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.

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.

Negative feedback is one method that organisms use to maintain homeostasis. In these systems, the rate of the process decreases as the amount of output increases. In other words, the output downregulates the process that created it. An example of this is the regulation of blood glucose. When blood glucose levels rise, the pancreas secretes more insulin to convert glucose to glycogen for storage. This lowers the blood glucose level. If the level falls too much, then the pancreas secretes more glucagon to convert glycogen to glucose, which raises blood glucose levels.

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.

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.

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.

The MAP kinase signaling system is a common method of cellular signaling that regulates transcription of genes within a cell. The pathway begins when a ligand binds to a membrane receptor. The activated receptor activates an associated Ras protein, which is a GTPase that is activated when bound to GTP. The active Ras protein then donates a phosphate group to a MAP kinase protein and activates it. This begins a MAP kinase phosphorylation cascade. MAP kinases phosphorylate other MAP kinases in a serial fashion, which allows for signal amplification. After a series of MAP kinase phosphorylation events, specific MAP kinases phosphorylate transcription factors, regulating their activity and gene expression. Thus, the pathway ends with transcription factor regulation.

When a protein is active in it’s native state, it is said to be constitutively active. In the case of protein receptors, a receptor is constitutively active when it is active without binding to a ligand.

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.