Ch04 Lecture Cells

7/31/2019 Ch04 Lecture Cells

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Cells: The Working

Units of Life

4


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Chapter 4 Cells: The Working Units of Life

Key Concepts 4.1 Cells Provide Compartments for

Biochemical Reactions

4.2 Prokaryotic Cells Do Not Have aNucleus

4.3 Eukaryotic Cells Have a Nucleus and

Other Membrane-Bound Compartments


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Chapter 4 Cells: The Working Units of Life

4.4 The Cytoskeleton Provides Strengthand Movement

4.5 Extracellular Structures Allow Cells to

Communicate with the External

Environment


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Chapter 4 Opening Question

What do the characteristics of moderncells indicate about how the first cells

originated?


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Concept 4.1 Cells Provide Compartments for Biochemical

Reactions

Cell theory was the first unifying theory ofbiology.

Cells are the fundamental units of life.

All organisms are composed of cells.

All cells come from preexisting cells.


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Reactions

Important implications of cell theory:

Studying cell biology is the same asstudying life.

Life is continuous.


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Reactions

Most cells are tiny, in order to maintain a

good surface area-to-volume ratio.

The volumeof a cell determines its

metabolic activity relative to time.

The surface areaof a cell determines thenumber of substances that can enter or

leave the cell.


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Figure 4.1 The Scale of Life


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Figure 4.2 Why Cells Are Small

C C C f


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Reactions

To visualizesmall cells, there are two typesof microscopes:

Light microscopesuse glass lenses and

light

Resolution = 0.2 m

Electron microscopeselectromagnetsfocus an electron beam

Resolution = 2.0 nm


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Figure 4.3 Microscopy

C t 4 1 C ll P id C t t f Bi h i l


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Reactions

Chemical analysisof cells involves breakingthem open to make a cell-free extract.

The composition and chemical reactions of

the extract can be examined.

The properties of the cell-free extract arethe same as those inside the cell.


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Figure 4.4 Centrifugation

C t 4 1 C ll P id C t t f Bi h i l


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Reactions

The plasma membrane:

Is a selectively permeable barrierthat allowscells to maintain a constant internal

environment

Is important in communicationand receivingsignals

Often has proteins forbinding and adheringto adjacent cells

4 1 C lC t 4 P id C t t f Bi h i l


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4.1 CelConcept 4. Provide Compartments for Biochemical

Reactions

Two types of cells: Prokaryotic andeukaryotic

Prokaryotes are without membrane-enclosed compartments.

Eukaryotes have membrane-enclosedcompartments called organelles, such as

the nucleus.


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In-Text Art, Ch. 4, p. 59


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Concept 4.2 Prokaryotic Cells Do Not Have a Nucleus

Prokaryotic cells:

Are enclosed by a plasma membrane

Have DNA located in the nucleoid

Therest of the cytoplasm consists of:

Cytosol (water and dissolved material)

and suspended particles Ribosomessites of protein synthesis

Fi 4 5 A P k ti C ll


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Figure 4.5 A Prokaryotic Cell


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Most prokaryotes have a rigid cell wall

outside the plasma membrane.

Bacteria cell walls contain peptidoglycans.

Some bacteria have an additional outer

membrane that is very permeable.

Other bacteria have a slimy layer of

polysaccharides, called the capsule.


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Some prokaryotes swim by means of

flagella, made of the protein flagellin.

A motor protein anchored to the plasma or

outer membrane spins each flagellum and

drives the cell.

Some rod-shaped bacteria have a network

of actin-like protein structures to helpmaintain their shape.

Fi 4 6 P k ti Fl ll (P t 1)


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Figure 4.6 Prokaryotic Flagella (Part 1)

Figure 4 6 Prokaryotic Flagella (Part 2)


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Figure 4.6 Prokaryotic Flagella (Part 2)

Concept 4 3 Eukaryotic Cells Have a Nucleus and Other


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Concept 4.3 Eukaryotic Cells Have a Nucleus and Other

Membrane-Bound Compartments

Eukaryotic cells have a plasma membrane,

cytoplasm, and ribosomesand also

membrane-enclosed compartments called

organelles.

