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Phospholipid bilayer is mainly a lipoidal membrane.

Ions being hydophillic (not lipid soluble) cannot cross the membrane directly. They are carried by specialized Ion transporters across the membrane.

These transporters can be

1. ION CHANNELS which are non specific( i.e. they can carry any ion) and work along the concentration gradient.

They are not dependent on energy for working.

2.PUMPS These are spefic for particular ions and can work against the concentration gradient.


NO. small ions cannot diffuse across

Charged particels (ions) do not easily diffuse through the bilayer, since the phospholipid tails are hydrophobic (=not attracted to water or charges). The ions would be attracted to the phosholipid heads which are hydriophilic (=attracted to water and charges). As a result, membranes must find a way to allow the ions to pass through the hydrophobicity of the bilayer, using ion channels or pores. The process could be facilitated diffusion, if it is a passive process or othwerwise, the concentration gradient favours ions moving from a highly concentrated area to a less concentrated area. If the process is an active one (e.g. Na+/K+ pump) and the concentration gradient does not favour ions moving to an area of fewer ions, but rather to a higher concentrated area, the process would be termed active 

Charged particels (ions) do not easily diffuce through the bilayer, since the phospholipid tails are hydrophobic (=not attracted to water or charges). The ions would be attracted to the phosholipid heads which are hydriophilic (=attracted to water and charges). As a result, membranes must find a way to allow the ions to pass through the hydrophobicity of the bilayer, using ion channels or pores. The process could be facilitated diffusion, if it is a passive process or othwerwise, the concentration gradient favours ions moving from a highly concentrated area to a less concentrated area. If the process is an active one (e.g. Na+/K+ pump) and the concentration gradient does not favour ions moving to an area of fewer ions, but rather to a higher concentrated area, the process would be termed active 

  • Small Ions can pass  through
  • Various ions diffuse through different ways
  • Such as water soluble molecules use transporter proteins.

the exact answer: 

Lipid bilayer The cell membrane consists primarily of a thin layer of amphipathic phospholipids which are arranged so that the hydrophobic "tail" regions are shielded from the surrounding polar fluid, causing the more hydrophilic "head" regions to associate with the cytosolic and extracellular faces of the resulting bilayer. This forms a continuous, spherical lipid bilayer. The arrangement of hydrophilic heads and hydrophobic tails of the lipid bilayer prevent polar solutes (e.g. amino acids, nucleic acids, carbohydrates, proteins, and ions) from diffusing across the membrane, but generally allows for the passive diffusion of hydrophobic molecules.

Passive transport means moving biochemicals and atomic or molecular substances across the cell membrane. Unlike active transport, this process does not involve chemical energy. The four main kinds of passive transport are diffusion, facilitated diffusion, filtration and osmosis. 

Depending on the location of the membrane yes, small permeable ions can diffuse across. However it also depends on the type of ion and thus the interactions it could cause for instance in the membranes of nerve axons, potassium ions can passively diffuse across the membrane in order to re-establish the neutrons resting potential yet that may not happen in the membrane of the gut lining. 

Other forms of ion transport across the membrane include:

facilitated diffusion - using a channel protein that is selectively permeable to certain ions so it allows them to flow into the cell or a carrier protein that alters its' shape when an ion binds causing a conformational change that allows it to release the ion into the cell or across the membrane. 

active transport - when ions are actively pumped against the concentration gradient into or out of the cell (occurs in respiration and photosynthesis)


hydrophobic and very small molecules can pass straight through the bilayer. other water soluble molecules require transporter proteins to cross the bilayer. These may be ion channels, symporters or antiporters

Small molecules that do not contain a charge, such as glucose,O2 and CO2, are able to pass the lipid layer through a process called passive diffusion. For a molecule to cross the layer it must be, 1) permeable to the membrane, 2) small, 3) hydrophobic, because the layer of the phospholipids is hydrophobic, so any small and covalent (no charge, molecule that are bond) are allow to cross the outer- layer to the inner- layer.

For an ion to cross the lipid layer, the layers most require an ion transporter (protein transporter) or ion pump to pass the membrane. This process will require energy (ATP) to take place and be successful. 

Ions, by definition, are charged molecules and therefore cannot diffuse across the membrane by themselves. They require special proteins embedded in the cell membrane to carry them across. There are two types of these proteins: 

(1) Channel proteins: Create an open channel in the membrane that provides an outlet for ions to diffuse through. Channel proteins are more than just holes in the membrane, as they exhibit specificity, and only allow certain ions to go through. E.g. The K+ (potassium) channel only allow potassium to enter by placing specific amino acids in the interior of the channel, which K+ will interact with (but, for instance, similarly charged Na+ cannot) 

(2) Carrier proteins: Exhibit an even higher amount of specificity, these proteins physically carry ions from one side of the membrane to the other via conformational changes E.g. the Na+-K+ transporter exchanges 3 Na+ for 2 K+ out. This also a form of active transport, in that the conformational changes in the protein that move these ions are the result of ATP hydrolysis

 Only small uncharged molecules can diffuse freely through phospholipid bilayers . Small nonpolar molecules, such as O2 and CO2, are soluble in the lipid bilayer and therefore can readily cross cell membranes. Small uncharged polar molecules, such as H2O, also can diffuse through membranes, but larger uncharged polar molecules, such as glucose, cannot. Charged molecules, such as ions, are unable to diffuse through phospholipid billayer regardless of size; even H+ ions cannot cross a lipid bilayer by free diffusion


Ions cross the phospholipid bilayer through the action of transmembrane proteins that act as transporters. No charged molecules can pass through the phosphpolipid bilayer by passive diffusion alone, so not even small ions can just diffuse across. 

No small ions cannot just diffuse across, no charged molecules can cross the phospholipid bilayer, they have to be passed through the membrane through the action of transmembrane proteins that act as transporters. Only small relatively hydrophobic molecules can pass through the phospholipid bilayer by passive diffusion. Hope that helps!

As ions are charged molecules they can only cross the phospholipid bilayer by facilitated diffusion where specific channel proteins allow the ions to cross along the concentration gradient. However other small uncharged molecules like water and carbon dioxide can diffuse without any protein channels.

Ions have charges and therefore in order to cross the phospholipid bilayer, they must have some kind of help to diffuse across. They cannot do this by themselves. There are proteins, specialised to perform certain jobs which can assist the ions  and therefore cannot diffuse across the membrane by themselves. 
Channel proteins open like a gate to make an open channel in the membrane. Once the gateway is open, the ions are able to diffuse through. However, they don't let any old ion through- Channel proteins are specific to certain ions, for example if you have a Cl- ion it will only be able to get through a channel protein which is a Cl- channel because specific amino acids are on the surface and need to interact with the ion. 
There are also carrier proteins which carry ions from one side of the membrane to the other. 

isn't it carrier proteins rather than channel proteins?

Ion transport across the bilayer is carried out by integral membrane proteins known as ion channels or ion pumps.

Ion pumps transport ions against the chemical gradient, i.e. from lower chemical potential to higher chemical potential, and need energy, typically ATP, to do this. They regulate ion concentration, pH etc and help build up the chemical gradients.

The ion channels allow transportation or flow of ions with the chemical gradient, i.e. from higher chemical potential to lower chemical potential, for the purpose of sending signals or doing a job etc. They have a trigger/mechanism to tell them when to allow ion flow and when not and as the ions are flowing down the chemical gradient, no energy is required.

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