4
Cells: The Working
Units of Life
Chapter 4 Cells: The Working Units of Life
Key Concepts
• 4.1 Cells Provide Compartments for
Biochemical Reactions
• 4.2 Prokaryotic Cells Do Not Have a
Nucleus
• 4.3 Eukaryotic Cells Have a Nucleus and
Other Membrane-Bound Compartments
Chapter 4 Cells: The Working Units of Life
• 4.4 The Cytoskeleton Provides Strength
and Movement
• 4.5 Extracellular Structures Allow Cells to
Communicate with the External
Environment
Chapter 4 Opening Question
What do the characteristics of modern
cells indicate about how the first cells
originated?
Concept 4.1 Cells Provide Compartments for Biochemical
Reactions
Cell theory was the first unifying theory of
biology.
Cells are the fundamental units of life.
All organisms are composed of cells.
All cells come from preexisting cells.
Concept 4.1 Cells Provide Compartments for Biochemical
Reactions
Important implications of cell theory:
Studying cell biology is the same as
studying life.
Life is continuous.
Concept 4.1 Cells Provide Compartments for Biochemical
Reactions
Most cells are tiny, in order to maintain a
good surface area-to-volume ratio.
The volume of a cell determines its
metabolic activity relative to time.
The surface area of a cell determines the
number of substances that can enter or
leave the cell.
Figure 4.1 The Scale of Life
Figure 4.2 Why Cells Are Small
Concept 4.1 Cells Provide Compartments for Biochemical
Reactions
To visualize small cells, there are two types
of microscopes:
Light microscopes—use glass lenses and
light
Resolution = 0.2 μm
Electron microscopes—electromagnets
focus an electron beam
Resolution = 2.0 nm
Figure 4.3 Microscopy
Concept 4.1 Cells Provide Compartments for Biochemical
Reactions
Chemical analysis of cells involves breaking
them open to make a cell-free extract.
The composition and chemical reactions of
the extract can be examined.
The properties of the cell-free extract are
the same as those inside the cell.
Figure 4.4 Centrifugation
Concept 4.1 Cells Provide Compartments for Biochemical
Reactions
The plasma membrane:
Is a selectively permeable barrier that allows
cells to maintain a constant internal
environment
Is important in communication and receiving
signals
Often has proteins for binding and adhering
to adjacent cells
4.1 CelConcept 4. Provide Compartments for Biochemical
Reactions
Two types of cells: Prokaryotic and
eukaryotic
Prokaryotes are without membraneenclosed compartments.
Eukaryotes have membrane-enclosed
compartments called organelles, such as
the nucleus.
In-Text Art, Ch. 4, p. 59
Concept 4.2 Prokaryotic Cells Do Not Have a Nucleus
Prokaryotic cells:
• Are enclosed by a plasma membrane
• Have DNA located in the nucleoid
The rest of the cytoplasm consists of:
• Cytosol (water and dissolved material)
and suspended particles
• Ribosomes—sites of protein synthesis
Figure 4.5 A Prokaryotic Cell
Concept 4.2 Prokaryotic Cells Do Not Have a Nucleus
Most prokaryotes have a rigid cell wall
outside the plasma membrane.
Bacteria cell walls contain peptidoglycans.
Some bacteria have an additional outer
membrane that is very permeable.
Other bacteria have a slimy layer of
polysaccharides, called the capsule.
Concept 4.2 Prokaryotic Cells Do Not Have a Nucleus
Some prokaryotes swim by means of
flagella, made of the protein flagellin.
A motor protein anchored to the plasma or
outer membrane spins each flagellum and
drives the cell.
Some rod-shaped bacteria have a network
of actin-like protein structures to help
maintain their shape.
Figure 4.6 Prokaryotic Flagella (Part 1)
Figure 4.6 Prokaryotic Flagella (Part 2)
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
Membrane-Bound Compartments
Eukaryotic cells have a plasma membrane,
cytoplasm, and ribosomes—and also
membrane-enclosed compartments called
organelles.
Each organelle plays a specific role in cell
functioning.
Figure 4.7 Eukaryotic Cells (Part 1)
Figure 4.7 Eukaryotic Cells (Part 8)
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
Membrane-Bound Compartments
Ribosomes—sites of protein synthesis:
They occur in both prokaryotic and
eukaryotic cells and have similar
structure—one larger and one smaller
subunit.
Each subunit consists of ribosomal RNA
(rRNA) bound to smaller protein
molecules.
