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. 

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.

The Golgi apparatus is an organelle that modifies and packages proteins for export. The Golgi receives proteins in vesicles from the endoplasmic reticulum on the cis face. Protein modification by enzymatic activity occurs within the Golgi. The modified proteins are then packaged into vesicles on the trans face, some of which are destined for extracellular transport via exocytosis. 

Prokaryotes, like eukaryotes, rely on cell signaling to coordinate metabolic activities and transduction pathways in response to other cells and the environment. Prokaryotes lack membrane-bound organelles; therefore, membrane trafficking is not the same as in eukaryotes. Prokaryotic membrane trafficking involves outer membrane vesicles, which hare nanoscale expansions of the periplasm that pinch off from bacterial cells and fuse to target cells. Cell signaling in prokaryotes also takes place at the host-pathogen interface, where host cells can recognize prokaryotic factors displayed on the plasma membrane. Additionally, prokaryotes take part in complex signaling pathways during quorum sensing to ascertain population density. 

Recognition of pathogens is an example of cell signaling and very important to the immune system’s ability to defend the cell from disease. Host cells have specific receptors that recognize pathogens; pattern recognition receptors bind pathogen-associated molecular patterns, B cell receptors bind epitopes, and cytokine receptors bind to cytokines. In this way, pathogens are recognized by host cells and begin signaling pathways to defend against them.

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. 

Agonists activate receptors; therefore, the receptor is activated when the agonist binds to it.

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. 

Antagonists are ligands that inhibit receptors; thus, they create a receptor blockade. Some antagonists are able to bind irreversibly to the receptor by covalent bonds, blocking the receptor. 

Intraspecies cell signaling is the communication between members of the same species on a cellular level. This occurs in both unicellular and multicellular organisms. Pheromones and quorum sensing are both common mechanisms used for intraspecies signaling. Pheromones are chemicals secreted by organisms. These signals trigger a number of social responses such as alerting others of danger, stimulating sexual attraction, and indicating territories. Pheromones are used by a variety of organisms. Quorum sensing is molecular signaling used to regulate population density in certain insect and bacteria species.