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The hydrophilic head includes a glycerol molecule attached to a single phosphate group, which is then attached to another small molecule. The phosphate group and its attachments are either polar or charged. Water molecules are polar and therefore are attracted to these charged regions of the phospholipid head. The hydrophobic tails are two fatty acids attached to the glycerol molecule of the head. Water molecules are not attracted to the tails because C-H bonds are relatively nonpolar and therefore do not result in charged regions in the tails.

Nonpolar Molecules

Small, nonpolar molecules are hydrophobic, so they can easily cross the phospholipid bilayer of the plasma membrane.

Polar Molecules

Polar molecules and ions are hydrophilic, so they cannot very easily cross the hydrophobic portion of the plasma membrane (formed by the phospholipid tails). Water is an unusual molecule because, despite the fact that it is polar, it is small enough to pass directly through the hydrophobic interior of the lipid bilayer, albeit slowly. Polar molecules and ions generally cross the plasma membrane with the help of transport proteins. For example, water crosses the bilayer rapidly via transport proteins called aquaporins.

Small nonpolar molecules

Small nonpolar (hydrophobic) molecules, such as dissolved gases (O2, CO2, N2) and small lipids, can pass directly through the membrane. They do so by interacting directly with the hydrophobic interior of the lipid bilayer.

Very small non polar molecules

Very small polar molecules such as water and glycerol can pass directly through the membrane, but much more slowly than small nonpolar molecules. The mechanism that permits small polar molecules to cross the hydrophobic interior of the lipid bilayer is not completely understood, but it must involve the molecules squeezing between the hydrophobic tails of the lipids that make up the bilayer.


In exocytosis, substances are transported to the plasma membrane in vesicles derived from the endomembrane system. These vesicles fuse with the plasma membrane, releasing the enclosed substances outside the cell.


In endocytosis, substances are taken into the cell by folding in of the plasma membrane and pinching off of the membrane to form a vesicle. Notice that in both exocytosis and endocytosis, the transported substances never actually cross the plasma membrane as they leave or enter the cell.

In facilitated diffusion, what is the role of the transport protein?

Transport proteins organize the phospholipids to allow the solute to cross the membrane.
Transport proteins provide a protein site for ATP hydrolysis, which facilitates the movement of a solute across a membrane.
Transport proteins provide a low-resistance channel for water molecules to cross the membrane.
Transport proteins provide a hydrophilic route for the solute to cross the membrane.
Transport proteins provide the energy for diffusion of the solute.

Transport proteins provide a hydrophilic route for the solute to cross the membrane.

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