A Tour of the Cell Chapter 6

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A Tour of the Cell (Chapter 6)
Figure 6.1
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Theodore Schwanns cell theory (1839)

• 1) All organisms are composed of one or more
  cells
• 2) Cells are the basic units of organization of
  all organisms
• 3) Cells arise only by division of a previously
  existing cell

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How do we study cells?

• Few cells are big enough to be viewed with the
  naked eye. Therefore, we use
• Cytology (microscopy) study of cell structure
• - magnification ratio of objects image size
  to its real size
• - resolution measure of clarity of the image,
  minimum distance two points can be separated and
  still be distinguished as two points
• - limits the effective study

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Microscopy

• Light microscopes (LMs)
• First used during the Renaissance (1600s)
• Pass visible light through a specimen
• Magnify cellular structures with lenses

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Smaller subcellular structures require higher
powered microscopes
1 micrometer 1 micron
0.000001 or 1/100,000 meter
1 nanometer
0.000000001 or 1/100,000,000 meter
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Microscopy

• Different methods for enhancing visualization of
  cellular structures

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Research Methods
Differential-interference-contrast (Nomarski).
Like phase-contrast microscopy, it uses optical
modifications to exaggerate differences
in density, making the image appear almost 3D.
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• Electron microscopes (EMs)
• Focus a beam of electrons through a specimen
  (TEM) or onto its surface (SEM)

• The scanning electron microscope (SEM)
• Provides for detailed study of the surface of a
  specimen

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TEM

• The transmission electron microscope (TEM)
• Provides for detailed study of the internal
  ultrastructure of cells

Longitudinal section of cilium
Cross section of cilium
1 µm
(b)
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.
Figure 6.4 (b)
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Electron micrographs of rabbit trachea cells
Transmission EM Scanning EM
Figure 6.4
- Beam through object
- beam on surface
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How do we study cells?

• 1) Microscopy
• 2) Cell fractionation
• - allows for determination of the function of
  subcellular structures
• Take cells apart and separate the major
  organelles from each other
• Done in a centrifuge spin tubes at various
  speeds
• Isolates cell components, based on size and
  density
• Ultracentrifuges most powerful, can spin as
  fast as 130K/minute applies forces on particles
  of more than 1 million times the force of gravity

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Cell Fractionation
Cells are homogenized in a blender to break them
up. The resulting mixture (cell homogenate) is
then centrifuged at various speeds and durations
to fractionate the cell components
Increasing speed (force)
Results 1)Researchers used microscopy to
identify the organelles in each pellet 2)
Researchers used biochemical methods to determine
the metabolic functions of organelles.
Big small Objects in the pellet
Figure 6.5
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Cells

• Two types of cells
• Prokaryotic
• Eukaryotic
• All cells have several basic features in common
• Bounded by a plasma membrane
• Contain a semifluid substance - cytosol
• Contain chromosomes
• Have ribosomes

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Two types of cells

• Prokaryotic cells basic, No interior membranes

• Circular DNA molecule (chromosome), not contained
  within a nucleus
• Have their DNA located in a region called the
  nucleoid
• Small in size ? 1-10 micrometers

2) Eukaryotic cells - Interior membranes

• Linear DNA molecules (chromosomes) Contained
  within a true nucleus, bounded by a membranous
  nuclear envelope
• 10-100 micrometers
• Have extensive and elaborately arranged internal
  membranes, which form organelles

