Principles of Life
Hillis • Sadava • Heller • Price
Instructor’s Manual
Chapter 4: Cells: The Working Units of Life
OVERVIEW
Chapter 4 examines the structure and function of both prokaryotic and eukaryotic cells.
The chapter opens with brief discussions of cell theory and limits to cell size. This is
followed by an introduction to light and electron microscopy and their applications to cell
biology. Structural features of prokaryotic cells are presented, including shared features
and more specialized organelles. The largest section of the chapter is devoted to
describing the organelles of eukaryotic cells, including the nucleus, the endomembrane
system, ribosomes, and mitochondria. The three components of the cytoskeleton are
explained in the context of both structural and motor functions. Chapter 4 concludes with
discussions of extracellular structures.
KEY CONCEPTS/ CHAPTER OUTLINE
4.1 Cells Provide Compartments for Biochemical Reactions
• Cell size is limited by the surface area-to-volume ratio
• Cells can be studied structurally and chemically
• The plasma membrane forms the outer surface of every cell
• Cells are classified as either prokaryotic or eukaryotic
Cells must maintain an efficient surface area-to-volume ratio in order to function.
Microscopes reveal cell features and allow study. The plasma membrane forms the outer
surface of every cell while cells are classified as either prokaryotic or eukaryotic based on
internal structures.
4.2 Prokaryotic Cells Do Not Have a Nucleus
• Prokaryotic cells share certain features
Prokaryotic cells share certain features, some of which are specialized.
4.3 Eukaryotic Cells Have a Nucleus and Other Membrane-Bound
Compartments
• Compartmentalization is the key to eukaryotic cell function
• Ribosomes are factories for protein synthesis
• The nucleus contains most of the DNA
• The endomembrane system is a group of interrelated organelles
• Some organelles transform energy
• Several other membrane-enclosed organelles perform specialized functions
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Eukaryotic cells function because of compartmentalization. Different organelles,
including ribosomes, the nucleus, and those in the endomembrane system, carry out
specialized functions. Some organelles are involved in the transfer of energy.
4.4 The Cytoskeleton Provides Strength and Movement
• Microfilaments are made of actin
• Intermediate filaments are diverse and stable
• Microtubules are the thickest elements of the cytoskeleton
• Cilia and flagella provide mobility
• Biologists manipulate living systems to establish cause and effect
The cytoskeleton has three types of filaments that provide structure and aid in movement
of cilia and flagella. Biologists study the relationship between structure and function by
manipulating one condition at a time.
4.5 Extracellular Structures Allow Cells to Communicate with the External
Environment
• The plant cell wall is an extracellular structure
• The extracellular matrix supports tissue functions in animals
• Cell junctions connect adjacent cells
Extracellular structures such as the plant cell provide support, are a barrier to infection,
and guide growth. In animal cells, an extracellular matrix holds cells together, contributes
to their properties, filters materials, and orients cell movement. Animal cells are often
connected by specialized cell junctions such as tight junctions, desmosomes, and gap
junctions.
LECTURE OUTLINE
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.
(See Chapters 2 and 3)
Important implications of cell theory:
Studying cell biology is the same as studying life.
Life is continuous.
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.
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FIGURE 4.1 The Scale of Life
FIGURE 4.2 Why Cells Are Small
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
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.
(See Chapter 5)
FIGURE 4.4 Centrifugation
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
(VIDEO 4.1 Amoeba with visible organelles)
(VIDEO 4.2 Newt epithelial cells)
(VIDEO 4.3 Cell Visualization: Membranes, hormones, and receptors)
Two types of cells: Prokaryotic and eukaryotic
Prokaryotes are without membrane-enclosed compartments.
Eukaryotes have membrane-enclosed compartments called organelles, such as the
nucleus.
(LINK: Eukaryotes arose from prokaryotes by endosymbiosis; see Concept 20.1)
IN-TEXT ART, 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
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(VIDEO 4.4 Cell Visualization: Cytoplasm and centrosome)
(See Chapter 3)
FIGURE 4.5 A Prokaryotic Cell
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.
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
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.
(ANIMATED TUTORIAL 4.1 Eukaryotic Cell Tour)
FIGURE 4.7 Eukaryotic Cells
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.
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.
(LINK Protein synthesis is described in more detail in Concept 10.4)
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.
(See Chapter 3 and Chapter 7)
The nucleus is surrounded by two membranes that form the nuclear envelope.
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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.
(See Figure 4.7)
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
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)
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.
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.
(VIDEO 4.5 Epidermal cells of an onion, with visible endoplasmic reticulum)
The Golgi apparatus is composed of flattened sacs (cisternae) and small membraneenclosed 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
(ANIMATED TUTORIAL 4.2: The Golgi Apparatus)
(See Figure 4.8)
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.
(VIDEO 4.6 A diatom, Surirella, with visible Golgi bodies)
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(VIDEO 4.7 Exocytosis of coccoliths in a marine golden alga, Pleurochrysis)
(See Figure 4.8)
Primary lysosomes originate from the Golgi apparatus.
They contain digestive enzymes, and are the site where macromolecules are hydrolyzed
into monomers.
(See Chapter 2)
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
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.
(APPLY THE CONCEPT 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.
(See Chapter 6)
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.
(VIDEO 4.8 Mitochondria)
(VIDEO 4.9 Cell Visualization: Mitochondria, microtubules, and motors)
IN-TEXT ART, p. 68(1)
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.
(See Chapter 6)
IN-TEXT ART, p. 68(2)
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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.
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.
IN-TEXT ART, p. 68(3)
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
(See Figure 5.3)
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
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
(VIDEO 4.10 Tradescantia stamen hair cell)
(VIDEO 4.11 Amoeba with visible nucleus and nucleolus)
The cytoskeleton has three components with very different functions:
• Microfilaments
• Intermediate filaments
• Microtubules
Microfilaments:
Help a cell or parts of a cell to move
Determine cell shape
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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.
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
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
(See Figure 4.18)
FIGURE 4.10 The Cytoskeleton
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
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.
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
(See Concept 4.2)
FIGURE 4.11 Cilia
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.
(VIDEO 4.9 Cell Visualization: Mitochondria, microtubules, and motors)
(VIDEO 4.12 A Paramecium uses cilia for feeding)
(VIDEO 4.13 Rotifers feeding via flagella-induced vortices)
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FIGURE 4.11 Cilia
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
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.
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
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
(See Figure 2.10)
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
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
Adjacent plant cells are connected by plasma membrane-lined channels called
plasmodesmata.
These channels allow movement of water, ions, small molecules, hormones, and some
(VIDEO 4.14 Cell walls and stomatal complexes in Tradescantia)
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(See Figure 4.7)
RNA and proteins.
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
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
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
Cell junctions are specialized structures that protrude from adjacent cells and “glue”
them together—seen often in epithelial cells:
• Tight junctions
• Desmosomes
• Gap junctions
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
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.
(See Figure 2.13)
FIGURE 4.19 A Protocell
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KEY TERMS
cell junction
cell theory
cell wall
chloroplast
cilia
collagen
cytoplasm
cytoskeleton
cytosol
dynamic instability
endomembrane system
endoplasmic reticulum (ER)
eukaryote
extracellular matrix
flagella
glyoxysome
Golgi apparatus
intermediate filament
microfilament
microtubule
mitochondrion
nucleoid
nucleolus
nucleus
organelle
peroxisome
plasma membrane
plasmodesmata
primary lysosome
prokaryote
proteoglycan
ribosome
rough endoplasmic reticulum (RER)
secondary lysosome
smooth endoplasmic reticulum (SER)
surface area-to-volume ratio
vacuole
vesicle
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