5
Cells: The Working
Units of Life
5 Cells: The Working Units of Life
5.1 What Features Make Cells the
Fundamental Units of Life?
5.2 What Features Characterize
Prokaryotic Cells?
5.3 What Features Characterize
Eukaryotic Cells?
5.4 What Are the Roles of Extracellular
Structures?
5.5 How Did Eukaryotic Cells Originate?
5 Cells: The Working Units of Life
Cells are the fundamental units of life.
Stem cells are able to differentiate
into any type of cell in the body and
are being investigated for treating a
host of human diseases.
Opening Question:
What is the status of stem cell
treatment for heart disease?
5.1 What Features Make Cells the Fundamental Units of Life?
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.
5.1 What Features Make Cells the Fundamental Units of Life?
Implications of the cell theory:
• Functions of all cells are similar.
• Life is continuous.
• Origin of life was the origin of cells.
5.1 What Features Make Cells the Fundamental Units of Life?
Cells are small (mostly).
Exceptions: Bird eggs, some algae,
and bacteria.
Figure 5.1 The Scale of Life (Part 1)
Figure 5.1 The Scale of Life (Part 2)
Figure 5.1 The Scale of Life (Part 1)
5.1 What Features Make Cells the Fundamental Units of Life?
Cells are small because a high surface
area-to-volume ratio is essential.
Volume determines the amount of
chemical activity in the cell per unit
time. Larger cells have more chemical
activity.
Cell surface area limits the amount of
resources and waste products that
can cross the cell boundary per unit
time.
Figure 5.2 Why Cells Are Small
5.1 What Features Make Cells the Fundamental Units of Life?
Most cells are < 200 μm in size. To see
them, we use microscopes:
Magnification: increases apparent size.
Resolution: clarity of magnified object—
minimum distance two objects can be
apart and still be seen as two objects.
5.1 What Features Make Cells the Fundamental Units of Life?
Two basic types of microscopes:
Light microscopes: use glass lenses
and light. Resolution = 0.2 μm
Electron microscopes: electromagnets
focus an electron beam. Resolution =
0.2 nm
Figure 5.3 Looking at Cells (Part 1)
Figure 5.3 Looking at Cells (Part 2)
Figure 5.3 Looking at Cells (Part 3)
5.1 What Features Make Cells the Fundamental Units of Life?
Pathology is a branch of medicine that
uses microscopy to analyze cells and
diagnose diseases.
Many methods are used, including
phase-contrast microscopy, staining
the cells with general or selective
dyes, and electron microscopy.
In-Text Art, Ch. 5, p. 79
5.1 What Features Make Cells the Fundamental Units of Life?
The plasma membrane is the outer
surface of every cell and has more or
less the same structure in all cells.
It is made of a phospholipid bilayer with
embedded proteins and other
molecules.
5.1 What Features Make Cells the Fundamental Units of Life?
The plasma membrane:
• is a selectively permeable barrier
• 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
5.1 What Features Make Cells the Fundamental Units of Life?
Two types of cells: Prokaryotic and
eukaryotic.
Bacteria and Archaea are prokaryotes.
They have no membrane-enclosed
internal compartments.
The first cells were probably
prokaryotic.
In-Text Art, Ch. 5, p. 81
5.1 What Features Make Cells the Fundamental Units of Life?
Eukarya are eukaryotes—cells with
membrane-enclosed compartments
called organelles.
The DNA is in a compartment called
the nucleus. Specific chemical
reactions occur in other organelles.
This “division of labor” was important in
the evolution of complex organisms.
5.2 What Features Characterize Prokaryotic Cells?
Prokaryotic cells are very small.
Individuals are single cells but often
form chains or clusters.
Prokaryotes are very successful; and
there is a huge diversity of species in
the Bacteria and Archaea domains.
5.2 What Features Characterize Prokaryotic Cells?
Characteristics of prokaryotic cells:
• Enclosed by a plasma membrane.
• DNA is contained in a region called
the nucleoid.
• Cytoplasm consists of cytosol (liquid
component) plus filaments and
particles.
5.2 What Features Characterize Prokaryotic Cells?
• Cytosol: water with dissolved ions,
small molecules, and soluble
macromolecules.
• Ribosomes: RNA and protein
complexes; sites of protein synthesis.
Figure 5.4 A Prokaryotic Cell
5.2 What Features Characterize Prokaryotic Cells?
Most prokaryotes have a rigid cell wall
outside the plasma membrane.
Bacterial cell walls contain
peptidoglycan.
Some bacteria have an additional
outer membrane.
Some bacteria have a slimy capsule of
polysaccharides.
5.2 What Features Characterize Prokaryotic Cells?
Photosynthetic bacteria have an
internal membrane system that
contains molecules necessary for
photosynthesis.
