Ch05 Lecture Cells

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5Cells: The Working Units

of Life



5 Cells: The Working Units of Life

• 5.1 What Features Make Cells theFundamental Units of Life?

• 5.2 What Features CharacterizeProkaryotic Cells?

• 5.3 What Features CharacterizeEukaryotic Cells?

• 5.4 What Are the Roles of ExtracellularStructures?

• 5.5 How Did Eukaryotic Cells Originate?



5.1 What Features Make Cells the Fundamental Units of Life?

Cell theory was the first unifying theoryof biology.

• Cells are the fundamental units of life

• All organisms are composed of cells

• All cells come from preexisting cells




Implications of cell theory:

• Functions of all cells are similar

• Life is continuous

• Origin of life was the origin of cells




Cells are small (mostly).

Exceptions: Bird eggs, neurons, somealgae, and bacteria cells



Figure 5.1 The Scale of Life (Part 1)



Figure 5.1 The Scale of Life (Part 2)




Cells are small because a high surfacearea-to-volume ratio is essential.

Volume determines the amount ofchemical activity in the cell per unit time.

Surface area determines the amount of

substances that can pass the cellboundary per unit time.



Figure 5.2 Why Cells Are Small (Part 1)



Figure 5.2 Why Cells Are Small (Part 2)




Most cells are < 200 μm in size. To seethem, we use microscopes:

Magnification : Increases apparent sizeResolution : Clarity of magnified object –

minimum distance two objects can be apartand still be seen as two objects.




Two basic types of microscopes:

Light microscopes —use glass lenses and

light. Resolution = 0.2 μmElectron microscopes —electromagnets focus

an electron beam. Resolution = 0.2 nm



Copyright © 2005 Pearson Prentice Hall, Inc.

Did you have breakfast this morning?

• What did you eat this morning?

• Where did that food go?

• Imagine a balloon inside a vat of mud.

• What’s inside and what’s not?

Did you have breakfast this morning?



Figure 5.3 Looking at Cells (Part 1)










Pathology is a branch of medicine thatuses microscopy to analyze cells anddiagnose diseases.

Many methods are used, includingphase-contrast microscopy, staining thecells with general or fluorescent dyes,

and electron microscopy.




The plasma membrane is the outersurface of every cell, and has more orless the same structure in all cells.

It is made of a phospholipid bilayer withproteins and other molecules embedded.

It is not rigid, but more like an oily fluid inwhich the proteins and lipids are inconstant motion.




The plasma membrane:• Is a selectively permeable barrier

• Allows cells to maintain a constantinternal environment

• Is important in communication andreceiving signals

• Often has proteins for binding andadhering to adjacent cells




Two types of cells : Prokaryotic andeukaryotic

Bacteria and Archaea are prokaryotes .

The first cells were probably prokaryotic.

Eukarya are eukaryotes the DNA is in

a membrane-enclosed compartmentcalled the nucleus .




Eukaryotes have other membrane-enclosed compartments in whichspecific chemical reactions occur.

This has allowed diversification offunctions in eukaryotic cells, and theirspecialization into tissues.



5.2 What Features Characterize Prokaryotic Cells?

Prokaryotic cells are very small.Individuals are single cells, but often

found in chains or clusters.

Prokaryotes are very successful —theycan live on a diversity of energy sources

and some can tolerate extremeconditions.




Prokaryotic cells:• Are enclosed by a plasma membrane

• The DNA is contained in the nucleoid • Cytoplasm consists of cytosol (water

and dissolved material) and suspended

particles

• Ribosomes — sites of protein synthesis

Figure 5 4 A Prokaryotic Cell



Figure 5.4 A Prokaryotic Cell




Most prokaryotes have a rigid cell walloutside the plasma membrane.

Bacteria cell walls contain peptidoglycan.

Some bacteria have an additional outermembrane.

Some bacteria have a slimy capsule ofpolysaccharides.




Photosynthetic bacteria have an internalmembrane system that containsmolecules necessary for photosynthesis.

Others have internal membrane folds thatare attached to the plasma membrane;they may function in cell division or inenergy-releasing reactions.




Some prokaryotes swim by means offlagella , made of the protein flagellin.

Some bacteria have pili —hairlikestructures projecting from the surface.They help bacteria adhere to other cells.

Some rod-shaped bacteria have acytoskeleton made of the protein actin.

Figure 5 5 Prokaryotic Flagella (Part 1)



Figure 5.5 Prokaryotic Flagella (Part 1)

Figure 5 5 Prokaryotic Flagella (Part 2)



Figure 5.5 Prokaryotic Flagella (Part 2)



5.3 What Features Characterize Eukaryotic Cells?

Eukaryotic cells are up to ten times largerthan prokaryotes.

Eukaryotic cells have membrane-enclosed compartments calledorganelles .

Each organelle has a specific role in cellfunctioning.




Compartmentalization allowed eukaryoticcells to specialize and form the tissuesand organs of multicellular organisms.

Figure 5.7 Eukaryotic Cells (Part 1)







g y ( )




g y ( )




g y ( )




Organelles were first studied using lightmicroscopy.

