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Chapter 6A Tour of the Cell
The Importance of Cells
All organisms are made of cellsThe cell is the simplest collection of living
matter; the smallest unit of life.
Figure 6.1
Microscopy
To study cells, biologists use microscopes and the tools of biochemistry
Scientists use microscopes to see cells too small to see with the naked eye
Types of microscopes◦Light microscopes◦Electron microscopes
Microscopy
Magnification – The ratio of an object’s image size to its actual size
Resolution – The measure of clarity of the object’s image; the minimum distance two distinct points can be separated by and still be distinguished as two points.
Microscopy
Light microscopes (LMs)◦Pass visible light through a specimen◦Magnify structures w/ lenses◦Limited magnification; cannot resolve detail
finer than 2 μm, or 200 nm.◦Up to 1000x magnification
Microscopy
Different types of microscopes◦Can be used to
visualize different sized cellular structures
◦Have other disadvantages and advantages associated with them
1 m
0.1 nm
10 m
0.1 m
1 cm
1 mm
100 µm
10 µ m
1 µ m
100 nm
10 nm
1 nm
Length of somenerve and muscle cells
Chicken egg
Frog egg
Most plant and Animal cells
Smallest bacteria
Viruses
Ribosomes
Proteins
Lipids
Small molecules
Atoms
NucleusMost bacteriaMitochondrion
Lig
ht m
icro
sco
pe
Ele
ctro
n m
icro
sco
pe
Ele
ctro
n m
icro
sco
pe
Figure 6.2
Human height
Measurements1 centimeter (cm) = 102 meter (m) = 0.4 inch1 millimeter (mm) = 10–3 m1 micrometer (µm) = 10–3 mm = 10–6 m1 nanometer (nm) = 10–3 mm = 10–9 m
MicroscopyTECHNIQUE RESULT
Brightfield (unstained specimen). Passes light directly through specimen. Unless cell is naturally pigmented or artificially stained, image has little contrast. [Parts (a)–(d) show a human cheek epithelial cell.]
(a)
Brightfield (stained specimen). Staining with various dyes enhances contrast, but most staining procedures require that cells be fixed (preserved).
(b)
Phase-contrast. Enhances contrast in unstained cells by amplifying variations in density within specimen; especially useful for examining living, unpigmented cells.
(c)
50 µm
Figure 6.3
Differential-interference-contrast (Nomarski). Like phase-contrast microscopy, it uses optical modifications to exaggerate differences indensity, making the image appear almost 3D.
Fluorescence. Shows the locations of specific molecules in the cell by tagging the molecules with fluorescent dyes or antibodies. These fluorescent substances absorb ultraviolet radiation and emit visible light, as shown here in a cell from an artery.
Confocal. Uses lasers and special optics for “optical sectioning” of fluorescently-stained specimens. Only a single plane of focus is illuminated; out-of-focus fluorescence above and below the plane is subtracted by a computer. A sharp image results, as seen in stained nervous tissue (top), where nerve cells are green, support cells are red, and regions of overlap are yellow. A standard fluorescence micrograph (bottom) of this relatively thick tissue is blurry.
50 µm
50 µm
(d)
(e)
(f)
Microsopcy
Electron microscopes (EMs) focus a beam of electrons either through or over the surface of a cell.◦Scanning Electron Microscope – Electrons pass
over surface of cell; resulting image is a three dimensional view of the surface of a cell.
◦Transmission Electron Microscope – Electrons pass through a cell (slice); resulting image shows internal structures of cell.
MicroscopyScanning electron microscope (SEM)
◦detail surface of a specimen (3D)
TECHNIQUE RESULTS
Scanning electron micro-scopy (SEM). Micrographs takenwith a scanning electron micro-scope show a 3D image of the surface of a specimen. This SEM shows the surface of a cell from a rabbit trachea (windpipe) covered with motile organelles called cilia. Beating of the cilia helps moveinhaled debris upward toward the throat.
(a)
Cilia1 µm
Figure 6.4 (a)
Microscopy
Transmission electron microscope (TEM)◦details internal ultrastructure of cells
Transmission electron micro-scopy (TEM). A transmission electron microscope profiles a thin section of a specimen. Here we see a section through a tracheal cell, revealing its ultrastructure. In preparing the TEM, some cilia were cut along their lengths, creating longitudinal sections, while other cilia were cut straight across, creating cross sections.
