Metabolic processes within cells are catalysed by protein enzymes. It was realized that the formation of macromolecules had to be catalysed by something, and unless life pulled itself up by its own bootstraps these catalysts could not be enzymes. So, it was proposed by A.G. Caines-Smith in 1985 that clay composed of silicates was the catalyst of protometabolism (see footnote).
It was soon realized that since the entire sea could not function as a metabolism, it had to be compartmentalized. It was observed that cell membranes are composed of phospholipids that naturally form spheres when placed in water. So, it was proposed that the protocell was bounded by a very simple lipid membrane that formed spontaneously. The problem with this simple model is that the protocell would be isolated from its environment. The phospholipid membrane of the hypothetical protocell would not allow molecules that fuel metabolism to enter the protocell, nor would it allow toxic waste products out. Isolation from the environment spells non-viability for cells.
The cells we know have a phospholipid bilayer plasma membrane with both embedded and peripheral proteins that regulate the entry and exit of substances into and out of the cytoplasm. The selectively permeable cell membrane maintains a steady internal environment within the cell. Water and some small molecules can cross the phospholipid membrane and follow their concentration gradient, but larger molecules such as glucose and amino acids, and also ions are assisted across the membrane by carrier proteins that are specific to each molecule or ion. Transmembrane proteins include channel proteins, carrier proteins, cell recognition proteins, receptor proteins and enzymatic proteins and transport may be active requiring the expenditure of energy donated by ATP.
The conclusion to this short section is that simple membranes would isolate a cell, making it unable to function as a cell, and complex membranes containing channels and carrier proteins that allow the cell to interact with its environment are complex.
 Cairns-Smith A.G. (1985) Seven Clues to the Origin of Life Cambridge University Press; Cairns-Smith A.G. (1993) Genetic Takeover: And the Mineral Origins of Life Cambridge University Press