Chapter 6
Cells: The Working Units of Life
PART II
AP Biology
Cells: The Working Units of Life
•
Key Concepts
•
Cells Provide Compartments for Biochemical
Reactions
•
Prokaryotic Cells Do Not Have a Nucleus
•
Eukaryotic Cells Have a Nucleus and Other
Membrane-Bound Compartments
•
The Cytoskeleton Provides Strength and Movement
•
Extracellular Structures Provide Support and
Protection For Cells and Tissues
Vacuoles: Diverse Maintenance
Compartments
• Vacuoles (mainly in plants and fungi) have several
functions:
• Storage of waste products and toxic compounds; some may deter
herbivores.
• Structure for plant cells —water enters the vacuole by osmosis,
creating turgor pressure.
• Food vacuoles: formed by phagocytosis
• Contractile vacuoles: pump excess water out of cells; found
in many freshwater protists
• Central vacuoles: hold organic compounds and water; found
in many mature plant cells
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Fig. 6-15
Central vacuole
Cytosol
Nucleus
Central
vacuole
Cell wall
Chloroplast
5 µm
Concept 6.5: Mitochondria and chloroplasts
change energy from one form to another
• Mitochondria: sites of cellular respiration, a metabolic
process that generates ATP
• Chloroplasts: found in plants and algae; site of
photosynthesis
• Mitochondria and chloroplasts
o
o
o
o
NOT part of the endomembrane system
Have a double membrane
Have proteins made by free ribosomes
Contain their own DNA
• Peroxisomes: oxidative organelles
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Mitochondria: Chemical Energy Conversion
• Mitochondria have two membranes:
• Outer membrane—very porous
• Inner membrane—extensive folds called
cristae increase surface area
• The inner membrane creates two compartments:
• intermembrane space
• mitochondrial matrix
• The fluid-filled matrix contains enzymes, DNA, and
ribosomes.
Mitochondria: Chemical Energy Conversion
Chloroplasts: Capture of Light Energy
• Chloroplast member of family of organelles called plastids
• Chloroplast: contains chlorophyll; site of
photosynthesis
o Photosynthesis converts light energy into chemical energy
(anabolic process).
o Chloroplasts found in leaves and other green organs of plants & algae
Chloroplasts: Capture of Light Energy
• Chloroplasts have two membranes, plus internal
membranes called thylakoids.
o Granum —a stack of thylakoids; light energy is converted to chemical
energy on these membranes.
o Stroma —aqueous matrix (fluid) around grana; contains ribosomes and
DNA; carbohydrates are synthesized here.
Chloroplasts: Capture of Light Energy
• Other plastids-
• Chromoplasts make and store red, yellow,
and orange pigments, especially in flowers
and fruits.
Peroxisomes: Oxidation
Other organelles perform specialized functions…
•Peroxisomes: collect and break down toxic byproducts of metabolism, such as hydrogen
peroxide (H2O2), using specialized enzymes.
Concept 6.6: The cytoskeleton is a network of fibers
that organizes structures and activities in the cell
• The cytoskeleton:
• Supports and maintains cell shape
• Holds organelles in position
• Moves organelles
• Involved in cytoplasmic streaming
• Interacts with extracellular structures
to anchor cell in place
Concept 6.6: The cytoskeleton is a network of fibers
that organizes structures and activities in the cell
• It is composed of three types of molecular structures:
o Microfilaments
o Intermediate filaments
o Microtubules
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Components of the Cytoskeleton
• Microfilaments: motility
o a.k.a: actin filaments
o thinnest components
• Help a cell or parts of a cell to move
• Determine cell shape
Cytoskeleton:
Microfilaments
Dynamic Instability: the
filaments can shorten
(more detachment) or
lengthen (more
assembly).
This allows for quick
assembly or
breakdown of the
cytoskeleton.
Cytoskeleton:
Intermediate Filaments
• Intermediate filaments:
• At least 50 different kinds in six
molecular classes
• Tough, ropelike protein assemblages;
more permanent than other filaments
and do not show dynamic instability
• Anchor cell structures in place
• Resist tension, maintain rigidity
Cytoskeleton: Intermediate Filaments
Cytoskeleton: Microtubules
• Microtubules:
• Thickest cytoskeleton elements.
