Chapter 6
A Tour of the Cell
The Importance of Cells
All organisms are made of cells
 The 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
10 m
1m
Human height
Length of some
nerve and
muscle cells
0.1 m
◦ Can be used to
visualize different
sized cellular
structures
◦ Have other
disadvantages and
advantages
associated with
them
Figure 6.2
1 cm
Frog egg
1 mm
100 µm
Most plant
and Animal cells
10 µ m
Nucleus
Most bacteria
Mitochondrion
1µm
100 nm
Smallest bacteria
Viruses
10 nm
Ribosomes
Proteins
1 nm
Lipids
Small molecules
0.1 nm
Atoms
Measurements
1 centimeter (cm) = 102 meter (m) = 0.4 inch
1 millimeter (mm) = 10–3 m
1 micrometer (µm) = 10–3 mm = 10–6 m
1 nanometer (nm) = 10–3 mm = 10–9 m
Electron microscope
Different types of
microscopes
Electron microscope

Chicken egg
Light microscope
Microscopy
Microscopy
RESULT
TECHNIQUE
(a) 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.]
(b) Brightfield (stained specimen).
Staining with various dyes enhances
contrast, but most staining procedures
require that cells be fixed (preserved).
(c) Phase-contrast. Enhances contrast
in unstained cells by amplifying
variations in density within specimen;
especially useful for examining living,
unpigmented cells.
Figure 6.3
50 µm
(d) Differential-interference-contrast (Nomarski).
Like phase-contrast microscopy, it uses optical
modifications to exaggerate differences in
density, making the image appear almost 3D.
(e) 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.
50 µm
(f) 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
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.
Microscopy

Scanning electron microscope (SEM)
◦ detail surface of a specimen (3D)
TECHNIQUE
RESULTS
1 µm
Cilia
(a) Scanning electron microscopy (SEM). Micrographs taken
with a scanning electron microscope 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 move
inhaled debris upward toward
the throat.
Figure 6.4 (a)
Microscopy

Transmission electron microscope (TEM)
◦ details internal ultrastructure of cells
Longitudinal
section of
cilium
(b) Transmission electron microscopy (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.
Figure 6.4 (b)
Cross section
of cilium
1 µm
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
Fractionation

Cell fractionation
◦ Takes cells apart & separates organelles from 1
another
◦ Centrifuge fractionates cells into their
component parts
Homogenization
Tissue
cells
1000 g
Homogenate
(1000 times the
force of gravity)
Differential centrifugation
10 min
Supernatant poured
into next tube
20,000 g
20 min
Pellet rich in
nuclei and
cellular debris
Figure 6.5
80,000 g
60 min
150,000 g
3 hr
Pellet rich in
mitochondria
(and chloroplasts if cells
are from a Pellet rich in
plant)
“microsomes”
(pieces of
plasma membranes and
Pellet rich in
cells’ internal ribosomes
membranes)
RESULTS
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 of
experiments, 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.
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 Cells





Plasma membrane
Cytoplasm
Cytosol
Chromosomes
Ribosomes
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 nucleus
 DNA is found in the nucleoid
 Nucleoid is not bound by a membrane

Pili: attachment structures on
the surface of some prokaryotes
Nucleoid: region where the
cell’s DNA is located (not
enclosed by a membrane)
Ribosomes: organelles that
synthesize proteins
Bacterial
chromosome
(a) A typical
rod-shaped bacterium
Figure 6.6 A, B
Plasma membrane: membrane
enclosing the cytoplasm
Cell wall: rigid structure outside
the plasma membrane
Capsule: jelly-like outer coating
of many prokaryotes
0.5 µm
Flagella: locomotion
organelles of
some bacteria
(b) A thin section through the
bacterium Bacillus coagulans
(TEM)
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
membrane
Surface area increases while
total volume remains constant
5
1
1
Figure 6.7
Total surface area
(height  width 
number of sides 
number of boxes)
6
150
750
Total volume
(height  width  length
 number of boxes)
1
125
125
Surface-to-volume
ratio
(surface area  volume)
6
12
6
Eukaryotic Cells

