponding to the cytosol of the
original bacterium, and contains one or more molecules of small circular DNA. It also contains
ribosomes; however most of its proteins are encoded by genes contained in the host cell nucleus,
with the protein products transported to the chloroplast.
Within the stroma are stacks of thylakoids, the sub-organelles, which are the site of
photosynthesis. The thylakoids are arranged in stacks called grana (singular: granum). A
thylakoid has a flattened disk shape. Inside it is an empty area called the thylakoid space orlumen. Photosynthesis takes place on the thylakoid membrane; as in mitochondrial oxidative
phosphorylation, it involves the coupling of cross-membrane fluxes with biosynthesis via the
dissipation of a proton electrochemical gradient.
In the electron microscope, thylakoid membranes appear as alternating light-and-dark bands,
each 0.01 m thick. Embedded in the thylakoid membrane are antenna complexes, each of which
consists of the light-absorbing pigments, including chlorophyll and carotenoids, as well as
proteins that bind the pigments. This complex both increases the surface area for light capture,
and allows capture of photons with a wider range of wavelengths. The energy of the incident
photons is absorbed by the pigments and funneled to the reaction centre of this complex throughresonance energy transfer. Two chlorophyll molecules are then ionised, producing an excited
electron, which then passes onto the photochemical reaction centre.
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Enclosed by two smooth membranes having distinct space known as Periplastidial Space(1030 nm)
Internal str. Can be differentiated into Grana (Light reaction) and Stroma (Dark reaction) Grana consist of lamellar system stroma is non-membranous and multienzyme complex Grana:Consist of closed thylacoids stacked together to form grana and connected together
with the help of intergranal lamellae
Granum or lamellae contain light absorbing pigments. The membrane consists Quantasomescapable of carrying out the photochemical reaction and each of them contains about 200
chlorophyll molecules.
FUNCTIONS
1. The chloroplast function is mainly the process through which the food inside the plant, thatis, the carbohydrate is transferred to all the parts of the plants. This is the basic chloroplast
function which helps the plant to survive. Once the function is performed properly in the
plants, the plant will grow well. The stomates are small holes which are present in the leaves.
2. These are generally located at the lower epidermis. The stomates absorb carbon dioxide andrelease oxygen. The food which is created is converted into organic compounds. Therefore,
these food substances are distributed and spread across the plant in various parts.
3. The presence of chloroplasts in the plants help in the process of food production easily andsmoothly. The functions and the procedures which is carried out by these chloroplasts in the
plants. Within the plant organelles the substance which is present in it are mainly proteins
which are called chloroplasts.