CBSE Class 11-science Answered

1. The lipid bilayer is impermeable to entry of polar molecules. Impermeable means that it does not allow molecules to freely pass across it. Only water and gases can easily pass through the bilayer. This property means that large molecules and small polar molecules cannot cross the bilayer, and thus the cell membrane, without the assistance of other structures. Polar molecules and large ions dissolved in water cannot diffuse freely across the plasma membranedue to the hydrophobic nature of the fatty acid tails of the phospholipids that make up the lipid bilayer.

2. Carrier proteins are proteins that carry substances from one side of a biological membrane to the other. The carrier proteins facilitate diffusion of molecules across the cell membrane. GLUT1 is a carrier protein found in almost all animal cell membranes that transports glucose across the bilayer.