Cells: The Working Units of Life

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Cells The WorkingUnits of Life
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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 a Nucleus
• 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 Strength and
  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 modern cells
  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 of
  biology.
• 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 as studying
  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 volume of a cell determines its metabolic
  activity relative to time.
• The surface area of a cell determines the number
  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
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Concept 4.1 Cells Provide Compartments for
Biochemical Reactions

• To visualize small cells, there are two types of
  microscopes
• Light microscopesuse glass lenses and light
• Resolution 0.2 µm
• Electron microscopeselectromagnets focus 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 analysis of cells involves breaking them
  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 are the
  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 barrier that allows
  cells to maintain a constant internal environment
• Is important in communication and receiving
  signals
• Often has proteins for binding and adhering to
  adjacent cells

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4.1 CelConcept 4. Provide Compartments for
Biochemical Reactions

• Two types of cells Prokaryotic and eukaryotic
• Prokaryotes are without membrane-enclosed
  compartments.
• Eukaryotes have membrane-enclosed compartments
  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
• The rest of the cytoplasm consists of
• Cytosol (water and dissolved material) and
  suspended particles
• Ribosomessites of protein synthesis

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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 help maintain
  their shape.

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Figure 4.6 Prokaryotic Flagella (Part 1)
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Figure 4.6 Prokaryotic Flagella (Part 2)
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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.

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Figure 4.7 Eukaryotic Cells (Part 1)
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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.

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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 nucleus is 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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Concept 4.3 Eukaryotic Cells Have a Nucleus and
Other Membrane-Bound Compartments

• The nucleus is surrounded by two membranes that
  form the nuclear envelope.
• Nuclear pores in the envelope control movement of
  molecules between nucleus and cytoplasm.
• In the nucleus, DNA combines with proteins to
  form chromatin in long, thin threads called
  chromosomes.

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Concept 4.3 Eukaryotic Cells Have a Nucleus and
Other Membrane-Bound Compartments

• The endomembrane system includes 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.

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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 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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Concept 4.3 Eukaryotic Cells Have a Nucleus and
Other Membrane-Bound Compartments

• Rough endoplasmic reticulum (RER) has ribosomes
  attached to begin protein synthesis.
• Newly made proteins enter the RER lumen.
• Once inside, proteins are chemically modified and
  tagged for delivery.
• The RER participates in the transport.
• All secreted proteins and most membrane proteins,
  including glycoproteins, which is important for
  recognition, pass through the RER.

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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 and
  steroids.

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Concept 4.3 Eukaryotic Cells Have a Nucleus and
Other Membrane-Bound Compartments

• The Golgi apparatus is 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
• Site of polysaccharide synthesis in plant cells

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Concept 4.3 Eukaryotic Cells Have a Nucleus and
Other Membrane-Bound Compartments

• The Golgi apparatus has three regions
• The cis region receives vesicles containing
  protein from the ER.
• At the trans region, vesicles bud off from the
  Golgi apparatus and travel to the plasma membrane
  or to lysosomes.
• The medial region lies in between the trans and
  cis regions.

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Concept 4.3 Eukaryotic Cells Have a Nucleus and
Other Membrane-Bound Compartments

• Primary lysosomes originate from the Golgi
  apparatus.
• They contain digestive enzymes, and are the site
  where macromolecules are hydrolyzed into monomers.

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Concept 4.3 Eukaryotic Cells Have a Nucleus and
Other Membrane-Bound Compartments

• Macromolecules may enter the cell by
  phagocytosispart of the plasma membrane encloses
  the material and a phagosome is formed.
• Phagosomes then fuse with primary lysosomes to
  form secondary lysosomes.
• Enzymes in the secondary lysosome hydrolyze the
  food molecules.

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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 Membrane-Bound Compartments

• Phagocytes are cells that take materials into the
  cell and break them down.
• Autophagy is the programmed destruction of cell
  components and lysosomes are where it occurs.
• Lysosomal storage diseases occurs when lysosomes
  fail to digest the components.

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Concept 4.3 Eukaryotic Cells Have a Nucleus and
Other Membrane-Bound 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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Concept 4.3 Eukaryotic Cells Have a Nucleus and
Other Membrane-Bound Compartments

• Mitochondria have two membranes
• Outer membranequite porous
• Inner membraneextensive folds called cristae, to
  increase surface area
• The fluid-filled matrix inside the inner membrane
  contains enzymes, DNA, and ribosomes.

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Figure 4.7 Eukaryotic Cells
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Concept 4.3 Eukaryotic Cells Have a Nucleus and
Other Membrane-Bound Compartments

• 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.

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Figure 4.7 Eukaryotic Cells
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Concept 4.3 Eukaryotic Cells Have a Nucleus and
Other Membrane-Bound Compartments

• Other organelles perform specialized functions.
• Peroxisomes collect and break down toxic
  by-products of metabolism, such as H2O2, using
  specialized enzymes.
• Glyoxysomes, found only in plants, are where
  lipids are converted to carbohydrates for growth.

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Concept 4.3 Eukaryotic Cells Have a Nucleus and
Other Membrane-Bound Compartments

• A chloroplast is enclosed within two membranes,
  with a series of internal membranes called
  thylakoids.
• A granum is a stack of thylakoids.
• Light energy is converted to chemical energy on
  the thylakoid membranes.
• Carbohydrate synthesis occurs in the stromathe
  aqueous fluid surrounding the thylakoids.

