Answer: Polarity and Size
Explanation: The cell membrane is known as selectively permeable because of the configuration of polar heads facing the intracellular and extracellular aqueous environment. These polar heads cling tightly together to form a physically impermeable barrier to most molecules. Some molecules small enough to fit through will slide between the gaps, but the size is only one of the requirements. Polarity can also make it difficult for molecules to pass through the membrane. The heads of the phospholipids that make up the membrane are polar molecules meaning they have a charge, and any other polar molecule will interact with it. It's like trying to push two magnets together. This makes other polar molecules unable to pass through because they actively repel the membrane. This means the membrane is only permeable by non-polar small molecules making it selectively permeable. Other molecules can travel through the membrane through the use of proteins embedded in the membrane but only a select few.
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