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Prokaryotic cells
Pili attachment structures on the surface of
some prokaryotes
Nucleoid region where the cells DNA is located
(not enclosed by a membrane)
Ribosomes organelles that synthesize proteins
Plasma membrane membrane enclosing the cytoplasm
Cell wall rigid structure outside the plasma
membrane
Capsule jelly-like outer coating of many
prokaryotes
Bacterialchromosome
0.5 µm
Flagella locomotion organelles of some bacteria
(b) A thin section through the bacterium
Bacillus coagulans (TEM)
(a) A typical rod-shaped bacterium
Figure 6.6
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Animal cell (eukaryotic)
Nuclear envelope
ENDOPLASMIC RETICULUM (ER)
NUCLEUS
Nucleolus
Smooth ER
Rough ER
Chromatin
Flagelium
Plasma membrane
Centrosome
CYTOSKELETON
Microfilaments
Intermediate filaments
Microtubules
Ribosomes
Microvilli
Golgi apparatus
Peroxisome
In animal cells but not plant cells Lysosomes Cen
trioles Flagella (in some plant sperm)
Lysosome
Mitochondrion
Figure 6.9
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Plant Cell (eukaryotic)
Nuclear envelope
Rough endoplasmic reticulum
Nucleolus
NUCLEUS
Chromatin
Smooth endoplasmic reticulum
Centrosome
Ribosomes (small brown dots)
Central vacuole
Tonoplast
Golgi apparatus
Microfilaments
Intermediate filaments
CYTOSKELETON
Microtubules
In plant cells (but not in Animal
cells) Chloroplasts Central vacuole and
tonoplast Cell wall Plasmodesmata
Mitochondrion
Peroxisome
Plasma membrane
Chloroplast
Cell wall
Plasmodesmata
Figure 6.9
Wall of adjacent cell
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Prokaryotes vs. Eukaryotes
Prokaryotes
Eukaryotes
Size 1-10 mm 10-100 mm Genetic circular
linear Material chromosome
chromosomes Plasma yes
yes Membrane Cell Wall yes some-plants,
etc. Ribosomes yes
yes Membrane-bound NO YES organelles
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Why are cells so small?

• 1) Diffusion
• main way of intracellular communication
• diffusion is very slow!
• 2) Surface area-to-volume ratio
• A smaller cell - has a higher surface to volume
  ratio, which facilitates the exchange of
  materials into and out of the cell
• volume increases more rapidly than surface area
• Cell must receive nutrients and expel wastes
  across the surface area of the cell
• Large volume greater nutrient need and more
  waste production
• consider a cube

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Surface area and volume
Surface area increases while total volume remains
constant
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)
125
125
1
Surface-to-volume ratio (surface area ? volume)
6
12
6
Area is proportional to the surface area
squared Volume is proportional to the surface
area cubed
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Plasma Membrane

• functions as a selective barrier
• Allows sufficient passage of nutrients and waste

Outside of cell
Carbohydrate side chain
Hydrophilic region
Inside of cell
0.1 µm
Hydrophobic region
Hydrophilic region
Phospholipid
Proteins
(b) Structure of the plasma membrane
Figure 6.8 A, B
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Nucleus

• Contains the eukaryotic cells genetic
  instructions
• Chromosomes, structures that carry genetic
  information
• Made of chromatin (a complex of protein and DNA)
• Each typical human cells has 46 chromosomes, 23
  pairs
• Fruit flies 8 chromosomes
• Dog 78
• Cat - 38
• Enclosed by the nuclear envelope (double
  membrane), separating its contents from the
  cytoplasm

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Nucleus-genetic information center
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
Nuclear lamina (TEM).
Pore complexes (TEM).
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Nuclear envelope

• Nucleus is surrounded by a double membrane
• Transport to-and-from cytoplasm via nuclear pores

Inner membrane lined with nuclear lamina
Nuclear Pore Complex
nucleus
cytoplasm
ER
Outer membrane
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Nucleolus

• Densely stained portion of the nucleus
• Site of ribosomal RNA (rRNA) synthesis
• Site of beginning of ribosomal subunit assembly

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Ribosomes - protein-producing machinery
Ribosomes
Cytosol
Endoplasmic reticulum (ER)
Free ribosomes
Bound ribosomes
Large subunit
Bound Ribosomes - make proteins for insertion
into membranes, packaging within certain
organelles, or export from the cell Free
Ribosomes produce proteins which function in
cytosol
Small subunit
0.5 µm
TEM showing ER and ribosomes
Diagram of a ribosome
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Ribosomes?site of protein synthesis
Large subunit
-composed of protein and ribosomal RNA (rRNA)
Small subunit
- assembled in the nucleolus - active in the
cytoplasm - prokaryotes have a smaller version
(70S) than eukaryotes (80S) - actively growing
or secreting eukaryotic cell may have millions of
ribosomes
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Endomembrane system

