Cell Details

7/29/2019 Cell Details.

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D R . S . D A M .

Cell anatomy.


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9/15/2013

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Dr. S. Dam slides Amity Teaching module.


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9/15/2013Dr. S. Dam slides Amity Teaching module.

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Endosymbiosis.


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Evidence supports eukaryotic cells are descendentsof separate prokaryotic cells that joined together in asymbiotic union.

Mitochondrion seems to be the "greatgranddaughter" of free-living bacterium that was

engulfed by another cell, as a meal, and ended upstaying permanent houseguest. Host cell profited from chemical energy

mitochondrion produced, mitochondrion benefitedfrom protected, nutrient-rich environmentsurrounding it.

This kind of "internal" symbiosis one organismtaking up permanent residence inside anotherevolving into a single lineage is calledendosymbiosis.


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Evidence for endosymbiosis


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Biologist Lynn Margulis first made the case forendosymbiosis in the 1960s, but for many years other

biologists were skeptical. Why should we think that a mitochondrion used to be a

free-living organism in its own right?

Decades ofaccumulated evidence, supports Margulis'sideas: endosymbiosis is best explanation for evolution ofeukaryotic cell.

Evidence for endosymbiosis applies not only tomitochondria, but also to other cellular organelles.

Chloroplast organelles were once free-living bacteria.

The endosymbiotic event that generated mitochondriahappened early , because all eukaryotes have them.

A similar event brought chloroplasts into some eukaryoticcells, creating the lineage that led to plants.


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Many striking similarities between prokaryotes and mitochondria.


7 MembranesCell membranes like prokaryotic cell .

DNA

Circular DNA genome like bacteria's genomebutmuch smaller. DNA passed from mitochondrion tooffspring separate from "host" cell's genome in nucleus.

ReproductionMultiply by pinching in half the sameprocess used by bacteria. Every new mitochondrion must

be produced from a parent mitochondrion in this way; if acell's mitochondria are removed, it can't build new onesfrom scratch.


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Gene function transfer.


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First eukaryotic cell evolved more than billionyears ago.

Since then, these organelles have becomecompletely dependent on their host cells.

E.g. many key proteins needed by mitochondrionare imported from rest of cell. Sometime during their long-standing

relationship, the genes that code for theseproteins were transferred from themitochondrion to its host's genome.

This mixing of genomes is irreversible at whichtwo independent organisms become a singleindividual.


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Cell Wall

A remarkable structure.

Most significant difference between plantand other eukaryotic cells.

It is rigid (many micrometers in thickness)gives plants defined shape.

Creates important difference between plantand animal cell functions.

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Cell wall.


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Layers of cell wall.


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Up to three strata or layers may be found in plant cellwalls:

The middle lamella, a layer rich in pectins. This outermost layer forms interface between

adjacent plant cells and glues them together. The primary cell wall, generally a thin, flexible and

extensible layer formed while the cell is growing. The secondary cell wall, a thick layer formed inside

the primary cell wall after the cell is fully grown. It is not found in all cell types. Some cells, such as the conducting cells in xylem,

possess a secondary wall containing lignin, whichstrengthens and waterproofs the wall.


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Material in the cell wall.


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Varies between species & cell type.

Composition, properties, and form of the cellwall may change during the cell cycle and dependon growth conditions.

Bacteria peptidoglycan. Archaean - glycoprotein S-layers, pseudopeptidoglycan,

or polysaccharides.

Fungi - glucosamine polymer chitin.

Algae - glycoproteins and polysaccharides. Diatoms -biogenic silica.

Accessory molecules found anchored to cell wall.


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Cell wall.

Composed of cellulose fibre, polysaccharides,and proteins.

In new cells cell wall is thin and pliable whichallows the young cell to grow.

First cell wall of growing cells is called primarycell wall. When fully grown, may retain primary wall,

sometimes thickening it, or deposits new layersof different material, called secondary cell wall.

Each cell's wall interacts with its neighbours toform a tightly bound plant structure. Chemical signals and cellular excretions pass

between cells.

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Permeability of cell wall.


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Primary cell wall of most plant cells is semi-permeable.

Permits passage of small molecules andsmall proteins, with size exclusion estimatedto be 30-60 kDa.