Each organelle plays a specific role in cell

functioning.

Figure 4 7 Eukaryotic Cells (Part 1)


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Figure 4.7 Eukaryotic Cells (Part 1)

Figure 4 7 Eukaryotic Cells (Part 8)


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Figure 4.7 Eukaryotic Cells (Part 8)



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Ribosomessites of protein synthesis:

They occur in both prokaryotic and

eukaryotic cells and have similar

structureone larger and one smaller

subunit.

Each subunit consists of ribosomal RNA

(rRNA) bound to smaller protein

molecules.



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Ribosomes translate the nucelotide

sequence of messenger RNA into a

polypeptide chain.

Ribosomes are not membrane-bound

organellesin eukaryotes, they are free in

the cytoplasm, attached to the

endoplasmic reticulum, or inside

mitochondria and chloroplasts.

In prokaryotic cells, ribosomes float freely in

the cytoplasm.



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The nucleusis usually the largest organelle.

It is the location of DNA and of DNA

replication.

It is the site where DNA is transcribed toRNA.

It contains the nucleolus, where ribosomes

begin to be assembled from RNA and

proteins.



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The nucleus is surrounded by two

membranesthat form the nuclearenvelope.

Nuclear poresin the envelope control

movement of molecules between nucleusand cytoplasm.

In the nucleus, DNA combines with proteins

to form chromatinin long, thin threadscalled chromosomes.



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The endomembrane system includes thenuclear envelope, endoplasmic reticulum,

Golgi apparatus, and lysosomes.

Tiny, membrane-surrounded vesiclesshuttle substances between the various

components, as well as to the plasma

membrane.

Figure 4 8 The Endomembrane System


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Figure 4.8 The Endomembrane System



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Endoplasmic reticulum (ER)network ofinterconnected membranes in the

cytoplasm, with a large surface area

Two types of ER:

Rough endoplasmic reticulum (RER)

Smooth endoplasmic reticulum (SER)



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Rough endoplasmic reticulum (RER) has

ribosomes attached to begin proteinsynthesis.

Newly made proteins enter the RER lumen.

Once inside, proteins are chemicallymodified and tagged for delivery.

The RER participates in the transport.

All secreted proteins and most membraneproteins, including glycoproteins, which isimportant for recognition, pass through theRER.



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Smooth endoplasmic reticulum (SER)more tubular, no ribosomes

It chemically modifies small molecules such

as drugs and pesticides.

It is the site of glycogen degradation in

animal cells.

It is the site of synthesis of lipids andsteroids.



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Concept 4.3 Eukaryotic Cells Have a Nucleus and OtherMembrane-Bound Compartments

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

Receives proteins from the RERcan

further modify them

Concentrates, packages, and sorts proteins

Adds carbohydrates to proteins

Site of polysaccharide synthesis in plant

cells



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Concept 4.3 Eukaryotic Cells Have a Nucleus and OtherMembrane-Bound Compartments

The Golgi apparatus has three regions:

The cisregion receives vesicles containingprotein from the ER.

At the transregion, vesicles bud off from theGolgi apparatus and travel to the plasma

membrane or to lysosomes.

The medialregion lies in between the transand cisregions.



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C p 3 y C OMembrane-Bound Compartments

Primary lysosomes originate from theGolgi apparatus.

They contain digestive enzymes, and are

the site where macromolecules are

hydrolyzed into monomers.



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p yMembrane-Bound Compartments

Macromolecules may enter the cell by

phagocytosispart of the plasmamembrane encloses the material and aphagosome is formed.

Phagosomes then fuse with primarylysosomes to form secondarylysosomes.

Enzymes in the secondary lysosomehydrolyze the food molecules.

Figure 4.9 Lysosomes Isolate Digestive Enzymes from the Cytoplasm (Part 1)


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Figure 4.9 Lysosomes Isolate Digestive Enzymes from the Cytoplasm (Part 2)


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Phagocytesare cells that take materials intothe cell and break them down.

