2.1. Cell Structure and Function of Organelles

8/11/2019 2.1. Cell Structure and Function of Organelles

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Lesson Objective: Describe the detailed structure andfunction of cell wall.

TOPIC 2.1: STRUCTURE AND FUNCTIONS OF ORGANELLES

In plants, the plasma membrane is surroundedby a rigid cellulose cell wall that distinguishesthem from animal cells.The cell wall protects the plant cell, maintains itsshape, and prevents excessive uptake of water .The basic composition of the walls aremicrofibrils that made of the polysaccharidecellulose & embedded in a matrix of otherpolysaccharides & protein.

A young plant cell first secretes a thin & flexiblewall called the primary cell wall.

2.1: a. CELL WALL


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Between primary walls of adjacent cells is themiddle lamella, a thin layer rich in stickypolysaccharides called pectin.The middle lamella glues the cells together.

After the cell stop growing, either new wallmaterial is secreted that thicken & solidifies theprimary wall or multiple layers of a secondarycell wall with a different chemical compositionare formed between the primary wall & theplasma membrane.


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The primary cell wall contains cellulosemicrofibrils loosely arranged. The microfibrils areembedded in an amorphous matrix ofhemicellulos, calcium & magnesium pectate.This arrangement produces spaces between

microfibrils. The cell wall is freely permeable towater & solutes.In secondary cell wall , the cellulose microfibrilsare more compact & successive layers arearranged at different angles.

Additional thickening such as lignin in tracheids& xylem vessels give mechanical strength &support.


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Suberin in cork cells & cutin on epidermalsurfaces reduce evaporation of water &entry of pathogens.The cell walls have pits . These pits enable

cytoplasmic strands (plasmodesmata) to pass through the connecting cytoplasmof one cell & its adjacent cell.Pit in lignified walls of tracheids & xylemvessels allow lateral transport to takeplace.


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Lesson Objective: Describe the structure, functionand location of cytoplasm.


Cytoplasm is the entire region between thenucleus & the plasma membrane.It consist of cytosol & organelles suspended init.Cytosol contains various component:

About 90% water & solutes such as sugars, aminoacids, enzymes, fatty acids, nucleotides, ATP &dissolved gases.

Large molecules such as proteins which form colloidsMicrofilaments & microtubules which form thecytoskeleton, providing support to the cell & areinvolved in cell mortality.

b. CYTOPLASM .


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Lesson Objective: Describe the detailed structure,function and location of organelles.


This is one of the specialize structures inthe cell that carry out specific functions,such as the mitochondria, Golgiapparatus, ribosomes; many organellesare membrane-bounded.The membranes that surround membrane-bounded organelles & the plasmamembrane have the same basic structure.

c. ORGANELLES


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Lesson Objective: Describe the structure, functionand location of nucleus.


Nucleus is the most prominent organelle in the cell.It is usually spherical or oval in shape & averages 5m indiameter.The nuclear envelope consists of 2 layers ofmembranes that separate the nuclear contents from thesurrounding cytoplasm.The spaces between the outer membrane & the innermembrane is known as perinucleus .

The nuclear envelope is perforated by numerous pores .Nuclear pores regulate the pass age of m ater ia lsbe tween n uc leop lasm & cy top lasm .

i. NUCLEUS



Most of the cells DNA is located inside thenucleus.DNA is associated with histone proteins, forminga complex known chromatin.

In dividing cells, the chromatin condenses &becomes visible, called chromosomes.Most nuclei have one or more compactstructures called nucleoli (sing.,nucleolus).

A nucleolus is not membrane-bounded.Here a special type of RNA called ribosomalRNA is synthesized.



Functions of nucleusChromosome contain the geneticmaterial of the cell.

The nuclear acts as the centre tocontrol cell activities and cell division.Production of ribosomes and RNAs

needed for protein and enzymesynthesis.


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Lesson Objective: Describe the structure, functionand location of ER.


The ER is a membranous system which iscontinuous with the outer membrane &scattered extensively in the cell.

There are 2 distinct types of ER:1. Rough Endoplasmic Reticulum (RER)2. Smooth Endoplasmic Reticulum (SER)

Although these regions have differentfunctions, the membranes are connectedand their internal spaces are continuous.

ii. ENDOPLASMIC RETICULUM (ER)



The RER consists of an interconnected systemof membrane-bounded flattened sacs called

sisternae .It is continuous with the outer membrane of thenucleus & has many ribosome on the outersurface of its membrane.RER is abundant in cells which are rapidlygrowing or secretory cells. Eg: pancreatic cells.

Rough Endoplasmic Reticulum

(RER)



Polypeptide chains synthesized byribosomes are modified by enzymes thatadd complex carbohydrates or lipids to

them forming glycoprotein or lipoprotein. The protein are then transferred to other

compartments by small transport vesicles.

