Cell Biology Junctions Overview

Questions and Answers

What are the three major types of junctions?

Desmosomes (correct)

Gap junctions (correct)

Tight junctions (correct)

Plasma junctions

What is a tight junction (zonula occludens)?

Encircles certain types of cells (epithelial cells) near their exposed apical surface and completely attaches each cell to its neighbors.

How do plasma membrane proteins interact in a tight junction?

The plasma membranes fuse at transverse seams parallel to the apical surface of the cell.

How do tight junctions affect substances entering the tight junctions?

The substances must pass through the epithelial cells, rather than between them. Signup and view all the answers

What is an example of tight junctions in the body?

Small intestines contain tight junctions that prevent corrosive digestive enzymes from moving between cells. Signup and view all the answers

Describe the structural shape that tight junction proteins form.

Tight junction proteins connect two plasma membranes as they wrap around a cell laterally, creating a woven shape. Signup and view all the answers

What are desmosomes?

They are a type of intercellular junction that holds cells together at a single point. Signup and view all the answers

What is another name for desmosomes?

Macula adherens. Signup and view all the answers

What is the function of desmosomes?

They provide resistance to mechanical stress at a single point. Signup and view all the answers

What do protein filaments anchor into on the inner surface of the cell in a desmosome?

They anchor into a thickened protein plaque located at the internal surface of the cell. Signup and view all the answers

What type of filament penetrates the small space that opens in a desmosome?

Intermediate filaments extend throughout the cell to provide support and strength. Signup and view all the answers

Where are desmosomes found in the human body?

In tissues exposed to stress, such as the external layer of the skin and cardiac muscles. Signup and view all the answers

What are hemidesmosomes?

They are 'half desmosomes' that anchor to the basal layer of cells of the epidermis. Signup and view all the answers

Why are hemidesmosomes called 'half desmosomes'?

Because they anchor to the underlying components of the basal layer, forming only 'one side' of a desmosome. Signup and view all the answers

What are gap junctions?

They are formed across the intercellular space between neighboring cells, bridged by six transmembrane proteins (connexins). Signup and view all the answers

How large are gap junctions?

About 2 nm in length. Signup and view all the answers

What are connexins?

They are transmembrane proteins that form tiny, fluid-filled tunnels or pores. Signup and view all the answers

How do gap junctions function?

They provide a direct passageway for substances to travel between neighboring cells. Signup and view all the answers

What passes through gap junctions?

Ions, glucose, amino acids, and other small solutes pass directly from the cytoplasm of one cell into the neighboring cell. Signup and view all the answers

Where are gap junctions found?

They are found where the flow of ions between cells would allow for the spread of electrical activity. Signup and view all the answers

Differentiate between a desmosome and a tight junction.

A desmosome has a space that opens between two cells; tight junctions do not allow as much mechanical action as desmosomes. Signup and view all the answers

Study Notes

Junctions Overview

Three major types of junctions: tight junctions, desmosomes, and gap junctions.

Tight Junctions (Zonula Occludens)

Encircle epithelial cells near the apical surface, entirely attaching neighboring cells.
Plasma membranes fuse at transverse seams parallel to the cell surface, forming a barrier.
Substances must pass through epithelial cells, preventing paracellular movement.
Examples include small intestines, where they protect against corrosive digestive enzymes.
Tight junction proteins wrap around the cell laterally, creating a woven or basket-shaped structure.

Desmosomes

Also known as macula adherens, these junctions hold epithelial cells together at single points like buttons.
Provide resistance to mechanical stress by anchoring cells at discrete points.
Protein filaments anchor into a thickened protein plaque on the cell’s inner surface.
Intermediate filaments extend through the cell, adding support and strength.
Commonly found in tissues exposed to stress, including the skin and cardiac muscles.

Hemidesmosomes

"Half desmosomes" that anchor to the basal layer of epidermal cells.
Named for their structure since they only contribute one side to a junction, lacking the counterpart from adjacent cells.

Gap Junctions

Formed by six transmembrane proteins called connexins, bridging intercellular spaces.
Approximately 2 nm in length, facilitating communication between neighboring cells.
Connexins create tiny, fluid-filled tunnels known as pores, allowing direct passageways for substances.
Permit the flow of ions, glucose, amino acids, and other small solutes between cell cytoplasms.
Often found in areas requiring rapid electrical activity transfer, such as cardiac muscle, ciliated cells, and neurons.

Comparison of Junction Types

Desmosomes allow space between cells with intermediate filaments anchoring them, offering mechanical support.
Tight junctions function more as a barrier, limiting paracellular movement and providing less mechanical flexibility than desmosomes.

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Description

Explore the three major types of cellular junctions: tight junctions, desmosomes, and gap junctions. This quiz delves into their structure, function, and examples, particularly focusing on their importance in epithelial tissues. Test your knowledge on how these junctions contribute to cellular integrity and resistance to stress.