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Biology 1
Mr. Greene
Unit 3
After viewing the list of items on the board, work
with a partner and make two new lists: those
items on the list comprised of cells and those
items not comprised of cells.
Give a rationale for each answer.
 How
were cells discovered?
 Why
does cell shape vary?
 What
enables eukaryotes to perform more
specialized functions than prokaryotes?





1665
used a crude compound
microscope to examine a
slice of cork from the
bark of an oak tree
observed tiny, hollow
boxes
called them cells
did not know that cells
are the basic unit of
living things
 cell
- the smallest
unit that can carry out
all of the activities
necessary for life
1
• All organisms are composed of 1 or more cells.
2
• The cell is the basic unit of organization of
organisms.
3
• All cells come from preexisting cells.
 Some
organisms are made up of many
cells and some are made up of only one
• unicellular - one celled organisms
• multicellular - many celled organisms
 do
everything living
organisms do
 both prokaryote and
eukaryote (some
algae and yeast)
 Colonial
• unicellular’s that live in
groups of individuals
• ex. Volvox
 interdependent
 each
has its own
function
 Cell
specialization
• separate roles for each
type of cell
Visual Concept: Cell Theory
 Cells
vary greatly in their size and shape.
A
cell’s shape reflects its function. Cell size is
limited by a cell’s surface area-to-volume ratio.
 Cells
can be branched, flat, round, or
rectangular.
 All
substances that enter or leave a cell must
cross the surface of the cell.
A
cell’s ability to move substances across its
surface can be estimated by finding its surface
area-to-volume ratio.
 Cells
with greater surface area-to-volume
ratios can exchange substances more
efficiently.
 When
comparing cells of the same shape, small
cells have greater surface area-to-volume
ratios than large cells.
 So, small
cells function more efficiently than
large cells.


All cells share common structural features, including a cell
membrane, cytoplasm, ribosomes, and DNA.
The cell membrane is the outer layer that covers a cell’s
surface and acts as a barrier between the outside
environment and the inside of the cell.
The cytoplasm is the
region of the cell
within the cell
membrane. The
cytoplasm includes
the fluid inside the cell
called the cytosol.

cell membrane
inside cell
outside cell
 Cell
membranes are composed of two
phospholipid layers.
• The cell membrane has two major functions.
 forms a boundary between inside and outside of the
cell
A
ribosome is a cellular structure that makes
proteins.
 The
DNA of a cell provides instructions for
making proteins, regulates cellular activities,
and enables cells to reproduce.
Features of Prokaryotic Cells
A
prokaryote is an organism made of a single
prokaryotic cell.
 Prokaryotic
cells do not have a nucleus or other
internal compartments. The genetic material of
a prokaryotic cell is a single loop of DNA.
 For
millions of years, prokaryotes were the only
organisms on Earth.
Features of Eukaryotic Cells
 A eukaryote is an organism made up of one or
more eukaryotic cells. All multicellular
organisms are made of eukaryotic cells.
 The
DNA of a eukaryotic cell is found in an
internal compartment of the cell called the
nucleus.
 All
eukaryotic cells have membrane-bound
organelles. An organelle is a small structure
found in the cytoplasm that carries out specific
activities inside the cell.
 Each
organelle in a eukaryotic cell performs
distinct functions.
 The
complex organization of eukaryotic cells
enables them to carry out more specialized
functions than prokaryotic cells.
Prokaryotic and Eukaryotic Cells
Cell membrane
Cytoplasm
Prokaryotic Cell
Cell
membrane
Cytoplasm
Nucleus
Eukaryotic Cell
Organelles
1 - Prokaryote - primitive - lacks an internal structure – single-celled (bacteria)
2 - Eukaryote – advanced – some unicellular/ some multicellular
Visual Concept: Comparing
Prokaryotes and Eukaryotes
 Microscope
observations of organisms led to
the discovery of the basic characteristics
common to all living things.
A
cell’s shape reflects its function. Cell size is
limited by a cell’s surface area-to-volume ratio.
 The
complex organization of eukaryotic cells
enable them to carry out more specialized
functions than prokaryotic cells.
Use a light microscope to view a slide of a
eukaryotic cell. Try to find the nucleus of the cell
and give your reason why you identified the
structure as the nucleus.
 What
 How
does the cytoskeleton do?
does DNA direct activity in the cytoplasm?