Transport vesicles containing modifiedproteins are budded off from RER &transported to the Golgi apparatus.

Functions of RER



SER is more tubular & does not haveribosomes bound to it, so its outermembrane surfaces appear smooth.SER is abundant in cells involved in lipid &steroid hormone synthesis, eg. cells in thetestes & ovaries or cells involved indetoxification, eg. liver cells.

Smooth Endoplasmic Reticulum (SER)


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Lesson Objective: Describe the structure, functionand location of Golgi body.


First described in 1898 by the Italianmicroscopist Camillo Golgi.Golgi apparatus consists of a stack of flattened,

membrane-bounded sacs called cisternae.However, unlike ER the internal spaces of Golgicomplex & the membranes are not continuous.

Each stack contains about 4-7 sisternae.Each Golgi stack has three areas referred to ascis & trans faces with a medial region between.

i i i .GOL GI A PPARA TUS/GOL GI B ODY


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Typically, the cis face is located nearest thenucleus & functions to receive materials fromtransport vesicles from the ER.The trans face , nearest to the plasmamembrane, packages molecules in vesicles &transports them out of the Golgi.The Golgi apparatus is continuously beingformed at the cis-end to replace the vesicleswhish bud off at the trans-end.Golgi apparatus is abundant in secretory cells &cells rapidly dividing e.g. Pancreatic cells, gobletcells, cells in testes & ovaries.


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The Golgi apparatus functions principally toprocess , sort , & modify proteins .The glycoproteins are synthesize in the RER &later in the Golgi.The glycoproteins are transported from the RERto the cis face of the Golgi apparatus in a smalltransport vesicles formed from the ERmembrane.

Products of the ER are usually modified duringtheir transit from the cis face to the trans face of the Golgi.

Functions of Golgi Apparatus .


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For e.g. various Golgi enzymes modify the

oligosaccharide portions of glycoproteins.Oligosaccharides are first added to proteins inthe RER.The Golgi removes some sugar monomers &

substitutes others, producing a large variety ofoligosaccharides.Glycoproteins are packaged in secretoryvesicles in the trans region.These vesicles pinch off from the Golgimembrane & transport their contents to aspecific destination.


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Vesicles transporting products for export from

the cell fuse with the plasma membrane.The vesicles becomes part of the plasmamembrane & the glycoproteins are secretedfrom the cells.Other vesicles may store glycoproteins forsecretion at a later time.In animal cells, the Golgi apparatus also

manufactures lysosomes.In plants cells, the Golgi secretespolysaccharides for the formation of cell plates &cell walls.


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Lesson Objective: Describe the structure, functionand location of lysosome


Lysosomes are produced by the Golgiapparatus.They are spherical, small sacs covered by a

single membrane.The membrane is resistant to digestion &prevents the release of enzymes which woulddestroy the cell contents.

Lysosomes contain a high concentration ofhydrolytic enzymes involved in intracellulardigestion such as protease, lipase.

iv. LYSOSOMES (SUICIDE BAG)


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Lesson Objective: Describe the structure, functionand location of lysosome


Primary lysosomes are formed by budding from

the Golgi apparatus.Their hydrolytic enzymes are synthesized in theRER.

One or more primary lysosomes fuse with thevesicle containing the ingested material, forminga larger vesicle called secondary lysosome .In the secondary lysosome the powerfulenzymes come in contact with the ingestedmolecules & degrade them into theircomponents.


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1) Digestion of ingested food.- Primary lysosome fuses with the food

vacuole forming a secondary lysosome,hyrolytic enzymes are released to breakdown the food into smaller particles forabsorption.

2. Autophagy

- The worn-out organelle, eg. mitochondriais surrounded by SER to formautophagosome .

Functions of lysosomes


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-Lysosome fuses with the autophagosome

& enzyme are then released to digest theorganelle .


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3. In cells that carry out phagocytosis e.g.

neutrophils & macrophages, the lysosomesfuse with the phagocytic vacuoles to breakdown the pathogens e.g. bacteria.

4. Autolysis- In old or damaged cells. Enzymes are

released to digest the cell. - Role in metamorphosis eg. The

reabsorption of tadpole tails, thedigestion of larval tissues to be replacedby new tissues.


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Lesson Objective: Describe the structure, functionand location of ribosomes.


Ribosomes are small, dense granules &made of ribosomal RNA (rRNA) & protein.Ribosome is composed of two subunits: alarge subunit & a small subunit.

v. RIBOSOMES


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Ribosomes are found attached to the RER

or occur freely within the cytoplasm.Ribosomes can occur singly or grouptogether to form polysomes.

