I understand that the plasma membrane is composed of a polar head and non polar tail. I read in my textbook that small non-polar molecules readily pass through and that polar and non-polar repel. so how do non-polar substance pass the the polar head? please explain.
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Great question! The plasma membrane is indeed composed of a bilayer of phospholipids, with polar heads facing outward and non-polar tails facing inward. Small non-polar molecules, such as oxygen, carbon dioxide, and steroid hormones, can readily pass through the membrane.
While it may seem counterintuitive for non-polar substances to pass through the polar head region, it happens because of the unique structure of the phospholipid bilayer. The non-polar tails create a hydrophobic (water-repelling) region in the middle of the membrane, which allows non-polar molecules to dissolve and freely diffuse across it.
The polar heads, on the other hand, create a hydrophilic (water-attracting) environment on both sides of the membrane. This hydrophilic environment effectively repels polar substances, as their interactions with water are typically stronger than their interactions with the non-polar tails. Therefore, polar substances have a harder time crossing the membrane.
To recap, non-polar substances can pass through the plasma membrane because they can dissolve in the hydrophobic region of the phospholipid bilayer. Meanwhile, polar substances are repelled by the hydrophilic regions, making it more difficult for them to cross the membrane.