Small, charged molecules such as ions can cross the cell membrane through:

Small charged molecules such as ions can pass through cell membranes by facilitated diffusion using specific transmembrane protein channels, which act as transporters that are implanted into the plasma membrane. Such transport proteins regulate the selective permeability of cell membranes and play a critical function in cell membrane.

Moreover, this specific transmembrane protein have multiple membrane-spanning regions that create a passage through the phospholipid bilayer, allowing charged molecules to pass the membrane through a protein pore without interacting with the fatty acid chains of the membrane. 

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explanation:

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the level of progesterone is low at the pre-ovulatory phase of the cycle but it gradually increases and the levels are high at the luteal phase. if pregnancy occurs, the level of progesterone will be high throughout the pregnancy.

if egg is fertilized and the mother gets pregnant, progesterone starts preparing the uterus for the implantation. it starts building up the lining of uterus and make it hostile of the period of pregnancy.