Cells that have high rates of proteinssynthesis have a particularly largenumber of ribosomes. E.g. a human

pancreas cells.


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Site of proteins synthesis.- Most of the proteins made by free

ribosomes will function within the cytosol,eg.are enzymes that catalyze the first step ofsugar break down.

- Bound ribosomes generally makes proteinsthat are destined either for insertion into

membrane, for packaging within certainorganelles such as lysosomes, or for exportfrom the cells (secretion).

Functions of ribosomes


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Mitochondria are found in nearly all eukaryoticcells, including those of plants, animals, fungi &protists.Mitochondria are about 1-10m long.The mitochondrion is enclosed by twomembranes; the outer membrane is smooth, butthe inner membrane is convoluted, withinfoldings called cristae .

Cristae greatly increase the surface area of theinner membrane, providing a surface for thechemical reactions.

vi. MITOCHONDRION

Lesson Objective: Describe the structure, functionand location of mitochondria


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Between the two membranes is the

intermembrane space.The inner membrane encloses a fluid-filledmatrix. In the matrix can be found a few circularstrands of DNA, 70S ribosomes & enzymes thatused in the Krebs cycle.DNA in the mitochondria enables themitochondria to replicate during cell division.

Function of mitochondriaMitochondrion are the sites of cellularrespiration, the catabolic process that generates

ATP.


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Chloroplasts are large plastids found in thephotosynthetic tissues of plant e.g. mesophyllcells & algae.Chloroplasts are organelles that containchlorophyll , a green pigment that traps energyfor photosynthesis.Chloroplasts also contain a variety of yellow &orange light-absorbing pigments known ascarotenoids .Chloroplast are typically disc-shaped structures,measuring about 5 to 10m in lengths.

vii. CHLOROPLASTS

Lesson Objective: Describe the structure, functionand location of chloroplast.


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Chloroplast is bounded by adouble membrane.The inner membrane enclosesa fluid-filled space called thestroma , which contains enzyme( catalyse light-independent

reactions), circular DNA,ribosomes, starch granules &lipid globules.

A series of interconnectingflattened membrane-boundedsacs, called the thylakoids , arefound in the stroma. Thylakoidmembranes may be stackedinto granum (plural, grana).


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Site of photosynthesis.- The grana & thylakoid membrane provide large

surface areas for chlorophyll & accessory

pigments to be located to trap light energy &carry out production of ATP & NADPH.

- The stroma contains enzymes to carry outlight-independent reaction ( Calvin cycle ) &the production of high energy organicmolecules e.g. sugar.

Functions of chloroplasts.


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Lesson Objective: Describe the structure, functionand location of centrioles.


Centrioles are found in animal cells.The centrioles are located outside the nucleus ina material of poorly defined structure called thecentrosome.

Centrioles are paired cylinders ( about 0.3-0.5m long & 0.24m diameter) & held at rightangle to each other.Each cylinder consists of nine triplets of

microtubules in a 9+0 arrangement. The centrioles separate & move to oppositeends of the cell before cell division.

vii. CENTRIOLES


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http://micro.magnet.fsu.edu/cells/animalcell.html


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Centrioles act as organizers ofspindle fibers & are involved in theseparation of chromosomes orchromatids during cell division.In some cells, centrioles divide to

produce basal bodies from whichflagella & cillia develop.

Functions of centrioles


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The cytoplasm contains a cytoskeletonwhich has 3 components:

- microtubules- microfilaments- intermediate filaments

The cytoskeleton gives mechanicalsupport, maintains cell shape & aids cellmotility .

c. THE CYTOSKELETON


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The structure & function of the cytoskeleton

Property Microtubules Microfilaments

Structure

Diameter

Hollow tubes; wallconsists of 13columns of tubulinMolecules.

25nm with 15nmLumen.

2 intertwinedstrands of actin.

7nm


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Protein

Subunits

Main

functions

Tubulin, consisting

of -tubulin &-tubulin.

Maintenance of cell

shape.Cell motility ( as incilia & flagella).Chromosome

movements in celldivision.Organelle movements.

Actin

Maintenance of cell

shape.Changes in cell shape.Muscle contraction.Cytoplasmic streaming.Cell motility ( as in

pseudopodia).Cell division.


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L Obj i D ib h f i


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Lesson Objective: Describe the structure, functionand location of vacuoles


Vacuoles are cavities bound by a single membraneYoung plant cells usually have many vacuoles whichfuse to form a single large central vacuole, the nucleus& cytoplasm are pushed to the cell periphery.

