Cells: The Working Unit of Life

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Cells: The Working

Units of Life

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Cells Provide Compartments for Biochemical Reactions

Cell theorywas 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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Most cells are tiny, in order to maintain agood surface area-to-volume ratio.

Thevolumeof a cell determines its

metabolic activity relative to time.

The surface areaof a cell determines the

number of substances that can enter or

leave the cell.


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The Scale of Life


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To visualizesmall cells, there are two types

of microscopes:

Light microscopesuse glass lenses and

light

Resolution = 0.2 m

Electron microscopeselectromagnets

focus an electron beam

Resolution = 2.0 nm


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Microscopy


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The plasma membrane:

Is a selectively permeable barrier that allows

cells to maintain a constant internal

environment

Is important in communicationand receiving

signals

Often has proteins for binding and adhering

to adjacent cells


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Two types of cells:Prokaryoticand

eukaryotic

Prokaryotesare without membrane-

enclosed compartments.

Eukaryotes have membrane-enclosed

compartments called organelles, such as

the nucleus.


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Prokaryotic Cells Do Not Have a Nucleus

Prokaryotic cells:

Are enclosed by a plasma membrane

Have DNA located in the nucleoid

Therest of thecytoplasmconsists of:

Cytosol(water and dissolved material)

and suspended particlesRibosomessites of protein synthesis


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A Prokaryotic Cell


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Most prokaryotes have a rigid cell wall

outside the plasma membrane.

Bacteria cell walls containpeptidoglycans.

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.


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Prokaryotic Flagella (Part 1)

E k ti C ll H N l d Oth M b B d


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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.


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Eukaryotic Cells (Part 1)

E k i C ll (P 8)


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Eukaryotic Cells (Part 8)

Eukaryotic Cells Have a Nucleus and Other Membrane Bound


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Compartments

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 proteinmolecules.



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Compartments

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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Compartments

The nucleusis usually the largest organelle.

It is the location of DNA and of DNA

replication.

It is the site where DNA is transcribed to

RNA.

It contains the nucleolus, where ribosomes

begin to be assembled from RNA and

proteins.



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Compartments

The nucleus is surrounded by twomembranesthat 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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Compartments

The endomembrane systemincludes the

nuclear envelope, endoplasmic reticulum,

Golgi apparatus, and lysosomes.

Tiny, membrane-surrounded vesicles

shuttle substances between the various

components, as well as to the plasma

membrane.

The Endomembrane System


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The Endomembrane System



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Compartments

Endoplasmic reticulum(ER)network of

interconnected 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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Compartments

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,whichare important for recognition, pass throughthe RER.


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Compartments

The Golgi apparatusis composed of

flattened sacs (cisternae) and small

membrane-enclosed vesicles.

Receives proteins from the RERcan

further modify them

Concentrates, packages, and sorts proteins

Adds carbohydrates to proteins



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Compartments

The Golgi apparatus has three regions:

Thecisregion receives vesicles containing

protein from the ER.

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

membrane or to lysosomes.

The medial region lies in between the trans

and cisregions.



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Compartments

Primary lysosomesoriginate from the

Golgi apparatus.

They contain digestive enzymes, and are

the site where macromolecules are

hydrolyzed into monomers.



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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.

Lysosomes Isolate Digestive Enzymes from the Cytoplasm (Part 1)


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Lysosomes Isolate Digestive Enzymes from the Cytoplasm (Part 1)



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Compartments

In eukaryotes, molecules are first broken

down in the cytosol.

The partially digested molecules enter the

mitochondriachemical energy is

converted to energy-rich ATP.

Cells that require a lot of energy often have

more mitochondria.



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Compartments

Mitochondria have two membranes:

Outer membranequite porous

Inner membraneextensive folds called

cristae, to increase surface area

The fluid-filled matrixinside the inner

membrane contains enzymes, DNA, and

ribosomes.

Eukaryotic Cells


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y



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Eukaryotic Cells Have a Nucleus and Other Membrane BoundCompartments

Plant and algae cells contain plastids that

can differentiate into organellessome

are used for storage.

A chloroplast contains chlorophyll and is

the site of photosynthesis.

Photosynthesis converts light energy into

chemical energy.

Figure 4.7 Eukaryotic Cells


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g y



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y C OCompartments

A chloroplast is enclosed within two

membranes, 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 the

thylakoids.



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yCompartments

Other organelles perform specialized

functions.

Peroxisomescollect and break down toxic

by-products of metabolism, such as H2

O2

,

using specialized enzymes.

Glyoxysomes, found only in plants, are

where lipids are converted to

carbohydrates for growth.

Eukaryotic Cells


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yCompartments

Vacuolesoccur in some eukaryotes, but

mainly in plants and fungi, and have

several functions:

Storageof waste products and toxic

compounds; some may deter herbivores

Structurefor plant cellswater enters the

vacuole by osmosis, creating turgor

pressure

EukaryoticCells Have a Nucleus and Other Membrane-Bound


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yCompartments

Vacuoles (continued):

Reproductionvacuoles in flowers and

fruits contain pigments whose colors

attract pollinators and aid seed dispersal

Catabolismdigestive enzymes in seeds

vacuoles hydrolyze stored food for early

growth


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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 actin


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Actin polymer(filament)Actin monomers

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

contraction.

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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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.

microtubule tubulin monomers

C S


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Microtubules line movable cell appendages.

Ciliashort, usually many present, move

with stiff power stroke and flexible

recovery stroke

Flagellalonger, usually one or two

present, movement is snakelike

Cilia (Part 1)


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


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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.


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


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Dyneina motor protein that drives the

sliding of doublets, by changing its shape

Nexinprotein that crosslinks doublets and

prevents sliding, so cilia bends

Kinesinmotor protein that binds to

vesicles in the cell and walks them along

the microtubule

A Motor Protein Moves Microtubules in Cilia and Flagella


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A Motor Protein Drives Vesicles along Microtubules


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Extracellular Structures Allow Cells to Communicate with theE ternal En ironment


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External Environment

Plant cell wallsemi-rigid structure outside

the plasma membrane

The fibrous component is the

polysaccharide cellulose.

The gel-like matrixcontains cross-linked

polysaccharides and proteins.

The Plant Cell Wall


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Extracellular Structures Allow Cells to Communicate with theExternal Environment


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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, and

some RNA and proteins.



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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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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.

An Extracellular Matrix (Part 1)


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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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Cell junctionsare specialized structures

that protrude from adjacent cells andglue them togetherseen often in

epithelial cells:

Tight junctions

Desmosomes

Gap junctions



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Tight junctions prevent substances from

moving through spaces between cells.

Desmosomeshold cells together but allow

materials to move in the matrix.

Gap junctionsare channels that run

between membrane pores in adjacent

cells, allowing substances to pass

between the cells.

Junctions Link Animal Cells (Part 1)


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Cells: The Working Unit of Life

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