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Figure 4.7 Eukaryotic Cells
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Concept 4.3 Eukaryotic Cells Have a Nucleus and
Other Membrane-Bound Compartments

• Vacuoles occur in some eukaryotes, but mainly in
  plants and fungi, and have several functions
• Storage of waste products and toxic compounds
  some may deter herbivores
• Structure for plant cellswater enters the
  vacuole by osmosis, creating turgor pressure

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Concept 4.3 Eukaryotic Cells Have a Nucleus and
Other Membrane-Bound Compartments

• 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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Concept 4.3 Eukaryotic Cells Have a Nucleus and
Other Membrane-Bound Compartments

• Contractile vacuoles in 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.

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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 to anchor
  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 actinwhich attaches to
  the plus end and detaches at the minus end of
  the filament
• The filaments can be made shorter or longer.

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Concept 4.4 The Cytoskeleton Provides Strength
and Movement

• Actin polymer(filament) ? Actin monomers
• Dynamic instability allows quick assembly or
  breakdown of the cytoskeleton.
• In muscle cells, actin filaments are associated
  with the motor protein myosin their
  interactions result in muscle contraction.

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Figure 4.10 The Cytoskeleton (Part 1)
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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

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Figure 4.10 The Cytoskeleton (Part 2)
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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 cells or
  regions
• Act as a framework for motor proteins to move
  structures in the cell

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Figure 4.10 The Cytoskeleton (Part 3)
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Concept 4.4 The Cytoskeleton Provides Strength
and Movement

• Microtubules are made from dimers of the protein
  tubulinchains of dimers surround a 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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Concept 4.4 The Cytoskeleton Provides Strength
and Movement

• 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

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Figure 4.11 Cilia (Part 1)
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Concept 4.4 The Cytoskeleton Provides Strength
and Movement

• Cilia and 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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Figure 4.11 Cilia (Part 2)
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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 and
  prevents sliding, so cilia bends
• Kinesinmotor protein that binds to vesicles in
  the cell and walks them along the microtubule

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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

• Cytoskeletal structure may be observed under the
  microscope, and function can be observed in a
  cell with that structure.
• Observations may suggest that a structure has a
  function, but correlation does not establish
  cause and effect.

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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 still
  occurs, then A does not cause B
• Mutationif genes for A are missing and B does
  not occurA probably causes B

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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 Communicate with the External Environment

• Extracellular structures 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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Concept 4.5 Extracellular Structures Allow Cells
to Communicate with the External Environment

• Plant cell wallsemi-rigid structure outside the
  plasma membrane
• The fibrous component is the polysaccharide
  cellulose.
• The gel-like matrix contains cross-linked
  polysaccharides and proteins.

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Figure 4.15 The Plant Cell Wall
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Concept 4.5 Extracellular Structures Allow Cells
to Communicate with the External Environment

• The plant cell wall has 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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Concept 4.5 Extracellular Structures Allow Cells
to Communicate with the External Environment

• Adjacent plant cells are connected by plasma
  membrane-lined channels called plasmodesmata.
• These channels allow movement of water, ions,
  small molecules, hormones, and some RNA and
  proteins.

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Concept 4.5 Extracellular Structures Allow Cells
to Communicate with the External Environment

• Many animal cells are surrounded by an
  extracellular matrix.
• The fibrous component is the protein collagen.
• The gel-like matrix consists of proteoglycans.
• A third group of proteins links the collagen and
  the matrix together.

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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 Communicate with the External Environment

• Role of extracellular matrices in animal cells
• Hold cells together in tissues
• Contribute to physical properties of cartilage,
  skin, and other tissues
• Filter materials
• Orient cell movement during growth and repair

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Concept 4.5 Extracellular Structures Allow Cells
to Communicate with the External Environment

• Proteins like integrin connect the extracellular
  matrix to the plasma membrane.
• Proteins bind to microfilaments in the cytoplasm
  and to collagen fibers in the extracellular
  matrix.
• For cell movement, the protein changes shape and
  detaches from the collagen.

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Figure 4.17 Cell Membrane Proteins Interact with
the Extracellular Matrix
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Concept 4.5 Extracellular Structures Allow Cells
to Communicate with the External Environment

• Cell junctions are specialized structures that
  protrude from adjacent cells and glue them
  togetherseen often in epithelial cells
• Tight junctions
• Desmosomes
• Gap junctions

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Concept 4.5 Extracellular Structures Allow Cells
to Communicate with the External Environment

• Tight junctions prevent substances from moving
  through spaces between cells.
• Desmosomes hold cells together but allow
  materials to move in the matrix.
• Gap junctions are channels that run between
  membrane pores in adjacent cells, allowing
  substances to pass between the cells.

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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

• Synthetic cell modelsprotocellscan demonstrate
  how cell properties may have originated.
• Combinations of molecules can produce a cell-like
  structure, with a lipid membrane and
  water-filled interior.
• As in modern cells, the membrane allows only
  certain things to pass, while RNA inside the cell
  can replicate itself.

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Figure 4.19 A Protocell