Re: What advantage does having internal membranes confer on eukaryotic cells ?

The simple answer is compartmentalization. To completely answer the question, I'll break the long answer down into the various intracellular compartments to show the importance of membranes for each.

• mitochondria and chloroplasts are organelles which evolved from symbiotic bacteria which used their membranes as centers for generating energy. Once internalized into the eukaryotic cell, their outer membranes became the inner membranes, which are still involved in energy production, of the organelles. because the ancestral bacteria required a specific environment to grow, the outer membrane of the organelle acts like an incubator to keep the inner organelles active.

• Lysosomes and peroxisomes are specialized vesicles which allow the cell to use harsh chemicals and acidic conditions to metabolize and neutralize many compounds brought into the cell by endocytosis. The membranes enclosing these vesicles prevent their contents from entering and degrading the cytosol, which would surely destroy the cell.

• The Endoplasmic Reticulum (ER) and Golgi Stacks do many of the jobs of the outer membranes of prokaryotes. But, by internalizing and encapsulating the outer membrane into separate domains, several modifications can be performed on secreted proteins using only a handful of enzymes that can be concentrated and recycled while only specific proteins are secreted. Also, the intimate relation of the Golgi to the cytoskeleton allows it to direct the movement of processed proteins to different parts of the cell, allowing the cell to have membrane domains with specific complements of proteins and phosholipids.

• The nuclear envelope is an extension of the ER which forms a barrier between the nucleoplasm and the cytosol. Beside containing the chromosomes, the nucleoplasm is enriched, relative to the cytosol, with nucleotides (the building blocks of DNA and RNA) and several specialized enzymes necessary for replication and communicating the DNA information to the rest of the cell. The separation of cytosol and nucleoplasm also allows the cell to regulate which proteins have access to the chromosomes and when - an important mechanism for gene regulation.

By compartmentalizing the cytoplasm, eukaryotic cells can perform several contrary functions simultaneously independent of their environment. Prokaryotes must commit completely to a single function depending upon what environment they find themselves in. There is clearly an advantage to the self-sufficience of the eukaryotes in so far as it gives them the ability to occupy more niches and adapt to new environments more easily.