1. Cell Communication

2. Overview: The Cellular Internet
Cell-to-cell communication is absolutely essential for multicellular organisms
Nerve cells must communicate pain signals to muscle cells (stimulus) in order for muscle cells to initiate a response to pain

3. Biologists have discovered some universal mechanisms of cellular regulation

4. Signal Transduction Pathway
External Signals
Signal Transduction Pathway

5. Yeast cells identify their mates by cell signaling (early evidence of signaling)
 factor
Receptor
Exchange of mating factors.
Each cell type
secretes a
mating factor
that binds to
receptors on
the other cell
type. 1
Mating. Binding
of the factors to
    receptors
induces changes
     in the cells that
    lead to their
    fusion.
New a/ cell.
The nucleus of
the fused cell
includes all the
genes from the
a and a cells. 2 3
Yeast cell,
mating type a
mating type  
a/ a

6. Signal Transduction Pathways
Convert signals on a cell’s surface into cellular responses
Are similar in microbes and mammals, suggesting an early origin

7. Local and Long-Distance Signaling
Cells in a multicellular organism (tissues, organs, systems) communicate via chemical messengers
A hormone is a chemical released by a cell in one part of the body, that sends out messages that affect cells in other parts of the organism
All multicellular organisms produce hormones
Plant hormones are also called phytohormones
Hormones in animals are often transported in the blood

8. 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
Figure 11.3
(a) Cell junctions. Both animals and plants have cell junctions that allow molecules
to pass readily between adjacent cells without crossing plasma membranes.

9. In local signaling, animal cells May communicate via direct contact
Figure 11.3
(b) Cell-cell recognition. Two cells in an animal may communicate by interaction
between molecules protruding from their surfaces.

10. diffuses across synapse
In other cases, animal cells
Communicate using local regulators
(a) Paracrine signaling. A secreting cell acts on nearby target cells by discharging molecules of a local regulator (a growth factor, for example) into the extracellular fluid.
(b) Synaptic signaling. A nerve cell releases neurotransmitter molecules into a synapse, stimulating the target cell.
Local regulator
diffuses through
extracellular fluid
Target cell
Secretory
vesicle
Electrical signal
along nerve cell
triggers release of
neurotransmitter
Neurotransmitter
diffuses across synapse
is stimulated
Local signaling

11. Long-distance signaling
In long-distance signaling
Both plants and animals use hormones (e.g. Insulin)
Hormone travels
in bloodstream
to target cells
(c) Hormonal signaling. Specialized endocrine cells secrete hormones into body fluids, often the blood Hormones may reach virtually all body cells.
Long-distance signaling
Blood
vessel
Target
cell
Endocrine cell
Figure 11.4 C

12. The Three Stages of Cell Signaling
Earl W. Sutherland
Discovered how the hormone epinephrine acts on cells

13. Sutherland’s Three Steps
Sutherland suggested that cells receiving signals went through three processes
Reception
Transduction
Response

14. Relay molecules in a signal transduction pathway
Overview of cell signaling
EXTRACELLULAR
FLUID
Receptor
Signal
molecule
Relay molecules in a signal transduction pathway
Plasma membrane
CYTOPLASM
Activation
of cellular
response
Figure 11.5
Reception 1
Transduction 2
Response 3

15. Step One - Reception
Reception occurs when a signal molecule (ligand) binds to a receptor protein, causing it to change shape
Receptor protein is on the cell surface

16. The binding between signal molecule (ligand) and receptor is highly specific
A conformational change in a receptor is often the initial transduction of the signal

17. Step Two - Transduction
The binding of the signal molecule alters the receptor protein in some way
The signal usually starts a cascade of reactions known as a signal transduction pathway
Multistep pathways can amplify a signal

18. Step Three - Response
Cell signaling leads to regulation of cytoplasmic activities or transcription
Signaling pathways regulate a variety of cellular activities

19. Example of Pathway Steroid hormones bind to intracellular receptors
(testosterone)
EXTRACELLULAR
FLUID
Receptor
protein
DNA
mRNA
NUCLEUS
CYTOPLASM
Plasma
membrane
Hormone-
receptor
complex
New protein
Figure 11.6 1
The steroid
hormone testosterone
passes through the
plasma membrane.
Testosterone binds
to a receptor protein
in the cytoplasm,
activating it. 2
The hormone-
receptor complex
enters the nucleus
and binds to specific
genes. 3
The bound protein
stimulates the
transcription of
the gene into mRNA. 4
The mRNA is
translated into a
specific protein. 5

20. Other pathways regulate genes by activating transcription factors that turn genes on or off
Reception
Transduction
Response
mRNA
NUCLEUS
Gene P
Active
transcription
factor
Inactive
DNA
Phosphorylation
cascade
CYTOPLASM
Receptor
Growth factor
Figure 11.14

21. Termination of the Signal
Signal response is terminated quickly by the reversal of ligand binding

22. Receptors in the Plasma Membrane
There are three main types of membrane receptors:
G-protein-linked
Tyrosine kinases
Ion channel

23. G-protein-linked receptors
Plasma Membrane
Enzyme
G-protein (inactive)
CYTOPLASM
Cellular response
Activated
enzyme
Activated Receptor
Signal molecule
Inactivate
Segment that
interacts with
G proteins
GDP
GTP
P i
Signal-binding site
Figure 11.7

24. Receptor tyrosine kinases
Signal molecule
Signal-binding site
CYTOPLASM
Tyrosines
Signal molecule
Helix in the
Membrane
Tyr
Dimer
Receptor tyrosine kinase proteins (inactive monomers) P
Cellular response 1
Inactive relay proteins
Activated relay proteins
Cellular response 2
Activated tyrosine-
kinase regions
(unphosphorylated
dimer)
Fully activated receptor
tyrosine-kinase
(phosphorylated
6 ATP ADP
Figure 11.7

25. Ion channel receptors Figure 11.7 Cellular response Gate open
Gate close
Ligand-gated
ion channel receptor
Plasma Membrane
Signal molecule (ligand)
Figure 11.7
Gate closed
Ions