• Form a rigid internal skeleton for some
cells or regions
• Act as a framework for motor proteins
to move structures in the cell
Cytoskeleton: Microtubules
Microtubules are
made from dimers
of the protein
tubulin—chains of
dimers surround a
hollow core
Table 6-1
10 µm
10 µm
10 µm
Column of tubulin dimers
Keratin proteins
Actin subunit
Fibrous subunit (keratins
coiled together)
25 nm
7 nm


Tubulin dimer
8–12 nm
Table 6-1a
10 µm
Column of tubulin dimers
25 nm


Tubulin dimer
Table 6-1b
10 µm
Actin subunit
7 nm
Table 6-1c
5 µm
Keratin proteins
Fibrous subunit (keratins
coiled together)
8–12 nm
Microtubules
• Microtubules: hollow rods
• Functions of microtubules:
o Shaping the cell
o Guiding movement of organelles
o Separating chromosomes during cell division
Centrosomes and Centrioles
• Centrosome: point
near nucleus from
which microtubules
grow
o “microtubule-organizing center”
• Animal cells: the centrosome
has a pair of centrioles, each
with nine triplets of microtubules arranged in a ring
Cytoskeleton Provides
Strength & Movement
• Microtubules form an internal skeleton for
moveable cellular appendages:
• Cilia —short, usually many present; move stiffly
to propel a cell, or move fluid over a stationary
cell
• Flagella —longer, usually one or two present;
push or pull cell through water
Cytoskeleton Provides
Strength & Movement
• Cilia and flagella microtubules are
arranged in a “9 + 2” pattern:
• Doublets —nine fused pairs of
microtubules form a cylinder
• One unfused pair in center
• Motion occurs as doublets slide past
each other.
A Motor Protein Moves
Microtubules in Cilia and Flagella
dynein causes microtubule doublets
to slide past one another
Cytoskeleton Provides
Strength & Movement
• Dynein —a motor protein that can
change shape and drives the sliding of
doublets
• Nexin —protein that crosslinks doublets
and prevents sliding, so cilia bend
• Kinesin —motor protein that binds to
vesicles in the cell and “walks” them
along the microtubule
Dynein
• How dynein “sliding” moves
flagella and cilia:
− Dynein arms alternately
grab, move, and release
the outer microtubules
o Protein nexin cross-links
limit sliding
o Forces exerted by dynein
arms cause doublets to
curve, bending the cilium
or flagellum
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 6-25a
Microtubule
doublets
ATP
Dynein
protein
(a) Effect of unrestrained dynein movement
Fig. 6-25b
ATP
Cross-linking proteins
inside outer doublets
Anchorage
in cell
(b) Effect of cross-linking proteins
1
3
2
(c) Wavelike motion
Cell Wall
• Plant cell wall: semi-rigid structure
outside the cell membrane
o Composed of the fibrous component,
polysaccharide cellulose.
o The gel-like matrix contains cross-linked
polysaccharides and proteins.
The Plant Cell Wall
The Plant Cell Wall
• The plant cell wall has three major roles:
• Provides support for the cell and limits
its volume by remaining rigid
• Acts as a barrier to infection
• Contributes to form during growth and
development
The Plant Cell Wall
• Adjacent plant cells are connected by cell
membrane-lined channels called
plasmodesmata.
• These channels allow movement of water, ions,
small molecules, hormones, and some RNA
and proteins.
The Extracellular Matrix (ECM) of Animal
Cells
• Extracellular matrices in animal cells:
• Hold cells together in tissues
• Contribute to physical properties of cartilage, skin, bone, and other tissues
• Help filter materials (e.g., in kidneys)
• Orient cell movement during development and tissue repair
• Made up of glycoproteins
o collagen,
o proteoglycans
o Fibronectin
Intercellular Junctions
• Neighboring cells in tissues/ organs/ organ systems
adhere/ interact/ communicate through direct
physical contact
• Intercellular junctions facilitate this contact
• Types
o
o
o
o
Plasmodesmata
Tight junctions
Desmosomes
Gap junctions
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Tight Junctions, Desmosomes, and Gap Junctions
in Animal Cells
• Tight junctions:
membranes of neighboring
cells pressed together,
preventing leakage of
extracellular fluid
• Desmosomes: anchoring
junctions; fasten cells
together into strong sheets
• Gap junctions:
communicating junctions;
provide cytoplasmic
channels between adjacent
cells
Tight Junctions, Desmosomes, and Gap Junctions in Animal Cells
Junctions Link Animal Cells
Junctions Link Animal Cells
Junctions Link Animal Cells
Fig. 6-32b
Tight junction
0.5 µm
Fig. 6-32c
Desmosome
1 µm
Fig. 6-32d
Gap junction
0.1 µm
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
• Example: Macrophage’s ability to destroy bacteria involves whole
cell, coordinating components such as the cytoskeleton,
lysosomes, and plasma membrane
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 6-UN1
Cell Component
Concept 6.3
The eukaryotic cell’s
genetic
instructions are housed in
the nucleus and carried out
by the ribosomes
Structure
Surrounded by nuclear
envelope (double membrane)
perforated by nuclear pores.
The nuclear envelope is
continuous with the
endoplasmic reticulum (ER).
Nucleus
Function
Houses chromosomes, made of
chromatin (DNA, the genetic
material, and proteins); contains
nucleoli, where ribosomal
subunits are made. Pores
regulate entry and exit of
materials.
