Why is a lipid bilayer semipermeable

Note: Water molecules can pass through these tails even though they are polar in nature because they are very small. Why are cell membranes considered semipermeable? Anuj Baskota. Oct 25, Because of the hydrophobic water hating tails of the phospholipids. Explanation: Cell membrane mostly consists of phospholipids which has hydrophobic tails. The proper way to state these features is to say that the membrane is highly permeable to lipid-soluble molecules , or that the membrane is not permeable to ions.

It may also be said that membrane permeability is high for lipid-soluble molecules, and that membrane permeability is low for ions and polar molecules. Another way of stating this is that lipid-soluble molecules are highly permeant , or that ions are impermeant i. Figure 1. Permeation through a pure lipid bilayer. Only a limited number of molecules can cross biological membranes without the aid of transport proteins.

Membrane impermeant molecules and ions require the aid of membrane transport proteins in order to cross the membrane. See text for details. This makes the phospholipid bilayer an excellent semipermeable membrane that allows cells to keep their contents separated from the environment and other cells. The concentration of the solution bound by a semipermeable membrane can be described by its tonicity as compared to the environment or other cells. Because biological membranes are permeable to water but not solutes, water tends to move into cells that are hypertonic to their environment, while water moves out of cells that are hypotonic.

If a point source e. Over time, the solute then diffuses from the point source outward in all avail- able directions, and eventually the concentration of solute is equal at any point in tea- cup-space. This behavior is governed by the Second Law of Thermodynamics. The solute is initially concentrated, which means that its constituent molecules are relatively orga- nized.

By the second law, these molecules will tend toward chaos, moving away from the constraints of the initial point toward an area with lower concentrations of the solute.

Now, imagine a temporary wall around the point source. The natural tendency is for the solutes to spread out, so by preventing that movement, you have bottled up some potential energy. Of course, this is only potential energy if there is some chance that the solutes can eventually go through the barrier e. If the solutes have absolutely zero chance of passing through, then there is no potential energy because there is no potential to get out and about. Recalling the Energy chapter in which the second law was introduced, the chemical potential energy of a solute is.

Now imagine this as something like a hydroelectric dam, where there is a great deal of pressure building up behind the dam, which can be utilized when some of the water is allowed through, powering turbines that generate electricity.

In the biological case, there is concentration pressure building up both inside and outside the cell because the natural thermodynamic tendency is to bring the inside and outside concentrations of each solute to equilibrium. When this pressure is released by allowing the ions or other molecules to ow across the membrane, energy is released, and may be captured and used.

The most direct example of this the proton-gradient-driven ATP synthase in the inner mitochondrial membrane Chapter 5 , which contains a direct molecular equivalent to the turning of a water wheel with the ow of water.