(b)
Longitudinalsection ofcilium
Cross sectionof cilium
1 µm
Figure 6.4 (b)
Microscopy
Biological samples stained with heavy metals
Electrons on cell surface become excited and can be detected electronically.
Uses electromagnets as lenses rather than glass.
Microscopy
Cytology – The study of cell structure
Brainstorm: Compare and contrast the different types of microscopes.What are some advantages to each type?What are some disadvantages?
Microscopy
What type of microscope would be used to study:◦Changes in shape of a living white blood cell◦Details of the surface structure of a hair◦The detailed structure of an organelle?
Isolating Organelles by Cell FractionationCell fractionation
◦Takes cells apart & separates organelles from 1 another
◦Centrifuge fractionates cells into their component parts
Tissuecells
Homogenization
Homogenate1000 g(1000 times theforce of gravity)
10 min Differential centrifugationSupernatant pouredinto next tube
20,000 g20 min
Pellet rich innuclei andcellular debris
Pellet rich inmitochondria(and chloro-plasts if cellsare from a plant)
Pellet rich in“microsomes”(pieces of plasma mem-branes andcells’ internalmembranes)
Pellet rich inribosomes
150,000 g3 hr
80,000 g60 min
Figure 6.5
In the original experiments, the researchers used microscopy to identify the organelles in each pellet, establishing a baseline for further experiments. In the next series ofexperiments, researchers used biochemical methods to determine the metabolic functions associated with each type of organelle. Researchers currently use cell fractionation to isolate particular organelles in order to study further details of their function.
RESULTS
Figure 6.5
Prokaryotic and Eukaryotic Cells
Review: What are the main differences between eukaryotic cells and prokaryotic cells?
Create a venn diagram to compare and contrast the structures that are found in eukaryotic and prokaryotic cells. Define all of the terms (including a description of the function of organelles). Using those definitions, write a brief explanation of the different capabilities of each type of cell.
Comparing Prokaryotic & Eukaryotic CellsPlasma membraneCytoplasmCytosolChromosomesRibosomes
Brainstorm: What is the function of ribosomes?
Hint: Look at the word. What other term does it remind you of?
Prokaryotic Cells
“Pro” + “karyon” (kernel)No nucleusDNA is found in the nucleoidNucleoid is not bound by a membrane
(b) A thin section through the bacterium Bacillus coagulans (TEM)
Pili: attachment structures onthe surface of some prokaryotes
Nucleoid: region where thecell’s DNA is located (notenclosed by a membrane)
Ribosomes: organelles thatsynthesize proteins
Plasma membrane: membraneenclosing the cytoplasm
Cell wall: rigid structure outsidethe plasma membrane
Capsule: jelly-like outer coatingof many prokaryotes
Flagella: locomotionorganelles ofsome bacteria
(a) A typical rod-shaped bacterium
0.5 µmBacterialchromosome
Figure 6.6 A, B
Eukaryotic Cells
“Eu” + “karyon”Membrane bound nucleus
◦Nuclear envelope
Prokaryotes vs Eukaryotes
Brainstorm: Why does the size of the cell matter?
Prokaryotes vs Eukaryotes
A smaller cell ◦Has higher surface area to volume ratio◦Makes it easier to transport enough through
membraneSurface area increases whiletotal volume remains constant
5
11
Total surface area (height width number of sides number of boxes)
Total volume (height width length number of boxes)
Surface-to-volume ratio (surface area volume)
6
1
6
150
125
12
750
125
6
Figure 6.7
Eukaryotic Cells
Plasma membrane acts as a selective barrier (semipermeable)◦Allows passage of nutrients
& wasteCarbohydrate side chain
Figure 6.8 A, B
Outside of cell
Inside of cell
Hydrophilicregion
Hydrophobicregion
Hydrophilicregion
(b) Structure of the plasma membrane
Phospholipid Proteins
TEM of a plasmamembrane. Theplasma membrane,here in a red bloodcell, appears as apair of dark bandsseparated by alight band.
(a)
0.1 µm
Eukaryotic CellsMembrane bound organelles
◦Compartmentalize the cell◦Membranes participate in metabolic activity
Brainstorm: Why is it important for eukaryotic cells to be compartmentalized?
