Cellular Communication Model 1 Types Of Signaling Answer Key
Urbano Bozman
When our senses perceive an environmental stress such as danger or a threat, cells in the nervous and endocrine systems work closelytogether to prepare the body for action. Often referred to as the fight or flight or stress response, this remarkable example ofcell communication elicits instantaneous and simultaneous responses throughout the body.
Sensory nerve cells pass the perception of a threat, or stress, from the environment to the hypothalamus in the brain. Neurosecretory cellsin the hypothalamus transmit a signal to the pituitary gland inciting cells there to release a chemical messenger into the bloodstream.Simultaneously, the hypothalamus transmits a nerve signal down the spinal cord. Both the chemical messenger and nerve impulse will travelto the same destination, the adrenal gland.
When chemical messengers arrive via the bloodstream, they dock on to receptors and begin a cell signaling cascade that results in theproduction of cortisol. Cortisol is released into the blood stream where it begins signaling cascades in several cell types, resultingin an increase in blood pressure, increase in blood sugar levels, and suppression of the immune system.
Signaling molecules from several origins work to provide an energetic boost in a variety of ways. When epinephrine binds to receptorson liver cells, it triggers a signaling cascade that produces glucose from larger sugar molecules. Circulating cortisol sets fatty acidsfree to be transformed into energy. These molecules are rapidly excreted into the bloodstream, supplying a boost of readily availableenergy for muscles throughout the body, priming them for exertion.
Often, it is necessary to take some artistic license when creating images of proteins and other molecules.So what do they really look like? Molecular models provide a glimpse into their tiny world and help us understand how they work. Take a lookat some scientifically accurate models of the molecules and proteins from the movie.
Epinephrine is an important cell signaling molecule in the fight or flight response. Also known as adrenaline, epinephrine is an efficientmessenger that signals many cell types throughout the body with many effects. In the lungs, epinephrine binds to receptors on smooth musclecells wrapped around the bronchioles. This causes the muscles to relax, dilating the bronchioles and allowing more oxygen into the blood.At the sino-atrial node of the heart, epinephrine stimulates pace maker cells to beat faster. This increases the rate at which other chemicalsignals, glucose and oxygen are circulated to the cells that need them. Epinephrine also contracts specific types of muscle cells below thesurface of the skin, causing beads of perspiration and raised hairs at the surface.
In fact, the initial perception of a threat or danger is also received by an area in the brain stem that beginsyet another axis of communication and response involving the release of the messenger norepinephrine. Likecortisol and epinephrine, norepinephrine travels throughout the body, triggering cell signaling cascades in a number of cell types.
Regardless of their kind, or point of origin, cell signaling molecules involved in the fight or flight response work closely together.Their overall effect is an increase in circulation and energy to certain body systems and a downshift of less important ones intomaintenance mode. In this way, the fight or flight response prepares the body for extreme action.
In plants, as in animals, cells are in constant communication with one another. Plant cells communicate to coordinate their activities in response to the changing conditions of light, dark, and temperature that guide the plant's cycle of growth, flowering, and fruiting. Plant cells also communicate to coordinate what goes on in their roots, stems, and leaves. In this final section, we consider how plant cells signal to one another and how they respond to light. Much less is known about the receptors and intracellular signaling mechanisms involved in cell communication in plants, and we shall concentrate mainly on how these differ from those used by animals. We discuss some of the details of plant development in Chapter 21.
Although plants and animals are both eucaryotes, they have had separate evolutionary histories for more than a billion years. Their last common ancestor was a unicellular eucaryote that had mitochondria but no chloroplasts. The plant lineage acquired chloroplasts after plants and animals diverged. The earliest fossils of multicellular animals and plants date from almost 600 million years ago. Thus, it seems that plants and animals evolved multicellularity independently, each starting from a different unicellular eucaryote, sometime between 1.6 and 0.6 billion years ago (Figure 15-75).
