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.

In a positive feedback system, outputs stimulate the system to create more of the same products. Uterine contractions, blood clotting, and lactation are all physiological examples of positive feedback systems. Oxytocin is a hormone that causes uterine contractions and also stimulates the hypothalamus to produce more oxytocin. In blood clotting, the activated platelets near a wound release signals to attract more platelets. During lactation, the nerve stimulation of suckling stimulates the hypothalamus to secrete prolactin, leading to increased milk production. On the other hand, blood glucose regulation is an example of a negative feedback system.

Mannose-6-phosphate is a special signal peptide that must be added to proteins destined for the lysosome in the Golgi apparatus. This ensures proper delivery to the lysosome so that it may carry out its digestive functions. KDEL is a target peptide sequence that prevents the protein from being secreted from the endoplasmic reticulum. If it is a functional KDEL motif, it will be retrieved by the Golgi apparatus via retrograde transport to the endoplasmic reticulum lumen. Galactosidase is a glycolipid enzyme, which if not present gives a lysosomal storage disease known as Fabry’s disease. Caspases are proteins that are involved in signaling apoptosis.

Phosphatidylserine is a phospholipid membrane component of the cell actively held facing the cytosolic side of the cell by an enzyme called flippase. This is an important distinction from all other phospholipids because all others are free to flip back and forth from both sides of the membrane. However, when a cell is undergoing the process of apoptosis, phosphatidylserine is no longer forced to stay on the cytosolic side; when it flips and is shown on the outer surface of the cell, it acts as a signal for the phagocytic cells to engulf and destroy the cell. 

A protein kinase phosphorylates proteins and activates them by adding a phosphate group. Phosphodiesterase breaks phosphodiester bonds. Phosphotases remove phosphate groups and dephosphorylate a protein. G proteins are usually bound to a receptor and can begin a cascade of reactions when activated. Adenylyl cyclase converts ATP to cAMP. 

Glycoproteins are attached to the cell membrane's lipid bilayer. They serve a unique role at the cellular level to help with cell to cell recognition. Glycoproteins serve as unique "markers" allowing nearby cells to know they have reached their destination. Cholesterol is embedded in the cell membrane to maintain fluidity.

In animal cells, gap junctions allow materials such as ions to flow directly from the cytoplasm of one cell to that of the next. They can be thought of as channels, or "bridges" between cells. Gap junctions are common in the muscle tissue of animals' hearts, for example, as they allow ions to pass rapidly from cell to cell to coordinate cardiac muscle contraction. Plasmodesmata connect plant cells to one another in the same way, and they enable rapid transport and communication between adjacent cells. 

Desmosomes are another type of cell junction found in animal cells, though they "rivet" cells together instead of forming a channel; they most frequently prevent epithelial and some muscle cells from shearing apart.

Tight junctions form an even tighter seal than those formed by desmosomes. They form a bond that is almost completely impermeable to fluid. Tight junctions join cells together, and also help regulate the movement of molecules and ions.

Vacuoles are not a type of cell junction at all. These organelles are found in plant cells and are used for storage, digestion, and the filling of space.