Desmosomes directly attach cells to one another by connecting their cytoskeletons. These aggressive junctions are found in cells that experience large amounts of stress, such as the skin. Cadherin proteins on adjacent cell membranes bind to one another in the extracellular space, and bind to intermediate filaments of the cytoskeleton in the cytosol.

Tight junctions blockade gaps between cell membranes, increasing the selectivity of an internal cavity by creating a strong barrier against fluids and ions. The blood brain barrier is formed from tight junctions. Gap junctions serve the opposite function, creating small perforations to connect adjacent cell cytosols. This allows for rapid transmission of cell signals and communication. Intercalated discs are specialized gap junctions found in cardiac muscle cells.

An action potential is a process that involves the rise and fall of membrane potential of a cell. The changes in membrane potential is associated with the movement of ions (especially sodium and potassium ions) into and out of the cell. Recall that gap junctions are a type of intercellular connections that create gaps (tunnels) between adjacent cells and facilitate movement of ions; therefore, cardiac cells must possess gap junctions to propagate action potentials.

Gap junctions do facilitate movement of fluid; however, fluid movement is irrelevant to action potential. Tight junctions are another type of intercellular connections that form a tight seal between adjacent cells. These tight seals do not allow movement of ions, molecules, or fluid.

There are two main characteristics that distinguish desmosomes from other cell junctions. First, a desmosome forms a connection between the cells’ cytoskeletons. A cell’s cytoskeleton is mostly made up of microfilaments, intermediate filaments, and microtubules. Adherens junctions form connections between the actin microfilaments of adjacent cells, while desmosomes form connections between the intermediate filaments. Second, desmosomes look like a patch and occur only at a single location. Other junctions, such as gap junctions and tight junctions, occur at multiple locations along the extracellular space; therefore, the researcher must have observed the connection at a single location.

Prevention of fluid exchange is not a characteristic of desmosomes; it is a characteristic of tight junctions. Tight junctions are tight seals that prevent the flow of water, molecules, and ions between cells.

There are four major types of connections between cells that facilitate intercellular communication and interaction: gap junctions, desmosomes, adherens junctions, and tight junctions.

Gap junctions are tunnels between cells, formed by perforations in the plasma membrane, that allow ions and molecules to pass between cells. Desmosomes connect the cytoskeletons of adjacent cells, assisting in force transduction. Adherens junctions use specialized proteins called cadherins and catenins to create a strong adhesion between adjacent cells. They are similar to desmosomes, but have different molecular components. Finally, tight junctions, as the name suggests, are sealed connections that do not permit exchange of fluid between cells. The question states that there is no exchange of water and ions between the cells; therefore, the connection between the cells must be a tight junction.

Plasmodesmata are similar to gap junctions, but they are only found in plant cell walls. They connect adjacent plant cells and facilitate intercellular communication and movement of nutrients between cells.

A cellular junction is made up of cytoskeletal filaments and cell adhesion molecules, which connect cytoskeletal elements between adjacent cells. There are three types of cytoskeletal filaments: microfilaments (actin filaments), intermediate filaments, and microtubules. A cellular junction usually utilizes connections with either actin filaments or intermediate filaments. Desmosomes and hemidesmosomes use intermediate filaments (particularly keratin) whereas tight junctions, adherens junctions, and focal adhesion junctions use actin filaments.

The question states that the autoimmune disease attacks occludin proteins. Recall that the occludin proteins are cell adhesion molecules found in tight junctions. Tight junctions, as the name implies, form sealing junctions that restrict the passage of molecules between adjacent cells. A lack of occludin proteins will decrease the amount of tight junctions and, subsequently, will increase the exchange of molecules between cells.

Cadherin proteins are also cell adhesion molecules, but they are found in adherens junctions. They do not depend on occludin proteins and, therefore, will not be affected by this disease. Tight junctions do require actin filaments and will be affected by this autoimmune disease; however, other actin utilizing junctions, such as adherens junctions and focal adhesions, don’t use occludin proteins and will not be affected.

Cell junctions are connections between a cell and its environment. There are two types of junctions: cell-cell and cell-matrix. Cell-cell junctions occur between adjacent cells and include tight junctions (or Zona Occludens), desmosomes, and adherens junctions. Cell-matrix junctions form between a cell and the extracellular matrix, and include hemidesmosomes and focal adhesions. 

Both desmosomes and hemidesmosmes use keratin, a type of intermediate filament; therefore, both use the same cytoskeletal filament. Tight junctions, focal adhesions, and adherens junctions utilize actin filaments, the other cytoskeletal filament found in cell junctions.

Desmosomes are spot-like junctions that occur between cells (cell-cell junction). Hemidesmosomes, or “half” desmosomes, are also spot-like junctions, but they occur between a cell and the extracellular matrix. Cell adhesion molecules (CAMs) are found in all cell junctions. They are typically found bound to the cytoskeletal filament, and function to hold the junction together. The CAM for desmosomes is a cadherin-like protein called desmoglein, whereas the CAM for hemidesmosomes is integrin. Epithelial cells typically have all types of cell junctions between adjacent cells, not just desmosomes. 

Cell walls are present in virtually all prokaryotic cells, but are also found in certain eukayotic domains (such as plants and fungi). As such, the presence of a cell wall cannot be used to distinguish between prokaryotic and eukaryotic cells.

Chyme from the stomach is transferred to the small intestine through the pyloric sphincter. The highly-acidic chyme is then neutralized in the duodenum of the small intestine. The majority of the small intestine has a pH between 6 and 7.