Eukaryotic Cells
Plant cells vs animal cells◦Have most of the same organelles◦More similar to each other than either are to
prokaryotic cells
Practice: Just like you did for prokaryotes vs eukaryotes, create a venn diagram comparing and contrasting plant and animal cells. Focus on organelles, and include a key that defines each of the terms.
A animal cell
Rough ER Smooth ER
Centrosome
CYTOSKELETON
Microfilaments
Microtubules
Microvilli
Peroxisome
Lysosome
Golgi apparatus
Ribosomes
In animal cells but not plant cells:LysosomesCentriolesFlagella (in some plant sperm)
Nucleolus
Chromatin
NUCLEUS
Flagelium
Intermediate filaments
ENDOPLASMIC RETICULUM (ER)
Mitochondrion
Nuclear envelope
Plasma membrane
Figure 6.9
A plant cell
In plant cells but not animal cells:ChloroplastsCentral vacuole and tonoplastCell wallPlasmodesmata
CYTOSKELETON
Ribosomes (small brwon dots)
Central vacuole
Microfilaments
Intermediate filaments
Microtubules
Rough endoplasmic reticulum Smooth
endoplasmic reticulum
ChromatinNUCLEUS
Nuclear envelope
Nucleolus
Chloroplast
PlasmodesmataWall of adjacent cell
Cell wall
Golgi apparatus
Peroxisome
Tonoplast
Centrosome
Plasma membrane
Mitochondrion
Figure 6.9
Eukaryotic Cells
Genetic instructions are housed in the nucleus and carried out by the ribosomes
Nucleus◦Genetic library of the cell◦Bound by nuclear envelope (double membrane)◦Usually the largest, most obvious organelle◦Protein studded surface Pore complex
Nuclear Lamina – maintains shapeChromosomes – made of chromatinNucleolus – cite of rRNA synthesis
Eukaryotic Cells nuclear envelope
◦Encloses the nucleus, separates contents from cytoplasm
Figure 6.10
Nucleus
NucleusNucleolus
Chromatin
Nuclear envelope:Inner membraneOuter membrane
Nuclear pore
Rough ER
Porecomplex
Surface of nuclear envelope.
Pore complexes (TEM). Nuclear lamina (TEM).
Close-up of nuclearenvelope
Ribosome
1 µm
1 µm0.25 µm
Eukaryotic Cells
Ribosomes◦Made of ribosomal RNA (rRNA)
and protein◦Carry out protein synthesis (translation)◦Free ribosomes – suspended in cytosol◦Bound ribosomes – attached to rough ER or
nuclear envelope◦Ribosomes can alternate roles
ER
Endoplasmic reticulum (ER)
Ribosomes Cytosol
Free ribosomes
Bound ribosomes
Largesubunit
Smallsubunit
TEM showing ER and ribosomes Diagram of a ribosome
0.5 µm
Figure 6.11
Eukaryotic Cells
Endomembrane system Regulates protein traffic and performs
metabolic functions in the cell◦Protein synthesis and transport◦Metabolism of lipids◦Detoxification
Structural differences mediate function◦Endoplasmic reticulum, Golgi apparatus,
lysosomes, vacuoles, plasma membrane.