 What
organelles are involved in protein
production?
 What
 How
are vesicles and vacuoles?
does the cell get energy?
 Eukaryotic
cells have an intricate network of
protein fibers called the cytoskeleton which
provides the interior framework of the cell.
 The
cytoskeleton helps the cell move, keep its
shape, and organize its parts.
 There
are three different kinds of cytoskeleton
fibers: microfilaments, microtubules, and
intermediate fibers.
 There
are little
structures called
organelles that
carry out specific
functions.
Click to animate the image.
A
B
C
D
 DNA
contains instructions for making proteins
which control most of the activity of the cell.
 The
DNA of eukaryotic cells is stored in the
nucleus.
 DNA
instructions are copied as RNA messages,
which leave the nucleus. In the cytoplasm,
ribosomes use the RNA messages to assemble
proteins.
Nucleus
 A double membrane called the nuclear
envelope surrounds the nucleus.
 Nuclear pores located on the nuclear envelope
act as channels to allow certain molecules to
move in and out of the nucleus.
 The nucleolus is a structure within the nucleus
where ribosome parts are made.
 These ribosome parts are transported out of
the nucleus into the cytoplasm where they are
assembled to form a complete ribosome.
Click to animate the image.
B
C
D
A
Ribosomes
 Each
ribosome in a cell is made of RNA and
many different proteins.
 Ribosomes
that are suspended in the cytosol
are called “free” ribosomes.
 Free
ribosomes make proteins that remain
inside the cell.
Ribosomes
 Ribosomes
that are attached to the membrane
of another organelle are called “bound”
ribosomes.
 Bound
ribosomes make proteins that are
exported from the cell.
 Ribosomes
can switch between being bound or
free, depending on what proteins the cell needs
to make.
 Some
proteins that a cell manufactures are
needed outside the cell that makes them.
 Proteins
that are sent outside the cell are
packaged in vesicles. Vesicles are small,
membrane-bound sacs that enclose the
proteins and keep them separate from the rest
of the cytoplasm.
 The
endoplasmic reticulum and Golgi
apparatus are organelles involved in preparing
proteins for extracellular export.
Endoplasmic Reticulum
 The
endoplasmic reticulum, or ER, is an
extensive system of internal membranes that
moves proteins and other substances through
the cell.
 The
membranes of the ER are connected to the
outer membrane of the nuclear envelope.
 The
endoplasmic reticulum is divided into two
portions: rough ER and smooth ER.
Endoplasmic Reticulum
 The portion of the ER with attached ribosomes is
called rough ER because it has a rough
appearance when viewed with an electron
microscope.
 The
portion of the ER with no attached
ribosomes is smooth ER because it has a smooth
appearance when viewed with a microscope.
 The
ribosomes on the rough ER make proteins
that are packaged into vesicles. Enzymes of the
smooth ER make lipids and break down toxic
substances.
Visual Concept:
ER and Ribosomes
Golgi Apparatus
 The
Golgi apparatus is a set of flattened,
membrane-bound sacs.
 The
Golgi apparatus helps modify, sort, and
package cell products for distribution.
Making and Exporting Proteins
 The ribosomes located on the rough ER make
proteins which then cross into the membranes
of the ER. The ER membrane then pinches off
and forms a vesicle around the proteins.
 Vesicles transport the proteins from the rough
ER to the Golgi apparatus, where they are
modified by enzymes and repackaged in new
vesicles.
 These new vesicles transport the modified
proteins to the cell membrane to be released
outside the cell.
Lysosomes
 Vesicles help maintain homeostasis by storing
and releasing a variety of substances as the cell
needs them.
A
lysosome is a vesicle produced by the Golgi
apparatus that contains enzymes that break
down large molecules.
 Lysosomes
recycle old or damaged organelles
and digest food particles to provide nutrients for
the cell.
Visual Concept: Lysosomes
Vacuoles
 A vacuole is a fluid-filled vesicle found in the
cytoplasm of many plant cells.
 Plant
cells contain a large compartment called
the central vacuole, which stores water, ions,
nutrients, and wastes.
 When
water fills the central vacuole, the cell
becomes rigid, allowing the plant to stand up.
When the vacuole loses water, the cell shrinks,
and the plant wilts.
Visual Concept: Vacuoles
Other Vacuoles
 Some protists have contractile vacuoles which
pump excess water out of the cell in order to
control the concentration of salts and other
substances.