• Extensive network of membranes
• Accounts for more than half of the total membrane
  in many eukaryotic cells
• Regulates protein synthesis and traffic
• Performs metabolic functions in the cell
• Metabolism and movement of lipids
• The endomembrane system
• Includes many different structures nuclear
  envelope, endoplasmic reticulum, Golgi apparatus,
  lysosomes, vacuoles

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• The endoplasmic reticulum (ER) membrane
• Is continuous with the nuclear envelope

Smooth ER
Nuclear envelope
Rough ER
ER lumen
Cisternae
Ribosomes
Transitional ER
Transport vesicle
200 µm
Smooth ER
Rough ER
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Endoplasmic reticulum (ER)Accounts for more than
half the total membrane in many eukaryotic cells
1) Smooth ER - no/few ribosomes - lipid
synthesis (example -testes and ovaries) -
carbohydrate metabolism - drug detoxification
(liver cells) - calcium ion storage 2) Rough
ER - ribosomes attached - continuous with
nuclear membrane - Produces proteins and
membranes, which are distributed by transport
vesicles (pancreatic cells) - protein
glycosylation (sugars added)
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Golgi apparatus

• Consists of flattened membranous sacs called
  cisternae
• Polarity opposite sides of the stack differ in
  thickness and molecular composition, cis face
  (receiving) and trans face (shipping)

• Functions
• Modification of the products of the rough ER
• Manufacture of certain macromolecules
• Receives many of the transport vesicles produced
  in the rough ER
• Products of ER are modified (glycosylation),
  stored, sent out
• Vesicles pinch off and carry cargo to appropriate
  locations in the cell

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Golgi Apparatus functions
cis face (receiving side of Golgi apparatus)
Vesicles move from ER to Golgi
Vesicles coalesce to form new cis Golgi
cisternae
0.1 0 µm
Vesicles also transport certain proteins
back to ER
Cisternae
Cisternal maturation Golgi cisternae move
in a cis- to-trans direction
Vesicles form and leave Golgi,
carrying specific proteins to other locations or
to the plasma mem- brane for secretion
Vesicles transport specific proteins backward to
newer Golgi cisternae
trans face (shipping side of Golgi apparatus)
Figure 6.13
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Lysosome acidic digestive compartments
1 µm
Nucleus

• Is a membranous sac of hydrolytic enzymes (acidic
  environments) from ER to Golgi to lysosome
• Lysosomes carry out intracellular digestion by
• Phagocytosis

Lysosome
Food vacuole fuses with lysosome
Hydrolytic enzymes digest food particles
Lysosome contains active hydrolytic enzymes
Digestive enzymes
Lysosome
Plasma membrane
Digestion
Food vacuole
(a) Phagocytosis lysosome digesting food
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Lysosome-acidic digestive compartment

• Autophagy
• Process of lysosomes which use their hydrolytic
  enzymes to recycle the cells own organic
  material
• Damaged organelle or small amount of cytosol
• Cell renews itself
• Tay-Sachs disease
• Lipid accumulation

Figure 6.14 B
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Vacuoles Diverse Maintenance Compartments

• A plant or fungal cell
• May have one or several vacuoles
• Food vacuoles
• Are formed by phagocytosis
• Contractile vacuoles
• Pump excess water out of
• protist cells
• Central vacuoles
• Are found in plant cells
• Hold reserves of important
• organic compounds and water
• Surrounded by tonoplast

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The endomembrane system

• Complex and dynamic
• Relationships among organelles of the
  endomembrane system

1
Nuclear envelope is 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
3
Golgi pinches off transport Vesicles and other
vesicles that give rise to lysosomes and
Vacuoles
Plasma membrane
trans Golgi
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5
Transport vesicle carries proteins to plasma
membrane for secretion
Lysosome available for fusion with
another vesicle for digestion
6
Plasma membrane expands by fusion of vesicles
proteins are secreted from cell
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Cell Power!