Key nutrients especially water and CO2distributed through from cell wall to cell wallin apoplastic flow.

The pH is an important factor governing thetransport of molecules through cell walls.


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Purpose of cell wall.


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Similar purpose in organisms that possessthem.

Gives cells rigidity and strength.

Offering protection against mechanicalstress.

Build and hold its shape (morphogenesis).

Limits entryof toxic large molecules.

Creation of a stable osmotic environmentbypreventing osmotic lysis and helping toretain water.


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What plants lost?


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Nervous systems.

Immune system.

Mobility.


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Cell Membranes A dynamic cell organelle.

The cell membrane defined the first cell.

Since then, it has evolved to become therepository of surface receptors and surfaceantigens.

It is abiological membrane separatingthe interior of cells from outsideenvironment.

Selectively permeable to ions and organicmolecules and controls traffic of substancesin and out of cells.

Protects cell from its surroundings.

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Cell processes.


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Cell membranes are involved in a variety ofcellular processes such as

1. Cell adhesion.

2.Ion conductivity.

3. Cell signaling.

4. Antigenic activity.

5. Attachment surface for several extracellular

structures, including the cell wall, glycocalyx,and intracellular cytoskeleton.

Can be artificially reassembled.


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Cell Membrane

Cell membrane is a flexible lipid bilayer. The lipid molecules (mostly phospholipids) that

make up the membrane have a polar hydrophilichead and two hydrophobic tails.

When the lipids are immersed in an aqueoussolution the lipids spontaneously bury the tailstogether and leave the hydrophilic headsexposed.

It is a handy membrane to use as it canautomatically fix itself when torn.

There are three different major classes of lipidmolecules - phospholipids, cholesterol, andglycolipids.

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Phospholipids.


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Class of lipids that are a major component of all cellmembranes as theycan form lipid bilayers.

Most phospholipids contain a diglyceride, a phosphate group,and simple organic molecule such as choline.

The first phospholipid identified as such in biological tissueswas lecithin or phosphatidylcholine in egg yolk.

Structure of the phospholipid molecule consists ofhydrophobictails & hydrophilic head. Biological membranes also contain another class of lipid sterol

interspersed among the phospholipids. Together they provide membrane fluidity and mechanical

strength.

Purified phospholipids are produced commercially and have foundapplications in nanotechnology and materials science.


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9/15/2013Dr. S. Dam slides Amity Teaching module. 21


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Phospholipid synthesis.


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Phospholipid synthesis occurs in cytosoladjacent to ER.

Studded with proteins that act in synthesis(GPAT and LPAAT acyl transferases,

phosphatase and choline phosphotransferase)and allocation (flippase and floppase).

Eventually avesicle will bud off from the ERcontaining phospholipids destined forcytoplasmic cellular membrane on its exteriorleaflet and phospholipids destined for theexoplasmic cellular membrane on its innerleaflet.


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Lipid bilayer.


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Lipid bilayer form through the process of self-assembly. Cell membrane consists ofamphipathic phospholipidswhich

spontaneously arrange. Amphipathic biomolecules : phospholipids,

cholesterol, glycolipids, fatty acids, bile acids, saponins, etc.

Amphipathic nature of phospholipids forms bilayers in cellmembrane.

They position their polar (hydrophilic) groups towards thesurrounding aqueous medium & nonpolar (hydrophobic) chainstowards inside of the bilayer, so two nonpolar region betweentwo polar ones.

Forces such as van der Waals, electrostatic, hydrogen bonds,

and noncovalent interactions all contribute to the formation ofthe lipid bilayer.

Hydrophobic interactions are the major driving force in theformation of lipid bilayers.

A f hi hi li id l l


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Arrangement of amphipathic lipid moleculesto form a lipid bilayer.


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Lipid bilayers arrangement.


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Generallyimpermeable to ions and polar molecules. Hydrophobic interior of lipidbilayer does not allow polar

molecules to enter e.g. amino acids, nucleic acids,carbohydrates, proteins & ions from diffusing but allow passivediffusion of hydrophobic molecules.

