4.5d Membrane Junctions

Differentiate between a desmosome and a tight junction.

A desmosome is a space that opens between two cells; intermediate filaments from each cytosol pass through the space to anchor the cells together. A tight junction is merely "stitching" that holds cells together, but it does not allow as much mechanical action as desmosomes do.

How large are gap junctions?

About 2 nm in length.

Why are hemidesmosomes called "half desmosomes?"

Because they anchor to the underlying components of the basal layer, forming only "1 side" of a desmosome. Each cell normally contributes half of a desmosome to complete a junction, and this does not happen with the basal layer.

Where are desmosomes found in the human body?

Cells of tisues exposed to stress, such as the external layer of the skin and cardiac muscles contain desmosomes.

Tight junction (zonula occludens)

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

Gap Junctions

Formed across the intercellular space between neighboring cells, bridged by six transmembrane proteins (connexions).

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

Intermediate filaments extend throughout the cell to provide support and strength.

What passes through gap junction?

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

What is another name for desmosomes?

Macula adherens

Desmosomes

One type of adhesion between two epithelial cells; a type of intercellular junction that holds cells together at a single point (like a button)

What is an example of tight junctions in the body?

Small intestines contain tight junctions that prevent corrosive digestive enzymes that are within the lumen of the intestine from moving between cells and damaging internal body structures.

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

The filaments anchor into a thickened protein plaque located at the internal surface of the cell.

How do plasma membrane proteins interact in a tight junction?

The plasma membranes fuse at transverse seems 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.

What are hemidesmosomes?

These are "half desmosomes" that anchor to the basal layer of cells of teh epidermis.

Where are gap junctions found?

They are found where the flow of ions between cells would allow for the spread of electrical activity, such as in cardiac muscle, coordinated activities such as beating of the cilia, or in neurons of the brain.

What are connexions?

They are transmembrane proteins that form tiny, fluid-filled tunnels or pores.

How do gap junctions function?

They provide a direct passageway for substances to travel between neighboring cells.

What is the function of desmosomes?

They provide resistance to mechanical stress at a single point.

What are the three major types of junctions?

Tight junctions, desmosomes, and gap junctions.

Describe the structural shape that tight function proteins form.

Tight protein junction proteins connect two plasma membranes as they wrap around a cell laterally, creating a woven shape (woven basket shaped).