CHAPTER 11 CELL COMMUNICATION
•
•
Overview:
Cell-to-cell communication
– Is absolutely essential for multicellular organisms
•
External signals are converted into responses within the cell
Evolution of Cell Signaling
• Yeast cells
– Identify their mates by cell signaling
– The yeast mating behavior is coordinated by chemical signaling.
Type “a” cells secrete an a – factor, while type “b” cells secrete b – factor BUT type a
cells are attracted to b- factors while type b cells are attracted to a-factors.
There are two types of chemical signals.
1. Local Regulator
2. Hormone
1. Local Regulator
This is a chemical signal that communicates between two nearby cells.
There are two types of local regulator
a. paracrine signaling
the cell secretes the signal into extracellular fluid and the signal acts on
nearby target cells. Example, growth factors (which stimulate the cells to
divide and grow.
b. synaptic signaling
a nerve cell releases a signal e.g. neurotransmitter into the synapse, which is
the narrow space between two nerve cells or a muscle cell.
2. Hormone (hormonal communication)
This is a chemical signal which communicates between cells some distance
apart.
Hormonal communication has been described in both plants (ethylene gas
which promotes growth and ripening of fruits) and animals ( secretion of
insulin which regulates blood glucose level)
Dr. Harold Kay- Bio 1406
Chapters 11 & 12
1
LOCAL AND LONG DISTANCE SIGNALING
Cells in a multicellular organism
–
Communicate via chemical messengers
Animal and plant cells
–
Have cell junctions that directly connect the cytoplasm of adjacent
cells
Plasma membranes
Plasmodesmata
between plant cells
Gap junctions
between animal cells
(a) Cell junctions. Both animals and plants have cell junctions that allow molecules
To pass readily between adjacent cells without crossing plasma membranes.
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Chapters 11 & 12
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In local signaling, animal cells
–
May communicate via direct contact
(b) Cell-cell recognition. Two cells in an animal may communicate by interaction
between molecules protruding from their surfaces.
•
In other cases, animal cells
–
Communicate using local regulators
Local
signaling
Target
cell
Secreto
ry
vesicle
Local
regulator
diffuses
through
(a)
Paracrine signaling. A secreting cell
extracellular
acts
fluid
on nearby target cells by discharging
molecules of a local regulator (a
growth
Electrical
signal
along nerve
cell
triggers
release of
neurotransmitt
Neurotransmit
er
ter
diffuses
across
synapse
Target cell
is
(b) Synaptic stimulated
signaling. A nerve
cell
releases neurotransmitter
molecules
into a synapse, stimulating the
target cell.
Dr. Harold Kay- Bio factor,
1406for example) into the
extracellular
Chapters 11 & 12 fluid.
3
In long-distance signaling
–
Both plants and animals use hormones
Long-distance
signaling
Endocrine
cell
Bloo
d
vess
el
Hormone
travels
in
bloodstream
to target cells
Targ
et
cell
(c) Hormonal signaling.
Specialized
endocrine cells secrete
hormones
into body fluids, often the
blood.
Hormones may reach
virtually all
body cells.
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Chapters 11 & 12
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There are Three Stages of Cell Signaling:
1. Signal reception:
This is when the signal binds to the surface membrane protein called
the receptor. signal reception:
The signal behaves like a ligand. This is when a large molecule binds to a
small molecule.
Most signal molecules cannot pass freely through the plasma membrane.
The receptors for such signal molecules are located on the plasma
membrane.
They are called plasma- membrane receptors.
2. Signal transduction:
This is after the binding of the signal to the receptor, a series of changes or
transductions take place. This converts the signal into specific responses.
3. Cellular response:
the transduction system will trigger specific cellular response.