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
Membrane-Bound Compartments
Ribosomes translate the nucelotide
sequence of messenger RNA into a
polypeptide chain.
Ribosomes are not membrane-bound
organelles—in eukaryotes, they are free in
the cytoplasm, attached to the
endoplasmic reticulum, or inside
mitochondria and chloroplasts.
In prokaryotic cells, ribosomes float freely in
the cytoplasm.
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
Membrane-Bound Compartments
The nucleus is usually the largest organelle.
It is the location of DNA and of DNA
replication.
It is the site where DNA is transcribed to
RNA.
It contains the nucleolus, where ribosomes
begin to be assembled from RNA and
proteins.
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
Membrane-Bound Compartments
The nucleus is surrounded by two
membranes that form the nuclear
envelope.
Nuclear pores in the envelope control
movement of molecules between nucleus
and cytoplasm.
In the nucleus, DNA combines with proteins
to form chromatin in long, thin threads
called chromosomes.
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
Membrane-Bound Compartments
The endomembrane system includes the
nuclear envelope, endoplasmic reticulum,
Golgi apparatus, and lysosomes.
Tiny, membrane-surrounded vesicles
shuttle substances between the various
components, as well as to the plasma
membrane.
Figure 4.8 The Endomembrane System
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
Membrane-Bound Compartments
Endoplasmic reticulum (ER)—network of
interconnected membranes in the
cytoplasm, with a large surface area
Two types of ER:
• Rough endoplasmic reticulum (RER)
• Smooth endoplasmic reticulum (SER)
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
Membrane-Bound Compartments
Rough endoplasmic reticulum (RER) has
ribosomes attached to begin protein
synthesis.
Newly made proteins enter the RER lumen.
Once inside, proteins are chemically
modified and tagged for delivery.
The RER participates in the transport.
All secreted proteins and most membrane
proteins, including glycoproteins, which is
important for recognition, pass through the
RER.
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
Membrane-Bound Compartments
Smooth endoplasmic reticulum (SER)—
more tubular, no ribosomes
It chemically modifies small molecules such
as drugs and pesticides.
It is the site of glycogen degradation in
animal cells.
It is the site of synthesis of lipids and
steroids.
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
Membrane-Bound Compartments
The Golgi apparatus is composed of
flattened sacs (cisternae) and small
membrane-enclosed vesicles.
Receives proteins from the RER—can
further modify them
Concentrates, packages, and sorts proteins
Adds carbohydrates to proteins
Site of polysaccharide synthesis in plant
cells
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
Membrane-Bound Compartments
The Golgi apparatus has three regions:
The cis region receives vesicles containing
protein from the ER.
At the trans region, vesicles bud off from the
Golgi apparatus and travel to the plasma
membrane or to lysosomes.
The medial region lies in between the trans
and cis regions.
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
Membrane-Bound Compartments
Primary lysosomes originate from the
Golgi apparatus.
They contain digestive enzymes, and are
the site where macromolecules are
hydrolyzed into monomers.
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
Membrane-Bound Compartments
Macromolecules may enter the cell by
phagocytosis—part of the plasma
membrane encloses the material and a
phagosome is formed.
Phagosomes then fuse with primary
lysosomes to form secondary
lysosomes.
Enzymes in the secondary lysosome
hydrolyze the food molecules.
Figure 4.9 Lysosomes Isolate Digestive Enzymes from the Cytoplasm (Part 1)
Figure 4.9 Lysosomes Isolate Digestive Enzymes from the Cytoplasm (Part 2)
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
Membrane-Bound Compartments
Phagocytes are cells that take materials into
the cell and break them down.
Autophagy is the programmed destruction of
cell components and lysosomes are where
it occurs.
Lysosomal storage diseases occurs when
lysosomes fail to digest the components.
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
Membrane-Bound Compartments
In eukaryotes, molecules are first broken
down in the cytosol.
The partially digested molecules enter the
mitochondria—chemical energy is
converted to energy-rich ATP.
Cells that require a lot of energy often have
more mitochondria.
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
Membrane-Bound Compartments
Mitochondria have two membranes:
Outer membrane—quite porous
Inner membrane—extensive folds called
cristae, to increase surface area
The fluid-filled matrix inside the inner
membrane contains enzymes, DNA, and
ribosomes.
Figure 4.7 Eukaryotic Cells
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
Membrane-Bound Compartments
Plant and algae cells contain plastids that
can differentiate into organelles—some
are used for storage.