Others have internal membrane folds
that are attached to the plasma
membrane; they may function in cell
division or in energy-releasing
reactions.
5.2 What Features Characterize Prokaryotic Cells?
Some prokaryotes swim by means of
flagella, made of the protein flagellin.
Some bacteria have pili—hairlike
structures projecting from the surface.
They help bacteria adhere to other
cells.
Fimbriae are shorter than pili; they help
cells adhere to surfaces such as
animal cells.
Figure 5.5 Prokaryotic Flagella
Figure 5.5 Prokaryotic Flagella (Part 1)
5.2 What Features Characterize Prokaryotic Cells?
Cytoskeleton: system of protein
filaments that maintain cell shape and
play roles in cell division.
5.3 What Features Characterize Eukaryotic Cells?
Eukaryotic cells are up to ten times
larger than prokaryotes.
Eukaryotic cells have membraneenclosed compartments called
organelles.
Each organelle has a specific role in
cell functioning.
5.3 What Features Characterize Eukaryotic Cells?
Compartmentalization allowed
eukaryotic cells to specialize and form
the tissues and organs of multicellular
organisms.
5.3 What Features Characterize Eukaryotic Cells?
To determine the functions of
organelles, they were first studied
using light microscopy and then
electron microscopy.
Cell fractionation separates organelles
by size or density for study by
chemical methods.
Figure 5.6 Cell Fractionation (Part 1)
Figure 5.6 Cell Fractionation (Part 2)
Figure 5.7 Eukaryotic Cells (Part 1)
Figure 5.7 Eukaryotic Cells (Part 2)
Figure 5.7 Eukaryotic Cells (Part 3)
Figure 5.7 Eukaryotic Cells (Part 4)
5.3 What Features Characterize Eukaryotic Cells?
Ribosomes: sites of protein synthesis.
Occur in both prokaryotic and
eukaryotic cells and have similar
structure.
Ribosomes consist of ribosomal RNA
(rRNA) and more than 50 different
protein molecules.
5.3 What Features Characterize Eukaryotic Cells?
In eukaryotes, ribosomes are free in
the cytoplasm, attached to the
endoplasmic reticulum, or inside
mitochondria and chloroplasts.
In prokaryotic cells, ribosomes float
freely in the cytoplasm.
5.3 What Features Characterize Eukaryotic Cells?
The nucleus is usually the largest
organelle.
• Contains the DNA
• Site of DNA replication
• Site where gene transcription is
turned on or off
• Assembly of ribosomes begins in a
region called the nucleolus
5.3 What Features Characterize Eukaryotic Cells?
The nucleus is surrounded by the
nuclear envelope. Many pores
control the movement of molecules
across the envelope.
In-text art, p. 9
5.3 What Features Characterize Eukaryotic Cells?
In the nucleus, DNA combines with
proteins to form chromatin in long,
thin threads called chromosomes.
Before cell division, chromatin
condenses, and individual
chromosomes are visible in the light
microscope.
Figure 5.8 The Nucleus, Chromatin, and Chromosomes
5.3 What Features Characterize Eukaryotic Cells?
The chromatin is attached to a protein
meshwork (the nuclear lamina), which
maintains the shape of the nucleus.
The outer membrane of the nuclear
envelope folds outward into the
cytoplasm and is continuous with the
endoplasmic reticulum.
5.3 What Features Characterize Eukaryotic Cells?
The endomembrane system includes
the plasma membrane, nuclear
envelope, endoplasmic reticulum,
Golgi apparatus, and lysosomes.
Tiny, membrane-surrounded vesicles
shuttle substances between the
various components.
Figure 5.9 The Endomembrane System (Part 1)
5.3 What Features Characterize Eukaryotic Cells?
Endoplasmic reticulum (ER): network
of interconnected membranes in the
cytoplasm; has large surface area.
Rough endoplasmic reticulum
(RER): ribosomes are attached.
Newly made proteins enter the RER
lumen where they are modified,
folded, and transported to other
regions.
5.3 What Features Characterize Eukaryotic Cells?
Smooth endoplasmic reticulum
(SER): more tubular, no ribosomes.
• Chemically modifies small molecules
such as drugs and pesticides
• Site of glycogen degradation in animal
cells
• Synthesis of lipids and steroids
5.3 What Features Characterize Eukaryotic Cells?
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, sorts
proteins
• In plant cells, polysaccharides for cell
walls are synthesized here
5.3 What Features Characterize Eukaryotic Cells?
• The cis region receives vesicles (a
piece of the ER that “buds” off) from
the ER.
• At the trans region, vesicles bud off
from the Golgi apparatus and are
moved to the plasma membrane or
other organelles.
Figure 5.9 The Endomembrane System (Part 2)
5.3 What Features Characterize Eukaryotic Cells?
Primary lysosomes originate from the
Golgi apparatus.