Plasma membrane acts as a selective
barrier (semipermeable)
◦ Allows passage of nutrients
Outside of cell
& waste
Carbohydrate side chain
Hydrophilic
region
Inside of cell
0.1 µm
Hydrophobic
region
Figure 6.8 A, B
(a) TEM of a plasma
membrane. The
plasma membrane,
here in a red blood
cell, appears as a
pair of dark bands
separated by a
light band.
Hydrophilic
region
Phospholipid
Proteins
(b) Structure of the plasma membrane
Eukaryotic Cells

Membrane 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
ENDOPLASMIC RETICULUM (ER)
Rough ER
Smooth ER
Nuclear envelope
Nucleolus
NUCLEUS
Chromatin
Flagelium
Plasma membrane
Centrosome
CYTOSKELETON
Microfilaments
Intermediate filaments
Ribosomes
Microtubules
Microvilli
Golgi apparatus
Peroxisome
Figure 6.9
Mitochondrion
Lysosome
In animal cells but not plant cells:
Lysosomes
Centrioles
Flagella (in some plant sperm)

A plant cell
Nuclear envelope
Nucleolus
Chromatin
NUCLEUS
Centrosome
Rough
endoplasmic
reticulum Smooth
endoplasmic
reticulum
Ribosomes (small brwon dots)
Central vacuole
Tonoplast
Golgi apparatus
Microfilaments
Intermediate
filaments
CYTOSKELETON
Microtubules
Mitochondrion
Peroxisome
Plasma membrane
Chloroplast
Cell wall
Plasmodesmata
Wall of adjacent cell
Figure 6.9
In plant cells but not animal cells:
Chloroplasts
Central vacuole and tonoplast
Cell wall
Plasmodesmata
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 shape
Chromosomes – made of chromatin
Nucleolus – cite of rRNA synthesis
Eukaryotic Cells

nuclear envelope
◦ Encloses the nucleus, separates contents from
cytoplasm
Nucleus
1 µm
Nucleolus
Chromatin
Nucleus
Nuclear envelope:
Inner membrane
Outer membrane
Nuclear pore
Pore
complex
Rough ER
Surface of nuclear
envelope.
1 µm
Ribosome
0.25 µm
Close-up of
nuclear
envelope
Figure 6.10
Pore complexes (TEM).
Nuclear lamina (TEM).
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
Ribosomes
ER
Cytosol
Endoplasmic reticulum (ER)
Free ribosomes
Bound ribosomes
Large
subunit
0.5 µm
TEM showing ER and ribosomes
Figure 6.11
Small
subunit
Diagram of a ribosome
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 ribosomes
 Smooth ER – Lacks ribosomes


ER membrane
◦ continuous with/attached to the nuclear envelope
Smooth ER
Rough ER
Nuclear
envelope
ER lumen
Cisternae
Ribosomes
Transitional ER
Transport vesicle
Smooth ER
Rough ER 200 µm
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/winter2014/express-shipping
 https://www.youtube.com/watch?v=jazs6sR
Q2og


http://www.ted.com/talks/david_bolinsky
_animates_a_cell
Golgi apparatus
Golgi
apparatus
cis face
(“receiving” side of
Golgi apparatus)
1 Vesicles move
2 Vesicles coalesce to
6 Vesicles also
form new cis Golgi cisternae
from ER to Golgi
transport certain
Cisternae
proteins back to ER
3 Cisternal
maturation:
Golgi cisternae
move in a cisto-trans
direction
Figure 6.13
5 Vesicles transport specific
proteins backward to newer
Golgi cisternae
0.1 0 µm
4 Vesicles form and
leave Golgi, carrying
specific proteins to
other locations or to
the plasma membrane for secretion
trans face
(“shipping” side of
Golgi apparatus)
TEM of Golgi apparatus
Lysosomes: Digestive
Compartments