Cell controls movement of these substances via transmembrane

protein complexes such as pores, channels and gates. Cells have devised means of transferring small polar molecules. Transport proteins each specialized for a certain molecule, can

transport polar molecules across the membrane. There are several types of membrane transport proteins. Uniports simply move solutes from one side to another.

Cotransport systemswork by simultaneously sending twosolutes across the lipid bilayer.


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Cotransport systems

1. Symport in which solutes are sent in the samedirection.

2. Antiport in which they are sent in opposite

directions. These transport proteins workpassivelyas the

cell doesn't have to expend energy sending thesolute in or out.

Solutes move in its natural direction - i.e. movingfrom more concentrated solution to less or frompositive to negative.

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Co Transport.

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Diverse functions of phospholipid bilayer.


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Regulate movement of materials into and outof cells.

Phospholipid bilayer structure with specific

membrane proteins accounts for the selectivepermeability of the membrane and passiveand active transport mechanisms.

Membranes in prokaryotes and in

mitochondria and chloroplasts of eukaryotesfacilitate the synthesis of ATP throughchemiosmosis.


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Chemiosmosis.


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Movement of ions across a selectively permeablemembrane down their electrochemical gradient.

It relates to generation of ATP by the movementof hydrogen ions across a membrane during

cellular respiration.

An Ion gradient has potential energyand can beused to power chemical reactions when the ionspass through a channel.

Hydrogen ions (protons) will diffuse from anarea of high proton concentration to an area oflower proton concentration.


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Chemiosmosis.


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An electrochemical concentration gradient ofprotons across a membrane could be harnessed tomake ATP.

This process is like to osmosis of water across amembrane, which is why it is called chemiosmosis.

ATP synthase is the enzyme that makes ATP bychemiosmosis.

Allows protons to pass through the membrane anduses the kinetic energy to phosphorylate ADP,

making ATP. The generation of ATP by chemiosmosis occurs in

chloroplasts and mitochondria as well as inmost bacteria and archaea.


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Chemiosmosis.


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Membrane proteins.

Different proteins are present on surface usedfor various functions such as cell surfacereceptors, enzymes, surface antigens, and

transporters. Many membrane-associated proteins have

hydrophilic and hydrophobic regions. Hydrophilic regions are used to help anchor

protein inside of the cell membrane. Some proteins extend across lipid bilayer others

cross bilayer several times.

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Cell membrane proteins.

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Functions of cell membrane.


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Physical separation of intracellular componentsfrom extracellular environment.

Anchors cytoskeleton providing shape to cell. Attaches to extracellular matrix and other cells

to group cells into tissues. Selectively permeable to regulate what

enters/exits, facilitating transport of materialsfor survival.

Movement of substances can be "passive"

(without energy) or active (expending energy). Membrane also maintains the cell potential. Employs a number of transport mechanisms.


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Cell potential.


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Membrane potential/transmembrane


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Membrane potential/transmembranepotential/membrane voltage


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Difference in electric potential between interior & exterior. Typical values - 40 mV to 80 mV. Serves as insulator & diffusion barrier to movement of ions. Ion transporter/pump proteins actively push ions across

membrane to establish concentration gradients. Ion channels allow ions to move across down concentration

gradients a process known as facilitated diffusion. Combination of ion pumps and ion channels is electrically

equivalent to a set of batteries and resistors inserted in themembrane creating a voltage difference between two sides.

All eukaryotic cells (including cells from animals, plants, and

fungi) maintain a nonzero transmembranepotential usually with a negative voltage in interior ascompared to exterior.


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Why membrane potential?


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Allows cell to function as battery providing power tooperate molecular devices embedded in membrane.

In electrically excitable cells as neurons and musclecells is used for transmitting signals betweendifferent parts of cell.

Signals generated by opening or closing of ionchannels at one point in the membrane producing alocal change in the membrane potential.

This change in electric field can quickly be detectedby either adjacent or more distant ion channels in

membrane. Those ion channels can then depolarize reproducing

& propagating the signal.


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1. Passive diffusion and osmosis.


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Some substances biochemical's &other atomic or molecules as CO2 & O2 canmove across by diffusion a passive transportprocess.

Unlike active transport does not requireinput of energy.

