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.

Kinases are enzymes that phosphorylate the substrates they bind to, meaning that the kinase transfers a phosphate group to the substrate. ATP donates the phosphate group used in this process. Phosphorylation by kinases can lead to a variety of effects including: activation, inhibition, stabilization, destabilization, and localization. Kinases are very important in cellular activities and metabolic processes. 

Receptors are proteins embedded in the plasma membrane that receive and transmit signals from extracellular and intracellular sources. Receptors can also be embedded in the plasma membrane of the nucleus. Receptors bind to ligands, which can elicit a variety of responses including: activation, partial activation, and inhibition.

Ligands are molecules that form complexes with other molecules, proteins, or genetic material. The binding of a ligand causes a conformational change to the complex that leads to a response. This response by associated molecules is not limited to activation, but also includes inhibition, partial activation, and inhibition of constitutive activity. Ligands can be either selective or non-selective, meaning that they can bind to only specific molecules or to many receptors, respectively.

Second messenger systems begin with an extracellular ligand that binds to a receptor on the cell surface. The receptor then activates intracellular primary effectors (proteins that transduce the signal from the plasma membrane to the cytosol). In the cytosol, effectors activate second messenger molecules, which regulate intracellular process including transcription, neurotransmitter release, and enzyme activation. Second messengers have several common characteristics: they are localized, are easily synthesized and degraded, and are intracellular. These systems are responsible for diverse cellular processes and are able to amplify signals through kinase cascades. Common second messenger systems are the cAMP system and the tyrosine kinase system.

The cAMP second messenger system is involved in many signaling pathways, such as the regulation of glycogen, growth hormone, and lipid metabolism. In the cAMP system, ligand binding activates a G protein-coupled receptor and the associated intracellular G protein. The activated G protein then stimulates the enzyme adenylate cyclase to produce cAMP second messenger molecules from ATP. The cAMP molecules activate protein kinases that, in turn, activate a variety of target proteins through phosphorylation. Cyclic AMP or cAMP systems are capable of transducing a variety of signals.