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Chapter 6
A tour of the cell
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Why study cells?
• Cell Theory
– All organisms are made of cells
– The cell is the basic unit of organization for
all organisms
• Cell = simplest collection of matter that
can live
– All cells come from pre-existing cells
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Why study cells?
• Biological diversity & unity
– Underlying the diversity
of life is a striking unity:
• DNA is the universal
genetic language
• Cells are the basic
units of structure
& function
You are here
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Why study cells?
• Activities of life
– Almost everything you think of a whole
organism needing to do, must be done at
the cellular level:
• Reproduction
• Growth & development
• Energy utilization
• Response to environment
• Homeostasis
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Why study cells?
• Cell structure is correlated to cellular function
10 µm
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
How do we study cells?
• Microscopes opened up the world of cells
– Robert Hooke (1665)
• the 1st cytologist
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
How do we study cells?
• Microscopes
– Light microscope (LM)
– Electron microscope (EM)
• Transmission electron
microscope (TEM)
• Scanning electron
microscope (SEM)
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
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Light microscopes
• visible light passes
through specimen
– use lenses to magnify
& focus cellular
structures
– 0.2 m resolution
(~size of a bacterium)
– can be used to study
live cells
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Light microscopes
– different methods for enhancing
visualization of cellular structures
TECHNIQUE
RESULT
(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.]
50 µm
(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.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Light microscopes
(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.
with confocal
enhancement
without confocal
enhancement
50 µm
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Electron microscope (EM)
– 1950s
– 2.0 nm resolution
– 100 times > light microscope
– reveals organelles
– for the most part, can
only use on dead cells
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Electron microscope (EM)
• Uses a beam of electrons
– focused using electromagnets; generates
computer image
– beam passes:
• through a
specimen
(TEM)
• onto its
surface
(SEM)
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transmission electron microscope (TEM)
• allows detailed study of the internal
ultrastructure of cells
• electron beam
passes through
thin section of
specimen
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Longitudinal
section of
cilium
Cross section
of cilium
1 µm
scanning electron microscope (SEM)
• allows detailed study of the surface of a
specimen
• sample surface
covered with
thin film of gold
• beam excites
surface electrons
• great depth of
field = image that
seems 3-D
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1 µm
Cilia
SEM images
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Isolating Organelles
• Cell fractionation
– takes cells apart &
separates major
organelles
– separation based on
variable density of
organelles
• ultracentrifuge
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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
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)
ultracentifuge
• spins up to 130,000 rpm
– force > 1 million X gravity (1,000,000g)
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microcentrifuge
• used in biotechnology research
– study cells at protein & DNA level
Figure 6.5
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
General cell characteristics
• All cells:
– are surrounded by plasma membrane
– have cytosol
• semi-fluid substance within the membrane
• cytoplasm = cytosol + organelles
– contain chromosomes which have genes in
the form of DNA
– have ribosomes
• tiny “organelles” that make proteins using
instructions contained in genes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Types of cells
• Prokaryotic vs. eukaryotic cells
– location of chromosomes
Prokaryotic cell
• DNA in nucleoid
region, without a
membrane
separating it from
rest of cell
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Eukaryotic cell
• Chromosomes in
nucleus, membraneenclosed organelle
Prokaryotic cells
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
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
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(b) A thin section through the
bacterium Bacillus coagulans
(TEM)
Eukaryotic Cells
• Have extensive and elaborately arranged
internal membranes, which form organelles
• Generally larger than prokaryotic cells
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Eukaryotic cells – 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
Mitochondrion
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Lysosome
Not in animal cells:
Chloroplasts
Central vacuole & tonoplast
Cell wall
Plasmodesmata
Eukaryotic cells – plant cell
Nuclear envelope
Nucleolus
Chromatin
NUCLEUS
Rough
endoplasmic
reticulum Smooth
Centrosome
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Not in plant cells:
Lysosomes
Centrioles
Flagella (in some plant
sperm)
Limits to cell size
• The logistics of carrying out cellular
metabolism sets limits on the size of cells
• Lower size limit: need enough DNA,
enzymes, & “equipment” to maintain
homeostasis & reproduce
– Smallest bacterial cells are 0.1 – 1.0 m (most
bacteria are 1-10 m)
• Upper size limit: surface area to volume
ratio
– Largest eukaryotic cells are generally 1-100 m
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Size limits??
