paper viii unit i- 11-12 cell wall and plasma membrane

7/31/2019 Paper VIII Unit I- 11-12 Cell Wall and Plasma Membrane

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B. Sc. IIIPaper VIII: UnitI : Cell Wall,Plasma Membrane, Cell Organelles Page 1 of33

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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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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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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B. Sc. IIIPaper VIII: UnitI : Cell Wall,Plasma Membrane, Cell Organelles Page 6of33


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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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
http://en.wikipedia.org/wiki/Protisthttp://en.wikipedia.org/wiki/Protisthttp://en.wikipedia.org/wiki/Protist


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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
http://en.wikipedia.org/wiki/Endoplasmic_reticulumhttp://en.wikipedia.org/wiki/Peroxisomal_targeting_signalhttp://en.wikipedia.org/wiki/C-terminushttp://en.wikipedia.org/wiki/C-terminushttp://en.wikipedia.org/wiki/C-terminushttp://en.wikipedia.org/wiki/C-terminushttp://en.wikipedia.org/wiki/N-terminushttp://en.wikipedia.org/wiki/N-terminushttp://en.wikipedia.org/wiki/Peroxinhttp://en.wikipedia.org/wiki/Peroxisome#cite_note-pmid17050007-16http://en.wikipedia.org/wiki/Peroxisome#cite_note-pmid17050007-16http://en.wikipedia.org/wiki/Peroxisome#cite_note-pmid17050007-16http://en.wikipedia.org/wiki/ATP_hydrolysishttp://en.wikipedia.org/wiki/ATP_hydrolysishttp://en.wikipedia.org/wiki/ATP_hydrolysishttp://en.wikipedia.org/wiki/PEX5http://en.wikipedia.org/wiki/PEX5http://en.wikipedia.org/wiki/PEX7http://en.wikipedia.org/wiki/PEX7http://en.wikipedia.org/wiki/Cytosolhttp://en.wikipedia.org/wiki/Enzymehttp://en.wikipedia.org/wiki/Catalasehttp://en.wikipedia.org/wiki/D-amino_acid_oxidasehttp://en.wikipedia.org/wiki/Uric_acid_oxidasehttp://en.wikipedia.org/wiki/Uric_acid_oxidasehttp://en.wikipedia.org/wiki/Gouthttp://en.wikipedia.org/wiki/Gouthttp://en.wikipedia.org/wiki/Uric_acid_oxidasehttp://en.wikipedia.org/wiki/Uric_acid_oxidasehttp://en.wikipedia.org/wiki/D-amino_acid_oxidasehttp://en.wikipedia.org/wiki/Catalasehttp://en.wikipedia.org/wiki/Enzymehttp://en.wikipedia.org/wiki/Cytosolhttp://en.wikipedia.org/wiki/PEX7http://en.wikipedia.org/wiki/PEX5http://en.wikipedia.org/wiki/ATP_hydrolysishttp://en.wikipedia.org/wiki/Peroxisome#cite_note-pmid17050007-16http://en.wikipedia.org/wiki/Peroxinhttp://en.wikipedia.org/wiki/N-terminushttp://en.wikipedia.org/wiki/C-terminushttp://en.wikipedia.org/wiki/C-terminushttp://en.wikipedia.org/wiki/Peroxisomal_targeting_signalhttp://en.wikipedia.org/wiki/Endoplasmic_reticulum


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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.

paper viii unit i- 11-12 cell wall and plasma membrane

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