MCAT Biology : Intercellular Junctions

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #1 : Intercellular Junctions

One component of the immune system is the neutrophil, a professional phagocyte that consumes invading cells. The neutrophil is ferried to the site of infection via the blood as pre-neutrophils, or monocytes, ready to differentiate as needed to defend their host.

In order to leave the blood and migrate to the tissues, where infection is active, the monocyte undergoes a process called diapedesis. Diapedesis is a process of extravasation, where the monocyte leaves the circulation by moving in between endothelial cells, enters the tissue, and matures into a neutrophil.

Diapedesis is mediated by a class of proteins called selectins, present on the monocyte membrane and the endothelium. These selectins interact, attract the monocyte to the endothelium, and allow the monocytes to roll along the endothelium until they are able to complete diapedesis by leaving the vasculature and entering the tissues.

The image below shows monocytes moving in the blood vessel, "rolling" along the vessel wall, and eventually leaving the vessel to migrate to the site of infection.

 

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Movement between cells, such as that carried out by monocytes in the passage, is typically blocked best by which kind of cell junction?

Possible Answers:

Adherens junctions

Gap junctions

Zona adherens

Zona occludens

Hemidesmosomes

Correct answer:

Zona occludens

Explanation:

Zona occludens block transport between cells by forming a zipper like boundary toward the apical surface of neighboring cells. These are also known as "tight junctions."

Zona adherens are also called adherens junctions or desmosones, and are designed to join cells together rather than to block transport between cells. Hemidesmosomes serve to link cells to an extracellular matrix or basement membrane, and gap junctions allow signal transduction between cells.

Example Question #2 : Intercellular Junctions

Which cellular junction involves the direct attachment of cytoskeletons between adjacent cells?

Possible Answers:

Desmosomes

Tight junctions

Intercalated discs

Gap junctions

Correct answer:

Desmosomes

Explanation:

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.

Example Question #3 : Intercellular Junctions

Action potential propagation in cardiac cells is very important for the proper functioning of the heart. Which of the following intercellular connections will be most effective in cardiac cells?

Possible Answers:

Gap junctions because they facilitate exchange of ions between cells

Tight junctions because they facilitate exchange of ions between cells

Gap junctions because they facilitate exchange of fluid between cells

Tight junctions because they facilitate exchange of fluid between cells

Correct answer:

Gap junctions because they facilitate exchange of ions between cells

Explanation:

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.

Example Question #2 : Intercellular Junctions

A researcher observes a sample of cells and concludes that they are connected by desmosomes. What observation(s) could have led to this conclusion?

I. He notices that there is no fluid exchange between the cells

II. There is a connection between the intermediate filaments of each cell

III. The connection occurs at a single location

Possible Answers:

II and III

I, II, and III

I and II

I only

Correct answer:

II and III

Explanation:

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.

Example Question #3 : Intercellular Junctions

A researcher notices a connection between two cells. Upon further analysis, he concludes that there is no exchange of water or ions between the cells. Which of the following could be the identity of the connection between the two cells?

Possible Answers:

Gap junction

Plasmodesmata

Tight junction

Desmosome

Correct answer:

Tight junction

Explanation:

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.

Example Question #4 : Intercellular Junctions

A patient is found to have a defect in his intermediate filaments. Which of the following cellular junctions will be absent in this patient?

I. Desmosomes

II. Gap junctions

III. Hemidesmosomes

Possible Answers:

I, II, and III

All three junction types will be present in this patient

I and II

I and III

Correct answer:

I and III

Explanation:

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.

Example Question #5 : Intercellular Junctions

A researcher is analyzing an autoimmune disease. His results indicate that the patient has antibodies that attack occludin proteins. Which of the following is likely true for this patient? 

Possible Answers:

There is an easy passage of molecules through the space between adjacent cells

The patient will lack adherens junctions

The patient’s cadherin proteins are also under attack

Cell junctions that require actin will be disrupted

Correct answer:

There is an easy passage of molecules through the space between adjacent cells

Explanation:

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.

Example Question #6 : Intercellular Junctions

Zona occludens are formed at __________ junctions and desmosomes are formed at __________ junctions. 

Possible Answers:

cell-cell . . . cell-matrix

cell-matrix . . . cell-matrix

cell-matrix . . . cell-cell

cell-cell . . . cell-cell

Correct answer:

cell-cell . . . cell-cell

Explanation:

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. 

Example Question #9 : Intercellular Junctions

Which of the following is true regarding desmosomes and hemidesmosomes? 

Possible Answers:

Desmosomes are cell-matrix junctions, whereas hemidesmosomes are cell-cell matrix

Only desmosomes are found in epithelial cells

The cell adhesion molecule integrin is used in both junctions

Both are made up of the same cytoskeletal filament

Correct answer:

Both are made up of the same cytoskeletal filament

Explanation:

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. 

Example Question #7 : Intercellular Junctions

Which of the following criteria does not help differentiate between prokaryotes and eukaryotes?

Possible Answers:

Size of the cell

Presence of a cell wall

A membrane-bound nucleus

Presence of mitochondria

Correct answer:

Presence of a cell wall

Explanation:

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.

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