Membranes: Reception, Communication Some of the proteins embedded in the phospholipid bilayer are receptors that receive a signal molecule. This molecule contains a message that is transmitted to another molecule, usually with instructions to perform some action. Some of the signal molecules pass through the membrane in ways described above, but many rely on membrane receptors to conduct the instructions to secondary messengers within the cell, a process of signal transduction (changing the molecular form of the signal). The secondary messengers then pass the transduced signal to the activator cells, which carry out the function. The calcium ion (Ca 2+ ) is the principal second messenger in plant cells and is stored in the vacuole. It moves out through special calcium channels in the tonoplast when a signal is received on the membrane and in the cytosol binds to calmodulin. (Some plants store Ca
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