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I was reading page 92 of Fundamentals of Microbiology, 4th edition, which states

In facilitated diffusion, the substance (glucose, for example) to be transported combines with a plasma membrane protein called a transporter.

The section of text that says this is in a section describing prokaryotic cell anatomy. As far as I understand human biology, (just to use one example) that facilitated diffusion is regulated by insulin, which, to use less than perfect language simply to streamline my question, uses insulin to "unlock" the plasma membrane proteins.

What's the difference when prokaryotic cells aren't dependent upon insulin to get glucose in?

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In prokaryotes the glucose transporter is always present in the cell membrane; in cells whose glucose uptake is insulin-regulated the transporter is only present in the plama membrane when hormone levels are high.

GLUT4 is the isulin-regulated glucose transporter found in muscle and adipose tissue. When insulin levels are low the GLUT4 protein is in the membrane of small vesicles inside the cell. The appearance of insulin signals the fusion of these vesicles with the plasma membrane. In this way the number of GLUT4 molecules in the plasma membrane is increased, raising the capacity of the cell for glucose transport. GLUT4 transports glucose into the cell by facilitated diffusion.

More information about the GLUT family of glucose transporters can be found here.

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Interestingly, bacterial and eukaryotic glucose transporters are similar in structure, though bacteria do not have insulin. ncbi.nlm.nih.gov/pubmed/23075985 –  shigeta May 16 '13 at 14:55

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