• Thiomargarita namibiensis ("Sulfur pearl of
Namibia")
– gram-negative coccus
Proteobacterium
– found in ocean
sediments
– largest bacterium ever
discovered
– up to 750 µm (0.75 mm);
easily visible to the
naked eye.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Limits to cell size
• As a cell gets bigger, its volume increases
faster than its surface area
Surface area increases while
total volume remains constant
– Smaller objects
have a greater
ratio of surface
area to volume
– facilitates the
exchange of
materials into &
out of the cell
5
1
1
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)
6:1
~1:1
6:1
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Limits to cell size
• A large cell cannot move material in & out
fast enough to support life
• How to get bigger?? Become multicellular!
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Cell Membrane
• Exchange organelle
– aka plasma membrane
Outside of cell
– Functions
as a selective
barrier
Carbohydrate side chain
– Allows
sufficient
passage
of O2,
nutrients
& waste
Hydrophilic
region
Inside of cell
0.1 µm
Hydrophobic
region
(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.
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Hydrophilic
region
Phospholipid
Proteins
(b) Structure of the plasma membrane
Nucleus – Genetic Library of the Cell
• about 5 m in diameter
• contains most of the genes in the
eukaryotic cell
• Where else are
genes found in
the cell?
mitochondria
&
chloroplasts
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Nucleus structure
Nucleus
Nucleus
1 µm
Nucleolus
Chromatin
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
Pore complexes (TEM).
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Nuclear lamina (TEM).
Nuclear envelope
• double membrane (separated by 20-40 nm
space)
• encloses the nucleus, separating its
contents from
the cytoplasm
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Nuclear envelope
• Perforated by pores (~100 nm diam.)
• Pores lined with pore complex (protein
structure) that regulates entry & exit of
large molecules & particles
• Examples?
RNA,
ribosomal subunits
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Nuclear contents
• Within nucleus, DNA organized into fibrous
material, chromatin
– Appears as diffuse mass in non-dividing
cell
• When cell prepares to divide, chromatin
fibers coil up as
distinct structures,
chromosomes
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Nuclear envelope
• Except at pores, nuclear side of envelope is
lined with nuclear lamina
– netlike array of protein filaments
(intermediate filaments; lamin polymers)
embedded in inner membrane
– maintains nuclear shape
– mutations in lamins &
lamin-binding proteins
cause a wide range of
human diseases, collectively
termed laminopathies including
muscular dystrophy & accelerated aging
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Nucleolus
• prominent in non-dividing cells
– number of nucleoli depends on species
• composed of densely staining granules &
fibers
• synthesis of ribosomal RNA
• assembly with imported
proteins to form ribosomal
subunits (exit through pores)
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Ribosomes
• Function
– protein synthesis
– cells that make many proteins have more
ribosomes (and nucleoli!)
• Ex. pancreas (digestive enzymes; insulin)
• Structure
– contain ribosomal RNA (rRNA) & protein
• rRNA (not protein) catalyzes peptide
bond formation in protein synthesis!
– composed of 2 subunits
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ribosomes
Ribosomes
Cytosol
Free ribosomes
Bound ribosomes
Large
subunit
0.5 µm
TEM showing ER and ribosomes
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Small
subunit
Diagram of a ribosome
Types of ribosomes
• Free ribosomes
– suspended in cytosol
– synthesize proteins that function within
cytosol
• Bound ribosomes
– attached to outside of endoplasmic
reticulum or nuclear envelope
– synthesize proteins for export or for
membranes
• Structurally identical! Can switch roles as
needed
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Eukaryotic vs. prokaryotic ribosomes
• Prokaryotes & eukaryotes have different
ribosomes
– different size
subunits
– different
proteins
– evolutionary
significance?