A
food vacuole is another type of vacuole. It is
formed when the cell membrane surrounds
food particles outside the cell and pinches off
to form a vesicle inside the cell.
 Cells
need a constant source of energy.
 The
energy for cellular functions is produced
by chemical reactions that occur in the
mitochondria and chloroplasts.
 In
both organelles, chemical reactions produce
adenosine triphosphate (ATP), the form of
energy that fuels almost all cell processes.
Chloroplasts
 A chloroplast is an organelle found in plant and
algae cells that uses light energy to make
carbohydrates from carbon dioxide and water.
 Chloroplasts
are surrounded by two
membranes and have several stacks of
flattened sacs where energy production takes
place.
 Plant
cells may have several chloroplasts.
Mitochondria
 Mitochondria are cell organelles that use
energy from organic compounds to make ATP.
 Most
of the ATP needed by a cell is produced
inside mitochondria. Both animal and plant
cells contain mitochondria.
A
smooth outer membrane and a folded inner
membrane surround a mitochondrion. ATP is
produced by enzymes on the folds of the inner
membrane.

a relatively inflexible
structure that surrounds
the plasma membrane
• provides support and
•
•
•
•

protection
much thicker
found in plants, fungi,
bacteria have cell walls
composed of cellulose in
plants
composed of chitin in fungi
animals have no cell
walls
cilia
 short hairlike
projections from the
plasma membrane
 organized in tightly
packed rows
 move like a "wave" in
a football stadium
flagella
 long, threadlike cable
projections
 move in a whiplike
motion
 one or two per cell that
have them
 consists of a circle of 9
microtubules around 2
central microtubules
 major means of
locomotion for
unicellular
 The
cytoskeleton helps the cell move, keep its
shape, and organize its parts
 DNA
instructions are copied as RNA messages,
which leave the nucleus. In the cytoplasm,
ribosomes use the RNA messages to assemble
proteins.
 The
endoplasmic reticulum and Golgi
apparatus are organelles involved in preparing
proteins for extracellular export.
 Vesicles
help maintain homeostasis by storing
and releasing a variety of substances as the
cell needs them.
 The
energy for cellular functions is produced
by chemical reactions that occur in the
mitochondria and chloroplasts.
What type of society would you prefer to live in:
one in which you must do everything for yourself,
including growing and gathering food, building
shelter, etc., or one in which each person does
the job that they do best?
What are some advantages to having each
person do a specialized job?
What are some advantages to doing everything
yourself?
 What
makes cells and organisms different?
 How
are cells organized in a complex
multicellular organism?
 What
makes an organism truly multicellular?
 Both
prokaryotic and eukaryotic cells can have
a variety of shapes and structures.
 The
function of a cell is determined by its
shape and the organelles found in the cell.
 The
different organelles and features of cells
enable organisms to function in unique ways in
different environments.
Diversity in Prokaryotes
 Prokaryotes can vary in shape, the way they
obtain and use energy, and their ability to
move.
 Many
prokaryotes have a flagellum, a long,
hair-like structure that grows out of the cell and
enables the cell to move through its
environment.
 Prokaryotes
may also have pili, short
outgrowths that allow the cell to attach to
surfaces or other cells.
B
C
D
A
E
F
Click to animate the image.
Eukaryotic Cell Specialization
 Eukaryotic cells can vary in shape and external
features.
 Depending
on their function, eukaryotic cells
can also vary in their internal organelles. For
example, muscle cells, which use large
amounts of energy, contain many mitochondria.
 Animal
and plant cells are two types of
eukaryotic cells. Plant cells also have
chloroplasts, a large vacuole, and a cell wall.
I
J
C
D
K
H
A
B
E
A
M
L
J
F
I
H
G
G
D
C
B
E
F
Click to animate the image.
 Plants
and animals have many highly
specialized cells that are arranged into tissues,
organs, and organ systems.
A
tissue is a distinct group of similar cells that
perform a common function.
 An
organ is a collection of tissues that work
together to form a structure which performs a
specific function.
 An
organ system is composed of a group of
organs that work together to perform major
body functions.
Click to animate the image.
 Unicellular
organisms can thrive independently
or live together in groups.
 Cells
that are permanently associated but do
not work together or integrate cell activities are
called colonial organisms.
