When inactive, ion gated receptors are closed. When a ligand binds, the channel undergoes a conformational change and opens: creating a tunnel. This conformational change does not last for a long period of time; the ligand soon dissociates and the ion channel closes.

Cell signaling is the process used by cells to communicate and control cellular activities. It can occur both within and between cells. The correct sequence of events that takes place during cell signaling is as follows: reception, transduction, and response. The reception stage is the detection of a signal, typically by a receptor on the cell surface. Next, transduction is characterized by the transmission of signals from the cell’s exterior to its interior by way of proteins. Finally, the response is the subsequent cellular reaction to the signaling. Cell signaling is critically important in normal cell function and widely diversified. 

G protein-coupled receptors are part of a large class of receptors involved in intercellular signaling. Structurally, G protein-coupled receptors have an extracellular N terminus, seven transmembrane helices, three intracellular loops, three extracellular loops, and an intracellular C terminus. The ligand-binding domain is within the transmembrane helices.

G protein-coupled receptors are only found in eukaryotes including yeast cells and animal cells. Bacteria cells are prokaryotes, and therefore do not contain G protein-coupled receptors. Even though yeast cells are single-celled, they possess all the characteristics of eukaryotic cells.

G proteins are a class of protein signaling molecules that are activated by G protein-coupled receptors (GPCRs). When a ligand binds to the transmembrane domain of GPCRs, the GPCR undergoes a conformational change. This conformational change activates the G protein, which binds to GTP rather than lower energy GDP. The active G protein can then dissociate and transmit the signal by interacting with other proteins.

Tyrosine kinase receptors exist as single monomers but possess the capability to polymerize. Tyrosine kinase receptors have a transmembrane domain, an extracellular N terminus, and an intracellular C terminus. When a ligand binds to the extracellular N terminus, the tyrosine kinase receptor dimerizes. There are multiple models of receptor dimerization. One of the models is that dimerization is aided by the ligand itself, which binds to the N termini of both tyrosine kinase receptors. Another is that dimerization occurs after each tyrosine kinase receptor monomer binds to a ligand. A final model postulates that the binding of a ligand induces a conformation change that allows dimerization.

Tyrosine kinase receptors are composed of a transmembrane domain, an extracellular N terminus, and an intracellular C terminus. Ligand binding to the extracellular N terminus stimulates receptor dimerization. The dimerization stimulates kinase activity on the intracellular C terminus, leading to the autophosphorylation of the tyrosine residues on the C terminus. This phosphorylation of tyrosine residues creates binding sites for relay proteins, which are then phosphorylation by the tyrosine kinase receptors. The phosphorylated relay proteins then transmit the signal to other cellular pathways. 

In animal cells, hormones mediate long distance cell signaling. Hormones are chemical messengers secreted by cells that travel through the circulatory system to the target cell receptors. Hormones communicate between diverse cell types and initiate diverse transduction pathways. Hormones are used for long distance cell signaling in both plant and animal cells. 

Hormones are chemical messengers responsible for long distance cell signaling. Hormones are involved in diverse signaling pathways and have a variety of effects on cellular activities; therefore, there are many types of hormones. Animal hormones can be organized into classes based on their chemical makeup—peptide hormones, steroid hormones, lipid-based hormones, and amino acid-derived hormones. Insulin and growth hormones are both in the class of peptide hormones, and they are responsible for promoting the absorption of glucose from the bloodstream and promoting cell growth and reproduction, respectively. Testosterone is a steroid hormone that controls the development of male reproductive features.

Plant hormones, like animal hormones, are involved in long distance cell signaling. Most plant hormones are involved in regulating plant growth and are secreted by plant cells. Because plants lack a circulatory system, plant hormones move through cells via passive transport. This demands that the chemical composition of plant hormones must be simple. Auxin is a plant hormone whose distribution controls plant growth in response to environmental conditions. Other common plant hormones include abscisic acid, cytokinin, ethylene, and gibberellin.