Dr. Harold Kay- Bio 1406
Chapters 11 & 12
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1. Signal reception
EXTRACELLULA
R
FLUID
1
Receptio
n
CYTOPLAS
M
Plasma
membrane
2
Transductio
n
Recepto
r
Relay molecules in a signal transduction
pathway
3
Respons
e
Activatio
n
of
cellular
response
Signal
molecul
e
Figure
11.5
. Reception: A signal molecule binds to a receptor protein, causing it to change
shape. The signal behaves as a ligand. This is a term for a small molecule that
binds to a larger molecule.
The binding between signal molecule (ligand)
–
And receptor is highly specific
Most signal molecules cannot pass freely through the plasma membrane. The
receptors for such signal molecules are located on the plasma memberane.
These are classified as three families of plasma-membrane receptors, namelya. G-protein- linked receptors
b. Tyrosine kinase receptors
c. Ion channel receptors
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Chapters 11 & 12
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Receptors in the Plasma Membrane
There are three families of membrane receptors
–
–
–
G-protein-linked
Tyrosine kinases
Ion channel
2. Signal Transduction Pathways
At each step in a pathway
–
The signal is transduced into a different form, commonly a
conformational change in a protein
–
Transduction: Cascades of molecular interactions relay signals from
receptors to target molecules in the cell
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Small Molecules and Ions as Second Messengers
Second messengers
–
Are small, nonprotein, water-soluble molecules or ions
1. Cyclic AMP (cAMP)
–
Is made from ATP
Many G- Proteins
–
Trigger the formation of cAMP, which then acts as a second
messenger in cellular pathways
2. Calcium ions and Inositol Triphosphate (IP3)
Calcium, when released into the cytosol of a cell
–
Acts as a second messenger in many different pathways
Calcium is an important second messenger
–
Because cells are able to regulate its concentration in the cytosol
3. Response:
Cell signaling leads to regulation of cytoplasmic activities or transcription
Must be:
a. Amplified
b. Specific
Dr. Harold Kay- Bio 1406
Chapters 11 & 12
8
The Cell Cycle – chapter 12
Unicellular organisms
–
Reproduce by cell division
Multicellular organisms depend on cell division for
–
–
–
Development from a fertilized cell
Growth
Repair
200 µm
(b) Growth and development.
This micrograph shows a
sand dollar embryo shortly
after the fertilized egg divided,
Figure 12.2 B, forming two cells (LM).
20 µm
(c) Tissue renewal. These dividing
bone marrow cells (arrow) will
give rise to new blood cells (LM).
C
Cell division results in genetically identical daughter cells
Cells duplicate their genetic material
–
Before they divide, ensuring that each daughter cell receives an
exact copy of the genetic material, DNA
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Chapters 11 & 12
9
Phases of the Cell Cycle
• The cell cycle consists of
– The mitotic phase
– Interphase
INTERPHASE
C
M yto
ito ki
s i ne
s si
s
G1
MI
(M TOT
) P IC
HA
SE
–
Figure 12.5
Phases of the Cell Cycle
• The cell cycle consists of
– The mitotic phase
– Interphase
INTERPHASE
S
(DNA synthesis)
C
M yto
ito ki
si ne
s s
is
G1
Dr. Harold Kay- Bio 1406
Chapters 11 & 12
Figure 12.5
MI
(M TOT
) P IC
HA
SE
10
G2
S
(DNA synthesis)
G2
Interphase can be divided into subphases
–
–
–
G1 phase
S phase
G2 phase
The mitotic phase
–
Is made up of mitosis and cytokinesis
Mitosis consists of five distinct phases:
-
Prophase
Prometaphase
Metaphase
Anaphase
telophase
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11
Cytokinesis: A Closer Look
•
In animal cells
–
Cytokinesis occurs by a process known as cleavage, forming a
cleavage furrow
Cleavage furrow
100 µm
Contractile ring of
microfilaments
Figure 12.9
A
Dr. Harold Kay- Bio 1406
Chapters 11 & 12
Daughter cells
(a) Cleavage of an animal cell (SEM)
12
In plant cells, during cytokinesis
–
A cell plate forms
Vesicles
forming
cell
plate
Figure 12.9 B
Dr. Harold Kay- Bio 1406
Chapters 11 & 12
13
Wall of
patent
cell
1 µm
Cell
plate
New cell
wall
Daughter
cells
(b) Cell plate formation in a plant cell
(SEM)
Binary Fission
Prokaryotes (bacteria)