Cell fractionation separates organelles forstudy by chemical methods.

Figure 5.6 Cell Fractionation



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Ribosomes —sites of protein synthesis.Occur in both prokaryotic and eukaryotic

cells and have similar structure.

Ribosomes consist of ribosomal RNA(rRNA) and more than 50 different

protein molecules.




In eukaryotes, ribosomes are free in thecytoplasm, attached to the endoplasmicreticulum, or inside mitochondria andchloroplasts.

In prokaryotic cells, ribosomes float freelyin the cytoplasm.




The nucleus is usually the largest organelle.

• Contains the DNA

• Site of DNA replication

• Site where gene transcription is turned onor off

• Assembly of ribosomes begins in a regioncalled the nucleolus






The nucleus is surrounded by twomembranes —the nuclear envelope .

Nuclear pores in the envelope controlmovement of molecules betweennucleus and cytoplasm.

Figure 5.8 The Nucleus Is Enclosed by a Double Membrane (Part 2)






Some large molecules (e.g., proteins)must have a certain amino acidsequence known as a nuclearlocalization signal (NLS ) to cross thenuclear envelope.




In the nucleus, DNA combines withproteins to form chromatin in long, thinthreads called chromosomes .

Before cell division, chromatincondenses, and individualchromosomes are visible in the light

microscope.

Figure 5.9 Chromatin and Chromosomes






Nucleoplasm surrounds the chromatin,and a network of structural proteins(nuclear matrix ) helps organize thechromatin.

The nuclear lamina attaches to both thechromatin and the nuclear envelope and

maintains nuclear shape.




The endomembrane system includesthe plasma membrane, nuclearenvelope, endoplasmic reticulum, Golgiapparatus, and lysosomes.

Tiny, membrane-surrounded vesiclesshuttle substances between the various

components.




Endoplasmic reticulum (ER ): network ofinterconnected membranes in thecytoplasm; has large surface area.

Rough endoplasmic reticulum (RER ):ribosomes are attached.

Newly made proteins enter the RER

lumen where they are modified, folded,and transported to other regions.

Figure 5.10 The Endomembrane System (Part 1)









Smooth endoplasmic reticulum (SER ):more tubular, no ribosomes

• Chemically modifies small moleculessuch as drugs and pesticides

• Hydrolysis of glycogen in animal cells

• Synthesis of lipids and steroids




The Golgi apparatus is composed offlattened sacs ( cisternae ) and smallmembrane-enclosed vesicles.

• Receives proteins from the RER —canfurther modify them

• Concentrates, packages, sorts proteins

• In plant cells, polysaccharides for cellwalls are synthesized here




The cis region receives vesicles (a pieceof the ER that ―buds‖ off) from the ER.

At the trans region, vesicles bud off fromthe Golgi apparatus and are moved tothe plasma membrane or otherorganelles.







h h k ll




Primary lysosomes originate from theGolgi apparatus.

They contain digestive enzymes —macromolecules are hydrolyzed intomonomers.

5 3 Wh F Ch i E k i C ll ?




Food molecules enter the cell by phagocytosis —a phagosome is formed.

Phagosomes fuse with primarylysosomes to form secondarylysosomes .

Enzymes in the secondary lysosomehydrolyze the food molecules.

Figure 5.11 Lysosomes Isolate Digestive Enzymes from the Cytoplasm (Part 1)



Figure 5.11 Lysosomes Isolate Digestive Enzymes from the Cytoplasm (Part 2)



5 3 Wh t F t Ch t i E k ti C ll ?




Lysosomes also digest cell materials(autophagy ).

Cell components are frequently destroyedand replaced by new ones.





In the mitochondria , energy in fuelmolecules is transformed to the bondsof energy-rich ATP: Cellular respiration .

Cells that require a lot of energy have alot of mitochondria.





Mitochondria have two membranes.The inner membrane folds inward to form

cristae . This creates a large surfacearea for proteins involved in cellularrespiration reactions.

The mitochondrial matrix containsenzymes, DNA, and ribosomes.

Figure 5.12 A Mitochondrion Converts Energy from Fuel Molecules into ATP



5 3 What Features Characterize Eukaryotic Cells?




Plastids occur only in plants and someprotists.

Chloroplasts : Site of photosynthesis —light energy is converted to the energy ofchemical bonds

Chloroplasts have a double membrane.

Figure 5.13 Chloroplasts Feed the World







Grana are stacks of thylakoids —made ofcircular compartments of the innermembrane.

Thylakoids contain chlorophyll and otherpigments that harvest light energy forphotosynthesis.

Stroma —fluid in which grana aresuspended. The stroma contains DNAand ribosomes.

Figure 5.14 Chloroplasts Are Everywhere







Other plastids:Chromoplasts contain red, orange, and

yellow pigments —gives color to flowers.

Leucoplasts store starches and fats.

Figure 5.15 Chromoplasts and Leucoplasts







Peroxisomes : Collect and break downtoxic byproducts of metabolism such asH2O 2, using specialized enzymes

Glyoxysomes : Only in plants —lipids areconverted to carbohydrates for growth





Plant and protist cells have vacuoles :

• Store waste products and toxiccompounds; some may deter herbivores

• Provide structure for plant cells — waterenters the vacuole by osmosis, creatingturgor pressure .





