Questions and Answers
What are the three major types of 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?
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What is an example of tight junctions in the body?
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Describe the structural shape that tight junction proteins form.
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What are desmosomes?
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What is another name for desmosomes?
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What is the function of desmosomes?
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What do protein filaments anchor into on the inner surface of the cell in a desmosome?
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What type of filament penetrates the small space that opens in a desmosome?
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Where are desmosomes found in the human body?
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What are hemidesmosomes?
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Why are hemidesmosomes called 'half desmosomes'?
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What are gap junctions?
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How large are gap junctions?
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What are connexins?
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How do gap junctions function?
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What passes through gap junctions?
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Where are gap junctions found?
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Differentiate between a desmosome and a tight junction.
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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.