–
•
Reproduce by a type of cell division called binary fission
In binary fission
–
–
The bacterial chromosome replicates
The two daughter chromosomes
actively move
Origin of
Cell wall
replication
apart
Plasma
Membrane
E. coli cell
Two copies
of origin
1 Chromosome replication begins.
Soon thereafter, one copy of the
origin moves rapidly toward the
other end of the cell.
2 Replication continues. One copy of
the origin is now at each end of
the cell.
Origin
3 Replication finishes. The plasma
membrane grows inward, and
new cell wall is deposited.
Figure 12.11 4 Two daughter cells result.
Dr. Harold Kay- Bio 1406
Chapters 11 & 12
14
Bacterial
Chromosome
Origin
The Cell Cycle Control System
•
The sequential events of the cell cycle
–
Are directed by a distinct cell cycle control
system, which is similar to a clock
G1
checkpoint
Control
system
G
1
M
M
checkpoint
Dr. Harold Kay- Bio 1406
Chapters 11 & 12
G2
checkpoint
15
G
2
S
The clock has specific checkpoints
–
Where the cell cycle stops until a go-ahead signal is received
G0
G1
checkpoint
G1
G1
Figure 12.15 A, B
(a) If a cell receives a go-ahead
signal at
the G1 checkpoint, the cell
continues
on in the cell cycle.
(b) If a cell does not receive a goahead
signal at the G1checkpoint, the cell
exits the cell cycle and goes into G0,
a
nondividing state.
The Cell Cycle Clock: Cyclins and
Cyclin-Dependent Kinases
Two types of regulatory proteins are involved in cell cycle control
Cyclins and cyclin-dependent kinases (Cdks)
Dr. Harold Kay- Bio 1406
Chapters 11 & 12
16
In density-dependent inhibition
–
Crowded cells stop dividing
Most animal cells exhibit anchorage dependence
–
In which they must be attached to a substratum to divide
(a) Normal mammalian cells. The
availability of nutrients, growth
factors, and a substratum for
attachment limits cell
density to a single layer.
Cells anchor to dish surface and
divide (anchorage dependence).
When cells have formed a complete single layer,
they stop dividing
(density-dependent inhibition).
If some cells are scraped away, the remaining cells
divide to fill the gap and then stop (density-dependent
inhibition).
Figure 12.18
A
25
µm
Cancer cells
–
Exhibit neither density-dependent inhibition nor anchorage
dependence
Cancer cells do not exhibit
anchorage dependence or
density-dependent inhibition.
(b) Cancer cells. Cancer cells usually
continue to divide well beyond a
single layer, forming a clump of
overlapping cells.
Figure 12.18
B
Dr. Harold Kay- Bio 1406
Chapters 11 & 12
25
µm
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Loss of Cell Cycle Controls in Cancer Cells
Cancer cells
–
–
Do not respond normally to the body’s control mechanisms
Form tumors
Malignant tumors invade surrounding tissues and can metastasize
–
Exporting cancer cells to other parts of the body where they may
form secondary tumors
A malignant tumor is invasive enough to impair normal function of one or more organs
of the body. Only an individual with a malignant tumor is said to have cancer.
Properties of malignant (cancerous) tumors
1. excessive proliferation
2. aberrant metabolism
3. may nnot attach to neighboring cells
4. unusual number of chromosomes
These cell may spread to other tissues and organs possibly entering the
blood and lymph circulation.
The spread of cancer cells beyond their original sites is called
METASTASIS.
TREATMENT
For Benign cancer cells
1. Excision
For Malignant cancer cells
1. Excision
2. Radiotherapy and Chemotherapy
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18