(ER)
Two subunits made of riboProtein synthesis
somal RNA and proteins; can be
free in cytosol or bound to ER
Ribosome
Concept 6.4
The endomembrane system
regulates protein traffic and
performs metabolic functions
in the cell
Concept 6.5
Mitochondria and chloroplasts change energy from
one form to another
Extensive network of
membrane-bound tubules and
sacs; membrane separates
lumen from cytosol;
continuous with
the nuclear envelope.
Smooth ER: synthesis of
lipids, metabolism of carbohydrates, Ca2+ storage, detoxification of drugs and poisons
Golgi apparatus
Stacks of flattened
membranous
sacs; has polarity
(cis and trans
faces)
Modification of proteins, carbohydrates on proteins, and phospholipids; synthesis of many
polysaccharides; sorting of Golgi
products, which are then
released in vesicles.
Lysosome
Membranous sac of hydrolytic
enzymes (in animal cells)
Vacuole
Large membrane-bounded
vesicle in plants
Digestion, storage, waste
disposal, water balance, cell
growth, and protection
Mitochondrion
Bounded by double
membrane;
inner membrane has
infoldings (cristae)
Cellular respiration
Endoplasmic reticulum
(Nuclear
envelope)
Chloroplast
Peroxisome
Rough ER: Aids in synthesis of
secretory and other proteins from
bound ribosomes; adds
carbohydrates to glycoproteins;
produces new membrane
Breakdown of ingested substances,
cell macromolecules, and damaged
organelles for recycling
Typically two membranes
Photosynthesis
around fluid stroma, which
contains membranous thylakoids
stacked into grana (in plants)
Specialized metabolic
compartment bounded by a
single membrane
Contains enzymes that transfer
hydrogen to water, producing
hydrogen peroxide (H2O2) as a
by-product, which is converted
to water by other enzymes
in the peroxisome
Fig. 6-UN1a
Structure
Cell Component
Concept 6.3
The eukaryotic cell’s
genetic
instructions are housed in
the nucleus and carried out
by the ribosomes
Nucleus
Function
Surrounded by nuclear
envelope (double membrane)
perforated by nuclear pores.
The nuclear envelope is
continuous with the
endoplasmic reticulum (ER).
Houses chromosomes, made of
chromatin (DNA, the genetic
material, and proteins); contains
nucleoli, where ribosomal
subunits are made. Pores
regulate entry and exit os
materials.
Two subunits made of ribosomal RNA and proteins; can be
free in cytosol or bound to ER
Protein synthesis
(ER)
Ribosome
Fig. 6-UN1b
Cell Component
Concept 6.4
Endoplasmic reticulum
The endomembrane system
(Nuclear
regulates protein traffic and
envelope)
performs metabolic functions
in the cell
Golgi apparatus
Lysosome
Vacuole
Structure
Function
Extensive network of
membrane-bound tubules and
sacs; membrane separates
lumen from cytosol;
continuous with
the nuclear envelope.
Smooth ER: synthesis of
lipids, metabolism of carbohydrates, Ca2+ storage, detoxification of drugs and poisons
Stacks of flattened
membranous
sacs; has polarity
(cis and trans
faces)
Rough ER: Aids in sythesis of
secretory and other proteins
from bound ribosomes; adds
carbohydrates to glycoproteins;
produces new membrane
Modification of proteins, carbohydrates on proteins, and phospholipids; synthesis of many
polysaccharides; sorting of
Golgi products, which are then
released in vesicles.
Breakdown of ingested subMembranous sac of hydrolytic stances cell macromolecules,
enzymes (in animal cells)
and damaged organelles for
recycling
Large membrane-bounded
vesicle in plants
Digestion, storage, waste
disposal, water balance, cell
growth, and protection
Fig. 6-UN1c
Cell Component
Concept 6.5
Mitochondrion
Mitochondria and chloroplasts change energy from
one form to another
Structure
Bounded by double
membrane;
inner membrane has
infoldings (cristae)
Function
Cellular respiration
Chloroplast
Typically two membranes
around fluid stroma, which
contains membranous thylakoids
stacked into grana (in plants)
Photosynthesis
Peroxisome
Specialized metabolic
compartment bounded by a
single membrane
Contains enzymes that transfer
hydrogen to water, producing
hydrogen peroxide (H2O2) as a
by-product, which is converted
to water by other enzymes
in the peroxisome
You should now be able to:
1.
2.
3.
4.
Distinguish between the following pairs of terms: magnification
and resolution; prokaryotic and eukaryotic cell; free and bound
ribosomes; smooth and rough ER
Describe the structure and function of the components of the
endomembrane system
Briefly explain the role of mitochondria, chloroplasts, and
peroxisomes
Describe the functions of the cytoskeleton
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
5.
6.
7.
8.
9.
Compare the structure and functions of microtubules,
microfilaments, and intermediate filaments
Explain how the ultrastructure of cilia and flagella relate to
their functions
Describe the structure of a plant cell wall
Describe the structure and roles of the extracellular matrix in
animal cells
Describe four different intercellular junctions
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings