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Ch. 15 - Cell Communication - Part 1 A summary of the chapter and lecture notes on cell communication. Includes illustrations and graphics from the textbook.

Course Code
BIOL 2021
Julie Clark

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BIOL 2021- April 9 2009
CHAPTER 15- Cell Communication I
- Communication between cells is mediated mainly by extracellular signal molecules.
- Figure 15-1: a simple intracellular signalling pathway activated by an extracellular signal
molecule. The signal molecule usually binds to a receptor protein that is embedded in the
plasma membrane of the target cell and activates one or more intracellular signalling pathways
mediated by a series of signalling proteins. Finally, one or more of the intracellular signalling
proteins alters the activity of effector proteins and thereby the behaviour of the cell.
Extracellular signal molecules
- Can include proteins, small peptides, amino acids, nucleotides, steroids, etc.
- Signal molecules can be released into the extracellular space by exocytosis from the signalling
cell or by diffusion through the signalling cells plasma membrane (Others are displayed on the
external surface of the cell and remain attached to it, providing a signal to other cells only when
they make contact- transmembrane proteins).
- Binds the signal molecule and then initiates a response in the target cell
- Usually transmembrane proteins on the target cell surface but can also be inside the cell where
signal molecule must enter the cell and bind to them

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Figure 15-3a: The binding of extracellular signal molecules to either the cell-surface or intracellular
receptors. A) Most signal molecules are hydrophilic and are therefore unable to cross the target
cell’s plasma membrane directly; instead they bind to cell-surface receptors, which in turn generate
signals inside the target cell.
Figure 15-3b. Some small signal molecules, by contrast, diffuse across the plasma membrane and
bind to receptor proteins inside the target cell- either in the cytosol or in the nucleus (as shown
above). Many of these small signal molecules are hydrophobic and nearly insoluble in aqueous
solutions; they are therefore transported in the bloodstream and other extracellular fluids bound to
carrier proteins, from which they dissociate before entering the target cell.
- Contact dependant signalling: signal molecules remain bound to the surface of the signalling cell
and influence only cells that contact it (15-3a)
Immune responses, development
Large distances
- Paracrine signalling (15-4b): short range cell-cell communication via secreted signal molecules
that act on neighbouring cells.
Act locally; signal molecules do not diffuse far because they are taken up by neighbouring
target cells, destroyed by extracellular enzymes or immobilized by the extracellular matrix.
Cell that produce signals that they themselves respond to are called autocrine signalling.
Endocrine cells
Secrete signal molecules called hormones into the blood stream which carries them to
target cells all over the body
Signalling over long distances

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Relies on diffusion and blood flow- relatively slow
Four forms of intercellular signalling
A) Contact-dependant signalling requires cells to be in direct membrane-membrane contact.
B) Pancrine signalling depends on signals that are released into the extracellular space and act
locally on neighboring cells
C) Synaptic signalling is performed by neurons that transmit signals electrically along their
axons and release neurotransmitters at synapses, which are often located far away from the
neuronal cell body
D) Endocrine signalling depends on endocrine cells, which secrete hormones into the
bloodstream for distribution throughout the body.
- Many of the same types of signalling molecules are used in paracrine, synaptic, and endocrine
signalling; the crucial differences lie in the speed and selectivity with which the signals are
delivered to their targets.
Differences between endocrine and synaptic signalling:
- Synaptic is much faster
- Endocrine is slow because it relies on blood flow
- Hormones are able to act at low concentrations (because they are so diluted in the blood,
whereas neurotransmitters are diluted much less and can achieve high local concentrations (and
receptors need lower affinity for ligand)
- Also after release, neurotransmitter is quickly removed from synaptic cleft
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