Transport of substances across cell membranes

This chapter is vaguely relevant to Section E(i) of the 2017 CICM Primary Syllabus, which expect the exam candidate to "explain mechanisms of transport of substances across cell membranes". Multiple CICM Part One SAQs explored this subject matter, making it the most popular subject in this otherwise underappreciated syllabus section. These questions were:

• Question 2 from the second paper of 2022
• Question 16 from the first paper of 2014
• Question 10 from the second paper of 2012
• Question 3(p.2) from the first paper of 2010

  In terms of what the college expected, from the cryptic remarks made by the examiners we can determine that some sort of structure was critical and that the bare minimum consisted of "some mention and description of exocytosis, endocytosis, ion channels, facilitated diffusion, passive diffusion, primary and secondary active transport". Examples were called for. Moreover, though diagrams were not specifically required, the trainees who did use diagrams were apparently rewarded.  This chapter (hopefully) produces both the desired structure examples and diagrams without drowning in detail.

In summary:

In terms of sources, the trainee under time pressure would find their needs are met by Chapter 4 ("Transport of Substances Through Cell Membranes") from Guyton & Hall (p. 47 of the 13^th edition). If for whatever reason one does not wish to actually purchase or read the official college textbook, one may make use of virtually any free online resource, because this fundamental topic is covered in basically the same way by almost every author. Literally, Wikipedia is enough. If some sort of authoritative peer-reviewed resource is desperately required, Yang & Hinner (2015) do a good job of covering the subject. If "covering the subject" for you means "exhaustively cover every aspect in minute detail", Wilfred Stein's Transport And Diffusion Across Cell Membranes (2012) will certainly satisfy any enthusiast's unhinged need for molecular biology.

Passive diffusion

Though there does not appear to be any official definition of passive diffusion which relates specifically to cell membranes, one might borrow from the official college reference text (Guyton & Hall, Ch.4):

"[passive diffusion is] kinetic movement of molecules or ions [which] occurs through a membrane opening or through intermolecular spaces without any interaction with carrier proteins in the membrane."

"Simple" or "passive" diffusion occurs by one main method. The molecules just brutally force themselves across the lipid bilayer, driven purely by their concentration gradient. This process is obviously governed by simple Fickian factors such as membrane thickness, concentration gradient, surface area, molecule size, temperature (velocity of kinetic molecular motion) and lipid solubility

One might summarise molecular susceptibility to passive diffusion as follows:

• Molecular gases cross very rapidly
• Small molecules cross without difficulty, especially if they are lipid soluble
• Larger molecules cross without difficulty only if they are lipid soluble
• For charged molecules (eg. ions), the lipid bilayer is virtually impermeable 

There is even a way to represent the ease of diffusion numerically, as a diffusion coefficient. Sensibly, the coefficient is essentially the rate of diffusion perpendicular to the surface of the membrane, in centimetres per second. The numbers below are quoted from Yang & Hinner (2015), who tirelessly scraped the literature for diffusion data. All the studies were done on either some sort of purified phosphatidylcholine membrane or an artificial lipid bilayer thought to mimic the properties of the real thing. 

Species	Molecule	Permeability coefficient (cm/s)
Ions	Na+ K+	5.0 × 10−14 4.7 × 10−14
Small molecules	O 2	2.3 × 101
	CO 2	3.5 × 10−1
	H 2 O	3.4 × 10−3
	EtOH	2.1 × 10−3
	Steroids	10−3 to 10−4
	Urea	4.0 × 10−6
	Glycerol	5.4 × 10−6
	Small molecule drugs	10−5 to 10−6
Peptides	Cyclosporin A	2.5 × 10−7
	TAT	2.7 × 10−9

So basically one can see the trend there.  Gases like molecular oxygen and carbon dioxide diffuse across the membrane as if it wasn't even there, at a rate of 2-3 mm/sec. Considering that the membrane is 5 nm thick, an average oxygen atom would only take 0.0025 msec to Brownian-motion its way across the bilayer. In short, gases have no problems. They don't even need to be particularly lipid-soluble, but it helps (hence the 50% faster rate of diffusion seen with CO²).

The next order of magnitude is occupied by small molecules like water, and larger lipid-soluble molecules like alcohol and steroids. Water is strongly polar but has the advantage of having a massive concentration (the concentration of water in water being 55.5 mol/L) which allows it to force its way across the lipid bilayer in answer to osmotic shifts. The lipid-soluble molecules benefit from their high lipophilicity and are able to penetrate the barrier easily in spite of their comparatively larger size (eg. ethanol actually diffuses faster than water).

The middle diffusion tier (we might call it "cross with difficulty") are substances which are either significantly larger or not particularly lipophilic. This group includes urea, various small-molecule drugs, peptides and glycerol. Relying on passive diffusion for their transport is probably unreasonable if rapid entry into the cells is therapeutically required, but - left to marinade - they will eventually make their way in. One clinically relevant example of this is the tendency of urea to gradually infiltrate the brain of a chronic renal failure patient, where it produces osmotic cerebral oedema by failing to rapidly equilibrate with extracellular fluid following dialysis.

Lastly, highly charged small molecules and ions have a lot of difficulties crossing the membrane. The rate of diffusion for charged sodium and potassium ions is fifteen orders of magnitude (one quadrillion) times slower than for uncharged molecular gases.  Working backwards from the diffusion coefficient, these ions take tens of thousands of seconds (i.e. hours) to cross a 5 nm lipid bilayer. Some sort of active help is therefore essential to mediate their transport.