Rate of transport depends on semi-

permeabilityof membrane which in turndepends on organization and characteristicsof the membrane lipids and proteins.


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Kinds of passive transport.


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Diffusion.

Facilitated diffusion.

Filtration.

Osmosis.


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Diffusion.


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Net movement of material from area of highconcentration to lower concentration.

Difference termed as concentration gradient

and diffusion continues till gradienteliminated.

Moving solutes down the concentrationgradient compared with active transport,

which moves material "against theconcentration gradient.


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Simple diffusion/Osmosis.


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Simple diffusion and osmosis are similar.

Simple diffusion is passive movement ofsolute until concentration of solute reaches

equilibrium. Osmosis like simple diffusion but specifically

describes movement of water (not thesolute)until there is equal concentration of

water and solute on both sides.


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Simple diffusion.


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Carrier-mediated diffusion where movementof molecules via special transport proteinsembedded in membrane.

Large molecules such as glucose are insoluble

in lipids and too large to fit through themembrane pores.

Will bind with specific carrier proteins &complex will be bonded to a receptor site andmoved through.

Facilitated diffusion is a passive process assolutes move down concentration gradient anddon't use energy.



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Filtration.


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Movement of water and solute due tohydrostatic pressure generatedby thecardiovascular system.

Depending on the size of the membrane pores,

only solutes of a certain size may pass through it.

E.g. membrane pores of Bowman's capsule arevery small, and only albumin has any chance ofbeing filtered through (prevent wastage).

Membrane pores of liver cells are extremelylarge to allow a variety of solutes to pass through& be metabolized (allow utilization).


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Filtration.


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Osmosis.


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Diffusion of water molecules across aselectively permeable membrane.

Net movement of water molecules from asolution of low water potential to an area ofhigh water potential.

Cell with a less negative water potential willdraw in water but this depends on other

factors as well such as solute potentialpressure in the cell created by solutes.


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Osmosis.


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2 Transmembrane protein


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2. Transmembrane proteinchannels and transporters.


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Nutrients as sugars or amino acids must enterthe cell and products of metabolism must leavethe cell.

Such molecules are pumped across membrane

by transmembrane transporters or diffusethrough protein channels such as Aquaporins inthe case of water (H2O).

These proteins, also called permeases, are quite

specific, recognizing and transporting only alimited group of chemicals, often only a singlesubstance.


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Transmembrane protein channels/Ion channels.


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Pore-forming membrane proteins whosefunctions include;

1. Establishing resting membrane potential.

2. Shaping action potentials.3. Shaping electrical signals by gating the flow

of ions.

4. Controlling the flow of ions and regulating

cell volume.

Present in membranes of all cells.


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Ion channel.


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Narrow water-filled tunnels that allow only ions of acertain size and/or charge to pass through. This characteristic called selective permeability. Channel pore isjust one or two atoms wide at its

narrowest point & is selective for specific ions suchas sodium or potassium.

Some channels may be permeable to more than one typeof ion sharing a common charge positive (cations) ornegative (anions).

Ions often move through segments of channel pore insingle file nearly as quickly as the ions move through freesolution.

In many ion channels, passage through the pore isgoverned by a "gate",which may be opened or closed inresponse to chemical or electrical signals, temperature, ormechanical force.

1.Channel domains typically four per channel.


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1.Channel domains typically four per channel.2. Outer vestibule.3. Selectivity filter .

4. Diameter of selectivity filter.5. Phosphorylation site.

6. Cell membrane.


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Sodium channels are integral membrane proteins forming ionchannels conducting Na+ through plasma membrane.


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VDCC.


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Voltage-dependent calcium channels are agroup of voltage-gated ion channels foundin membrane of excitable cells(e.g., muscle, glial cells, neurons, etc with

permeability to Ca2+.

These channels are slightly permeableto sodium ions so are called Ca2+-

Na

+

channels. Permeability to calcium is 1000-fold greater

than sodium.


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VDCC.


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VGKC.


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Voltage-gated potassiumchannelsare transmembrane channelsspecific for potassium & sensitive to voltagechanges in cell's membrane potential.

During action potentials they play a crucialrole in returning depolarized cell to restingstate.


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VGKC.


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3. Endocytosis.


