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?
5.1 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 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, neurons, some
algae, and bacteria cells
Figure 5.1 The Scale of Life (Part 1)
Figure 5.1 The Scale of Life (Part 2)
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.
Surface area determines the amount of
substances that can pass the cell
boundary per unit time.
Figure 5.2 Why Cells Are Small (Part 1)
Figure 5.2 Why Cells Are Small (Part 2)
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 fluorescent dyes,
and electron microscopy.
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
proteins and other molecules embedded.
It is not rigid, but more like an oily fluid in
which the proteins and lipids are in
constant motion.
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.
The first cells were probably prokaryotic.
Eukarya are eukaryotes—the DNA is in
a membrane-enclosed compartment
called the nucleus.
5.1 What Features Make Cells the Fundamental Units of Life?
Eukaryotes have other membraneenclosed compartments in which
specific chemical reactions occur.
This has allowed diversification of
functions in eukaryotic cells, and their
specialization into tissues.
5.2 What Features Characterize Prokaryotic Cells?
Prokaryotic cells are very small.
Individuals are single cells, but often
found in chains or clusters.
Prokaryotes are very successful—they
can live on a diversity of energy sources
and some can tolerate extreme
conditions.
5.2 What Features Characterize Prokaryotic Cells?
Prokaryotic cells:
• Are enclosed by a plasma membrane
• The DNA is contained in the nucleoid
• Cytoplasm consists of cytosol (water
and dissolved material) and suspended
particles
• Ribosomes—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.
Bacteria 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.
Some rod-shaped bacteria have a
cytoskeleton made of the protein actin.
Figure 5.5 Prokaryotic Flagella (Part 1)
Figure 5.5 Prokaryotic Flagella (Part 2)
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.
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?
Organelles were first studied using light
microscopy.
Cell fractionation separates organelles for
study by chemical methods.
Figure 5.6 Cell Fractionation
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
Figure 5.8 The Nucleus Is Enclosed by a Double Membrane (Part 1)
5.3 What Features Characterize Eukaryotic Cells?
The nucleus is surrounded by two
membranes—the nuclear envelope.
Nuclear pores in the envelope control
movement of molecules between
nucleus and cytoplasm.
Figure 5.8 The Nucleus Is Enclosed by a Double Membrane (Part 2)
5.3 What Features Characterize Eukaryotic Cells?
Some large molecules (e.g., proteins)
must have a certain amino acid
sequence known as a nuclear
localization signal (NLS) to cross the
nuclear envelope.
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 5.9 Chromatin and Chromosomes
5.3 What Features Characterize Eukaryotic Cells?
Nucleoplasm surrounds the chromatin,
and a network of structural proteins
(nuclear matrix) helps organize the
chromatin.
The nuclear lamina attaches to both the
chromatin and the nuclear envelope and
maintains nuclear shape.
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.
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.
Figure 5.10 The Endomembrane System (Part 1)
Figure 5.10 The Endomembrane System (Part 2)
5.3 What Features Characterize Eukaryotic Cells?
Smooth endoplasmic reticulum (SER):
more tubular, no ribosomes
• Chemically modifies small molecules
such as drugs and pesticides
• Hydrolysis of glycogen 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 5.10 The Endomembrane System (Part 1)
Figure 5.10 The Endomembrane System (Part 2)