A TOUR OF THE CELL

the fundamental unit of lifethe fundamental unit of life

The discovery and early study of cells The discovery and early study of cells progressed with the invention and progressed with the invention and improvement of microscopes in the 17th improvement of microscopes in the 17th century.century. Robert HookeRobert Hooke Anton van LeeuwenhoekAnton van Leeuwenhoek

Microscopes are a major tool in Microscopes are a major tool in cytologycytology, the , the study of cell structuresstudy of cell structures Cytology coupled with Cytology coupled with biochemistrybiochemistry, the study of molecules , the study of molecules

and chemical processes in metabolism, developed modern and chemical processes in metabolism, developed modern cell biology.cell biology.

Microscopes - windows to the Microscopes - windows to the world of the cellworld of the cell

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

From Monk’s Room to From Monk’s Room to Unit of Life Unit of Life

Robert Hooke Robert Hooke thought what he thought what he was looking at was looking at resembled the resembled the rooms monk’s rooms monk’s occupied…..occupied…..

CellsCells

The InstrumentsThe Instruments

D arkfie ld M icroscopy

P hase C ontrast M icroscopy

D ifferentia l In terference C ontrast M icroscopy

F luorescence M icroscopy

C onfocal M icroscopy

C om pound L ight M icroscope

S canning E lectron M icroscopy

Transm iss ion E lectron M icroscopy

A tom ic F orce E lectron M icroscopy

E lectron M icroscopes

Compound Light Compound Light MicroscopeMicroscope

Uses visible lightUses visible light Has at least 2 sets Has at least 2 sets

of lensesof lenses Can achieve Can achieve

maximum 2000X maximum 2000X magnificationmagnification

Resolution of Resolution of objects as small as objects as small as 0.2 0.2 mm

Light MicroscopyLight Microscopy

In a In a light microscope light microscope visible lightvisible light passes through the specimen and passes through the specimen and then through glass lenses.then through glass lenses. The lenses refract light such that the The lenses refract light such that the

image is magnified into the eye or a image is magnified into the eye or a video screen.video screen.

Brightfield IlluminationBrightfield Illumination

Usual operationsUsual operations Specimens must be Specimens must be

stained for viewingstained for viewing Best magnification Best magnification

and resolution with and resolution with the oil immersion the oil immersion objectiveobjective

Oil has same Oil has same refractive index as refractive index as glassglass

Microscopes vary in magnification and Microscopes vary in magnification and resolving power.resolving power.

MagnificationMagnification is the ratio of an object’s is the ratio of an object’s image to its real size.image to its real size.

Resolving powerResolving power is a measure of image is a measure of image clarity.clarity. It is the minimum distance two points It is the minimum distance two points

can be separated and still viewed as two can be separated and still viewed as two separate points.separate points.

Resolution is limited by the shortest Resolution is limited by the shortest wavelength of the source, in this case wavelength of the source, in this case light.light.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Light MicroscopesLight Microscopes

The minimum resolution The minimum resolution of a light microscope is of a light microscope is about 2 microns, the size about 2 microns, the size of a small bacteriumof a small bacterium

Light microscopes can Light microscopes can magnify effectively to magnify effectively to about 1,000 times the about 1,000 times the size of the actual size of the actual specimen.specimen. At higher At higher

magnifications, the magnifications, the image blurs.image blurs.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 7.1

Resolution of Light Resolution of Light MicroscopesMicroscopes

Electron MicroscopyElectron Microscopy

Beam of electrons has shorter Beam of electrons has shorter so so gives better resolution than visible gives better resolution than visible lightlight

Electromagnetic lenses rather than Electromagnetic lenses rather than glassglass

Done in a vacuumDone in a vacuum Can resolve to 0.5nm and magnify up Can resolve to 0.5nm and magnify up

to 100,000 times.to 100,000 times. Specimen must be dry….deadSpecimen must be dry….dead

Electron micrographsElectron micrographs

The goal of The goal of cell fractionationcell fractionation is to separate is to separate the major organelles of the cells so that their the major organelles of the cells so that their individual functions can be studiedindividual functions can be studied..