A chloroplast contains chlorophyll and is
the site of photosynthesis.
Photosynthesis converts light energy into
chemical energy.
Figure 4.7 Eukaryotic Cells
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
Membrane-Bound Compartments
Other organelles perform specialized
functions.
Peroxisomes collect and break down toxic
by-products of metabolism, such as H2O2,
using specialized enzymes.
Glyoxysomes, found only in plants, are
where lipids are converted to
carbohydrates for growth.
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
Membrane-Bound Compartments
A chloroplast is enclosed within two
membranes, with a series of internal
membranes called thylakoids.
A granum is a stack of thylakoids.
Light energy is converted to chemical
energy on the thylakoid membranes.
Carbohydrate synthesis occurs in the
stroma—the aqueous fluid surrounding the
thylakoids.
Figure 4.7 Eukaryotic Cells
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
Membrane-Bound Compartments
Vacuoles occur in some eukaryotes, but
mainly in plants and fungi, and 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
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
Membrane-Bound Compartments
Vacuoles (continued):
Reproduction—vacuoles in flowers and
fruits contain pigments whose colors
attract pollinators and aid seed dispersal
Catabolism—digestive enzymes in seeds’
vacuoles hydrolyze stored food for early
growth
Concept 4.3 Eukaryotic Cells Have a Nucleus and Other
Membrane-Bound Compartments
Contractile vacuoles in freshwater protists
get rid of excess water entering the cell
due to solute imbalance.
The contractile vacuole enlarges as water
enters, then quickly contracts to force
water out through special pores.
Concept 4.4 The Cytoskeleton Provides Strength and Movement
The cytoskeleton:
• Supports and maintains cell shape
• Holds organelles in position
• Moves organelles
• Is involved in cytoplasmic streaming
• Interacts with extracellular structures to
anchor cell in place
Concept 4.4 The Cytoskeleton Provides Strength and Movement
The cytoskeleton has three components
with very different functions:
• Microfilaments
• Intermediate filaments
• Microtubules
Concept 4.4 The Cytoskeleton Provides Strength and Movement
Microfilaments:
• Help a cell or parts of a cell to move
• Determine cell shape
• Are made from the protein actin—which
attaches to the “plus end” and detaches at
the “minus end” of the filament
• The filaments can be made shorter or
longer.
Concept 4.4 The Cytoskeleton Provides Strength and Movement
Actin polymer(filament) ⇌ Actin monomers
Dynamic instability allows quick assembly
or breakdown of the cytoskeleton.
In muscle cells, actin filaments are
associated with the “motor protein”
myosin; their interactions result in muscle
contraction.
Figure 4.10 The Cytoskeleton (Part 1)
Concept 4.4 The Cytoskeleton Provides Strength and Movement
Intermediate filaments:
At least 50 different kinds in six molecular
classes
Have tough, ropelike protein assemblages,
more permanent than other filaments and
do not show dynamic instability
Anchor cell structures in place
Resist tension, maintain rigidity
Figure 4.10 The Cytoskeleton (Part 2)
Concept 4.4 The Cytoskeleton Provides Strength and Movement
Microtubules:
The largest diameter components, with two
roles:
• Form rigid internal skeleton for some cells
or regions
• Act as a framework for motor proteins to
move structures in the cell
Figure 4.10 The Cytoskeleton (Part 3)
Concept 4.4 The Cytoskeleton Provides Strength and Movement
Microtubules are made from dimers of the
protein tubulin—chains of dimers surround
a hollow core.
They show dynamic instability, with (+) and
(-) ends:
microtubule ⇌ tubulin monomers
Polymerization results in a rigid structure—
depolymerization leads to collapse.
Concept 4.4 The Cytoskeleton Provides Strength and Movement
Microtubules line movable cell appendages.
Cilia—short, usually many present, move
with stiff power stroke and flexible
recovery stroke
Flagella—longer, usually one or two
present, movement is snakelike
Figure 4.11 Cilia (Part 1)
Concept 4.4 The Cytoskeleton Provides Strength and Movement
Cilia and flagella appear in a “9 + 2”
arrangement:
• Doublets—nine fused pairs of
microtubules form a cylinder
• One unfused pair in center
Motion occurs as doublets slide past each
other.
