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Diffusion and passive transport

Covers selective permeability of membranes, diffusion, and facilitated diffusion (including channels and carrier proteins).

Introduction

Have you been through airport security lately? If you have, you’ve probably noticed that it’s carefully designed to let some things in (such as passengers with tickets) and to keep others out (such as weapons, explosives, and bottled water). Flight attendants, captains, and airport personnel travel through quickly via a special channel, while regular passengers pass through more slowly, sometimes with a long wait in line.
In many ways, airport security is a lot like the plasma membrane of a cell. Cell membranes are selectively permeable, regulating which substances can pass through, as well as how much of each substance can enter or exit at a given time. Selective permeability is essential to cells’ ability to obtain nutrients, eliminate wastes, and maintain a stable interior environment different than that of the surroundings (maintain homeostasis).
The simplest forms of transport across a membrane are passive. Passive transport does not require the cell to expend any energy and involves a substance diffusing down its concentration gradient across a membrane. A concentration gradient is a just a region of space over which the concentration of a substance changes, and substances will naturally move down their gradients, from an area of higher to an area of lower concentration.
In cells, some molecules can move down their concentration gradients by crossing the lipid portion of the membrane directly, while others must pass through membrane proteins in a process called facilitated diffusion. Here, we’ll look in more detail at membrane permeability and different modes of passive transport.

Selective permeability

The phospholipids of plasma membranes are amphipathic: they have both hydrophilic (water-loving) and hydrophobic (water-fearing) regions. The hydrophobic core of the plasma membrane helps some materials move through the membrane, while it blocks the movement of others.
Image modified from OpenStax Biology.
Polar and charged molecules have much more trouble crossing the membrane. Polar molecules can easily interact with the outer face of the membrane, where the negatively charged head groups are found, but they have difficulty passing through its hydrophobic core. Water molecules, for instance, cannot cross the membrane rapidly (although thanks to their small size and lack of a full charge, they can cross at a slow rate).
Additionally, while small ions are the right size to slip through the membrane, their charge prevents them from doing so. This means that ions like sodium, potassium, calcium, and chloride cannot cross membranes to any significant degree by simple diffusion, and must instead be transported by specialized proteins (which we’ll discuss later). Larger charged and polar molecules, like sugars and amino acids, also need help from proteins to efficiently cross the membrane.

Diffusion

In the process of diffusion, a substance tends to move from an area of high concentration to an area of low concentration until its concentration becomes equal throughout a space. For example, think about someone opening a bottle of cleaning ammonia in the middle of a room. The ammonia molecules will initially be most concentrated right where the person opened the bottle, with few or no molecules at the edges of the room. Gradually, the ammonia molecules will diffuse, or spread, away from the place where they were released, and eventually you’ll be able to smell ammonia at the edges of the room. Ultimately, if the bottle is capped and the room is closed, the ammonia molecules will become evenly distributed throughout its volume.
The same will happen with molecules of any type: as a population, they tend to move from an area where they’re more concentrated to an area where they’re less concentrated. To understand this, imagine that there’s an area where molecules are more concentrated (such as where ammonia has just been opened) and an area where they’re less concentrated (the surrounding room). Since there are lots of ammonia molecules in the concentrated area, it’s pretty likely that one will move from there into the non-concentrated area. But since there are few molecules of ammonia in the non-concentrated area, it’s pretty unlikely that the reverse will happen.
Thus, over time, the net movement of molecules will be out of the more concentrated area and into the less concentrated one, until the concentrations become equal (at which point, it’s equally likely for a molecule to move in either direction). This process does not require any energy input; in fact, a concentration gradient itself is a form of stored (potential) energy, and this energy is used up as the concentrations equalize.
Image credit: OpenStax Biology, modified from original work by Mariana Ruiz Villareal.
Molecules can move through the cell’s cytosol by diffusion, and some molecules also diffuse across the plasma membrane (as shown in the picture above). Each individual substance in a solution or space has its own concentration gradient, independent of the concentration gradients of other materials, and will diffuse according to that gradient. Other factors being equal, a stronger concentration gradient (larger concentration difference between regions) results in faster diffusion. Thus, in a single cell, there can be different rates and directions of diffusion for different molecules. For example, oxygen might move into the cell by diffusion, while at the same time, carbon dioxide might move out in obedience to its own concentration gradient.

