The Fluid Mosaic Model was first proposed in 1972 to explain the structure of the plasma membrane.Therefore, we don’t know what’s going on! Many theories and models were created to describe the structure of the biological membranes, the most acceptable and famous one being the Fluid Mosaic Model. At this size, it is very hard to see the exact structure, even with an electron microscope. For comparison, human red blood cells, visible via light microscopy, are approximately 8 µm wide, or approximately 1,000 times wider than a plasma membrane. Plasma membranes range from 5 to 10 nm in thickness.
Therefore, we also describe a cell membrane as ‘selectively permeable’ – or only allowing certain substances to pass through.
Things are brought in and things are shipped out of all cells, but it would be dangerous to let just anything pass through them without the right credentials. Because of this, a cell membrane provides a barrier to separate the cytoplasm from the extracellular space. All cells have a fluid but defining cell membrane that encircles the cytoplasm, like an old city would have a fluid, protective moat. 
Not much inside it is static or stationary, including the cell membrane that surrounds it. Like any town, a cell needs to create boundaries and to protect itself.
Cells are often likened to a tiny metropolis. The cytoplasm is the intracellular space within the cell containing all these structures and components. It contains numerous important structures and components that function together, and all parts interact with each other. A cell is the basic fundamental unit of all life on Earth. Keep reading for more detailed A-level Biology revision! The fluid mosaic model is a widely accepted model of the cell membrane structure, which describes the membrane as a fluid lipid bilayer embedded with various proteins and other biomolecules that can move laterally within the membrane. Which substances easily cross the cell membrane?. What is the role of proteins in the plasma membrane?. What is meant by a selectively permeable membrane?. \): In an aqueous solution, phospholipids tend to arrange themselves with their polar heads facing outward and their hydrophobic tails facing inward.
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