Cell FractionationCell Fractionation

Fractionation begins with homogenization, begins with homogenization, gently disrupting the cell gently disrupting the cell

Then, the homogenate is spun in a Then, the homogenate is spun in a centrifuge to separate heavier pieces into centrifuge to separate heavier pieces into the pellet while lighter particles remain in the pellet while lighter particles remain in the supernatant. the supernatant.

As the process is repeated at higher speeds As the process is repeated at higher speeds and longer durations, smaller and smaller and longer durations, smaller and smaller organelles can be collected in subsequent organelles can be collected in subsequent pelletspellets ultracentrifuge ultracentrifuge

can spin at up to 130,000 revolutions per minute can spin at up to 130,000 revolutions per minute and apply forces more than 1 million times and apply forces more than 1 million times gravity (1,000,000 gravity (1,000,000 gg).).

The ProcedureThe Procedure

Cell fractionation prepares quantities of Cell fractionation prepares quantities of specific cell components.specific cell components. enables the functions of these organelles to be isolated, enables the functions of these organelles to be isolated,

especially by the reactions or processes catalyzed by their especially by the reactions or processes catalyzed by their proteins.proteins.

For example, one cellular fraction is enriched in For example, one cellular fraction is enriched in enzymes that function in cellular respirationenzymes that function in cellular respiration..

Electron microscopy reveals that this fraction is Electron microscopy reveals that this fraction is rich in the organelles called mitochondria.rich in the organelles called mitochondria.

Cytology and biochemistry complement each Cytology and biochemistry complement each other in connecting cellular structure and other in connecting cellular structure and functionfunction..

Why Look at the Parts Why Look at the Parts rather than the Wholerather than the Whole

Fluorescent stain of cellFluorescent stain of cell

Cell Structure and FunctionCell Structure and Function

Mid 1600s - Robert Hooke observed Mid 1600s - Robert Hooke observed

and described cells in corkand described cells in cork

Late 1600s - Antony van Leeuwenhoek Late 1600s - Antony van Leeuwenhoek

observed sperm, microorganismsobserved sperm, microorganisms

1820s - Robert Brown observed and 1820s - Robert Brown observed and

named nucleus in plant cellsnamed nucleus in plant cells

Early DiscoveriesEarly Discoveries

Cell TheoryCell Theory

Schleiden and SchwannSchleiden and Schwann Every organism is composed of one or more Every organism is composed of one or more

cellscells

Cell is smallest unit having properties of lifeCell is smallest unit having properties of life

VirchowVirchow All exisiting cells arise from pre-existing All exisiting cells arise from pre-existing

cells.cells.

Smallest unit of lifeSmallest unit of life

Can survive on its own or has Can survive on its own or has

potential to do sopotential to do so

Is highly organized for metabolismIs highly organized for metabolism

Senses and responds to environmentSenses and responds to environment

Has potential to reproduceHas potential to reproduce

CellCell

Measuring Measuring

Cells Vary in SizeCells Vary in Size

Why Are Cells So Small?Why Are Cells So Small?

Surface-to-volume ratioSurface-to-volume ratio

The bigger a cell is, the less surface The bigger a cell is, the less surface

area there is per unit volumearea there is per unit volume

Above a certain size, material cannot Above a certain size, material cannot

be moved in or out of cell fast be moved in or out of cell fast

enoughenough

Size is Limited Size is Limited

Fig. 7.5

Metabolic requirements also set an upper limit to the size of Metabolic requirements also set an upper limit to the size of a single cell.a single cell.

As a cell increases in size its volume increases faster than its As a cell increases in size its volume increases faster than its surface area.surface area. Smaller objects have a greater Smaller objects have a greater

ratio of surface area to volume.ratio of surface area to volume.

