BIOD27H3 Chapter 6: Chapter 6 Study Guide
CHAPTER 6
CELL-TO-CELL COMMUNICATION
- there are two basic types of physiological signals: electrical and chemical.
- Electrical signals ± FKDQJHVLQDFHOO¶VPHPEUDQHSRWHQWLDO
- Chemical signals ± molecules secreted by cells into the extracellular fluid. They are responsible for most
communication within the body
- Target cells / targets± cells that receive electrical or chemical signals
- Our bodies use 4 basic methods of cell-to-cell communication:
1) Gap junctions, which allow direct cytoplasmic transfer of electrical and chemical signals
between adjacent cells
2) Contact-dependent signals, which occur when surface molecules on one cell membrane bind to
surface molecules on another cell membrane
3) Local communication by chemicals that diffuse through the extracellular fluid; and
4) Long-distance communication through a combination of electrical signals carried by nerve
cells and chemical signals transported in the blood. ( a given molecule can act close to the cell
that released it (local communication) as well as in distant parts of the body (long-distance
communication)
Gap junctions create cytoplasmic brigdes
- gap junctions are protein channels that create cytoplasmic bridges between adjacent cells
- formed from union of membrane-spanning proteins, called connexins, on 2 adjacent cells. This creates a
protein channel, connexion that can open and close
- ions and small molecules, such as amino acids, ATP, cAMP diffuse directly from the cytoplasm of one cell
to the cytoplasm of the next. (larger molecules caQ¶WSDVVWKURXJK
- only means through which electrical signals can pass directly from cell to cell
- movement of molecules can be modulated or shut off completely
- are not all alike
Contact-Dependent Signals Require Cell-to-Cell Contact
- requires that surface molecules on one cell membrane bind to a membrane protein of another cell ± occurs in
immune system and during growth and development
- CAMs, cell adhesion molecules, act as receptors in cell-to-cell signalling. They are linked to cytoskeleton
and to intracellular enzymes. Hence, they transfer signals in both directions across cell membranes
Paracrines and Autocrine Signals Carry Out Local Communication
- paracrine signal ± a chemical that acts on cells in the immediate vicinity (surrounding area) of the cell that
secreted the signal
- autocrine signal ± a chemical signal that acts on cell that secreted it
- these signal molecules reach their target cells by diffusing through the interstitial fluid (chp 3)
- since distance is a limiting factor for diffusion, the effective range of paracrine signals is restricted to
adjacent cells (e.g. of a paracrine molecules is histamine)
Neural Signals, Hormones, and Neurohormones Carry Out Long-Distance Communication
- nervous and endocrine systems are responsible for long-distance communication
- Endocrine system communicates by using hormones ± chemical signals that are secreted into blood and
distributed all over the body by circulation. Only those cells with receptors for hormone are target cells
- Nervous system uses a combination of chemical and electrical signals to communicate over long distances
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- An electrical signal travels along the neuron until it reaches the end of the cell, where it is translated into a
chemical signal secreted by neuron ± a neurocrine
- If a neurocrine molecule diffuses from the neuron across a narrow extracellular space to a target cell and has
a rapid effect, it is called a neurotransmitter
- If a neurocrine acts more slowly as an autocrine or paracrine signal, it is called a neuromodulator
- if a neurocrine released by a neuron diffuses into the blood for distribution, it is called a neurohormone
Cytokines May Act as Both Local and Long-Distance Signals
- function as autocrine or paracrine signals in development and differentiation
- in stress and inflammation, some may act on relatively distant targets and may be transported through the
circulation just as hormones are
- act on a broader spectrum of target cells (unlike hormones)
- are made on demand ( not produced by specialized glands the way hormones are )
SIGNAL PATHWAYS
- chemical signals bind to the target-cell receptor proteins
- a cell cannot respond to a chemical signal if the cell lacks the appropriate receptor proteins for that signal
- all signal pathways share the following common features
1) the signal molecule is a ligand that binds to a receptor. The ligand is also known as a first
messenger because it brings info to its target cell
2) ligand-receptor binding activates the receptor
3) the receptor in turn activates one or more intracellular signal molecules
4) the last signal molecule in the pathway initiates synthesis of target proteins or modifies existing
target proteins to create a response
Receptor Proteins Are Located Inside the Cell or on the Cell Membrane
- chemical signals fall into 2 categories based on their lipid solubility: lipophilic or lipophobic
- target-cell receptors may be found in nucleus, in cytosol, or on the cell membrane as integral proteins
-
- lipophilic signal molecules can diffuse through the phospholipid bilayer of cell membrane and bind to
cytosolic receptors or nuclear receptors. This is a relatively slow process related to gene activity (receptor
activation can turn on/off gene activity. (e.g. hormones Æ lipophilic signal molecules)
- lipophobic signal molecules FDQ¶WGLIIXVHWKURXJKSKRVSholipid bilayer and therefore remain in the
extracellular fluid and bind to receptor proteins on cell membrane. Responses can be very rapid and can be
seen within milliseconds to minutes
- membrane receptors can be grouped into 4 major categories:
1) ligand-gated ± simplest receptors that are chemically gated ion channels called receptor-
channels. Ligand binding opens or closes the channel and alters ion flow across the membrane
2) receptor-enzyme
3) G protein-coupled receptors, and
4) Integrin receptor
- for the last three receptors, info from signal molecule must be passed across membrane to initiate an
intracellular response
- signal transduction ± transmission of info from one side of a membrane to other using membrane proteins
Membrane Proteins Facilitate Signal Transduction
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