Answer:
The plasma membrane is a selectively permeable barrier that separates the cell from its environment. This means that while some substances can pass through the membrane freely, others cannot. The selective permeability of the plasma membrane is due to its structure, which is composed of a lipid bilayer with hydrophilic heads facing outward and hydrophobic tails facing inward.
The hydrophilic heads of the phospholipids are attracted to water, while the hydrophobic tails repel it. This creates a barrier that is impermeable to water-soluble substances, such as ions and polar molecules. Additionally, the phospholipid bilayer is relatively impermeable to larger nonpolar molecules, such as proteins and carbohydrates.
Small nonpolar molecules, such as oxygen and carbon dioxide, can diffuse easily across the plasma membrane due to their ability to dissolve in the hydrophobic interior of the lipid bilayer. However, larger polar molecules, such as glucose and amino acids, cannot pass freely through the membrane.
One reason for this is that the polar molecules are attracted to water, which is repelled by the hydrophobic tails of the phospholipids. This makes it difficult for the polar molecules to cross the lipid bilayer without assistance from specialized transport proteins, such as channels and carriers.
Furthermore, ions have an electrical charge that makes it difficult for them to cross the hydrophobic lipid bilayer. The charged particles are repelled by the hydrophobic tails of the phospholipids, and therefore, cannot pass through the membrane freely.
In summary, the selective permeability of the plasma membrane is due to the chemistry of the phospholipids, which create a hydrophobic barrier that is impermeable to water-soluble substances. Larger polar molecules and ions cannot freely cross the lipid bilayer because they are attracted to water and have an electrical charge that repels them from the hydrophobic interior of the membrane.