5
Cells: The Working Units
of Life
Figure 4.15 The Origin of Life
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:
•  Life is continuous: all cells are derived from
pre-existing cells
•  Origin of life was the origin of cells
5.1 What Features Make Cells the Fundamental Units of Life?
Cellule
membrane
water +
thousand of molecules
used to
•  transform matter and energy
•  respond to their environments
•  reproduce
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?
Cellule
membrane
water +
thousand of molecules
used to
•  transform matter and energy
•  respond to their environments
•  reproduce
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?
Two basic types of microscopes:
Light microscopes—use glass lenses and light.
Resolution = 0.2 µm
=> Visualisation of cells and some internal structures
Electron microscopes—electromagnets focus
an electron beam. Resolution = 0.2 nm
=> Visualisation of subcellular structures
Figure 5.3 Looking at Cells (Part 1)
Figure 5.3 Looking at Cells (Part 3)
5.1 What Features Make Cells the Fundamental Units of Life?
Cellule
membrane
water +
thousand of molecules
used to
•  transform matter and energy
•  respond to their environments
•  reproduce
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.
5.2 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.
http://www.larousse.fr/encyclopedie/data/cartes/1309193-Classification_des_espèces_vivantes.HD.jpg
prokaryotes
http://www.larousse.fr/encyclopedie/data/cartes/1309193-Classification_des_espèces_vivantes.HD.jpg
Figure 5.4 A Prokaryotic Cell
5.2 What Features Characterize Prokaryotic Cells?
Prokaryotic cells:
• Are enclosed by a plasma membrane
• The DNA is contained in the nucleoid
• Cytoplasm comprise the rest of the
cell’s content (water, biomolecules and
ribosomes)
• Ribosomes—sites of protein synthesis
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 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.
Figure 5.5 Prokaryotic Flagella (Part 1)
eukaryotes
http://www.larousse.fr/encyclopedie/data/cartes/1309193-Classification_des_espèces_vivantes.HD.jpg
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.
Figure 5.7 Eukaryotic Cells (Part 1)
Figure 5.7 Eukaryotic Cells (Part 1)
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)
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
Figure 5.7 Eukaryotic Cells (Part 1)
5.3 What Features Characterize Eukaryotic Cells?
The endomembrane system includes
the nuclear envelope, endoplasmic
reticulum, Golgi apparatus, and
lysosomes.
Tiny, membrane-surrounded vesicles
shuttle substances between the various
components.
Figure 5.10 The Endomembrane System (Part 2)
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.
http://www.mhhe.com/biosci/genbio/enger/student/olc/art_quizzes/genbiomedia/0064.jpg
5.3 What Features Characterize Eukaryotic Cells?
Ribosomes—sites of protein synthesis.
Ribosomes consist of ribosomal RNA (rRNA)
and more than 50 different protein molecules.
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.
Figure 5.10 The Endomembrane System (Part 2)
5.3 What Features Characterize Eukaryotic Cells?
The Golgi apparatus is composed of
flattened sacs and small membraneenclosed vesicles.
• Receives proteins from the RER—can
further modify them
• Concentrates, packages, sorts proteins
Figure 5.10 The Endomembrane System (Part 2)
The Golgi receives
vesicles (a piece of the
ER that “buds” off)
from the ER.
Vesicles bud off from the Golgi apparatus
and are moved to the plasma membrane
or other organelles.
Figure 5.11 Lysosomes Isolate Digestive Enzymes from the Cytoplasm (Part 1)
lysosomes
originate from
the Golgi
apparatus.
They contain
digestive
enzymes—
macromolecules
are hydrolyzed
into monomers.
Figure 5.7 Eukaryotic Cells (Part 1)
5.3 What Features Characterize Eukaryotic Cells?
In the mitochondria, energy in fuel
molecules is transformed into energyrich ATP with O2: Cellular respiration.
Cells that require a lot of energy have a
lot of mitochondria.
Figure 5.7 Eukaryotic Cells (Part 1)
5.3 What Features Characterize Eukaryotic Cells?
The cytoskeleton:
• Supports and maintains cell shape
• Holds organelles in position
• Moves organelles
• Involved in cytoplasmic streaming
Figure 5.17 The Cytoskeleton
Figure 5.7 Eukaryotic Cells (Part 3)
5.3 What Features Characterize Eukaryotic Cells?
Chloroplasts: Site of photosynthesis—light
energy is converted to the energy of chemical
bonds
Chloroplasts have a double membrane like
mitochondria
Figure 5.7 Eukaryotic Cells (Part 3)
5.3 What Features Characterize Eukaryotic Cells?
Plant and protist cells have vacuoles:
• Store waste products and toxic
compounds; some may dissuade
herbivores
• Provide structure for plant cells—water
enters the vacuole by osmosis, creating
turgor pressure.