Membranous sacs
Full of digestive enzymes
Internally acidic
Synthesized by the rough ER, processed
by the Golgi
Phagocytosis – intracellular digestion
◦ Cells engulf smaller objects (sometimes
organisms), forming a food vacuole
◦ Food vacuole fuses with lysosome
Lysosomes
1 µm
Nucleus
Lysosome
Lysosome contains
active hydrolytic
enzymes
Food vacuole
fuses with
lysosome
Hydrolytic
enzymes digest
food particles
Digestive
enzymes
Lysosome
Plasma membrane
Digestion
Food vacuole
Figure 6.14 A
(a) Phagocytosis: lysosome digesting food
Lysosomes

Autophagy- self
eating (vesicle
surrounds
damaged
organelle)
Lysosome containing
two damaged organelles
1µm
Mitochondrion
fragment
Peroxisome
fragment
Lysosome fuses with
vesicle containing
damaged organelle
Hydrolytic enzymes
digest organelle
components
Lysosome
Vesicle containing
damaged mitochondrion
Digestion
(b) Autophagy: lysosome breaking down damaged organelle
Figure 6.14 B
Vacuoles: Diverse Maintenance
Compartments





Plant or fungal cells have at least one vacuole
Food vacuoles – surround ingested materials
so that they can be broken down
Contractile vacuoles – pump excess water out
of a cell
Central vacuole – enclosed by tonoplast, also
works to maintain homeostasis.
Cell sap – solution inside of the central
vacuole
Central Vacuole
Central vacuole
Cytosol
Tonoplast
Nucleus
Central
vacuole
Cell wall
Chloroplast
Figure 6.15
5 µm
The Endomembrane System: A
Review



Compartmental organization of a cell
System of interconnected membranes
Function, composition, and contents
modified with progression from ER to
vacuoles
Brainstorm: How do transport vesicles
serve to integrate the endomembrane
system?
Relationships within the
endomembrane system
Nuclear
envelope is
1
connected to rough ER,
which is also continuous
with smooth ER
Nucleus
Rough ER
2
Membranes
and proteins
produced by the ER flow in
the form of transport vesicles
to the Golgi
Smooth ER
cis Golgi
Nuclear envelop
Golgi
3 pinches off transport
Vesicles and other vesicles
that give rise to lysosomes and
Vacuoles
Plasma
membrane
trans Golgi
4
Lysosome available
for fusion with another
vesicle for digestion
Figure 6.16
5 Transport vesicle carries 6
proteins to plasma
membrane for secretion
Plasma membrane expands
by fusion of vesicles; proteins
are secreted from cell
Mitochondria and Chloroplasts
Change energy from one form to another
 Mitochondria

◦ Sites of cellular respiration

Chloroplasts
◦ 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
matrix
Number 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
Outer
membrane
Free
ribosomes
in the
mitochondrial
matrix
Inner
membrane
Cristae
Matrix
Mitochondrial
DNA
100 µm
Brainstorm: Why is the inner membrane folded?
Figure 6.17
Chloroplasts: Capture of Light
Energy

Plastids – 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 algae
 Chloroplast structure:

◦ Thylakoids- membranous sacs; flattened
◦ Stroma- internal fluid; contains chloroplast DNA
Chloroplast
Ribosomes
Stroma
Chloroplast
DNA
Inner and outer
membranes
Granum
Figure 6.18
1 µm
Thylakoid
Peroxisomes: Oxidation
◦ Make H2O2 (hydrogen peroxide) and convert it to H2O
◦ Single membrane; transfers hydrogen
◦ Break down fatty acids to transfer to mitochondria
Chloroplast
Peroxisome
Mitochondrion
Brainstorm: What role would
a peroxisome in a liver cell
have?
1 µm
Figure 6.19
The Cytoskeleton
A network of fibers that organizes
structures and activities in the cell
Microtubule
Figure 6.20
0.25 µm
Microfilaments
Roles of the Cytoskeleton:
Support, Motility, and Regulation

Cytoskeleton
◦ Gives mechanical support to the cell
◦ involved in cell motility, using motor proteins
ATP
Vesicle
Receptor for
motor protein
Motor protein
Microtubule
(ATP powered)
of cytoskeleton
(a) Motor proteins that attach to receptors on organelles can “walk”
the organelles along microtubules or, in some cases, microfilaments.
Vesicles
Microtubule
0.25 µm
Figure 6.21 A, B
(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.)

