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Answer:
Some molecules, such as carbon dioxide and oxygen, can diffuse across the plasma membrane directly, but others need help to cross its hydrophobic core. In facilitated diffusion, molecules diffuse across the plasma membrane with assistance from membrane proteins, such as channels and carriers.
Molecules that are too large to flow across the membrane need help from integral proteins.
Some molecules are too large to diffuse through the membrane channels and too lipid insoluble to diffuse through the phospholipid layer.
Such is the case with glucose and some other monosaccharides,these substances, however, can cross the plasma membrane through the facilitated diffusion process.
The transport of these molecules depends on integral membrane proteins that act as transporters, transferring the molecules to one side of the membrane and the other without coming into contact with its hydrophobic interior.
Intrinsic or integral proteins constitute more than 70% of the total and are totally or partially included within the lipid matrix of the membrane maintained by hydrophobic interactions.
When they are arranged along the membrane, they are called transmembrane, they can pass totally or partially through the bilayer, sticking out one or both of its surfaces.
There are pores and channels in the membrane to pass substances to the interior and exterior of the cells formed by transmembrane proteins.
The passage of ions through ion channels is faster than through "carrier" proteins, since it does not require the union of the ion with the pore protein.
Therefore, we can conclude that molecules that are too large to flow across the membrane need help from integral proteins.
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