Facilitated diffusion

Some molecules, such as carbon dioxide and oxygen, can diffuse across the plasma membrane directly, but others need help to cross its hydrophobic core. In facilitated diffusion, molecules diffuse across the plasma membrane with assistance from membrane proteins, such as channels and carriers.
A concentration gradient exists for these molecules, so they have the potential to diffuse into (or out of) the cell by moving down it. However, because they are charged or polar, they can't cross the phospholipid part of the membrane without help. Facilitated transport proteins shield these molecules from the hydrophobic core of the membrane, providing a route by which they can cross. Two major classes of facilitated transport proteins are channels and carrier proteins.

Channels

Channel proteins span the membrane and make hydrophilic tunnels across it, allowing their target molecules to pass through by diffusion. Channels are very selective and will accept only one type of molecule (or a few closely related molecules) for transport. Passage through a channel protein allows polar and charged compounds to avoid the hydrophobic core of the plasma membrane, which would otherwise slow or block their entry into the cell.
_Image modified from "Scheme facilitated diffusion in cell membrane," by Mariana Ruiz Villareal (public domain)._
Aquaporins are channel proteins that allow water to cross the membrane very quickly, and they play important roles in plant cells, red blood cells, and certain parts of the kidney (where they minimize the amount of water lost as urine).
Some channel proteins are open all the time, but others are “gated,” meaning that the channel can open or close in response to a particular signal (like an electrical signal or the binding of a molecule). Cells involved in the transmission of electrical signals, such as nerve and muscle cells, have gated ion channels for sodium, potassium, and calcium ions in their membranes. The opening and closing of these channels, and the resulting shifts in ion levels inside the cell, play an important role in electrical transmission along membranes (in nerve cells) and in muscle contraction (in muscle cells).

Carrier proteins

Another class of transmembrane proteins involved in facilitated transport consists of the carrier proteins. Carrier proteins can change their shape to move a target molecule from one side of the membrane to the other.
_Image modified from "Scheme facilitated diffusion in cell membrane," by Mariana Ruiz Villareal (public domain)._
Like channel proteins, carrier proteins are typically selective for one or a few substances. Often, they will change shape in response to binding of their target molecule, with the shape change moving the molecule to the opposite side of the membrane. The carrier proteins involved in facilitated diffusion simply provide hydrophilic molecules with a way to move down an existing concentration gradient (rather than acting as pumps).
Channel and carrier proteins transport material at different rates. In general, channel proteins transport molecules much more quickly than do carrier proteins. This is because channel proteins are simple tunnels; unlike carrier proteins, they don’t need to change shape and “reset” each time they move a molecule. A typical channel protein might facilitate diffusion at a rate of tens of millions of molecules per second, whereas a carrier protein might work at a rate of a thousand or so molecules per second1.

Want to join the conversation?