The plant vacuole contains cell sap & is surroundedby a single membrane called tonoplast .The cell sap is a solution containing dissolved organicmolecules eg. sugars & amino acids, pigments,mineral ions, oxygen, carbon dioxide & some wasteproducts.The vacuoles in animal cells are smaller & temporary.They are formed by phagocytosis or pinocytosis

VACUOLES


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Fun ct ion o f vacu oles

In plants the vacuolefunctions to store foodsubstances e.g. sugars,amino acids & mineral ions.Vacuoles of some cells eg.petals of flowers containcolored pigments to attractinsects for pollination.Vacuoles in leaves

accumulate some wasteproducts eg. tannins & areremoved whe.n the leavesfall.

Lesson Objective:Describe the structure, function and location ofplasma membrane. Description of Fluid Mosaic


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Model is required.


All cell are covered by a thin plasma membrane(cell surface membrane) which separates thecell contents from the extracellular environment.

The plasma membrane & the organelle

membrane have the same basic structure. In 1972, S. J. Singer & G.L.Nicolson proposedthe fluid-mosaic model of cell membraneconsisting of a bilayer of phospholipids withvarious protein molecules embedded & attachedto it.

The membrane is about 7 - 7.5nm thick.

PLASMA MEMBRANE


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A phospholipid is an amphipathic molecule,meaning it has both a hydrophilic region & ahydrophobic region.

The cell membrane consists of a phospholipid

bilayer. The polar hydrophilic heads ofphospholipids point outwards , attracted to theaqueous surrounding & form hydrogen bondswith water molecules. The non-polar,

hydrophobic tails face inwards , & areattracted to one another by hydrophobicinteractions & Van der Waals forces.


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There are 2 major classes of membraneproteins, integral proteins (intrinsic protein) & peripheral proteins (extrinsic protein).

Peripheral proteins are not embedded in

the lipid bilayer at all, they are looselybound to the outer & inner surface of themembrane.

The integral proteins are embeddedpartially in the membrane & somepenetrate through the membrane.


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The protein part that contains hydrophobicamino acids interact with the hydrophobicfatty acid chains to expel the water. Thehydrophilic part of the protein is attracted

to the aqueous. Some of the proteins & phospholipidsconjugate with short branchedoligosaccharides to form glycoproteins &glycolipids on the outer surface. Theseare important for cell recognition .


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Membrane fluidity

Cell membrane are not static sheets ofmolecules locked rigidly in place. It is dynamic,fluid structures.

Most of the lipids & some of the proteins can

move laterally or change places that is in theplane of the membrane give the membranefluidity .

Unsaturated fatty acids have double bonds

which form kinks. These prevent close packingof the phospholipid molecules, increase fluidity &promote movement of substances across themembrane


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The steroid cholesterol, which is foundbetween phospholipid molecules, hasdifferent effects on membrane fluidity cholesterol makes the membrane less fluidby restraining the movement ofphospholipids


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The proteins are scattered in the fluidphospholipid bilayer giving a mosaic

appearance & have different structure &functions.

Membranes are mosaics of structure

& function

Lesson Objective: Explain the roles of phospholipid


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j p p p pand protein in membrane.


Surface membrane forms a protective barrier between cell contents & external environment, &determine the shape of the cell.Form membranous organelles , for eg.

mitochondria, chloroplast & ER.Membranes are selectively permeable &regulate movement of substances in & out of thecell.

- small molecules such as oxygen, CO2

&hydrophobic molecules diffuse throughphospholipid bilayers down a concentrationgradient.

Fun c t ions o f m em brane.


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- Protein channels & carriers move polar

ions, molecules, for e.g. glucose & aminoacids across membranes by facilitateddiffusion.

- The protein pumps actively transportsubstances against a concentrationgradient.

Some membrane proteins act as electroncarriers in the electron transport chain.Glycoproteins with branching oligosaccharidesact as antigens


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Glycolipids are involved in cell-cellrecognition.Cell adhesion proteins join cells togetherforming tissue which carries out specificfunction.Cholesterol molecules help to stabilize themembrane structure & reduce the entry orexit of polar molecules through themembrane.


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Cell- cell recognition, a cells ability to distinguish onetype of neighboring cell from another.It is important, for eg. in the sorting of cells into tissues &organs.It also the basis for the rejection of foreign cells( including those of transplanted organ) by immunesystem.

The oligosaccharides on the external side of the plasmamembrane vary from species to species, amongindividuals of the same species, & even from one celltype to another in a single individual.

Membrane carbohydrates are important for cell ce

recognition.


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The diversity of the molecules & theirlocation on the cells surface enableoligosaccharides to function as markers that distinguish one cell from another, fore.g. the human blood groups designated

A, B, AB & O reflect variation in theoligosaccharide on the surface of the

blood cells.

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2.1. Cell Structure and Function of Organelles