The Endoplasmic Reticulum: Biosynthetic Factory
Endoplasmic reticulum accounts for more than half of the total membrane in eukaryotic cells
Net-like structure made up of membranous tubules and cisternae (sacs of liquid)
Rough ER – Covered in ribosomesSmooth ER – Lacks ribosomes
ER membrane◦continuous with/attached to the nuclear envelope
Smooth ER
Rough ER
ER lumenCisternae
RibosomesTransport vesicle
Smooth ER
Transitional ER
Rough ER 200 µm
Nuclearenvelope
Figure 6.12
Functions of Smooth ER
◦Synthesizes lipids◦Metabolizes carbohydrates◦Stores Ca+
Releases into cytosol in response to nerve impulse, causing muscle contraction
◦Detoxifies poison (adds hydroxyl groups to drugs)
Functions of Rough ER
◦Studded with ribosomes◦Makes proteins and membranes, distributed by
transport vesicles (sacs of membrane) Secreted proteins Membrane-bound proteins
◦Glycoproteins – secretory proteins, covalently bonded to carbohydrates
◦Synthesizes membranes Phospholipids Embeds proteins directly into membrane
The Golgi Apparatus: Shipping and Receiving Center Golgi apparatus
◦gets transport vesicles from rough ER◦flattened membranous sacs called cisternae◦Modifies products of rough ER◦Makes certain macromolecules
http://www.hhmi.org/bulletin/winter-2014/express-shipping
https://www.youtube.com/watch?v=jazs6sRQ2og
http://www.ted.com/talks/david_bolinsky_animates_a_cell
Golgiapparatus
TEM of Golgi apparatus
cis face(“receiving” side ofGolgi apparatus)
Vesicles movefrom ER to Golgi Vesicles also
transport certainproteins back to ER
Vesicles coalesce toform new cis Golgi cisternae
Cisternalmaturation:Golgi cisternaemove in a cis-to-transdirection
Vesicles form andleave Golgi, carryingspecific proteins toother locations or tothe plasma mem-brane for secretion
Vesicles transport specificproteins backward to newerGolgi cisternae
Cisternae
trans face(“shipping” side ofGolgi apparatus)
0.1 0 µm16
5
2
3
4
Golgi apparatus
Figure 6.13
Lysosomes: Digestive CompartmentsMembranous sacsFull of digestive enzymesInternally acidicSynthesized by the rough ER, processed
by the GolgiPhagocytosis – intracellular digestion
◦Cells engulf smaller objects (sometimes organisms), forming a food vacuole
◦Food vacuole fuses with lysosome
Lysosomes
Figure 6.14 A(a) Phagocytosis: lysosome digesting food
1 µm
Lysosome containsactive hydrolyticenzymes
Food vacuole fuses with lysosome
Hydrolyticenzymes digestfood particles
Digestion
Food vacuole
Plasma membraneLysosome
Digestiveenzymes
Lysosome
Nucleus
Lysosomes
Autophagy- self eating (vesicle surrounds damaged organelle)
Figure 6.14 B
(b) Autophagy: lysosome breaking down damaged organelle
Lysosome containingtwo damaged organelles 1 µ m
Mitochondrionfragment
Peroxisomefragment
Lysosome fuses withvesicle containingdamaged organelle
Hydrolytic enzymesdigest organellecomponents
Vesicle containingdamaged mitochondrion
Digestion
Lysosome
Vacuoles: Diverse Maintenance CompartmentsPlant or fungal cells have at least one
vacuoleFood vacuoles – surround ingested
materials so that they can be broken downContractile vacuoles – pump excess water
out of a cellCentral vacuole – enclosed by tonoplast,
also works to maintain homeostasis.Cell sap – solution inside of the central
vacuole
Central Vacuole
Central vacuole
Cytosol
Tonoplast
Centralvacuole
Nucleus
Cell wall
Chloroplast
5 µmFigure 6.15
The Endomembrane System: A ReviewCompartmental organization of a cellSystem of interconnected membranesFunction, composition, and contents
modified with progression from ER to vacuoles
Brainstorm: How do transport vesicles serve to integrate the endomembrane system?
Plasma membrane expandsby fusion of vesicles; proteinsare secreted from cell
Transport vesicle carriesproteins to plasma membrane for secretion
Lysosome availablefor fusion with anothervesicle for digestion
4 5 6
Nuclear envelope isconnected to rough ER, which is also continuouswith smooth ER
Nucleus
Rough ER
Smooth ERcis Golgi
trans Golgi
Membranes and proteinsproduced by the ER flow inthe form of transport vesiclesto the Golgi Nuclear envelop
Golgi pinches off transport Vesicles and other vesicles that give rise to lysosomes and Vacuoles
1
3
2
Plasmamembrane
Relationships within the endomembrane system
Figure 6.16
Mitochondria and Chloroplasts
Change energy from one form to anotherMitochondria
◦Sites of cellular respirationChloroplasts
◦Only in plants & plantlike protists; photosynthesis
Mitochondria
Chemical energy conversion – site of cellular respiration
Generates ATP from fuel sources (sugars, fats, etc…)
Double membrane◦Outer = smooth◦ Inner = convoluted; infoldings called cristae
Intermembrane space and mitochondrial matrixNumber of mitochondria related to metabolic
activity of a cell
Mitochondria; enclosed by 2 membranes◦smooth outer membrane◦inner membrane folded into cristae
Mitochondrion
Intermembrane space
Outermembrane
Freeribosomesin the mitochondrialmatrix
MitochondrialDNA
Innermembrane
Cristae
Matrix
100 µm
Figure 6.17
Brainstorm: Why is the inner membrane folded?