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Endomembrane system
• Includes many different structures:
– Endoplasmic reticulum, Golgi apparatus,
Lysosomes, Vacuoles, Vesicles
• Related either thru direct contact or by
transfer of vesicles
• Plays key role in synthesis
(& hydrolysis) of
macromolecules in cell
(biosynthesis)
• Various “players” modify
macromolecules for various functions
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Endomembrane system
• Relationships among organelles of the
endomembrane system
1
Nuclear envelope is
connected to rough
& smooth ER
Nucleus
Rough ER
Membranes
& proteins
2
Made by ER flow to Golgi
in transport vesicles
Smooth ER
cis Golgi
Golgi pinches off transport
Vesicles3& other vesicles
that give rise to lysosomes &
Vacuoles
trans Golgi
Lysosome can fuse with
Another vesicle for digestion
Transport vesicle carries
proteins to plasma
membrane for secretion
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Plasma
membrane
Plasma membrane expands
by fusion of vesicles; proteins
are secreted from cell
Endoplasmic Reticulum: Biosynthetic Factory
• Function
– Makes membranes
– Performs many biosynthesis functions
• Structure
– Membrane connected to nuclear envelope
& extends throughout cell
– Accounts for ~50% of total membrane in
many eukaryotic cells
• Rough ER = w/ bound ribosomes
• Smooth ER = w/o bound ribosomes
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Endoplasmic reticulum
• Consists of membranous tubules & sacs
called cisternae
Smooth ER
• ER separates
internal
compartment
from cytosol
(cisternal space)
Nuclear
envelope
Rough ER
ER lumen
Cisternae
Ribosomes
Transport vesicle
Smooth ER
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Transitional ER
Rough ER
200 µm
Endoplasmic reticulum
• Functions of smooth ER
– Synthesizes lipids
• Oils, phospholipids, steroids
– STF: ovaries & testes are rich in smooth
ER . . . Why??
– Metabolizes carbohydrates
• Hydrolysis of glycogen in liver cells
– 1st produces glucose phosphate which
can’t cross membrane; ER enzymes
remove PO4 so it can enter bloodstream
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Endoplasmic reticulum
• Functions of smooth ER (cont’d)
– Detoxifies drugs & poisons
• Usually by adding OH–; makes toxin
more soluble
– Cell responds to increased drug use by
making more smooth ER = tolerance
– Stores calcium ions for muscle contraction
• Pumps Ca2+ from cytosol to cisternal
space
– When Ca 2+ rushes back out, muscle
contracts
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Endoplasmic reticulum
• Functions of rough ER
– Membrane factory
• Grows in place by adding proteins &
phospholipids (assemble by enzymes in ER)
– as ER membrane expands, it buds off &
transfers to other cell parts that need it
• Membrane bound proteins synthesized
directly into membrane
– anchored by hydrophobic portions
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Endoplasmic reticulum
• Functions of rough ER (contd.)
