How does the structureof the cell membrane the cell regulate water

"The major purpose of the cell membrane is to regulate the types of substances that enter the cells and leave the cell, thus, it is a semi permeable structure.
The structure of the cell membrane help the cell in regulating the amount of the water inside the cell because, when the optimum amount of water is inside the cell, the membrane will not allow the pasage of water molecules into the cell again.
And in a situation, where the concentration of substance inside the cell is above the normal concentration, the cell membrane will allow the required amount of water to flow into the cell in order to reduce the concentration.
Also, the phospholipid bilayers that are found in the cell membrane help to regulate water homeostasis of the cell by using its hydrophobic tail to repel water molecules and thus keep them out or inside of the cells."
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The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.