EM 010 Plasma Membrane

Plasma Membrane

Freeze fracture of the plasma membranes of adjacent cells as seen by scanning electron microscopy (SEM). At this high magnification, the narrow extracellular space that passes diagonally between the top and bottom cells is visible.

The fracture usually splits membranes through the hydrophobic core of the lipid bilayer.

• Most proteins (or complexes of proteins) remain embedded in the inner half of the plasma membrane adjacent to the cytoplasm. This is known as the protoplasmic face (or P-Face). The P-face of the plasma membrane for the top cell is seen (cyan).
• Fewer particles are found in the outer half of the plasma membrane adjacent to the extracellular space. This is known as the extracellular face (or E-Face). The E-face of the plasma membrane for the bottom cell is seen (yellow).

Unit Membrane

At the highest magnification, examine the right side of the image for the plasma membrane of the lower cell cut in cross section.

The plasma membrane is a lipid bilayer composed of phospholipids. The hydrophilic groups on each side of the lipid bilayer bind the heavy metals used to stain the specimen. The hydrophobic core of the membrane is unstained.

This three-layered structure has been designated the unit membrane.

Mitochondria

In the lower left corner of the bottom cell, a mitochondria can be seen.

The fracture passed underneath the mitochondria leaving only fragments of the inner and outer mitochondrial membranes and cristae. (Cristae are infoldings of the inner mitochondrial membrane that extend into the matrix of the mitochondria.)