Vacuoles (continued):

• Store anthocyanins (pink and bluepigments) in flowers and fruits; the colors

attract pollinators• Vacuoles in seeds have digestive enzymes

to hydrolyze stored food for early growth

Figure 5.16 Vacuoles in Plant Cells Are Usually Large







Freshwater protists may have contractilevacuoles to expel excess water.

The vacuoles take in excess water that

enters the cell by osmosis; then expel itby contracting, forcing water out througha pore.





The cytoskeleton:

• Supports and maintains cell shape

• Holds organelles in position

• Moves organelles

• Involved in cytoplasmic streaming

• Interacts with extracellular structures tohold cell in place





The cytoskeleton has three components:

• Microfilaments

• Intermediate filaments

• Microtubules

Figure 5.17 The Cytoskeleton







Microfilaments :

• Help a cell or parts of a cell to move

• Determine cell shape

• Made from the protein actin

• Actin has + and – ends and polymerizesto form long helical chains (reversible)




. W y ?

In muscle cells, actin filaments areassociated with the ―motor protein‖myosin ; interactions between the tworesult in muscle contraction.

Microfilaments are also involved in theformation of pseudopodia ( pseudo ,

―false‖; podia , ―feet‖).

Figure 5.18 Microfilaments and Cell Movements






y

In some cells, microfilaments form ameshwork just inside the plasmamembrane.

This provides structure, for example inthe microvilli that line the humanintestine.

Figure 5.19 Microfilaments for Support






y

Intermediate filaments :

• Many different kinds in six molecularclasses

• Tough, ropelike protein assemblages

• Anchor cell structures in place

• Resist tension




y

Microtubules :

• Form rigid internal skeleton in some cells

• Act as a framework for motor proteins

• Made from the protein tubulin —a dimer

• Have + and – ends

• Can change length rapidly by adding orlosing dimers




y

Cilia and eukaryotic flagella are made ofmicrotubules in ―9 + 2‖ array.

• Cilia—short, usually many present,

move with stiff power stroke and flexiblerecovery stroke

• Flagella —longer, usually one or twopresent, movement is snakelike

Figure 5.20 Cilia






The nine microtubule doublets extendinto the basal body in the cytoplasm.

In the basal body, each doublet is joined

by another microtubule, making ninetriplets.




Centrioles are identical to basal bodies.

Involved in formation of the mitoticspindle — to which chromosomes attach

during cell division.




Motor proteins : undergo reversibleshape changes powered by ATPhydrolysis.

Dynein binds to microtubule doublets andallows them to slide past each other.

Nexin can cross-link the doublets and

prevent them from sliding, and the ciliumbends.

Figure 5.21 A Motor Protein Moves Microtubules in Cilia and Flagella






The motor protein kinesin binds to avesicle and ―walks‖ it along by changingshape.

Figure 5.22 A Motor Protein Drives Vesicles along Microtubules






Experiments to determine the function ofcellular components fall into twocategories:

Inhibition : A drug that inhibits a structureor process — does the function stilloccur?

Mutation : Examine a cell that lacks thegene for the structure or process

Figure 5.23 The Role of Microfilaments in Cell Movement —Showing Cause and Effect in Biology



5.4 What Are the Roles of Extracellular Structures?



Extracellular structures are secreted tothe outside of the plasma membrane.

Example: The peptidoglycan cell wall of

bacteria.

In eukaryotes, extracellular structureshave a prominent fibrousmacromolecule in a gel-like medium.




Plant cell walls : Cellulose fibers areembedded in other complexpolysaccharides and proteins.

Adjacent plant cells are connected byplasma membrane-lined channels calledplasmodesmata .

Figure 5.24 The Plant Cell Wall



Plasmodesmata






Many animal cells are surrounded by anextracellular matrix , composed offibrous proteins such as collagen , gel-like proteoglycans (glycoproteins), andother proteins.

Figure 5.25 An Extracellular Matrix






The extracellular matrix:

• Holds cells together in tissues

• Contributes to properties of bone,

cartilage, skin, etc.• Filters materials passing between

different tissues

• Orients cell movements in developmentand tissue repair

• Plays a role in chemical signaling

5.5 How Did Eukaryotic Cells Originate?



The endomembrane system and cellnucleus may have originated from theinward folds of plasma membrane ofprokaryotes.

Enclosed compartments would haveallowed chemicals to be concentrated

and chemical reactions to proceed moreefficiently.

Figure 5.26 The Origin of Organelles (A)






Some organelles may have arose bysymbiosis (―living together‖).

The endosymbiosis theory proposes

that mitochondria and plastids arosewhen one cell engulfed another cell.

Figure 5.26 The Origin of Organelles (B)






Support for the endosymbiosis theory :

The discovery of a a single-celledeukaryote, Hatena , that ingests a green

alga, Nephroselmis

The green alga loses most of itsstructures and acts as a chloroplast

Figure 5.27 Endosymbiosis in Action



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Ch05 Lecture Cells