A
multicellular organism is composed of many
individual, permanently associated cells that
coordinate their activities with each other. True
multicellularity occurs only in eukaryotes.
Visual Concept: Comparing
Unicellular and Multicellular
 In
a multicellular body, cells are
interdependent. Distinct types of cells have
specialized functions to help the organism
survive.
 The
individual cells in a multicellular organism
cannot survive alone and are dependent on the
other cells of the organism.
 Must
multicellular organisms begin as a single
cell, which divides to form more cells. These
cells then grow and become specialized in a
process called differentiation.
Visual Concept:
Differentiation
 The
different organelles and features of cells
enable organisms to function in unique ways in
different environments.
 Plants
and animals have many highly
specialized cells that are arranged into tissues,
organs, and organ systems.
A
multicellular organism is composed of many
individual, permanently associated cells that
coordinate their activities with each other.
Humans must have their body temperature at a
constant temperature. Make a list of ways the
body responds when it gets cold.
 How
does the cell membrane help a cell
maintain homeostasis?
 How
does the cell membrane restrict the
exchange of substances?
 What
are some functions of membrane
proteins?
 All
living things respond to their environments.
These reactions help our bodies maintain
homeostasis.
 Homeostasis
is the maintenance of stable
internal conditions in a changing environment.
Individual cells, as well as organisms, must
maintain homeostasis in order to live.
 One
way that a cell maintains homeostasis is by
controlling the movement of substances across
the cell membrane.
 Cells
are suspended in a fluid environment.
Even the cell membrane is fluid. It is made up
of a “sea” of lipids in which proteins float.
 By
allowing some materials but not others to
enter the cell, the cell membrane acts as a
gatekeeper.
 The
cell membrane also provides structural
support to the cytoplasm, recognizes foreign
material, and communicates with other cells, all
of which contribute to maintaining
homeostasis.
 The
cell membrane is made of phospholipids.
A
phospholipid is a specialized lipid made of a
phosphate “head” and two fatty acid “tails.”
 The
phosphate head is polar and is attracted to
water.
 The
fatty acid tails are nonpolar and are
repelled by water.
Click above to play the video
Structure
 Because there is water inside and outside the
cell, the phospholipids form a double layer
called the lipid bilayer.
 The
nonpolar tails, repelled by water, make up
the interior of the lipid bilayer.
 The
polar heads are attracted to the water, so
they point toward the surfaces of the lipid
bilayer. One layer of polar heads faces the
cytoplasm, while the other layer is in contact
with the cell’s immediate surroundings.
Click above to play the video
Click above to play the video
Barrier
 Only certain substances can pass through the
lipid bilayer.
 The
phospholipids form a barrier through
which only small, nonpolar substances can
pass.
 Ions
and most polar molecules are repelled by
the nonpolar interior of the lipid bilayer.
 Various
proteins can be found in the cell
membrane. Some proteins face inside the cell,
and some face outside. Other proteins may
stretch across the lipid bilayer and face both
inside and outside.
 Proteins
are made of amino acids. Some amino
acids are polar, and others are nonpolar.
 The
attraction and repulsion of polar and
nonpolar parts of the protein to water help hold
the protein in the membrane.
Types of Proteins
 Proteins in the cell membrane include cellsurface markers, receptor proteins, enzymes,
and transport proteins.
 Cell-surface
markers act like a name tag. A
unique chain of sugars acts as a marker to
identify each type of cell.
Types of Proteins
 Receptor proteins enable a cell to sense its
surroundings by binding to certain substances
outside the cell. When this happens, it causes
changes inside the cell.
 Many
substances that the cell needs cannot
pass through the lipid bilayer. Channel or
transport proteins aid the movement of these
substances into and out of the cell.
 One
way that a cell maintains homeostasis is by
controlling the movement of substances across
the cell membrane.
 The
lipid bilayer is selectively permeable to
small, nonpolar substances.
 Proteins
in the cell membrane include cellsurface markers, receptor proteins, enzymes,
and transport proteins.
Write three sentences using the word diffuse or
one of its conjugates. Consult a dictionary if you
have trouble thinking of how the word is used.
 What
determines the direction in which passive
transport occurs?
 Why
 How
is osmosis important?
do substances move against their
concentration gradients?
 In
a solution, randomly moving molecules tend
to fill up a space. When the space is filled
evenly, a state called equilibrium is reached.