Structure of CellsStructure of Cells Two or Three types of cellsTwo or Three types of cells

Archeo - cell typeArcheo - cell type There is much evidence that this is a third cell typeThere is much evidence that this is a third cell type

ProkaryoticProkaryotic EukaryoticEukaryotic

All cells have:All cells have: Plasma membrane Plasma membrane Region where DNA is storedRegion where DNA is stored Cytoplasm Cytoplasm ribosomesribosomes

Archeo-cell type and Archeo-cell type and ProkaryotesProkaryotes

No nucleusNo nucleus Nucleoid area where DNA residesNucleoid area where DNA resides

No membrane bound organelles.No membrane bound organelles. 70s ribosomes70s ribosomes Cell walls contain petidoglycan Prokaryotic Cell walls contain petidoglycan Prokaryotic

OrganismsOrganisms EubacteriaEubacteria CyanobacteriaCyanobacteria

Archeo-cell typeArcheo-cell type Pseudomurein rather than peptidoglycanPseudomurein rather than peptidoglycan Organisms belong to the ArcheobacterOrganisms belong to the Archeobacter

A prokaryotic cellA prokaryotic cell

E. coliE. coli

Eukaryotic CellsEukaryotic Cells

Have a nucleus and Have a nucleus and other organellesother organelles

Eukaryotic organismsEukaryotic organisms ProtistansProtistans Fungi Fungi PlantsPlants AnimalsAnimals

Overview of a plant cellOverview of a plant cell

Overview of an animal cellOverview of an animal cell

contains most of the genes in a contains most of the genes in a eukaryotic cell.eukaryotic cell. Some genes are located in mitochondria Some genes are located in mitochondria

and chloroplasts.and chloroplasts. The nucleus averages about 5 microns The nucleus averages about 5 microns

in diameter.in diameter. The nucleus is separated from the The nucleus is separated from the

cytoplasm by a double membrane.cytoplasm by a double membrane. These are separated by 20-40 nm.These are separated by 20-40 nm.

Where the double membranes are Where the double membranes are fused, a pore allows large fused, a pore allows large macromolecules and particles to pass macromolecules and particles to pass through.through.

The nucleus contains a The nucleus contains a eukaryotic cell’s genetic libraryeukaryotic cell’s genetic library

NucleolusNucleolus

In the nucleus is a region of densely stained In the nucleus is a region of densely stained fibers and granules adjoining chromatin, the fibers and granules adjoining chromatin, the nucleolusnucleolus. . ribosomal RNA (rRNA) is synthesized ribosomal RNA (rRNA) is synthesized assembled with proteins from the assembled with proteins from the

cytoplasm to form ribosomal subunits.cytoplasm to form ribosomal subunits. The subunits pass from the nuclear pores The subunits pass from the nuclear pores

to the cytoplasm where they combine to to the cytoplasm where they combine to form ribosomes.form ribosomes.

The nucleus and its envelope The nucleus and its envelope 

Keeps the DNA molecules of Keeps the DNA molecules of eukaryotic cells separated from eukaryotic cells separated from metabolic machinery of cytoplasmmetabolic machinery of cytoplasm

Makes it easier to organize DNA and Makes it easier to organize DNA and to copy it before parent cells divide to copy it before parent cells divide into daughter cells into daughter cells

Functions of NucleusFunctions of Nucleus

The nucleus and its envelope The nucleus and its envelope 

Group of related organelles in which lipids Group of related organelles in which lipids are assembled and new polypeptide are assembled and new polypeptide chains are modifiedchains are modified

Products are sorted and shipped to various Products are sorted and shipped to various destinationsdestinations

Components of the cytomembrane systemComponents of the cytomembrane system Endoplasmic reticulumEndoplasmic reticulum Golgi apparatusGolgi apparatus vesiclesvesicles

Cytomembrane SystemCytomembrane System

Endoplasmic ReticulumEndoplasmic Reticulum

In animal cells, continuous with In animal cells, continuous with

nuclear membranenuclear membrane

Extends throughout cytoplasmExtends throughout cytoplasm

Two regions - rough and smoothTwo regions - rough and smooth

Rough ERRough ER

Arranged into flattened sacsArranged into flattened sacs Ribosomes on surface give it a rough Ribosomes on surface give it a rough

appearanceappearance Some polypeptide chains enter rough Some polypeptide chains enter rough