Chloroplasts: Capture of Light EnergyPlastids – family of plant organelles
◦Amyloplast – colorless; stores starch◦Chromoplast – pigmented; gives color to
flowers◦Chloroplast
Chloroplast◦has chlorophyll – green pigment◦Enzymes and molecules that function in
photosynthetic production
Chloroplasts
Leaves and green algaeChloroplast structure:
◦ Thylakoids- membranous sacs; flattened◦ Stroma- internal fluid; contains chloroplast DNA
Chloroplast
ChloroplastDNA
RibosomesStroma
Inner and outermembranes
Thylakoid
1 µm
GranumFigure 6.18
Peroxisomes: Oxidation
◦ Make H2O2 (hydrogen peroxide) and convert it to H2O◦ Single membrane; transfers hydrogen◦ Break down fatty acids to transfer to mitochondria
ChloroplastPeroxisome
Mitochondrion
1 µm
Figure 6.19
Brainstorm: What role woulda peroxisome in a liver cellhave?
The Cytoskeleton
A network of fibers that organizes structures and activities in the cell
Figure 6.20
Microtubule
0.25 µm MicrofilamentsFigure 6.20
Roles of the Cytoskeleton: Support, Motility, and Regulation
Cytoskeleton◦Gives mechanical support to the cell◦involved in cell motility, using motor proteins
VesicleATP
Receptor formotor protein
Motor protein(ATP powered)
Microtubuleof cytoskeleton
(a) Motor proteins that attach to receptors on organelles can “walk”the organelles along microtubules or, in some cases, microfilaments.
Microtubule Vesicles 0.25 µm
(b) Vesicles containing neurotransmitters migrate to the tips of nerve cell axons via the mechanism in (a). In this SEM of a squid giant axon, two vesicles can be seen moving along a microtubule. (A separate part of the experiment provided the evidence that they were in fact moving.)Figure 6.21 A, B
3 types of fibers make up the cytoskeleton
Table 6.1
The wacky history of cell theory - Lauren Royal-Woods | TED-Ed
Microtubules
1.) Microtubules◦Shape the cell◦Guide movement of organelles◦Help separate chromo copies (mitosis)
Centrosomes and Centrioles
Centrosome- “microtubule-organizing center”
Cilia and Flagella
Cilia & flagella◦have specialized arrangements of microtubules◦locomotor appendages of some cells
Brainstorm: How are cilia and flagella similar? How are they different?
Flagella beating pattern
(a) Motion of flagella. A flagellum usually undulates, its snakelike motion driving a cell in the same direction as the axis of the flagellum. Propulsion of a human sperm cell is an example of flagellatelocomotion (LM).
1 µm
Direction of swimming
Figure 6.23 A
Ciliary motion
(b) Motion of cilia. Cilia have a back- and-forth motion that moves the cell in a direction perpendicular to the axis of the cilium. A dense nap of cilia, beating at a rate of about 40 to 60 strokes a second, covers this Colpidium, a freshwater protozoan (SEM).
Figure 6.23 B
15 µm
Cilia and flagella share a common ultrastructure
(a)
(c)
(b)
Outer microtubuledoublet
Dynein arms
Centralmicrotubule
Outer doublets cross-linkingproteins inside
Radialspoke
Plasmamembrane
Microtubules
Plasmamembrane
Basal body
0.5 µm
0.1 µm
0.1 µm
Cross section of basal body
Triplet
Figure 6.24 A-C
The protein dynein◦Is responsible for the bending movement of cilia
and flagella
Microtubuledoublets ATP
Dynein arm
Powered by ATP, the dynein arms of one microtubule doublet grip the adjacent doublet, push it up, release, and then grip again. If the two microtubule doublets were not attached, they would slide relative to each other.
(a)
Figure 6.25 A
Outer doubletscross-linkingproteins
Anchoragein cell
ATP
In a cilium or flagellum, two adjacent doublets cannot slide far because they are physically restrained by proteins, so they bend. (Only two ofthe nine outer doublets in Figure 6.24b are shown here.)
(b)
Figure 6.25 B
Localized, synchronized activation of many dynein arms probably causes a bend to begin at the base of the Cilium or flagellum and move outward toward the tip. Many successive bends, such as the ones shown here to the left and right, result in a wavelike motion. In this diagram, the two central microtubules and the cross-linking proteins are not shown.