All of these numbers and measurements are relevant mainly to some sort of idealised model of a purified deproteinated lipid bilayer. In real life, ions water and small molecules can also make their way across the membrane using protein channels as conduits. As with direct-through-the-bilayer diffusion, this is a purely passive process - i.e. nothing is done to help the solutes get across the membrane.

Facilitated diffusion

All of the abovementioned passive diffusion numbers and measurements are relevant mainly to some sort of idealised model of a purified deproteinated lipid bilayer. In real life, ions water and small molecules can also make their way across the membrane using protein channels as conduits. As with direct-through-the-bilayer diffusion, this is a purely passive process - i.e. nothing is done to help the solutes get across the membrane. This is facilitated diffusion. Again, for lack of any official body to take control of definitions and nomenclature, we may turn to Guyton & Hall for a definition:

"Facilitated diffusion requires interaction of a carrier protein. The carrier protein aids passage of the molecules or ions through the membrane by binding chemically with them and shuttling them through the membrane in this form."

Though cumbersome, this definition is essentially valid, though for mnemonic purposes of the exam candidate one might summarise facilitated diffusion as "membrane transport which uses proteins as channels but which otherwise gets no chemical help whatsoever".  

The rate of this is obviously going to be quite variable from cell to cell, depending on the expression of porins and channels on its surface. Famously, Guyton & Hall report that "the total amount of water that diffuses in each direction through the red blood cell membrane during each second is about 100 times as great as the volume of the red blood cell itself", apparently owing to pore proteins - though no reference is offered in support of this. However, unlike simple passive diffusion, facilitated diffusion usually has some practical limits to its capacity, which is basically related to the rate at which those molecules can percolate through those channels. That is to say, as concentration gradient increases, so the rate of passive diffusion increases, but facilitated diffusion hits a wall beyond which no additional changes in concentration can produce any increase in the membrane permeability.  

Ion channels

The proteins which permit facilitated diffusion can exert some degree of control over what precisely is permitted entry into the cell; this is accomplished by a combination of pore size and charge selectivity. The upshot of this is that usually, diffusion-facilitating protein channels are strongly selective for a specific molecule. 

The diffusion of ions across these channels usually needs to be regulated in some way in order for the diffusion to be useful in some meaningful sense. Concentration gradients are there to be used for various cellular projects, and so it would be pointless to just allow ions to equilibrate on either side of the membrane. Thus, ion channels are usually gated. Gating can be by effects of some substance the binding of which triggers the protein to open or close (ligand gating) or it can be managed by changes in membrane potential (voltage-gating).

Primary active transport

Thus far the discussion has been limited to the various ways molecules can dumbly barge their way into the cell, whether through proteins channels or by lipid bilayer penetration. Active transport is by far more elegant. Particularly, primary active transport is the process of using chemical energy (usually stored in ATP) to facilitate the transport of ions from one side of the membrane to the other. Often, against the concentration gradient of that ionic species.   

These sort of active ATP-powered protein transporters are often colloquially referred to as "pumps". An excellent (ubiquitous) example is the sodium-potassium exchange pump, which cracks an ATP molecule to move 3 sodium ions out of the cell in exchange for 2 potassium ions entering, both ionic movements occurring in opposition to the concentration gradients of those ions. Without going into an insane amount of detail (well over a page is spent on this in Guyton & Hall) it suffices to say that this process is energy-expensive. It costs energy to pump something uphill; and the more a substance is concentrated on one side of the membrane, the more energy-expensive it is to concentrate it further. The G&H chapter goes on to mention that the cells which are routinely tasked with concentrating something by several orders of magnitude (eg. renal tubular cells and glands) expend 90% of their energy purely on powering the pump activity (which explains to some extent the oxygen extraction ratio of those tissues).

Secondary active transport

According to the college examiners, "in a number of answers, there was confusion between facilitated diffusion and secondary active transport." This is somewhat unexpected, as the two are rather different. Secondary active transport, like facilitated diffusion, uses a protein carrier to mediate transport, but beyond that, the similarity stops. The reason it is called secondary is that instead of direct ATP-burning chemical energy, this mode of transport uses a concentration gradient which has already been created by another primary active transport system.  For a more official definition from Guyton & Hall: 

"In secondary active transport, the energy is derived secondarily from energy that has been stored in the form of ionic concentration differences of secondary molecular or ionic substances between the two sides of a cell membrane, created originally by primary active transport."

In short, proteins involved in this mode of transport are usually referred to as "exchangers" rather than pumps. This group includes co-transporters and counter-transporters. In co-transport, the concentration gradient pushing an ion across a cell membrane is used to pull another molecule in the same direction (eg. sodium and glucose co-transport into the cell). Counter-transport is the exchange of molecules, where the concentration gradient of one participant is used to move the other in the opposite direction (and often against its own concentration gradient).

Endocytosis and exocytosis

Wherever molecules are of a substantial size, the use of an exchange protein or ATP-powered pump to move them becomes impractical. Consider, that the typical ATP-powered pump is a 100 kDa protein. If what you are trying to move is also a 100 kDA protein, obviously another method is required. That method is endocytosis. On some fundamental level one might classify endocytosis as a type of primary active transport. However, instead of an ATP-cracking protein undergoing a conformal change and pushing a molecule through the membrane, the entire membrane undergoes a conformal change in order to envelop and consume the molecule, bringing it into the cell in the form of an endosome. 

This is not a peculiarity of transporting molecules from outside of the cell into the inside. Vesicular transport is the main method of getting a large molecule from A to B within the cell, and most organelles exchange their substrates in this fashion. The process of internalising molecules in this fashion is complex, requiring many steps. It is well described in such freely available online resources as this chapter of The Cell. In summary, a whole array of molecular machinery is deployed to manipulate these vesicles, and the process is relatively energy-expensive.