Autophagyis the programmed destruction ofcell components and lysosomes are where

it occurs.

Lysosomal storage diseasesoccurs whenlysosomes fail to digest the components.



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In eukaryotes, molecules are first broken

down in the cytosol.

The partially digested molecules enter the

mitochondriachemical energy isconverted to energy-rich ATP.

Cells that require a lot of energy often have

more mitochondria.



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Mitochondria have two membranes:

Outer membranequite porous

Inner membraneextensive folds called

cristae, to increase surface area

The fluid-filled matrixinside the innermembrane contains enzymes, DNA, and

ribosomes.

Figure 4.7 Eukaryotic Cells


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Plant and algae cells contain plastids thatcan differentiate into organellessome

are used for storage.

A chloroplast contains chlorophyll and isthe site of photosynthesis.

Photosynthesis converts light energy into

chemical energy.



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Other organelles perform specialized

functions.

Peroxisomes collect and break down toxicby-products of metabolism, such as H2O2,

using specialized enzymes.

Glyoxysomes, found only in plants, arewhere lipids are converted to

carbohydrates for growth.



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A chloroplast is enclosed within twomembranes, with a series of internal

membranes called thylakoids.

A granumis a stack of thylakoids.

Light energy is converted to chemical

energy on the thylakoid membranes.

Carbohydrate synthesis occurs in thestromathe aqueous fluid surrounding thethylakoids.



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Vacuoles occur in some eukaryotes, butmainly in plants and fungi, and have

several functions:

Storageof waste products and toxiccompounds; some may deter herbivores

Structurefor plant cellswater enters thevacuole by osmosis, creating turgor

pressure

Concept 4.3EukaryoticCells Have a Nucleus and Other


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Vacuoles (continued):

Reproductionvacuoles in flowers andfruits contain pigments whose colors

attract pollinators and aid seed dispersal

Catabolismdigestive enzymes in seedsvacuoles hydrolyze stored food for early

growth

Concept 4.3 Eukaryotic Cells Have a Nucleus and OtherC


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Contractile vacuolesin freshwater protists

get rid of excess water entering the cell

due to solute imbalance.

The contractile vacuole enlarges as water

enters, then quickly contracts to force

water out through special pores.

C t 4 4 Th C t k l t P id St th d M t


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Concept 4.4 The Cytoskeleton Provides Strength and Movement

The cytoskeleton:

Supports and maintains cell shape

Holds organelles in position

Moves organelles

Is involved in cytoplasmic streaming

Interacts with extracellular structures toanchor cell in place



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The cytoskeleton has three components

with very different functions:

Microfilaments

Intermediate filaments

Microtubules



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Microfilaments:

Help a cell or parts of a cell to move

Determine cell shape

Are made from the protein actinwhichattaches to the plus end and detaches atthe minus end of the filament

The filaments can be made shorter orlonger.



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Actin polymer(filament) Actin monomers

Dynamic instability allows quick assemblyor breakdown of the cytoskeleton.

In muscle cells, actin filaments areassociated with the motor proteinmyosin; their interactions result in musclecontraction.

Figure 4.10 The Cytoskeleton (Part 1)


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Intermediate filaments:

At least 50 different kinds in six molecular

classes

Have tough, ropelike protein assemblages,more permanent than other filaments and

do not show dynamic instability

Anchor cell structures in place

Resist tension, maintain rigidity



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Concept 4 4 The Cytoskeleton Provides Strength and Movement


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Microtubules:

The largest diameter components, with two

roles:

Form rigid internal skeleton for some cellsor regions

Act as a framework for motor proteins to

move structures in the cell



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Microtubules are made from dimersof the

protein tubulin

chains of dimers surrounda hollow core.

They show dynamic instability, with (+) and

(-) ends:

microtubule tubulin monomers

Polymerization results in a rigid structuredepolymerization leads to collapse.



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Microtubules line movable cell appendages.