– Produces proteins to be distributed by
transport vesicles
• mainly secretory proteins (for export)
– mostly glycoproteins (= proteins that are
covalently bonded to carbohydrates)
• carbohydrate (oligosaccharides) attached
to protein in ER membrane
• growing polypeptide chain threads into
cisternal space (through pore) where it folds
into native shape
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Golgi Apparatus : receiving, processing, &
shipping
• first reported by Camillo Golgi (1843-1926)
– at a meeting of the Medical Society of Pavia
on 19 April 1898
– he named it the 'internal
reticular apparatus'
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Golgi apparatus
cisGolgi
face
apparatus
(“receiving” side)
1 Vesicles move 2 Vesicles coalesce to
6 Vesicles also
0.1 0 µm
from ER to Golgi form new cis Golgi cisternae
transport certain
Cisternae
proteins back to ER
3
Cisternal
maturation:
move from cisto-trans
4
Vesicles bud & carry
proteins to other locations
or to plasma membrane
for secretion
5
Vesicles transport some
trans face
proteins backward to newer
(“shipping”)
Golgi cisternae
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TEM of Golgi apparatus
Golgi Apparatus
• Structure
– consists of flattened membranous sacs called
cisternae
• look like stack of pita bread
• usually 3-8 sacs per set
– can be as few as 1 in animal cells
– can be hundreds in some plant cells
• specialized secretory cells have more sets
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Golgi Apparatus
• Structure (contd.)
– Usually near
nucleus
– 2 sides =
2 functions
(polarity!)
• Cis = “receiving” = receives material by
fusing with vesicles from ER
• Trans = “shipping” = buds off vesicles
that travel to other sites (transport)
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Golgi Apparatus
• Functions
– Finishes, sorts, & ships cell products
– Extensive in cells specialized for secretion
– Which cells
have a lot
of Golgi?
Secretory cells
(pancreatic,
intestinal goblet,
salivary)
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Golgi apparatus
• Functions (contd.)
– modification of rough ER products
• done by changing sugar monomers
• allows wide variety of oligosaccharides
– manufacture of certain macromolecules
• pectins & non-cellulose polysaccharides
– sorting macromolecules & targeting for
proper destination
• done by adding molecular ID tags (ex.
phosphate groups)
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Lysosomes: Digestive Compartments
• membranous sacs of hydrolytic enzymes
– can digest all kinds of macromolecules
• examples?
Polysaccharides
Polypeptides/proteins
Lipids/fatty acids
Nucleic acids
– enzymes & membrane of lysosomes are
synthesized by rough ER & transferred to
Golgi
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lysosomes
• fuse with vacuoles containing ingested food of
cell parts to be recycled
• polymers are digested into monomers
– pass to cytosol to become nutrients of cell
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Lysomes
• Help carry out intracellular digestion of
engulfed particles. What is this process
called?
1 µm
Nucleus
Phagocytosis
• what kinds of cells
do this?
Lysosome
Protists (ameba)
Macrophages (WBCs)
Lysosome contains
active hydrolytic
enzymes
Food vacuole
fuses with
lysosome
Hydrolytic
enzymes digest
food particles
Digestive
enzymes
Lysosome
Plasma membrane
Digestion
Food vacuole
(a) Phagocytosis: lysosome digesting food
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lysosomes
• Also use hydrolytic enzymes to recycle cell’s
own organic material . . . called?
Lysosome containing
two damaged organelles
1µm
Autophagy
A human liver cell
recycles half of its
macromolecules
each week!
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
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Lysosomal enzymes
• Work best at pH 5 (cytosol pH ~7.3)
– organelle creates custom pH
• How?
Proteins in lysosomal membrane pump
H+ ions from cytosol into lysosome
• Why?
• Enzymes are proteins
• Proteins are sensitive to pH – affects structure
• Digestive enzymes won’t work well if they leak
into cytosol – helps prevent autodigestion
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When things go wrong . . .
• If lysosomal enzymes don’t function,
– biomolecules collect & accumulate in
lysosomes
• Lysosomes grow larger & larger,
eventually disrupting cell & organ
function
• “Lysosomal storage diseases” are
usually fatal
• Ex. Tay-Sachs disease
– Lipids build up in
brain cells
– Child dies before age 5
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Sometimes, it’s supposed to work that way . .