  • blobby green style avatar for user AkashdeepKar2015
    Why no energy is spent on switching the carrier proteins?
    (31 votes)
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    • leafers tree style avatar for user Sam
      These carrier proteins are gated trans-membrane proteins and do not require ATP (adenosine triphosphate) to function. The gate is activated due to the concentration gradient of its target molecule. The target molecule binds to the gated carrier protein and, in response, the carrier protein opens up - this allows the target molecule to enter. The carrier protein then changes shape and releases the target molecule into the cell. It waits in its closed position, once again, until it is activated by the binding of its target molecule (outside of the cell). Therefore, no energy is spent switching shapes. The shape change only occurs due to the binding of the carrier protein's target molecule, in accordance with a concentration gradient.
      (43 votes)
  • blobby green style avatar for user Andrea Petersen
    What is the difference between simple diffusion and facilitated diffusion?
    (8 votes)
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  • mr pants teal style avatar for user ujalakhalid01
    if particles moves from low concentration to higher concentration can we call it the concentration gradient of that substance as the concentration changes?
    (2 votes)
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    • old spice man green style avatar for user Matt B
      Careful: it moves from HIGH to LOW, not the other way around.
      That said, this is also the definition of a gradient: it changes as you move from high to low.
      Using a real life example of gradient: lets say you are in a dark room and then you turn on one lightbulb. The light will have a gradient too because the closer to are to the lightbulb, the brighter it becomes (high concentration of light) and the further you move away, the less light there is (low concentration of light). The differences in light intensity is a gradient.
      (20 votes)
  • marcimus orange style avatar for user Mango
    Can carrier proteins move molecules from the inside to the outside? Also, is it possible to replace carrier proteins with channel proteins that are specific to the same molecules? Channel proteins is faster, and you can still precisely modulate how much goes in and out by gating.
    (7 votes)
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    • blobby green style avatar for user sl;adkf sdfl;k
      Yes and it depends but generally yes
      the big thing to keep in mind is that many of these gates are evolving randomly. So getting the most efficient solution while inevitable is not going to happen quickly so if it works it works. Also keep in mind that for larger molecules (think an enzyme or a long carbon chain) the channel would have to be very large meaning that a lot of things could flow out on accident due purely to the size of the pipe. So using a gate instead keeps internal pressure while also heavily regulating the release of the molecules.
      (5 votes)
  • male robot johnny style avatar for user Artemy
    I noticed that according to the quiz (Practice: Passive transport) sodium, potassium, and calcium can't move through the channel proteins. In this article mentioned nerve and muscle cells in which channels can pass sodium, potassium, and calcium. Maybe it was about active transport, but this article about passive transport. It is strange, I had some troubles in the quiz because of it. Can somebody explain this stuff?
    (4 votes)
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    • blobby green style avatar for user carlsagancosmos101
      Yes , i also think that question is wrong. There are channel proteins in the body for transport of those ions.
      Sodium : Voltage gated Channel Proteins in Neurons for propagation of nerve Impulse.
      Potassium : There are Leaky Channels inside nerve cells ...refer Nerve trnsmission videos of Khan Academy itself.
      Calcium : In smooth muscles of the Body...there are Voltage Gated Calcium Channels Present.
      (6 votes)
  • mr pants teal style avatar for user ujalakhalid01
    the topic states above that "a concentration gradient itself is a form of stored (potential) energy" please explain this?
    (4 votes)
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  • mr pants teal style avatar for user ujalakhalid01
    What is faster, a simple diffusion (of oxygen, for example), or a facilitated one (of water through aquaporines)?
    (3 votes)
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    • old spice man green style avatar for user Matt B
      I don't think there is a general rule (leaning toward facilitated but I have counter examples too).
      However, you cannot use passive diffusion to move things from low concentration to high concentration but you can do this with facilitated/active. Depending on the difference in concentration, the simple/passive diffusion will vary while facilitated diffusion can move against concentration gradients and if affected by other conditions too.
      (5 votes)
  • duskpin ultimate style avatar for user edgewaterah
    If a molecule wanted to diffuse across the plasma membrane, but wasn't able to make it all the way through, what would happen to it? Would it eventually work its way in, or would it get stuck in the membrane?
    (4 votes)
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    • female robot grace style avatar for user tyersome
      Interesting question, I don't know if anyone has looked into whether membranes might get "gummed up" by material getting stuck part way through.

      One interesting example I was able to find are the persistent organic pollutants known as PCBs (polychlorinated biphenyls) — these bioaccumulate and at least in some cases are known to disrupt membranes:
      https://academic.oup.com/toxsci/article/80/1/54/1674897

      Membranes are incredibly dynamic. Vesicles are constantly leaving and joining each membrane. In addition, membrane molecules (e.g. phospholipids) move between the two layers§ — for example the lipid composition of the inner and outer layer of the plasma membrane is quite different on most cells . This very tight control of membrane composition suggests to me that there are probably mechanisms for clearing out bad/contaminated sections of membrane.

      If this sort of thing interests you, you might want to consider going to grad school :-)

      §Note: This is done by enzymes called flippases and floppases, just in case you were wondering if biologists had a sense of humor!
      (3 votes)
  • blobby green style avatar for user Thisbishjuju
    Which of these cells can help to move and transport blood
    (2 votes)
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  • blobby green style avatar for user lawaschristine621
    What is osmosis
    (3 votes)
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