Ciliashort, usually many present, movewith stiff power stroke and flexible

recovery stroke

Flagellalonger, usually one or twopresent, movement is snakelike

Figure 4.11 Cilia (Part 1)


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Ciliaand flagella appear in a 9 + 2

arrangement:

Doubletsnine fused pairs of

microtubules form a cylinder

One unfused pair in center

Motion occurs as doublets slide past each

other.

Figure 4.11 Cilia (Part 2)


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Dyneina motor protein that drives the

sliding of doublets, by changing its shape

Nexinprotein that crosslinks doublets andprevents sliding, so cilia bends

Kinesinmotor protein that binds to

vesicles in the cell and walks them along

the microtubule

Figure 4.12 A Motor Protein Moves Microtubules in Cilia and Flagella


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Figure 4.13 A Motor Protein Drives Vesicles along Microtubules


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Cytoskeletal structure may be observed

under the microscope, and function can beobserved in a cell with that structure.

Observations may suggest that a structure

has a function, but correlation does notestablish cause and effect.



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Two methods are used to show links

between structure (A) and function (B):

Inhibitionuse a drug to inhibit Aif B stilloccurs, then A does not cause B

Mutationif genes for A are missing and Bdoes not occurA probably causes B

Figure 4.14 The Role of Microfilaments in Cell Movement: Showing Cause and Effect in Biology (Part 1)


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Figure 4.14 The Role of Microfilaments in Cell Movement: Showing Cause and Effect in Biology (Part 2)


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Concept 4.5 Extracellular Structures Allow Cells to Communicatewith the External Environment


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with the External Environment

Extracellularstructures are secreted to the

outside of the plasma membrane.

In eukaryotes, these structures have two

components:

A prominent fibrous macromolecule

A gel-like medium with fibers embedded



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Plant cell wallsemi-rigid structure outside

the plasma membrane

The fibrouscomponent is thepolysaccharide cellulose.

The gel-like matrixcontains cross-linkedpolysaccharides and proteins.

Figure 4.15 The Plant Cell Wall


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The plant cell wallhas three major roles:

Provides support for the cell and limits

volume by remaining rigid

Acts as a barrier to infection

Contributes to form during growth and

development



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Adjacent plant cells are connected by

plasma membrane-lined channels calledplasmodesmata.

These channels allow movement of water,

ions, small molecules, hormones, andsome RNA and proteins.



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Many animal cells are surrounded by an

extracellular matrix.

The fibrous componentis the proteincollagen.

The gel-like matrixconsists ofproteoglycans.

A third group of proteins links the collagenand the matrix together.

Figure 4.16 An Extracellular Matrix (Part 1)


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Figure 4.16 An Extracellular Matrix (Part 2)


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Role of extracellular matrices in animal

cells:

Hold cells together in tissues

Contribute to physical properties ofcartilage, skin, and other tissues

Filter materials

Orient cell movement during growth and

repair



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Proteins like integrinconnect the

extracellular matrix to the plasmamembrane.

Proteins bind to microfilaments in the

cytoplasm and to collagen fibers in theextracellular matrix.

For cell movement, the protein changes

shape and detaches from the collagen.

Figure 4.17 Cell Membrane Proteins Interact with the Extracellular Matrix


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Cell junctions are specialized structures

that protrude from adjacent cells andglue them togetherseen often in

epithelial cells:

Tight junctions

Desmosomes

Gap junctions



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Tight junctionsprevent substances from

moving through spaces between cells.

Desmosomeshold cells together but allowmaterials to move in the matrix.

Gap junctionsare channels that runbetween membrane pores in adjacent

cells, allowing substances to pass

between the cells.

Figure 4.18 Junctions Link Animal Cells (Part 1)


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Answer to Opening Question


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Synthetic cell modelsprotocellscan

demonstrate how cell properties may haveoriginated.

Combinations of molecules can produce a

cell-like structure, with a lipid membraneand water-filled interior.

As in modern cells, the membrane allows

only certain things to pass, while RNAinside the cell can replicate itself.

Figure 4.19 A Protocell


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Ch04 Lecture Cells