• Apoptosis = programmed cell death
– Auto-destruct mechanism
– Some cells must die in an organized
fashion, especially during development
• Ex. Space between your fingers
• Ex. Self-destruction of damaged cells
before they can become cancerous
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Vacuoles
• Diverse maintenance compartments
– Food vacuoles
• formed by phagocytosis
– Contractile vacuoles
• Pump excess water out of protist cells
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vacuoles
• Central vacuoles (in plant cells)
– Functions:
• Storage, waste
disposal, protection,
growth
– hold reserves of
important organic
compounds & water
– increase surface area
(internally)
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Central vacuole
Cytosol
Tonoplast
Nucleus
Central
vacuole
Cell wall
Chloroplast
5 µm
Peroxisomes: Oxidation
• Peroxisomes
– Do not bud from endomembrane system
• Membrane grows in place using proteins
& lipids in cytosol
– Produce
hydrogen
peroxide &
convert it to
water
– (2H2O2  2H2O + O2)
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Chloroplast
Peroxisome
Mitochondrion
1 µm
Mitochondria & chloroplasts
• Energy conversion!
– not “creation” of energy; 1st law of
thermodynamics!
• Mitochondria
– sites of cellular respiration
• Chloroplasts
– sites of photosynthesis
– found only in plants (& algae)
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Mitochondria: Chemical Energy Conversion
• found in nearly all eukaryotic cells
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mitochondria
• enclosed by two membranes
• smooth outer membrane
• inner membrane folded into cristae
– STF: what is the value of christae?
• interior filled with matrix
– contains enzymes, mitochondrial DNA,
ribosomes, proteins
– Example of compartmentalization!
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mitochondria
Mitochondrion
Intermembrane space
Outer
membrane
Free
ribosomes
in the
mitochondrial
matrix
Inner
membrane
Cristae
Matrix
Mitochondrial
DNA
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100 µm
Chloroplast: Capture of Light Energy
• specialized member of a family of closely
related plant organelles called plastids
• contains chlorophyll
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chloroplasts
• found in leaves and other green organs of
plants and in algae
Chloroplast
Ribosomes
Stroma
Chloroplast
DNA
Inner and outer
membranes
Granum
1 µm
Thylakoid
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chloroplast
• structure includes
– Thylakoids, membranous disc-shaped sacs
• Stacked to form granum (pl. = grana)
– Stroma, the internal fluid surrounding
thylakoids
• Contains chloroplast DNA, ribosomes,
proteins, enzymes
• Example of compartmentalization!
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cytoskeleton
• network of fibers extending throughout the
cytoplasm
Microtubule
0.25 µm
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Microfilaments
Cytoskeleton
• functions
– mechanical support
– motility
– regulation
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cytoskeleton
– motor proteins interact with cytoskeleton to
provide motility
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
(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.)
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Components of the Cytoskeleton
• three main types of fibers make up the
cytoskeleton
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Microtubules
• Microtubules
– Hollow; resist compression
– Shape the cell
– Guide movement of organelles
– Help separate the chromosome copies in
dividing cells
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Centrosomes & Centrioles
• centrosome
– “microtubule-organizing center”
Centrosome
– in animal cells,
contains a
pair of
centrioles
Microtubule
Centrioles
0.25 µm
Longitudinal section
of one centriole
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Microtubules Cross section
of the other centriole
Cilia and Flagella
• Cilia and flagella
– Contain specialized arrangements of
microtubules
– Are locomotor appendages of some cells
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• 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
1 µm
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• 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).
15 µm
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Cilia & flagella
• share common ultrastructure
9+2
doublets
Outer microtubule
doublet
Dynein arms
0.1 µm
Plasma
membrane
Central
microtubule
Outer doublets
cross-linking
proteins inside
Microtubules
Radial
spoke
Plasma
membrane
Basal body
(b)
0.5 µm
0.1 µm
(a)
Triplet
9 triplets
(c)
Cross section of basal body
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• 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.
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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.)
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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.
