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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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Concept 4.1 Cells Provide Compartments for Biochemical
Reactions
Important implications of cell theory:
Studying cell biology is the same asstudying life.
Life is continuous.
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Concept 4.1 Cells Provide Compartments for Biochemical
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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Concept 4.1 Cells Provide Compartments for Biochemical
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
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Concept 4.1 Cells Provide Compartments for Biochemical
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
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Concept 4.1 Cells Provide Compartments for Biochemical
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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Concept 4.2 Prokaryotic Cells Do Not Have a Nucleus
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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Concept 4.2 Prokaryotic Cells Do Not Have a Nucleus
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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Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
Membrane-Bound 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 protein
molecules.
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
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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Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
Membrane-Bound 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 toRNA.
It contains the nucleolus, where ribosomes
begin to be assembled from RNA and
proteins.
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
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.
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
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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Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
Membrane-Bound Compartments
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)
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
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.
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
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.
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
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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.
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
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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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Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
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p yMembrane-Bound Compartments
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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p yMembrane-Bound Compartments
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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p yMembrane-Bound Compartments
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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Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
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p yMembrane-Bound Compartments
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.
Figure 4.7 Eukaryotic Cells
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Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
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Membrane-Bound Compartments
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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Membrane-Bound Compartments
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.
Figure 4.7 Eukaryotic Cells
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Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
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Membrane-Bound Compartments
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
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Membrane-Bound Compartments
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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Membrane-Bound Compartments
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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Concept 4.4 The Cytoskeleton Provides Strength and Movement
The cytoskeleton has three components
with very different functions:
Microfilaments
Intermediate filaments
Microtubules
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Concept 4.4 The Cytoskeleton Provides Strength and Movement
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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Concept 4.4 The Cytoskeleton Provides Strength and Movement
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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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
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
Figure 4.10 The Cytoskeleton (Part 2)
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Concept 4 4 The Cytoskeleton Provides Strength and Movement
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Concept 4.4 The Cytoskeleton Provides Strength and Movement
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
Figure 4.10 The Cytoskeleton (Part 3)
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Concept 4 4 The Cytoskeleton Provides Strength and Movement
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Concept 4.4 The Cytoskeleton Provides Strength and Movement
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.
Concept 4 4 The Cytoskeleton Provides Strength and Movement
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Concept 4.4 The Cytoskeleton Provides Strength and Movement
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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Concept 4 4 The Cytoskeleton Provides Strength and Movement
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Concept 4.4 The Cytoskeleton Provides Strength and Movement
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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Concept 4 4 The Cytoskeleton Provides Strength and Movement
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Concept 4.4 The Cytoskeleton Provides Strength and Movement
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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Concept 4 4 The Cytoskeleton Provides Strength and Movement
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Concept 4.4 The Cytoskeleton Provides Strength and Movement
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.
Concept 4 4 The Cytoskeleton Provides Strength and Movement
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Concept 4.4 The Cytoskeleton Provides Strength and Movement
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
Concept 4.5 Extracellular Structures Allow Cells to Communicatewith the External Environment
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with the External Environment
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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Concept 4.5 Extracellular Structures Allow Cells to Communicatewith the External Environment
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with the External Environment
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
Concept 4.5 Extracellular Structures Allow Cells to Communicatewith the External Environment
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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.
Concept 4.5 Extracellular Structures Allow Cells to Communicatewith the External Environment
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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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Concept 4.5 Extracellular Structures Allow Cells to Communicatewith the External Environment
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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
Concept 4.5 Extracellular Structures Allow Cells to Communicatewith the External Environment
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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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Concept 4.5 Extracellular Structures Allow Cells to Communicatewith the External Environment
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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
Concept 4.5 Extracellular Structures Allow Cells to Communicatewith the External Environment
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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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Figure 4.18 Junctions Link Animal Cells (Part 2)
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Figure 4.18 Junctions Link Animal Cells (Part 3)
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Figure 4.18 Junctions Link Animal Cells (Part 4)
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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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