Figure 5.24 The Plant Cell Wall
5.5 How Did Eukaryotic Cells Originate?
Some organelles may have arose by
symbiosis (“living together”).
The endosymbiosis theory proposes
that mitochondria and chloroplast arose
when one cell engulfed another cell.
Figure 5.26 The Origin of Organelles (B)
image of Bacillus subtilis cells in an early stage of biofilm formation
http://www.cell.com/cell_picture_show-biofilms
cytoskeleton
http://upload.wikimedia.org/wikipedia/commons/0/09/FluorescentCells.jpg
Neurons in cerebral cortex of a six day old rat
http://www.nikonsmallworld.com/subjects/image/neuron/1
A collage of a
wide variety of
mammalian cells
http://
www.microscopyu
.com/smallworld/
gallery/contests/
2011/photos/
winners/19-2011large.jpg
5
Cell Membranes
Figure 3.20 Phospholipids (Part 2)
Figure 6.1 The Fluid Mosaic Model
6.1 What Is the Structure of a Biological Membrane?
The general structure of membranes is
known as the fluid mosaic model.
Phospholipids form a bilayer which is like
a “lake” in which a variety of proteins
“float.”
6.1 What Is the Structure of a Biological Membrane?
Membranes may vary in lipid composition.
Phospholipids vary in fatty acid chain
length, degree of saturation, and
phosphate groups.
Membranes may be up to 25 percent
cholesterol.
6.1 What Is the Structure of a Biological Membrane?
Membranes also contain proteins; the
number of proteins varies with cell
function.
Two types of membrane proteins:
• Peripheral membrane proteins lack
exposed hydrophobic groups and do not
penetrate the bilayer.
• Integral membrane proteins have
hydrophobic and hydrophilic regions or
domains.
Figure 6.3 Interactions of Integral Membrane Proteins
6.1 What Is the Structure of a Biological Membrane?
The proteins and lipids in the membrane
are independent and only interact
noncovalently.
6.1 What Is the Structure of a Biological Membrane?
Membranes are dynamic and are constantly forming,
transforming, fusing, and breaking down.
6.1 What Is the Structure of a Biological Membrane?
Membranes also have carbohydrates on
the outer surface that serve as
recognition sites for other cells and
molecules.
Glycolipids—carbohydrate + lipid
Glycoproteins—carbohydrate + protein
6.3 What Are the Passive Processes of Membrane Transport?
Membranes have selective permeability—
some substances can pass through, but
not others.
Passive transport—no outside energy
required (diffusion).
Active transport—energy required.
6.3 What Are the Passive Processes of Membrane Transport?
Simple diffusion: Small molecules pass
through the lipid bilayer.
Lipid-soluble molecules can diffuse
across the membrane.
Electrically charged and polar molecules
can not pass through easily.
6.3 What Are the Passive Processes of Membrane Transport?
Facilitated diffusion of polar molecules
(passive):
• Channel proteins have a central pore
lined with polar amino acids.
• Carrier proteins—membrane proteins
that bind some substances and speed
their diffusion through the bilayer.
6.3 What Are the Passive Processes of Membrane Transport?
Ion channels: Specific channel proteins
with hydrophilic pores.
Most are gated—can be closed or open to
ion passage.
Gate opens when protein is stimulated to
change shape. Stimulus can be a molecule
(ligand-gated) or electrical charge resulting
from many ions (voltage-gated).
Figure 6.11 A Gated Channel Protein Opens in Response to a
Stimulus
6.3 What Are the Passive Processes of Membrane Transport?
All cells maintain an imbalance of ion
concentrations across the plasma
membrane; thus a small voltage
potential exists.
Rate and direction of ion movement
through channels depends on the
concentration gradient and the
distribution of electrical charge.
Figure 6.14 A Carrier Protein Facilitates Diffusion (Part 1)
6.4 What Are the Active Processes of Membrane Transport?
Active transport: Moves substances
against a concentration and/or
electrical gradient—requires energy.
The energy source is often adenosine
triphosphate (ATP).
Figure 6.15 Three Types of Proteins for Active Transport
• Uniporters
• Symporters
• Antiporters
Figure 6.16 Primary Active Transport: The Sodium–Potassium
Pump
6.5 How Do Large Molecules Enter and Leave a Cell?
Macromolecules (proteins,
polysaccharides, nucleic acids) are too
large to cross the membrane.
They can be taken in or secreted by
means of membrane vesicles.
Figure 6.18 Endocytosis and Exocytosis (A)
Endocytosis: Processes that bring
molecules and cells into a eukaryotic cell.
The plasma membrane folds around the
material, forming a vesicle.
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