• Mitochondria and chloroplasts change energy from
  one form to another
• Enclosed by membranes
• not part of the endomembrane system
• Mitochondria
• cellular respiration
• Chloroplasts
• photosynthesis

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Mitochondria Chemical Energy Conversion

• Mitochondria are enclosed by two membranes
• Smooth outer membrane
• Inner membrane folded into cristae
• Matrix contains enzymes,
• mitochondrial
• DNA, and
• ribosomes
• divide and
• partition
• into daughter
• cells

Figure 6.17
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Chloroplasts Capture of Light Energy

• Chloroplast - Contains chlorophyll
• Specialized member of a family of plant
  organelles called plastids
• Found in leaves/other green organs of plants and
  algae

Chloroplast
Stroma the internal fluid
Ribosomes
Chloroplast DNA
Inner and outer membranes
Granum
1 µm
Thylakoid - membranous sacs
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Peroxisomes

• Removal of electrons and hydrogen - oxidation
• Produce hydrogen peroxide as a byproduct, by
  transferring hydrogen from substrates to oxygen
• Hydrogen peroxide is broken down to water
• Break down fatty acids, detoxify alcohol/toxins
• highly reactive and very destructive to cell
• Contain catalase
• 2 H2O2 2 H2O O2

catalase
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Peroxisomes Oxidation
Often have a granular or crystal core - thought
to be a dense collection of enzymes molecules
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Cytoskeleton Support, Motility, and Regulation

• Network of fibers - organizes structures and
  activities in the cell
• Gives mechanical support to the cell

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Cytoskeleton Support, Motility, and Regulation

• Involved in cell motility, which utilizes motor
  proteins
• Railroad tracks
• for the cell,
• Trafficking of
• vesicles and
• organelles

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Cytoskeleton
Three main types of fibers make up the
cytoskeleton
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Cytoskeleton
Table 6.1
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Microtubules

• Microtubules
• Shape the cell
• Guide movement of organelles
• Help separate the chromosome copies in dividing
  cells
• The centrosome
• Is considered to be a microtubule-organizing
  center

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Centrosomes and Centrioles
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Cilia and Flagella

• Specialized arrangements of microtubules
• Are locomotor appendages of some cells

Direction of organisms movement
Direction of Active stroke
Direction of Recovery stroke
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Cilia and flagella share a common ultrastructure
Cilia shorter than flagella - more
abundant/cell than flagella

• Cilia shorter than flagella
• more abundant/
• cell than flagella

Outer microtubule doublet
Plasma membrane
0.1 µm
Dynein arms
Central microtubule
Outer doublets cross-linking proteins inside
Microtubules
Radial spoke
Plasma membrane
(b) Cross-section through the Cilium shows 92
arrangement Of microtubules
Basal body
0.5 µm
0.1 µm
(a) A longitudinal section of cilium shows
microtubules running the length of the structure
Triplet
Figure 6.24 A-C
(c) Basel Body (anchor) nine outer doublets of
a cilium or flagellum exend into the basel body
where each doublet joins another microtubule to
form a ring of nine triplets
Cross section of basal body
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Microfilaments (Actin Filaments)

• Found in microvilli and muscle cells
• For cellular motility - Contain the protein
  myosin in addition to actin

Microvillus
Plasma membrane
Microfilaments (actin filaments)
Intermediate filaments
0.25 µm
Figure 6.26
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Microfilaments

• Cytoplasmic streaming
• Is another form of locomotion created by
  microfilaments

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Special plant structures

• Cell Wall
• Protects the cell
• Provides rigidity to the cell
• Made of cellulose (polysaccharide)
• Different from bacterial cell wall
• Vacuole
• Storage area for the cell
• Water, sugars, ions

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Plant cell walls
Cellulose fibers embedded in other
polysaccharides and protein a) Rigid structure-
support b) Prevents excessive H2O uptake c)
Acts against gravity d) Protects the cell
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The Extracellular Matrix (ECM) of Animal Cells

• Animal cells - Covered by a matrix, the ECM
• Is made up of glycoproteins and other
  macromolecules
• Functions of the ECM support, adhesion,
  movement, regulation

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Intercellular Junctions - Plants

• Plasmodesmata
• Are channels that perforate plant cell walls

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Intercellular junctions (animals)
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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

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Prokaryotes vs. Eukaryotes
Prokaryotes
Eukaryotes
Size 1-10 mm 10-100 mm Genetic circular
linear Material chromosome
chromosomes Plasma yes
yes Membrane Cell Wall yes some-plants,
etc. Ribosomes yes
yes Membrane-bound NO YES organelles