In terms of permeability, water, dissolved gases such as carbon dioxide and oxygen and lipid solid molecules simply diffuse across the phospholipid bilayer because the fluidity permits them.

Water soluble anions (negative ions) generally pass through small horns less than .8 nm in diameter (.0000000008m; 1m=1000mm, 1mm=1000µ-micrometer, 1µm= 1000nm). However, all other larger molecules require carrier molecules or proteins to transport them through the membrane.


The following are an overview of the four different ways in which atoms and molecules cross the biomembrane, which is also called transmembrane transport:


  • Simple diffusion; is the process by which small molecules and ions simply, because of their tendency to spontaneously move around, especially from a region where they are highly concentrated towards a region where they are less, diffuse through the phospholipid bilayer of the biomembrane.


  • Facilitated diffusion; is the process by which, apart from water and ions, specific molecules such as monosaccharides and amino acids, diffuse through channel proteins down their concentration gradient. This assisted diffusion provides them with a special hydrophilic pathway since some molecules might resist the hydrophobic core of the phospholipid bilayer. Some specific ion channels remain open much of the time and are called nongated channels and others only open in response to specific chemical or electrical signals and referred to as gated channels, including the voltage-gated channels of the nervous system which transmit action potentials (nerve impulses) to the brain.


  • Active Transport; is the transmembrane transport that occurs when channel proteins, such as the enzyme ATPases (enzymes usually have the suffix –ase) use the energy of ATP hydrolysis (breaking one phosphate group from the nucleotide Adenosine triphosphate) to move ions or small molecules across a membrane against their chemical concentration gradient or electric potential gradient or both.


General Principle of ATP hydrolysis. Breaking bonds usually give off energy.


ATP-powered pumps which requires energy, is coupled to the hydrolysis of ATP and the overall reaction– ATP hydrolysis and the “uphill” movement of ions or small molecules- is energetically favorable. All ATP-powered pumps are transmembrane proteins with one or more binding sites for ATP located on the cytosolic face of the membrane.

Sodium/Potassium ATP Powered Pump.

Although these proteins commonly are called ATPases, they normally do not hydrolyze ATP into ADP and Pi unless ions or other molecules are simultaneously transported. Because of this tight coupling between ATP hydrolysis and transport, the energy stored in the phosphahydride bond (bond between two phosphate groups) is not dissipated but rather used to move ions or other molecules uphill against an electrochemical gradient.


  • Cotransporter; are also enzymes. They mediate coupled reactions in which an energetically unfavorable reaction, such as the movement of molecules or ions against their concentration gradient, is coupled to an energetically favorable reaction such as the passive transport of molecules down their concentration gradient. The co-transporter will use the energy of the passive movement to actively transport other molecules up their concentration gradient. Therefore, unlike ATP, two molecules have to always move through the co-transporter at one time.


This Cotransporter is a Symporter. More below. Click on image for credit.

 The co-transporter is sometimes referred to as secondary-active transporter because it uses the energy stored into an electrochemical gradient.

Cotransporters fall into two types:

1-            Antiporter: in which the movement of both molecules (energy favorable and unfavorable) are in opposite directions.

2-            Symporter: in which the movement of both molecules are in the same direction (see above image). This should not confuse you because the same principle applies- one molecule will be moving down its concentration gradient, and the energy in this will be used to actively move another molecule up its concentration gradient but in the same direction, similar to the Sodium/Potassium pump but without the need for ATP hydrolysis because the Cotransporters are smarter and more efficient than Active transporters or ATPases.