 The
amount of a particular substance in a given
volume is called the concentration of the
substance.When one area has a higher
concentration than another area does, a
concentration gradient exists.
 The
movement of substances down a
concentration gradient is called diffusion.
 The
cell membrane separates the cytoplasm
from the fluid outside the cell.
 Some
substances enter and leave the cell by
diffusing across the cell membrane.
 The
direction of movement depends on the
concentration gradient and does not require
energy.
 In
passive transport,
substances cross the cell
membrane down their
concentration gradient.
 Some
substances diffuse
through the lipid bilayer.
 Other
substances diffuse
through transport proteins.
Simple Diffusion
 Small, nonpolar molecules can pass directly through
the lipid bilayer. This type of movement is called simple
diffusion.


Oxygen moves down its concentration gradient into the
cell. Carbon dioxide diffuses out of the cell.
Natural steroid hormones, which are nonpolar and fat
soluble, can also diffuse across the lipid bilayer.
Facilitated Diffusion
 Many ions and polar molecules that are
important for cell function do not diffuse easily
through the nonpolar lipid bilayer.
 During
facilitated diffusion, transport proteins
help these substances diffuse through the cell
membrane.
 Two
types of transport proteins are channel
proteins and carrier proteins.
Facilitated Diffusion
 Ions, sugars, and amino acids can diffuse
through the cell membrane through channel
proteins.
 These
proteins, sometimes called pores, serve
as tunnels through the lipid bilayer.
 Each
channel allows the diffusion of specific
substances that have the right size and charge.
Click above to play the video.
Facilitated Diffusion
 Carrier proteins transport substances that fit
within their binding site.
A
carrier protein binds to a specific substance
on one side of the cell membrane. This binding
causes the protein to change shape.
 As
the protein’s shape changes, the substance
is moved across the membrane and is released
on the other side.
Click above to play the video.
 Water
can diffuse across a selectively
permeable membrane in a process called
osmosis.
 Osmosis
in cells is a form of facilitated
diffusion. Polar water molecules do not diffuse
directly through the bilayer. But the cell
membrane contains channel proteins that only
water molecules can pass through.
 Osmosis
allows cells to maintain water balance
as their environment changes.
 When
ions and polar substances dissolve in
water, they attract and bind some water
molecules. The remaining water molecules are
free to move around.
 If
a concentration gradient exists across a
membrane for solutes, a concentration gradient
also exists across the membrane for free water
molecules.
 Osmosis
occurs as free water molecules move
down their concentration gradient into the
solution that has the lower concentration of free
water molecules.
Click above to play the video.
 The
direction of water movement in a cell
depends on the concentration of the cell’s
environment.
 If
the solution is hypertonic, or has a higher
solute concentration than the cytoplasm does,
water moves out of the cell. The cell loses water
and shrinks.
 If
the solution is isotonic, or has the same solute
concentration that the cytoplasm does, water
diffuses into and out of the cell at equal rates.
The cell stays the same size.
 If
the solution is hypotonic, or has a lower solute
concentration than the cytoplasm does, water
moves into the cell. The cell gains water and
expands in size.
 If
left unchecked, the swelling caused by a
hypotonic solution could cause a cell to burst.
 The
rigid cell walls of plants and fungi prevent
the cells of these organisms from expanding
too much. In fact, many plants are healthiest in a
hypotonic environment.
 Some
unicellular eukaryotes have contractile
vacuoles, which collect excess water inside the
cell and force the water out of the cell.
 Animal
cells have neither cell walls nor
contractile vacuoles.
 Many
animal cells can avoid swelling caused
by osmosis by actively removing solutes from
the cytoplasm.
 In
order to move substances against their
concentration gradients, cells must use energy.
 Active
transport requires energy to move
substances against their concentration
gradients.
 Most
often, the energy needed for active
transport is supplied directly or indirectly by
ATP.
Click above to play the video.
Pumps
 Many active transport processes use carrier
proteins to move substances.
 In
facilitated diffusion, the carrier proteins do
not require energy.
 In
active transport, the carrier proteins do
require energy to “pump” substances against
their concentration gradient.
 The
sodium-potassium pump is a carrier
protein that actively transports three sodium
ions out of the cell and two potassium ions into
the cell.
 This
pump is one of the most important carrier
proteins in animal cells. It prevents sodium ions
from building up in the cell, resulting in
osmosis into the cell.