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Microfilaments (actin)
• Solid; tension-bearing
• Network gives cortex (outer cytoplasmic
layer) gel consistency
– vs. sol consistency of cytosol
• Alternate with myosin to contract
– muscle contraction
– contracting band forms cleavage furrow in
cytokinesis
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microfilaments
– found in microvilli
Microvillus
Plasma membrane
Microfilaments (actin
filaments)
Intermediate filaments
0.25 µm
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microfilaments
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microfilaments
• Those that function in cellular motility
– contain the protein myosin in addition to
actin
Muscle cell
Actin filament
Myosin filament
Myosin arm
(a) Myosin motors in muscle cell contraction.
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• Amoeboid movement
– Involves the contraction of actin and
myosin filaments
Cortex (outer cytoplasm):
gel with actin network
Inner cytoplasm: sol
with actin subunits
Extending
pseudopodium
(b) Amoeboid movement
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• Cytoplasmic streaming
– another form of locomotion created by
microfilaments
Nonmoving
cytoplasm (gel)
Chloroplast
Streaming
cytoplasm
(sol)
Parallel actin
filaments
(b) Cytoplasmic streaming in plant cells
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Cell wall
Intermediate fibers
• Solid; intermediate diameter
• Tension-bearing
• Diverse class of elements
– in keratin family of proteins
• More permanent
• Maintain overall shape & anchor some
organelles
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Cell Walls of Plants
– extracellular structure of plant cells that
distinguishes them from animal cells
– made of cellulose fibers embedded in other
polysaccharides and protein
– may have multiple layers
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Plant cell walls
Plasma
membrane
Central
vacuole
of cell
Secondary
cell wall
Primary
cell wall
Central
vacuole
of cell
Middle
lamella
1 µm
Central vacuole
Cytosol
Plasma membrane
Plant cell walls
Plasmodesmata
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Extracellular matrix (ECM)
• Main ingredients are glycoproteins (collagen,
proteoglycans, & fibronectins)
– Fibronectins bind to integrins (receptor
proteins)
EXTRACELLULAR FLUID
Collagen
A proteoglycan
complex
Polysaccharide
molecule
Carbohydrates
Core
protein
Fibronectin
Plasma
membrane
Integrin
Integrins
Microfilaments
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CYTOPLASM
Proteoglycan
molecule
Exctracellular matrix
• Functions of the ECM include
– Support
– Adhesion
– Movement
Collagen fibers
– Regulation
• New research
suggests that
changes in ECM
can alter internal
cell structure & function
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Collagen fibrils
Intercellular Junctions
• Integrate cells into higher levels of
structure & function
– Organize animal & plant cells into tissues,
organs, and organ systems
• Special patches of direct physical contact
– Allow neighboring cells adhere, interact, &
communicate
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Intercellular Junctions in plants
• plasmodesmata
– channels that perforate cell walls
– plasma membrane lines channels
– cytosol flows between cells
Cell walls
– unifies most of plant into one living continuum
Interior
of cell
Interior
of cell
0.5 µm
Plasmodesmata
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Plasma membranes
Intercellular junctions in animals
• integrate cells in various ways
• three types:
– Tight junctions
– Desmosomes
– Gap junctions
• structure fits function!
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Intercellular junctions in animals
TIGHT JUNCTIONS
Tight junctions prevent
fluid from moving
across a layer of cells
Tight junction
•Neighboring cells fused
•Form continuous belts
•Prevent leakage
0.5 µm
DESMOSOMES
•Anchoring junctions
•Rivets fasten cells into sheets
•Reinforced w/ intermediate fibers
Tight junctions
Intermediate
filaments
Desmosome
Gap
junctions
Extracellular
matrix
Plasma membranesGap junction
of adjacent cells
Space
between cells
1 µm
GAP JUNCTIONS
•Communication junctions
•Provide cytoplasmic channels
•Common in animal embryos
0.1 µm
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Cell functions are emergent properties
• the cell is a living unit greater than the sum of
its parts
5 µm
• How do cell parts interact in this function?
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Any questions??
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