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 2+ in the lumen of the endoplasmic reticulum.) The Ca 2+‐calmodulin then activates enzymes in the membranes.
Auxins (the plant growth substances similar to the hormones of animals) have several receptors on the membranes, each of which activates a different set of reactions. Receptors receive signals from other organisms also. These signals trigger interactions between plants as hosts or as prey to fungi, bacteria, protists, or to other species of plants. A group of receptors named lectins are scattered throughout the membrane system. They are proteins and glycoproteins that recognize, among other things, the specific bacteria that form root nodules with legumes in the nitrogen‐fixing symbiosis. Other lectins are involved in recognition of the pollen that falls on the stigmas of flowers. Pollen tubes are initiated only if pollen and stigma are of compatible species.