ER and are modifiedER and are modified Cells that specialize in secreting Cells that specialize in secreting

proteins have lots of rough ERproteins have lots of rough ER

Smooth ERSmooth ER

A series of interconnected tubules A series of interconnected tubules No ribosomes on surfaceNo ribosomes on surface Lipids assembled inside tubulesLipids assembled inside tubules Smooth ER of liver inactivates Smooth ER of liver inactivates

wastes, drugswastes, drugs Sarcoplasmic reticulum of muscle is Sarcoplasmic reticulum of muscle is

a specialized forma specialized form

Endoplasmic reticulum (ER)Endoplasmic reticulum (ER)

Golgi BodiesGolgi Bodies

Put finishing touches on proteins and Put finishing touches on proteins and lipids that arrive from ERlipids that arrive from ER

Package finished material for Package finished material for shipment to final destinationsshipment to final destinations

Material arrives and leaves in vesiclesMaterial arrives and leaves in vesicles

The Golgi apparatusThe Golgi apparatus

VesiclesVesicles

Membranous sacs that Membranous sacs that

move through the move through the

cytoplasmcytoplasm

LysosomesLysosomes

PeroxisomesPeroxisomes

LysosomesLysosomes

Review: relationships among organelles of the Review: relationships among organelles of the endomembrane systemendomembrane system  

The mitochondrion, site of cellular The mitochondrion, site of cellular respirationrespiration

PlastidsPlastids

Central VacuoleCentral Vacuole

Specialized Plant Specialized Plant OrganellesOrganelles

The chloroplast, site of The chloroplast, site of photosynthesisphotosynthesis

Other PlastidsOther Plastids

Chromoplasts Chromoplasts No chlorophyllNo chlorophyll Abundance of carotenoidsAbundance of carotenoids Color fruits and flowers red-to-yellowColor fruits and flowers red-to-yellow

AmyloplastsAmyloplasts No pigmentsNo pigments Store starchStore starch

The plant cell vacuole The plant cell vacuole 

Organelles with no Organelles with no MembranesMembranes

RibosomesRibosomes Function in protein synthesisFunction in protein synthesis

CytoskeletonCytoskeleton Function in maintenance of cell shape Function in maintenance of cell shape

and positioning of organellesand positioning of organelles Centrioles (animals only)Centrioles (animals only)

Function during cell divisionFunction during cell division

Figure 7.10 RibosomesFigure 7.10 Ribosomes

Ribosomes contain rRNA and protein.Ribosomes contain rRNA and protein. A ribosome is composed of two subunits that A ribosome is composed of two subunits that

combine to carry out protein synthesis.combine to carry out protein synthesis.

Ribosomes build a cell’s Ribosomes build a cell’s proteinsproteins

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 7.10

Present in all eukaryotic cellsPresent in all eukaryotic cells

Basis for cell shape and internal Basis for cell shape and internal organizationorganization

Allows organelle movement within Allows organelle movement within cells and, in some cases, cell motilitycells and, in some cases, cell motility

CytoskeletonCytoskeleton

Cytoskeletal ElementsCytoskeletal Elements

microtubule

microfilament

intermediatefilament

MicrotubulesMicrotubules

Largest elementsLargest elements

Composed of the protein tubulinComposed of the protein tubulin

Arise from microtubule organizing Arise from microtubule organizing

centers (MTOCs)centers (MTOCs)

Polar and dynamicPolar and dynamic

Involved in shape, motility, cell Involved in shape, motility, cell

divisiondivision

MicrofilamentsMicrofilaments

Thinnest cytoskeletal elementsThinnest cytoskeletal elements

Composed of the protein actin Composed of the protein actin

Polar and dynamicPolar and dynamic

Take part in movement, formation Take part in movement, formation

and maintenance of cell shapeand maintenance of cell shape

Intermediate FilamentsIntermediate Filaments

Present only in animal cells of certain Present only in animal cells of certain tissuestissues

Most stable cytoskeletal elementsMost stable cytoskeletal elements Six known groupsSix known groups Different cell types usually have 1-2 Different cell types usually have 1-2

different kindsdifferent kinds

Cell-to-Cell JunctionsCell-to-Cell Junctions

Plants Plants Plasmodesmata Plasmodesmata

Animals Animals Tight junctionsTight junctions Adhering Adhering

junctionsjunctions Gap junctionsGap junctions

plasmodesma

Animal Cell JunctionsAnimal Cell Junctions

tightjunctions

adhering junction

gapjunction