They contain digestive enzymes which
hydrolyze macromolecules into
monomers.
5.3 What Features Characterize Eukaryotic Cells?
Food molecules enter the cell by
phagocytosis—a phagosome is
formed.
Phagosomes fuse with primary
lysosomes to form secondary
lysosomes.
Enzymes in the secondary lysosome
hydrolyze the food molecules.
Figure 5.10 Lysosomes Isolate Digestive Enzymes from the Cytoplasm
5.3 What Features Characterize Eukaryotic Cells?
Lysosomes also digest cell materials
(autophagy).
Cell components are frequently
destroyed and replaced by new ones.
5.3 What Features Characterize Eukaryotic Cells?
In the mitochondria, energy in fuel
molecules is transformed to the bonds
of energy-rich ATP (cellular
respiration).
Cells that require a lot of energy have
many mitochondria.
5.3 What Features Characterize Eukaryotic Cells?
Mitochondria have two membranes.
The inner membrane folds inward to
form cristae. This creates a large
surface area for the proteins involved
in cellular respiration reactions.
The mitochondrial matrix contains
enzymes, DNA, and ribosomes.
Figure 5.11 A Mitochondrion Converts Energy from Fuel Molecules into ATP
5.3 What Features Characterize Eukaryotic Cells?
Plastids occur only in plants and some
protists.
Chloroplasts: Site of photosynthesis—
light energy is converted to the energy
of chemical bonds.
Chloroplasts have a double membrane.
Figure 5.12 Chloroplasts Feed the World
5.3 What Features Characterize Eukaryotic Cells?
Grana are stacks of thylakoids—
circular compartments of the inner
membrane.
Thylakoids contain chlorophyll and
other pigments that harvest light
energy for photosynthesis.
Stroma—fluid in which grana are
suspended. The stroma contains DNA
and ribosomes.
5.3 What Features Characterize Eukaryotic Cells?
Other plastids:
• Chromoplasts contain red, orange,
and yellow pigments—gives color to
flowers.
5.3 What Features Characterize Eukaryotic Cells?
• Leucoplasts store starches and fats.
5.3 What Features Characterize Eukaryotic Cells?
Peroxisomes: collect and break down
toxic byproducts of metabolism such
as H2O2, using specialized enzymes.
Glyoxysomes: only in plants—lipids
are converted to carbohydrates for
growth.
5.3 What Features Characterize Eukaryotic Cells?
Plant and protist cells have vacuoles:
• Store waste products and toxic
compounds; some may deter
herbivores.
• Provide structure for plant cells—
water enters the vacuole by osmosis,
creating turgor pressure.
5.3 What Features Characterize Eukaryotic Cells?
• Store anthocyanins (pink and blue
pigments) in flowers and fruits; the
colors attract pollinators.
Vacuoles in seeds have digestive
enzymes to hydrolyze stored food for
early growth.
Figure 5.13 Vacuoles in Plant Cells Are Usually Large
5.3 What Features Characterize Eukaryotic Cells?
Freshwater protists may have
contractile vacuoles to expel excess
water.
The vacuoles take in excess water that
enters the cell by osmosis; then expel
it by contracting, forcing water out
through a pore.
5.3 What Features Characterize Eukaryotic Cells?
The cytoskeleton:
• Supports and maintains cell shape
• Holds organelles in position
• Moves organelles
• Involved in cytoplasmic streaming
• Interacts with extracellular structures
to hold cell in place
5.3 What Features Characterize Eukaryotic Cells?
The cytoskeleton has three
components:
• Microfilaments
• Intermediate filaments
• Microtubules
Figure 5.14 The Cytoskeleton (Part 1)
Figure 5.14 The Cytoskeleton (Part 2)
Figure 5.14 The Cytoskeleton (Part 3)
5.3 What Features Characterize Eukaryotic Cells?
Microfilaments:
• Help a cell or parts of a cell to move
• Determine cell shape
• Made from the protein actin
• Actin has + and – ends and
polymerizes to form long helical
chains (reversible)
5.3 What Features Characterize Eukaryotic Cells?
In muscle cells, actin filaments are
associated with the “motor protein”
myosin; interactions between the two
result in muscle contraction.
Microfilaments are also involved in the
formation of pseudopodia (pseudo,
“false”; podia, “feet”).
Figure 5.15 Microfilaments and Cell Movements
5.3 What Features Characterize Eukaryotic Cells?
In some cells, microfilaments form a
meshwork just inside the plasma
membrane.
This provides structure, for example in
the microvilli that line the human
intestine.
