Monday, March 28, 2011
Ancient Cellular Telephones
On page 119, Shubin discusses how in order for multi-cellular organisms to evolve, cells needed be able to communicate with one another. Various mechanisms were needed first to allow for the flow of such communication. Using your knowledge of primitive multi-cellular organisms such as sponges as well as your knowledge of more advanced multi-cellular organisms such as humans or zoo animals, compare and contrast the mechanisms available for communication and the ways in which these mechanisms are used. What selective advantage do some of these mechanisms provide that has allowed certain species to evolve? sami_kopinsky@yahoo.com Sami Kopinsky
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Shubin said that the transition from single-celled organisms to organisms with body necessitated cells to be able to “stick together” and “talk to each other” (119). Organisms use many different methods to do this: to stick together, cells make different types of cell junctions; to talk to each other, cells utilize those cell junctions to send cellular signals of different kinds.
ReplyDeleteCell junctions come in different shapes depending on what purpose they serve as well as the type of organism they are in. In plants, for example, plasmodesmata are used to provide transport of molecules between cells for communication, where molecules can move via the cell wall (apoplastic movement) or the cytoplasm (symplastic movement) to transport substances throughout the plant (Campbell 771). In vertebrates, there are three types of junctions that allow organisms to have cellular communication: tight junctions, gap junctions, and desmosomes. Tight junctions are used to closely hold cells together in order to act as protective barriers (http://www.zonapse.net/tight_junction/id2.html). By preventing the passage of molecules or ions through intercellular space, tight junctions force all substances to actually pass through the cell before travelling throughout the body, preventing harmful substances from travelling around in an organism - similar to the Casparian strip in plants, which prevents all materials from passively moving into endodermal cells through apoplastic movement (Campbell 772). Organisms that adapted to gain this characteristic were able to keep harmful materials out of their cells and survive longer, whereas organisms that did not have such barriers had trouble not only preventing harmful materials, but also maintaining the polarity and salinity of their cells and thus were at a selective disadvantage. Gap junctions are another type of cell junction that are similar to plasmodesmata, allowing molecules to travel along cytoplasmic channels to allow cells to communicate in tissues like heart muscle and embryonic tissue, while desmosomes hold cells together in strong sheets, acting as anchors to hold them together (Campbell 121). Such junctions facilitate faster movement of molecules throughout the body as opposed to simple diffusion or active transport from interstitial fluid, so organisms that grew larger without the ability to have such cellular communication would be at a selective disadvantage and die out.
Cell communication is done by a network of signals, in which molecules are sent between cells (either single-celled organisms or cells inside a multi-cellular organism) to trigger certain reactions. In unicellular organisms, signals can be exchanged by releasing molecules into the environment; in yeast, mating preparation can be triggered by the release of pheromones into the environment (http://mcb.asm.org/cgi/content/abstract/24/5/2041). In multicellular organisms, cells can communicate differently depending on how far away they are. Juxtacrine signaling is a type of signaling found in cells that are in direct contact, where molecules are transported through the connexons of gap junctions between cells. Paracrine signaling is the type where the target cell and secreting cell are near each other; examples are growth and clotting factors as well as responses to allergens. Endocrine signaling, seen in the endocrine system, is the type in which hormones are sent throughout the body, to cells far away. All these types of signaling allow the body to self-regulate more efficiently, keeping homeostasis without having to use up very much energy. Paracrine signaling, for example, allows for necessary molecules to move quickly from a source to a target cell nearby, making clotting and allergic responses work faster. Organisms that do not have such ability have more trouble dealing with injuries and harmful substances, so signaling that works efficiently through cellular communication serves as a selective advantage to organisms that do have that ability.
Eugene Bulkin (doubleaw002@gmail.com)
On a small cell-to-cell scale, as Eugene thoroughly discussed, cells can communicate in a variety of ways. On a larger, organism scale, however, several additional and different mechanisms take place that enable not only different cells of the body but different entire parts of the body to communicate. Perhaps the most evolutionarily important development that allows for this is the nervous system.
ReplyDeleteThe nervous system is made up of circuits of neurons and related supporting cells. The nerve system has sensory neurons that, upon receiving a stimulus, transmit an electrical signal along their axon via an electrical action potential generated by varying concentrations of Na+ and K+ from the neuron’s dendrites to its axon terminals, where the signal is transmitted by the release of neurotransmitters to the next neuron, called the postsynaptic cell (Campbell 1067). This type of cell-to-cell communication is much faster than other methods of communication, like hormones in the endocrine system. Thus, organisms that benefit from rapidly responding to stimuli in their environment had much to gain from developing this type of quick response.
Sponges, our distant ancestors that do not significantly respond to their environment quickly, are so primitive that they have no nervous system at all (http://faculty.washington.edu/chudler/invert.html). In contrast, vertebrates have highly developed nervous systems. Before diving into detail about the human nervous system, however, let’s first look at a few examples of the evolution of the nervous system.
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ReplyDeleteThe simplest nervous systems can be found in Cnidarians, which are radially symmetrical and have nerve networks that transmit signals from sensory cells to muscle cells (http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20102/bio%20102%20lectures/nervous%20system/nervous1.htm). These nervous systems are neither centralized nor sophisticated. More developed and specialized nervous systems are found in organisms with more organized body plans, particularly those with bilateral symmetry.
These organisms, like planarians, leeches, arthropods, mollusks and ultimately vertebrates, exhibit cephalization, a clustering of sensory neurons and interneurons at the anterior end of the organism (Campbell 1065). Along with cephalization comes the development of a central nervous system and also that of a peripheral nervous system. This development is an evolutionary advantage in that organisms with a well-defined head can locate their sensory organs on the increasingly important front of their bodies, which allows their nervous system-based brain to receive and process information from the peripheral nervous system more efficiently (http://www.madsci.org/posts/archives/2002-12/1039119710.Dv.r.html). Organisms with a functional anterior end also have advantages in other ecological ways, like in their ability to consume food in front of them and excrete waste behind them, which makes getting food into their mouths and keeping waste away easier.
The full development of vertebral nervous systems as opposed to invertebrate nervous systems can be most generally distinguished by its level of complexity and sophistication. As vertebrates have evolved, their ability to detect and respond to other organisms and their environment has become increasingly important from a natural selection perspective, even more so than in invertebrates. One quantifiable indicator of this development is the presence of cerebrospinal fluid, which contains mineral salts and small amounts of protein and sugar, in vertebrates’ nerve cord and brain (http://www.daviddarling.info/encyclopedia/V/vertebrate_nervous_system.html). This fluid, much more necessary in the highly developed nervous systems of vertebrates than of invertebrates, helps nurture the nervous tissue.
Possibly the only true indicator of the human nervous system, however, is the development of consciousness. Biologically speaking, the human condition of sentience is only existent in chemical and electrical signals in the brain. Since there remains much research to be done in this field, and because the nature of perceived human existence is so controversial, it is difficult to directly compare human intelligence to that of other animals. In some degree, however, humans’ ability to think and reason is what distinguishes us from all our prior ancestors.
- Vincent Fiorentini
(vincent@panatechcomputer.com)