If multicellularity evolved independently in plants and animals, the molecules and mechanisms used for cell communication will have evolved separately and would be expected to be different. Some degree of resemblance is expected, however, as both plant and animal genes diverged from the set of genes contained by the unicellular eucaryote that was the last common ancestor of plants and animals. Nitric oxide and Ca2+ are widely used for signaling in both plants and animals. However, because the genome of Arabidopsis thaliana, a widely studied small flowering plant, has been completely sequenced, we know that there are no homologs of Wnt, Hedgehog, Notch, Jak/STAT, TGF-β, Ras, or the nuclear receptor family in this organism. Similarly, cyclic AMP has not been definitively implicated in intracellular signaling in plants, although cyclic GMP has.
Much of what is known about the molecular mechanisms involved in signaling in plants has come from genetic studies on Arabidopsis. Although the specific molecules used in cell communication in plants often differ from those used in animals, the general strategies are frequently very similar. Enzyme-linked cell-surface receptors, for example, are used in both lineages, as we now discuss.
Like animals, plants make extensive use of cell-surface receptors. Whereas most cell-surface receptors in animals are G-protein-linked, most found so far in plants are enzyme-linked. Moreover, whereas the largest class of enzyme-linked receptors in animals is receptor tyrosine kinases, this type of receptor is extremely rare in plants, even though they contain many cytoplasmic tyrosine kinases, and tyrosine phosphorylation and dephosphorylation have important roles in plant cell signaling. Instead, plants seem to rely on a great diversity of transmembrane receptor serine/threonine kinases, which are distinct from this type of receptor used by animal cells. Like the animal receptors, however, they have a typical serine/threonine kinase cytoplasmic domain and an extracellular ligand-binding domain. The most abundant types identified so far have a tandem array of extracellular leucine-rich repeats (Figure 15-76) and are therefore called leucine-rich repeat (LRR) proteins.
There are about 80 LRR receptor kinases encoded by the Arabidopsis genome. One of the best-studied examples is CLAVATA 1 (CLV1), which was originally identified in genetic studies. Mutations that inactivate the protein cause the production of flowers with extra floral organs and a progressive enlargement of both the shoot and floral meristems, which are groups of self-renewing stem cells producing the cells that give rise to stems, leaves, and flowers (discussed in Chapter 21). The extracellular signal molecule for the receptor is thought to be a small protein called CLV3, which is secreted by neighboring cells. The binding of CLV3 to its receptor, CLV1, suppresses meristem growth, either by inhibiting cell division there or, more probably, by stimulating cell differentiation (Figure 15-77A). The intracellular signaling pathway from CLV1 to the cell response is largely unknown, but it includes a serine/threonine protein phosphatase that inhibits CLV1 signaling; also involved is a small GTP-binding protein of the Rho class and a nuclear gene regulatory protein that is distantly related to homeodomain proteins. Mutations that inactivate this gene regulatory protein have the opposite effect of mutations that inactivate CLV1: cell division is greatly decreased in the shoot meristem, and the plant produces flowers with too few organs. Thus, the intracellular signaling pathway activated by CLV1 is thought to normally stimulate cell differentiation by inhibiting the gene regulatory protein that normally inhibits cell differentiation (Figure 15-77B).
A different LRR receptor kinase called BRI1 acts as a cell-surface steroid hormone receptor in Arabidopsis. Plants synthesize a class of steroids called brassinosteroids, because they were originally identified in the mustard family Brassicaceae, which includes Arabidopsis. During development, these plant growth regulators stimulate cell expansion and help mediate responses to darkness. Mutant plants that are deficient in the BRI1 receptor kinase are insensitive to brassinosteroids. Normally, Arabidopsis plants grown in darkness are white and gangly as a result of brassinosteroid signaling; in the absence of brassinosteroid signaling, they become green, as though they were growing in light, and the mature plant is severely dwarfed. As for the other known LRR receptor kinases in plants, the nature of the signal transduction pathway that leads from the receptor to the response remains a mystery.
The LRR receptor kinases are only one of many classes of transmembrane receptor serine/threonine kinases in plants. There are at least six additional families, each with its own characteristic set of extracellular domains. The lectin receptor kinases, for example, have extracellular domains that bind carbohydrate signal molecules. The Arabidopsis genome encodes over 300 receptor serine/threonine kinases, which makes this family of receptors the largest one known in plants. Many of these are involved in defense responses against pathogens.
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