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Chapter 9: Cell Communication

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Learning Objectives 

  1. Compare and contrast the five common methods of cell-to-cell communication.
  2. Describe the three major types of cell surface receptors, and give examples of each.
  3. Outline the basic mechanism of different types of signal transduction pathways.
  4. Identify the chemical nature of ligands for intracellular receptors.
  5. Compare signaling through cell surface receptors with signaling that occurs through intracellular receptors.
  6. Explain the significance of kinase cascades.
  7. Relate the function of second messengers to signal transduction pathways.
  8. Provide examples of programmed cell death that demonstrate the importance of process in living organisms.
  9. Explain the relationship between cell signaling mechanisms and programmed cell death.


Chapter Summary

Cells of multicellular organisms must communicate with one another so that they behave as a coordinated group of cells rather than just a bunch of independent ones. Recall the cliche “the left hand doesn’t know what the right hand is doing.” The signals to which a cell responds are dependent on the kinds of receptor proteins associated with that cell. If the signal is just the right molecular shape, it and the receptor bind eliciting a response somewhere in the cell. There are four main types of cell signaling. Direct contact and paracrine signaling are important in early organismal development. Endocrine signaling via hormones provides widespread response in both plants and animals. Synaptic signaling found in animal nervous systems produces more localized responses at the chemical synapse between the neuron and the receptor cell.


Intracellular receptors are small molecules that are able to pass through the plasma membrane of the target cell. Nitrous oxide is one example. It activates the enzyme that catalyzes synthesis of cyclic GMP. A superfamily of steroid hormone receptors have specific DNA binding sites normally occupied by an inhibitory protein. When the signal molecule binds to another site on the receptor the inhibitor is released. The receptor then binds to DNA to activate or suppress a certain gene.


There are three superfamilies of cell surface receptors. The signals associated with these receptors bind to receptor proteins on the cell surface. Thus an extracellular signal is converted to an intracellular signal. In chemically-gated ion channels the receptor is a transmembrane protein that winds back and forth through the membrane several times (called a multi-pass protein). The center of this protein forms a channel through which specific ions can pass. Enzymatic receptors are linked to enzymes or themselves act as enzymes. Most of these are protein kinases; they add phosphate groups to proteins. They are single pass transmembrane proteins where the region that binds with the signal molecule is located outside the cell and the portion that initiates enzymatic activity is found within it. G protein-linked receptors are seven-pass transmembrane proteins that become activated when they bind to GTP. These receptors are important because they provide the mechanism of action for over half of the therapeutic drugs currently in use.


Second messengers are small molecules or ions that alter the shape and behavior of receptor proteins to relay the signal message to enzymes or genes within a cell. cAMP is the most widely used second messenger in animal cells. Ultimately the cAMP binds to A-kinase, activating it to phosphorylate certain cell proteins. Calcium is normally sequestered outside a cell or within its endoplasmic reticulum. With the proper G protein signal inositol triphosphate is eventually produced, which opens calcium channels in the ER membrane. This influx of calcium triggers many activities. In most cases, a cellular signal is too insignificant to result in an adequate cellular response. Protein kinase cascades amplify the signal. In vision for example, a single light-activated rhodopsin molecule activates many transducin molecules that further split 105 cyclic GMP molecules.


Apoptosis is a form of programmed cell death important in the normal life cycle of many multicellular organisms. This process can be triggered by extracellular stimuli acting through specific signal transduction pathways; it can also be triggered cell-autonomously by intracellular cues such as damaged DNA or worn out organelles.


Virtual Lectures  

  1. cAMP as a second messenger
  2. Ca2+ as a second messenger
  3. Cellular Response: Adreniline 


** make sure to press OKwhen the LMS implementation screen appears.


PowerPoint Presentations

Chapter 9 PowerPoint (.pdf)


Reading Assignments and Homework

Please access the ConnectPlus site for Health Science Academy to access our reading assignments and homework.




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