 The
concentration gradients of sodium ions and
potassium ions also help transport other
substances, such as glucose, across the cell
membrane.
Click above to play the video.
Vesicles
 Many substances, such as proteins and
polysaccharides, are too large to be
transported by carrier proteins.
 Instead, they
cross the cell membrane in
vesicles, which are membrane-bound sacs.
 The
vesicle membrane is a lipid bilayer, like
the cell membrane. Therefore, vesicles can bud
off from the membrane, fuse with it, or fuse with
other vesicles.
Vesicles
 The movement of a large substance into a cell
by means of a vesicle is called endocytosis.
 During
endocytosis the cell membrane forms a
pouch around the substance.
 The
pouch then closes up and pinches off from
the membrane to form a vesicle inside the cell.
 Vesicles
that form by endocytosis may fuse
with lysosomes or other organelles.
Vesicles
 The movement of material out of a cell by
means of a vesicle is called exocytosis.
 During
exocytosis, vesicles inside the cell fuse
with the cell membrane. From the cell
membrane, the contents of the vesicle are
released to the outside of the cell.
 Cells
use exocytosis to export proteins
modified by the Golgi apparatus. Some cells
also use exocytosis to remove bacteria or other
microbes.
 In
passive transport, substances cross the cell
membrane down their concentration gradient.
 Osmosis
allows cells to maintain water balance
as their environment changes.
 Active
transport requires energy to move
substances against their concentration
gradients.
Write several sentences that describe where
hormones are produced, how they reach the cells
they stimulate, and how those target cells
recognize the hormones.
 How
do cells use signal molecules?
 How
do cells receive signals?
 How
do cells respond to signaling?
 We
communicate in many ways to share
information.
 Cells
in both multicellular and unicellular
organisms need to communicate in order to
coordinate activities.
 Cells
use various methods of communication.
 These
methods vary depending on whether the
target is specific or general. They also depend
on whether the target is nearby or far away.
 Cells
communicate and coordinate activity by
sending chemical signals that carry
information to other cells.
A
signaling cell produces a signal, often a
molecule, that is detected by the target cell.
 Typically, target
cells have specific proteins
that recognize and respond to the signal.
 Neighboring
cells can communicate through
direct contact between their membranes.
 Short-distance
signals may act locally, a few
cells away from the originating cell.
 Long-distance
signals are carried by hormones
and nerve cells.
 Hormones
.
are signal molecules that are made
in one part of the body.
 Hormones
are distributed widely in the
bloodstream throughout the body, but they
affect only specific cells.
 Nerve
cells also signal information to distant
locations in the body, but their signals are not
widely distributed.
 While
most signal molecules originate within
the body, some signals come from outside.
A
target cell is bombarded by hundreds of
signals. But it recognizes and responds only to
the few signals that are important for its
function.
 This
response to some signals, but not to
others, is made possible by receptor proteins,
such as the ones in the cell’s membrane.
A
receptor protein binds specific substances,
such as signal molecules.
 The
outer part of the receptor protein is folded
into a unique shape, called the binding site. A
receptor protein binds only to signals that
match the specific shape of its binding site.
 Only
the “right” shape can fit into the receptor
protein while the “wrong” shape have no effect
on that particular receptor protein.
A
cell may also have receptor proteins that
bind to molecules in its environment.
 Receptor
proteins enable a cell to respond to
its environment.
 Once
it binds the signal molecule, the receptor
protein changes its shape in the membrane.
 This
change in shape relays information into
the cytoplasm of the target cell.
 When
a signal molecule binds to a receptor
protein, the protein changes shape, which
triggers changes in the cell membrane.
 The
cell may respond to a signal by changing
its membrane permeability, by activating
enzymes, or by forming a second messenger.
 Transport
proteins may open or close in
response to a signal.
 Some
receptor proteins are enzymes or they
activate enzymes in the cell membrane.
Enzymes trigger chemical reactions in the cell.
 Binding
of a signal molecule outside the cell
may cause a second messenger to form. The
second messenger acts as a signal molecule
within the cell and causes changes in the
cytoplasm and nucleus.
 Cells
communicate and coordinate activity by
sending chemical signals that carry
information to other cells.
A
receptor protein binds only to the signals that
match the specific shape of its binding site.
 The
cell may respond to a signal by changing
its membrane permeability, by activating
enzymes, or by forming a second messenger.