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Cells absorb molecules by engulfing them. Plasma membrane creates a small deformation

inward, called an invagination in which thesubstance to be transported is captured.

The deformation then pinches off from the

membrane on the inside of the cell creating avesicle containing the captured substance.

Endocytosis is a pathway for internalizing solidparticles (cell eating or phagocytosis), smallmolecules and ions (cell drinking or

pinocytosis), and macromolecules. Endocytosis requires energy and is thus active

transport.


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Endocytosis.


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Low density lipoprotein, transferrin, growth


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factors, antibodies.


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Constitute 1/3 of membrane area of some tissues being abundantin smooth muscle type I pneumocytes fibroblasts adipocytes


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in smooth muscle, type I pneumocytes, fibroblasts, adipocytes,& endothelial cells.


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Bacteria, dead tissue, cell & small mineral particles


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, , pare phagocytosed.


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EM of a phagocyte phagocytosing anthrax


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bacilli.


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Exocytosis.


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Extruding contents to the surroundingmedium.

Occurs in various cells

1. Remove undigested residues of substances.2. Secrete substances as hormones and

enzymes.

3. Transport a substance completely across

cellular barrier.

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Process of Exocytosis.


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Undigested waste-containing food vacuole orsecretory vesicle budded from Golgiapparatus is moved by cytoskeleton frominterior to the surface.

Vesicle membrane comes in contact with theplasma membrane.

The lipid molecules of the two bilayersrearrange themselves and the two

membranes are thus fused.A passage is formed in fused membrane and

vesicles discharges its contents outside.

E i


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Exocytosis.


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Membrane Receptors.


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The cell membrane is pocketed with receptorsand antigens.

Molecules targeted toward that specific cell willbind with the cell surface receptor, which binds

the signalling molecule and sends a signal thatalters the behaviour of the target cell.

Antigens are used to tell the cell whether foreignmaterials are present.

If any foreign materials are detected the immunesystem will mobilize its killer T-cells to destroythe foreign cell.

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Cell surface receptorsmembraneb


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receptors, transmembrane receptors.


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Specialized integral membrane proteins thattake part in communication between cell and theoutside world.

Extracellular signaling molecules -hormones, neurotransmitters, cytokines, growth

factors or cell recognition molecules attach toreceptor, triggering changes in function of cell. Process is called signal transduction: Binding

initiates a chemical change on intracellular sideof membrane.

Receptors play a unique and important role incellular communications and signaltransduction.

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Receptors.


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Structure and mechanism.


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Many transmembrane receptors are composed of two or more proteinsubunits which operate collectively and may dissociate when ligands bind, fall off,or at another stage of their "activation" cycles.

Classified based on their molecular structure, or because the structure isunknown in any detail for all but a few receptors, based on their hypothesized(and sometimes experimentally verified) membrane topology.

The polypeptide chains of the simplest are predicted to cross the lipid bilayer onlyonce, while others cross as many as seven times (for example, the so-called G-protein coupled receptors).

There are various kinds, such as glycoprotein and lipoprotein.

Hundreds of different receptors are known and many more are yet to bediscovered.

Almost all known membrane receptors are transmembrane proteins. A certain cell membrane can have several membrane receptors with various

amounts on its surface. A certain receptor may also exist at varying concentrations on different

membrane surfaces, depending on the membrane and cell function. Since receptors usually form clusters on the membrane surface, the distribution

of receptors on membrane surface is mostly heterogeneous.

R t d l ti


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Receptor modulation.


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Extracellular signal molecule: Produced by one cell and iscapable of traveling to neighboring cells or to cells thatmay be far away.

Receptor protein: Surface receptor proteins bind to signalmolecule and communicate its presence inward into thecell.

Intracellular signaling proteins: Distribute signal toappropriate parts of the cell. Binding of signal molecule toreceptor protein will activate intracellular signalingproteins initiating a signaling cascade (a series ofintracellular signaling molecules that act sequentially).

Target proteins: Conformations or other properties of

target proteins are altered when a signaling pathway isactive and changes the behavior of the cell.

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Receptor modulation.


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C t l


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Cytoplasm.

Area of the cytoplasm outside of the individualorganelles is called the cytosol a gel likematerial.