(c)
1 3
2
Figure 6.25 C
Microfilaments (Actin Filaments)
Microfilaments are twisted double chains of actin◦Actin = globular protein◦Branching network connected by proteins◦Tension bearing (pulling forces)◦Connected with myosin (motor protein) ◦Ameoboid movement (pseudopodia) and
cytoplasmic streaming
Microfilaments
0.25 µm
Microvillus
Plasma membrane
Microfilaments (actinfilaments)
Intermediate filaments
Figure 6.26
Microfilaments
Microfilaments that function in cellular motility◦Contain the protein myosin in addition to actin
Actin filament
Myosin filament
Myosin motors in muscle cell contraction. (a)
Muscle cell
Myosin arm
Figure 6.27 A
Amoeboid movement◦Involves the contraction of actin & myosin
filamentsCortex (outer cytoplasm):gel with actin network
Inner cytoplasm: sol with actin subunits
Extendingpseudopodium
(b) Amoeboid movementFigure 6.27 B
Cytoplasmic streaming◦Is another form of locomotion created by
microfilaments
Nonmovingcytoplasm (gel)
Chloroplast
Streamingcytoplasm(sol)
Parallel actinfilaments Cell wall
(b) Cytoplasmic streaming in plant cellsFigure 6.27 C
Intermediate Filaments
Support cell shapeFixes organelles in spaceNo set structure or function (diverse class
of cytoskeletal elements)◦Constructed from proteins from specific family
(e.g. keratin)More permanent than microtubules and
microfilaments
Cell Walls of Plants
An extracellular structure of plant cells that distinguishes them from animal cells
Cellulose fibers embedded in other polysaccharides and protein
Middle lamina (sticky)Multiple layers
◦Primary cell wall◦Secondary cell wall
The Extracellular Matrix (ECM) of Animal CellsAnimal cells do not have a cell wallInstead are connected by extracellular
matrixComposed mostly of glycoproteins
◦Collagen◦Proteoglycans◦Fibronectin
Connected to proteins embedded in plasma membrane called integrins
Collagen
Fibronectin
Plasmamembrane
EXTRACELLULAR FLUID
Micro-filaments
CYTOPLASM
Integrins
Polysaccharidemolecule
Carbo-hydrates
Proteoglycanmolecule
Coreprotein
Integrin
Figure 6.29
A proteoglycan complex
Extracellular Matrix
Functions◦Support◦Adhesion◦Movement◦Regulation
Intercellular Junctions
Plants: Plasmodesmata
Plasmodesmata◦channels in plant cell walls
Interiorof cell
Interiorof cell
0.5 µm Plasmodesmata Plasma membranes
Cell walls
Figure 6.30
Animals: Tight Junctions, Desmosomes, and Gap Junctions
Animals have 3 types of intercellular junctions◦Tight junctions◦Desmosomes◦Gap junctions
Types of intercellular junctions in animals
Tight junctions prevent fluid from moving across a layer of cells
Tight junction
0.5 µm
1 µm
Spacebetweencells
Plasma membranesof adjacent cells
Extracellularmatrix
Gap junction
Tight junctions
0.1 µm
Intermediatefilaments
Desmosome
Gapjunctions
At tight junctions, the membranes ofneighboring cells are very tightly pressedagainst each other, bound together byspecific proteins (purple). Forming continu-ous seals around the cells, tight junctionsprevent leakage of extracellular fluid acrossA layer of epithelial cells.
Desmosomes (also called anchoringjunctions) function like rivets, fastening cellsTogether into strong sheets. IntermediateFilaments made of sturdy keratin proteinsAnchor desmosomes in the cytoplasm.
Gap junctions (also called communicatingjunctions) provide cytoplasmic channels fromone cell to an adjacent cell. Gap junctions consist of special membrane proteins that surround a pore through which ions, sugars,amino acids, and other small molecules maypass. Gap junctions are necessary for commu-nication between cells in many types of tissues,including heart muscle and animal embryos.
TIGHT JUNCTIONS
DESMOSOMES
GAP JUNCTIONS
Figure 6.31
The Cell: A Living Unit Greater Than the Sum of Its Parts
Cells rely on the integration of structures and organelles in order to function
5 µ
m
Figure 6.32