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).
Direction of swimming
Figure 6.23 A
1 µm

Ciliary motion
(b) Motion of cilia. Cilia have a backand-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
Outer microtubule
doublet
Dynein arms
0.1 µm
Central
microtubule
Outer doublets
cross-linking
proteins inside
Microtubules
Radial
spoke
Plasma
membrane
Basal body
(b)
0.5 µm
(a)
0.1 µm
Triplet
(c)
Figure 6.24 A-C
Cross section of basal body
Plasma
membrane

The protein dynein
◦ Is responsible for the bending movement of cilia
and flagella
Microtubule
doublets
ATP
Dynein arm
(a) 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.
Figure 6.25 A
ATP
Outer doublets
cross-linking
proteins
Anchorage
in cell
(b) In a cilium or flagellum, two adjacent doublets cannot slide far because
they are physically restrained by proteins, so they bend. (Only two of
the nine outer doublets in Figure 6.24b are shown here.)
Figure 6.25 B
1
3
2
(c) 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.
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
Microvillus
Plasma membrane
Microfilaments (actin
filaments)
Intermediate filaments
Figure 6.26
0.25 µm
Microfilaments

Microfilaments that function in cellular
motility
◦ Contain the protein myosin in addition to actin
Muscle cell
Actin filament
Myosin filament
Myosin arm
Figure 6.27 A
(a) Myosin motors in muscle cell contraction.

Amoeboid movement
◦ Involves the contraction of actin & myosin
filaments
Cortex (outer cytoplasm):
gel with actin network
Inner cytoplasm: sol
with actin subunits
Extending
pseudopodium
Figure 6.27 B
(b) Amoeboid movement

Cytoplasmic streaming
◦ Is another form of locomotion created by
microfilaments
Nonmoving
cytoplasm (gel)
Chloroplast
Streaming
cytoplasm
(sol)
Parallel actin
filaments
Figure 6.27 C
(b) Cytoplasmic streaming in plant cells
Cell wall
Intermediate Filaments
Support cell shape
 Fixes organelles in space
 No 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 Cells
Animal cells do not have a cell wall
 Instead are connected by extracellular
matrix
 Composed mostly of glycoproteins

◦ Collagen
◦ Proteoglycans
◦ Fibronectin

Connected to proteins embedded in
plasma membrane called integrins
EXTRACELLULAR FLUID
Collagen
A proteoglycan
complex
Polysaccharide
molecule
Carbohydrates
Core
protein
Fibronectin
Plasma
membrane
Integrin
Figure 6.29
Integrins
Microfilaments
CYTOPLASM
Proteoglycan
molecule
Extracellular Matrix

Functions
◦
◦
◦
◦
Support
Adhesion
Movement
Regulation
Intercellular Junctions
Plants: Plasmodesmata

Plasmodesmata
◦ channels in plant cell walls
Cell walls
Interior
of cell
Interior
of cell
Figure 6.30
0.5 µm
Plasmodesmata
Plasma membranes
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
Tight junction
Tight junctions prevent
fluid from moving
across a layer of cells
0.5 µm
At tight junctions, the membranes of
neighboring cells are very tightly pressed
against each other, bound together by
specific proteins (purple). Forming continuous seals around the cells, tight junctions
prevent leakage of extracellular fluid across
A layer of epithelial cells.
DESMOSOMES
Desmosomes (also called anchoring
junctions) function like rivets, fastening cells
Together into strong sheets. Intermediate
Filaments made of sturdy keratin proteins
Anchor desmosomes in the cytoplasm.
Tight junctions
Intermediate
filaments
Desmosome
Gap
junctions
Space
between Plasma membranes
cells
of adjacent cells
Figure 6.31
1 µm
Extracellular
matrix
Gap junction
0.1 µm
GAP JUNCTIONS
Gap junctions (also called communicating
junctions) provide cytoplasmic channels from
one 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 may
pass. Gap junctions are necessary for communication between cells in many types of tissues,
including heart muscle and animal embryos.
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 µm

Figure 6.32