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. 

Tight junctions commonly line organs such as the urinary bladder, as they prevent unwanted fluid from leaking into or out of these organs. They are most notable for their barrier function: they are almost completely impermeable to fluid.

Though desmosomes also join cells together, they are more commonly found connecting cells that are subject to mechanical stress, such as muscles or the outer layer of skin. They are comparable to a button or a rivet, and do not connect cells as tightly to one another as tight junctions do. Gap junctions are essentially channels between cells that enable the rapid transport of ions and other solutes between the cytoplasm of neighboring cells; they would not help keep urine within the urinary bladder. Plasmodesmata are only found in plants (they are analogous to gap junctions) and would therefore have no business forming a human bladder.

Within the digestive tract, it is important for nutrients and toxins to remain contained within the organs to allow for proper absorption. Furthermore, the acidity and basicity of various regions of the digestive tract must be kept partitioned from the rest of the body. Tight junctions are found between most cells of the digestive tract. This type of cell junction prevents movement of material between cells, requiring it to actually move into cells via diffusion or active transport in order to pass through tissue.

Gap junctions are intercellular junctions that connect the cytoplasm of adjacent cells and are particularly important for signaling. Desmosomes are structures specialized for cell-to-cell adhesion and force transmission. Plasmodesmata are transport and communication channels in plant cells, analogous to gap junctions.

Cardiac myocytes (muscle cells) need to be able to contract in unison in order to ensure proper pumping of blood. This is accomplished by specialized junctions called gap junctions that allow cells to communicate with one another very quickly. These junctions join the cytoplasms of adjacent cells, allowing ions to flow between them. When one cardiac cell is depolarized, the gap junctions found in intercalated discs allow the depolarization to jump from cell to cell.

Plasmodesmata are structurally analogous to gap junctions, but are located in plant cells.

Gap junctions are formed by connexons—rings of proteins that spans the membranes of two adjacent cells. The ring surrounds a pore that allows ions, among other substances, to pass between the cells uninterrupted. The result is a quick transmission of the electrochemical signal for cardiac muscle cell contraction, so that the cells contract essentially together. Gap junctions can also be found in numerous other tissue types, but not in skeletal muscle.