The parietal cells of the stomach are vital for both food digestion and as a defense mechanism against pathogens. When the parietal cells are not functioning properly, diseases such sepsis due to Clostridium difficile and malnutrition may occur. To keep the digestive system healthy, proper nutrition as well as a balanced diet is vital.

The parietal cells of the stomach secrete hydrochloric acid via the hormone gastrin. Gastrin is released when the stomach distends, via the presence of proteins and/or indirectly by the vagus nerve from the parasympathetic nervous system. Hydrochloric acid breaks down certain ingested food as well as activates certain zymogens for further digestion of macromolecules. The high acidity of the stomach due to the release of hydrochloric acid by parietal cells also destroys most pathogens. When the parietal cell is not functioning properly, opportunistic pathogens may create health problems.

Parietal cells also secrete intrinsic factor, a glycoprotein which binds to vitamin B12 to prevent destruction of the vitamin by the hydrochloric acid. Down the gastrointestinal tract, the vitamin is absorbed by the ileum of the small intestine. Vitamin B12 is essential for red blood cell production. A diet low in vitamin B12 may lead to anemia.

When the acidity of the stomach is too high, somatostatin is secreted by the D cells of the stomach. somatostatin inhibits the parietal cells from releasing more hydrochloric acid. What type of feedback is this?

The cellular membrane is a very important structure. The lipid bilayer is both hydrophilic and hydrophobic. The hydrophilic layer faces the extracellular fluid and the cytosol of the cell. The hydrophobic portion of the lipid bilayer stays in between the hydrophobic regions like a sandwich. This bilayer separation allows for communication, protection, and homeostasis. 

One of the most utilized signaling transduction pathways is the G protein-coupled receptor pathway. The hydrophobic and hydrophilic properties of the cellular membrane allows for the peptide and other hydrophilic hormones to bind to the receptor on the cellular surface but to not enter the cell. This regulation allows for activation despite the hormone’s short half-life. On the other hand, hydrophobic hormones must have longer half-lives to allow for these ligands to cross the lipid bilayer, travel through the cell’s cytosol and eventually reach the nucleus. 

Cholesterol allows the lipid bilayer to maintain its fluidity despite the fluctuation in the body’s temperature due to events such as increasing metabolism. Cholesterol binds to the hydrophobic tails of the lipid bilayer. When the temperature is low, the cholesterol molecules prevent the hydrophobic tails from compacting and solidifying. When the temperature is high, the hydrophobic tails will be excited and will move excessively. This excess movement will bring instability to the bilayer. Cholesterol will prevent excessive movement.

Epinephrine binds to its receptor on the surface of the cell. Molecule B also binds to the same receptor but when bound, all of the receptors on the cell's surface are upregulated. What type of regulation is this?