passing through the semipermeable membrane

[drgshahbaz1193]

Hi I have a question about moving things through a eukaryotic phospholipid membrane. The textbook says there are three ways in which this occurs; simple diffusion, cell mediated transport and endo/exocytosis. What about the intrinsic proteins that can serve as channels, where do they fit in with the other three options?

[calena7178147]

The lipid bilayer is permeable to water molecules and a few other small, uncharged, molecules like oxygen (O2) and carbon dioxide (CO2). These enter and leave the cell by diffusion. Lipid bilayers are not permeable to: ions (K+, Na+, Ca+, Cl-), small hydrophilic molecules like glucose, and macromolecules like proteins and RNA.

Trans-membrane proteins or intrinsic proteins which are embedded in the phospholipid membrane create a water-filled pore through which ions and some small hydrophilic molecules can pass by diffusion. The channels can be opened (or closed) according to the needs of the cell---this is facilitated diffusion. The channels are “gated”, or they are opened in response to different types of stimuli. Types of gated channels are: ligand-gated, mechanically-gated, voltage-gated, light-gated.

Trans-membrane proteins, called transporters, use the energy of ATP to force ions or small molecules through the membrane against their concentration gradient. This is active transport.