Largest structure - 54% volume. Milieu where metabolic functions occur. Contains thousands of enzymes for metabolic

activity as glycolysis & gluconeogenesis &biosynthesis of sugars, fatty acids & amino acids.

Cytosol takes molecules & breaks them down foruse by organelles.

E.g. - Glucose is ingested and broken down intopyruvate in the cytosol, for use inthe mitochondria.

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C t l


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Cytosol.


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Complex mixture of cytoskeleton filaments, dissolvedmolecules, and water that fills much of the volume of acell.

Also contains the protein filaments that make upthe cytoskeleton, as well as soluble proteins and smallstructures such as ribosomes, proteasomes, and the

mysterious vault complexes.

The inner, granular and more fluid portion of thecytoplasm is referred to as endoplasm.

Due to this network of fibres and high concentrations ofdissolved macromolecules, such as proteins, an effectcalled macromolecular crowding occurs and the cytosol

does not act as an ideal solution. This crowding effect alters how the components of the

cytosol interact with each other.

C t l i l i


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Cytosol inclusions.


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The inclusions are small particles of insoluble substancessuspended in the cytosol. Range of inclusions in different cell types as crystals

of calcium oxalate or silicon dioxide in plants to granulesof energy-storage materials such

as starch, glycogen, or polyhydroxybutyrate. Lipid droplets are spherical droplets composed of lipids

and proteins that are used in both prokaryotes andeukaryotes as a way of storing lipids such as fattyacids and sterols.

Lipid droplets make up much of the volumeof adipocytes, which are specialized lipid-storage cells,

but they are also found in a range of other cell types.

Cytoplasm


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Cytoplasm.


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All contents of prokaryotes organisms are contained incytoplasm.

Inside nucleus is nucleoplasm. 70% to 90% water & colorless. It is within the cytoplasm that most cellular activities

occur, such as many metabolic pathways includingglycolysis, and processes such as cell division.

The inner, granular mass is called the endoplasm and theouter, clear and glassy layer is called the cell cortex orthe ectoplasm.

Movement of calcium ions in and out of the cytoplasm isthought to be a signaling activity for metabolic processes. In plants, movements of the cytoplasm around vacuoles

are known as cytoplasmic streaming.

Cytoskeleton


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Cytoskeleton.

Eukaryotic cells have variety of distinctshapes and internal organizations.

Cells are capable of changing shape, movingorganelles & move from place to place.

Requires a network a protein filamentsplaced in cytoplasm known as cytoskeleton.

Two most important protein filaments are

called the actin filaments & the microtubules.Actin is for contraction (like in muscles) &

microtubules for structural strength.

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Cytoskeleton


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Cytoskeleton.

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Cytoskeleton


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Cytoskeleton.


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Cytoskeleton


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Cytoskeleton.


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Nucleus.


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It is the cellular control centre and exists onlyin eukaryotes.

Contains the genetic information for the cell, in the formof DNA and RNA.

The genetic information is surrounded by a two-layernuclear envelope and it is generally found at the centre ofthe cell.

Nucleus is responsible for communicating with otherorganelles in the cytoplasm (the gel-like space

surrounding the nucleus). Messages from inside the nucleus travel through pores

on the nuclear envelope to enter the cytoplasm.

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Nucleus.


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Inside the nucleus, called the nucleoplasm, DNA isbound by histone proteins and organized intochromatin.

During replication, the chromatins are condensed to

form highly organized structurescalled chromosomes.

Besides the genetic information, most nuclei also

contain one or more spherical-shaped organellescalled the nucleoli.

The nucleolus is where ribosomes are assembled.

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Nucleic Acids / DNA / RNA


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Nucleic Acids / DNA / RNA

At the core of each cell are molecules that store theblueprint of life, deoxyribonucleic acid (DNA). DNA is composed of four types of molecules known

as nucleic acids or nucleotides. The four nucleotides are:

1. Adenine (A),2. Guanine (G),3. Cytosine (C),4. Thymine (T). Classified into two families:

Adenine (A) and guanine (G) are known as purineand cytosine (C) and thymine (T) are known aspyrimidine.

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Double helix of DNA


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Double helix of DNA.