Figure 4.11 Cilia (Part 2)
Concept 4.4 The Cytoskeleton Provides Strength and Movement
Dynein—a motor protein that drives the
sliding of doublets, by changing its shape
Nexin—protein that crosslinks doublets and
prevents sliding, so cilia bends
Kinesin—motor protein that binds to
vesicles in the cell and “walks” them along
the microtubule
Figure 4.12 A Motor Protein Moves Microtubules in Cilia and Flagella
Figure 4.13 A Motor Protein Drives Vesicles along Microtubules
Concept 4.4 The Cytoskeleton Provides Strength and Movement
Cytoskeletal structure may be observed
under the microscope, and function can be
observed in a cell with that structure.
Observations may suggest that a structure
has a function, but correlation does not
establish cause and effect.
Concept 4.4 The Cytoskeleton Provides Strength and Movement
Two methods are used to show links
between structure (A) and function (B):
Inhibition—use a drug to inhibit A—if B still
occurs, then A does not cause B
Mutation—if genes for A are missing and B
does not occur—A probably causes B
Figure 4.14 The Role of Microfilaments in Cell Movement: Showing Cause and Effect in Biology (Part 1)
Figure 4.14 The Role of Microfilaments in Cell Movement: Showing Cause and Effect in Biology (Part 2)
Concept 4.5 Extracellular Structures Allow Cells to Communicate
with the External Environment
Extracellular structures are secreted to the
outside of the plasma membrane.
In eukaryotes, these structures have two
components:
• A prominent fibrous macromolecule
• A gel-like medium with fibers embedded
Concept 4.5 Extracellular Structures Allow Cells to Communicate
with the External Environment
Plant cell wall—semi-rigid structure outside
the plasma membrane
The fibrous component is the
polysaccharide cellulose.
The gel-like matrix contains cross-linked
polysaccharides and proteins.
Figure 4.15 The Plant Cell Wall
Concept 4.5 Extracellular Structures Allow Cells to Communicate
with the External Environment
The plant cell wall has three major roles:
• Provides support for the cell and limits
volume by remaining rigid
• Acts as a barrier to infection
• Contributes to form during growth and
development
Concept 4.5 Extracellular Structures Allow Cells to Communicate
with the External Environment
Adjacent plant cells are connected by
plasma membrane-lined channels called
plasmodesmata.
These channels allow movement of water,
ions, small molecules, hormones, and
some RNA and proteins.
Concept 4.5 Extracellular Structures Allow Cells to Communicate
with the External Environment
Many animal cells are surrounded by an
extracellular matrix.
The fibrous component is the protein
collagen.
The gel-like matrix consists of
proteoglycans.
A third group of proteins links the collagen
and the matrix together.
Figure 4.16 An Extracellular Matrix (Part 1)
Figure 4.16 An Extracellular Matrix (Part 2)
Concept 4.5 Extracellular Structures Allow Cells to Communicate
with the External Environment
Role of extracellular matrices in animal
cells:
• Hold cells together in tissues
• Contribute to physical properties of
cartilage, skin, and other tissues
• Filter materials
• Orient cell movement during growth and
repair
Concept 4.5 Extracellular Structures Allow Cells to Communicate
with the External Environment
Proteins like integrin connect the
extracellular matrix to the plasma
membrane.
Proteins bind to microfilaments in the
cytoplasm and to collagen fibers in the
extracellular matrix.
For cell movement, the protein changes
shape and detaches from the collagen.
Figure 4.17 Cell Membrane Proteins Interact with the Extracellular Matrix
Concept 4.5 Extracellular Structures Allow Cells to Communicate
with the External Environment
Cell junctions are specialized structures
that protrude from adjacent cells and
“glue” them together—seen often in
epithelial cells:
• Tight junctions
• Desmosomes
• Gap junctions
Concept 4.5 Extracellular Structures Allow Cells to Communicate
with the External Environment
Tight junctions prevent substances from
moving through spaces between cells.
Desmosomes hold cells together but allow
materials to move in the matrix.
Gap junctions are channels that run
between membrane pores in adjacent
cells, allowing substances to pass
between the cells.
Figure 4.18 Junctions Link Animal Cells (Part 1)
Figure 4.18 Junctions Link Animal Cells (Part 2)
Figure 4.18 Junctions Link Animal Cells (Part 3)
Figure 4.18 Junctions Link Animal Cells (Part 4)
Answer to Opening Question
Synthetic cell models—protocells—can
demonstrate how cell properties may have
originated.
Combinations of molecules can produce a
cell-like structure, with a lipid “membrane”
and water-filled interior.
As in modern cells, the membrane allows
only certain things to pass, while RNA
inside the cell can replicate itself.
Figure 4.19 A Protocell