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
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
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
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 4.17 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)
Chapter 5 Cell Membranes and Signaling
Key Concepts
5.1 Biological Membranes Have a
Common Structure and Are Fluid
5.2 Some Substances Can Cross the
Membrane by Diffusion
5.3 Some Substances Require Energy to
Cross the Membrane
Chapter 5 Cell Membranes and Signaling
5.4 Large Molecules Cross the Membrane
via Vesicles
5.5 The Membrane Plays a Key Role in a
Cell’s Response to Environmental
Signals
5.6 Signal Transduction Allows the Cell to
Respond to Its Environment
Figure 5.1 Membrane Molecular Structure
Figure 8.7 The structure of a transmembrane protein
Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid
Membranes may differ in lipid composition as
there are many types of phospholipids.
Phospholipids may differ in:
Fatty acid chain length
Degree of saturation
Kinds of polar groups present
Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid
Two important factors in membrane fluidity:
Lipid composition—types of fatty acids can
increase or decrease fluidity
Temperature—membrane fluidity decreases
in colder conditions
Figure 5.2 Rapid Diffusion of Membrane Proteins
Concept 5.2 Some Substances Can Cross the Membrane by Diffusion
Biological membranes allow some
substances, and not others, to pass.
This is known as selective
permeability.
Two processes of transport:
Passive transport does not require
metabolic energy.
Active transport requires input of
metabolic energy.
Concept 5.2 Some Substances Can Cross the Membrane by Diffusion
Osmosis is the diffusion of water across
membranes.
It depends on the concentration of solute
molecules on either side of the
membrane.
Water passes through special membrane
channels.
Concept 5.2 Some Substances Can Cross the Membrane by Diffusion
When comparing two solutions separated
by a membrane:
A hypertonic solution has a higher solute
concentration.
Isotonic solutions have equal solute
concentrations.
A hypotonic solution has a lower solute
concentration.
Figure 8.11 Osmosis
Figure 5.3A Osmosis Can Modify the Shapes of Cells
Figure 5.3B Osmosis Can Modify the Shapes of Cells
Figure 5.3C Osmosis Can Modify the Shapes of Cells
Figure 8.12 The water balance of living cells
Figure 36.3 Water potential and water movement: a mechanical model
Figure 36.4 Water relations of plant cells
Concept 5.2 Some Substances Can Cross the Membrane by Diffusion
The concentration of solutes in the
environment determines the direction of
osmosis in all animal cells.
In other organisms, cell walls limit the
volume that can be taken up.
Turgor pressure is the internal pressure
against the cell wall—as it builds up, it
prevents more water from entering.
Concept 5.2 Some Substances Can Cross the Membrane by Diffusion
Diffusion may be aided by channel
proteins.
Channel proteins are integral membrane
proteins that form channels across the
membrane.
Substances can also bind to carrier
proteins to speed up diffusion.
Both are forms of facilitated diffusion.
Concept 5.2 Some Substances Can Cross the Membrane by Diffusion
Ion channels are a type of channel
protein—most are gated, and can be
opened or closed to ion passage.
A gated channel opens when a stimulus
causes the channel to change shape.
The stimulus may be a ligand, a
chemical signal.
Concept 5.2 Some Substances Can Cross the Membrane by Diffusion
A ligand-gated channel responds to its
ligand.
A voltage-gated channel opens or closes
in response to a change in the voltage
across the membrane.
Figure 5.4 A Ligand-Gated Channel Protein Opens in Response to a Stimulus
Figure 5.5 Aquaporins Increase Membrane Permeability to Water (Part 1)
Figure 5.6 A Carrier Protein Facilitates Diffusion (Part 1)
Concept 5.2 Some Substances Can Cross the Membrane by Diffusion
Transport by carrier proteins differs from
simple diffusion, though both are driven
by the concentration gradient.
The facilitated diffusion system can
become saturated—when all of the
carrier molecules are bound, the rate of
diffusion reaches its maximum.
Table 5.1 Membrane Transport Mechanisms
Concept 5.3 Some Substances Require Energy to Cross the Membrane
Two types of active transport:
Primary active transport involves
hydrolysis of ATP for energy.
Secondary active transport uses the
energy from an ion concentration
gradient, or an electrical gradient.
Concept 5.3 Some Substances Require Energy to Cross the Membrane
The sodium–potassium (Na+–K+) pump
is an integral membrane protein that
pumps Na+ out of a cell and K+ in.
One molecule of ATP moves two K+ and
three Na+ ions.
Figure 5.7 Primary Active Transport: The Sodium–Potassium Pump
Concept 5.3 Some Substances Require Energy to Cross the Membrane
Secondary active transport uses energy
that is “regained,” by letting ions move
across the membrane with their
concentration gradients.
Secondary active transport may begin
with passive diffusion of a few ions, or
may involve a carrier protein that
transports both a substance and ions.
Figure 8.16 Review: passive and active transport compared
Figure 5.8 Endocytosis and Exocytosis (Part 1)
Figure 5.8 Endocytosis and Exocytosis (Part 2)
Concept 5.4 Large Molecules Cross the Membrane via Vesicles
In phagocytosis (“cellular eating”), part
of the membrane engulfs a large
particle or cell.
A food vacuole (phagosome) forms and
usually fuses with a lysosome, where
contents are digested.
Concept 5.4 Large Molecules Cross the Membrane via Vesicles
In pinocytosis (“cellular drinking”),
vesicles also form.
The vesicles are smaller and bring in
fluids and dissolved substances, as in
the endothelium near blood vessels.
Figure 5.9 Receptor-Mediated Endocytosis (Part 1)
Figure 5.9 Receptor-Mediated Endocytosis (Part 2)
Concept 5.4 Large Molecules Cross the Membrane via Vesicles
Exocytosis moves materials out of the
cell in vesicles.
The vesicle membrane fuses with the
plasma membrane and the contents are
released into the cellular environment.
Exocytosis is important in the secretion of
substances made in the cell.