Figure 5.16 Microfilaments for Support
5.3 What Features Characterize Eukaryotic Cells?
Intermediate filaments:
• 50 different kinds in six molecular
classes
• Tough, ropelike protein assemblages
• Anchor cell structures in place
• Resist tension
5.3 What Features Characterize Eukaryotic Cells?
Microtubules:
• Long, hollow cylinders
• Form rigid internal skeleton in some cells
• Act as a framework for motor proteins
• Made from the protein tubulin—a dimer
• Have + and – ends
• Can change length rapidly by adding or
losing dimers
5.3 What Features Characterize Eukaryotic Cells?
Cilia and eukaryotic flagella are made
of microtubules in “9 + 2” array.
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 5.17 Cilia
5.3 What Features Characterize Eukaryotic Cells?
The motion of cilia and flagella results
from the sliding of the microtubule
doublets past one another.
Dynein binds to microtubule doublets
and allows them to slide past each
other.
Nexin can cross-link the doublets and
prevent them from sliding, and the
cilium bends.
Figure 5.18 A Motor Protein Moves Microtubules in Cilia and Flagella
5.3 What Features Characterize Eukaryotic Cells?
The motor protein kinesin moves
vesicles or organelles from one part of
a cell to another.
It binds to a vesicle and “walks” it along
by changing shape.
Figure 5.19 A Motor Protein Pulls Vesicles along Microtubules
5.3 What Features Characterize Eukaryotic Cells?
Experiments to determine the function
of cellular components fall into two
categories:
• Inhibition: A drug that inhibits a
structure or process—does the
function still occur?
• Mutation: Examine a cell that lacks
the gene for the structure or process.
Figure 5.20 The Role of Microfilaments in Cell Movement—Showing Cause and Effect in Biology
(Part 1)
Figure 5.20 The Role of Microfilaments in Cell Movement—Showing Cause and Effect in Biology
(Part 2)
Working with Data
The drug cytochalasin B was found to
block the polymerization of actin
monomers into microfilaments.
Thus, it was used in experiments to
determine which cellular processes
involved microfilaments.
Several control experiments using
other chemicals were also conducted
using Amoeba proteus.
Working with Data 5.1, Table 1
Working with Data 5.1: The Role of Microfilaments in Cell
Movement
Question 1:
Explain the reasoning behind each
experiment.
Why were these controls important?
Working with Data 5.1: The Role of Microfilaments in Cell
Movement
Question 2:
Interpret the results of each
experiment.
What can you conclude about Amoeba
and the cytoskeleton?
5.4 What Are the Roles of Extracellular Structures?
Extracellular structures are secreted to
the outside of the plasma membrane.
Example: The peptidoglycan cell wall of
bacteria.
In eukaryotes, extracellular structures
have a prominent fibrous
macromolecule in a gel-like medium.
5.4 What Are the Roles of Extracellular Structures?
Plant cell walls: Cellulose fibers are
embedded in other complex
polysaccharides and proteins.
Adjacent plant cells are connected by
plasma membrane-lined channels
called plasmodesmata.
Figure 5.21 The Plant Cell Wall
5.4 What Are the Roles of Extracellular Structures?
Many animal cells are surrounded by
an extracellular matrix, composed of
fibrous proteins such as collagen,
gel-like proteoglycans
(glycoproteins), and other proteins.
Figure 5.22 An Extracellular Matrix
5.4 What Are the Roles of Extracellular Structures?
The extracellular matrix:
• Holds cells together in tissues
• Contributes to properties of bone,
cartilage, skin, etc.
• Filters materials passing between
different tissues
• Orients cell movements in
development and tissue repair
• Plays a role in chemical signaling
5.5 How Did Eukaryotic Cells Originate?
Eukaryotic cells first appeared about
1.5 billion years ago.
The advent of compartmentalization
was a major event in the history of life.
How did compartmentalization arise?
5.5 How Did Eukaryotic Cells Originate?
The endomembrane system and cell
nucleus may have originated from the
inward folds of plasma membrane of
prokaryotes.
Enclosed compartments would have
allowed chemicals to be concentrated
and chemical reactions to proceed
more efficiently.
5.5 How Did Eukaryotic Cells Originate?
Some organelles may have arisen by
symbiosis (“living together”).
The endosymbiosis theory proposes
that mitochondria and plastids arose
when one cell engulfed another cell.
Figure 5.23 The Origins of Organelles
5.5 How Did Eukaryotic Cells Originate?
Support for the endosymbiosis
theory:
• The discovery of a single-celled
eukaryote, Hatena, that ingests a
green alga, Nephroselmis.
• The green alga loses most of its
structures and acts as a chloroplast.
Figure 5.24 Endosymbiosis in Action
5 Answer to Opening Question
In China, stem cells are collected from
umbilical cords at childbirth and stored
for later use.
These stem cells have been
successfully used to repair heart
muscle and the blood vessels that
supply it.