Transition from Rough to Smooth Endoplasmic Reticulum - Thought Questions

This exercise is not about what we know, but about what we do not know.

This TEM shows a high magnification view of a section of a liver cell that contains both rough and smooth endoplasmic reticulum. In the picture identify rough endoplasmic reticulm and identify smooth endoplasmic reticulum. Now find areas of transition between these two forms of ER. Find areas where the membrane is continuous from rough to smooth ER. Click on the figure to toggle an overlay, if you need help. The green lines point out the membranes and the pink regions are the areas of transition between rough and smooth endoplasmic reticulum. This exercise continues below the picture.

Think about these transitional areas. Notice that RER and SER have different morphology. RER usually consists of flattened, sheet-like, cisternae, while SER is a complex of anastomosing tubules. RER is covered with ribosomes on the cytosol surface and SER is not.

• What accounts for these differences?
• Do you think that the primary difference is likely to lie in the composition of the lipids in the membrane bilayer?
• What else might form the basis for these differences?
• What sort of things would have to be occurring to give rise to the observed differences in the morphology of these membranes? Can you come up with any sort of model to account for these differences? Hint: you might think about which form of membrane, tht in RER or in SER is closer to a minimum energy conformation for membranes.