The unit membrane model, proposed by J. David Robertson in 1959, describes the structure common to many biological membranes, particularly the plasma membrane and organelle membranes.

Understanding the Unit Membrane Model Structure

According to the unit membrane model, a biological membrane consists of three distinct layers visible under an electron microscope:

• Two outer, electron-dense layers (appearing dark).
• A central, electron-transparent layer (appearing light).

This structure is interpreted as a lipid bilayer (the light central layer) sandwiched between two layers of protein (the dark outer layers). The term "dense band" in the context of this model refers to these outer protein layers.

Thickness of the Unit Membrane Dense Bands

In the unit membrane model, the dimensions of these layers were estimated based on electron microscopy:

• The central lipid bilayer is estimated to be approximately 35 Angstroms thick.
• Each of the outer protein layers, or dense bands, is estimated to be approximately 20 Angstroms thick.

Therefore, the total thickness of the unit membrane is approximately $20\text{\AA} + 35\text{\AA} + 20\text{\AA} = 75\text{\AA}$.

The question asks specifically for the thickness of each dense band (the protein layer). Based on the unit membrane model proposed by Robertson, the thickness of each dense band is approximately 20 Angstroms.

Using LaTeX for the Angstrom symbol: $\text{\AA}$