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Double helix


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Double helix.

The chemical structures of the four nucleotides are planardue to the delocalized electrons in the five- and six-membered rings, each having a thickness of 3.4angstroms.

When the nucleotides form the double helix structure, A-Tand G-C arejoined together by a hydrogen bond to form a

base pair. The base pairs are then joined together by Pentose sugarbonds to form the helix.

X-ray data shows that there are 10 base pairs per turn ofthe helix.

The helical model of DNA also explains the theory of

genetic replication. James Watson once described it as the "pretty molecule"

because the method of replication is so self evident in thisstructure.

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DNA replication


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DNA replication.

During replication, the hydrogen bonds between nucleotidesbreak and allow each single strand of DNA to serve as a templatefor replication of the other half.

The two identical copies of newly synthesized of DNA are thendistributed to two new daughter cells.

Because during each cycle of replication half of the old DNA ispreserved, DNA replication is said to be semi-conservative.

Although DNA contains the genetic blueprint of life, it requiresthe assistance of ribonucleic acid (RNA) to be functional.

RNA also consists of strands of nucleic acids joined together bysugar-phosphate bonds.

Unlike DNA, RNA substitutes the nucleotide thymine (T) withuracil (U) and exists as single strands.

After DNA is converted into strands of RNA, the messenger RNA(mRNA) is sent to the ribosome to direct the synthesis ofproteins.

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Chromosome


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Chromosome. A cell's genetic information, in the form of DNA, is

stored in the nucleus. Space inside the nucleus is limited and has to contain

billions of nucleotides that compose the cell's DNA. Therefore, the DNA has to be highly organized. There are several levels to the DNA packaging. DNA starts out as single strand double helices and

continues to be condensed until it reaches thechromosomal level. At the finest level, the nucleotides are organized in the

form of linear strands of double helices. DNA strand is wrapped around histones, a form of DNA

binding proteins. Each unit of DNA wrapped around a histone molecule is

called a nucleosome. The nucleosomes are linked together by the long strand

of DNA.

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To further condense the DNA material, nucleosomes are compactedtogether to form chromatin fibers.

The chromatin fibers then fold together into large looped domain. During the mitotic cycle, the looped domains are organized into distinct

structures called the chromosomes. Chromosomes are also used as a way of referring to the genetic basis of

an organism as either diploidor haploid.

Many eukaryotic cells have two sets of the chromosomes and are calleddiploid. Other cells that only contain one set of the chromosomes arecalled haploid.

The chromosome also plays an important role in cell death-related agingphenomena.

At the tips of chromosomes are segments called telomeres. As a cell's DNA is damaged, the telomeres are shortened.

Once the telomeres have been reduced to a level, the cell decides that itcan no longer repair itself and initiates apoptosis, the cellular deathprocess.

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Mitochondria - cellular power plant


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Mitochondria cellular power plant.

0.5 to 1.0 micrometer m. Number of mitochondria in a cell varies widely

by organism and tissue type.

Many cells have a single mitochondrion, whereas

others can contain several thousand. Consists of four major sections :

1. Outer membrane.

2. Intermembrane space.

3. Inner membrane.

4. Matrix.

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EM of Mitochondria.
https://en.wikipedia.org/wiki/Micrometrehttps://en.wikipedia.org/wiki/Micrometrehttps://en.wikipedia.org/wiki/Micrometre


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EM of Mitochondria.

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Structure of mitochondria


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Structure of mitochondria.

The Outer Membrane Contains a number of largetransport proteins which allow for large molecules toenter.This membrane includes proteins that can convert lipidsubstrates into forms that can be used by the matrix.

Intermembrane Space

Contains enzymes that use ATP to phosphorylate othernucleotides.

Inner MembraneHighly convoluted forming many folds called cristae.Greatly increase surface area allowing more work done in

a smaller space. MatrixThe Krebs Cycle takes place here.

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Other tasks of Mitochondria.


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Other tasks of Mitochondria.95

Signaling. Cellular differentiation.

Cell death.

Control of the cell cycle.

Cell growth.

Ageing.

Implicated in several human diseases like

mitochondrialdisorders & cardiac dysfunction and may playa role in the aging process.

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