Diffusion is one of those topics that pops up at all levels of biology, and each time gets a little more complicated. But it is still the basic process of substances evenly distributing themselves. It is just a little more complex when it happens across a cell membrane.
Simple diffusion
Firstly, lets recap the basics of diffusion. It is the net movement of particles from an area of high concentration to an area of low concentration i.e down a concentration gradient. Net movement means that the particles will move in both directions, but overall there is more movement from high to low. This means that eventually the particles will be evenly distributed in the area they are contained in. It is a passive process, meaning that no energy is needed.
Some particles are small enough to pass straight through the cell membrane by simple diffusion. These include small non-polar molecules such as oxygen and carbon dioxide. If a molecule is non-polar it is soluble in lipids, so it can dissolve in the phospholipid bilayer that makes up the cell membrane. So gas exchange in the lungs happens by simple diffusion.
Facilitated diffusion
But what about polar molecules that have partial positive and partial negative charges? These are dissolved in water so can’t get through the hydrophobic region of the phospholipid bilayer. And what about molecules that a too large to fit between the phospholipids? Both these types of molecules need a bit of help. We saw in the last article that the cell membrane also contains proteins. Some of these proteins help to transport charged particles and large molecules across the cell membrane. This is called facilitated diffusion – the particles are still moving down a concentration gradient, there is still no energy input needed (a passive process), but a membrane protein is needed.
Let’s start with the large molecules such as glucose. These use carrier proteins. The molecule attaches to the carrier protein on one side of the membrane, then the protein changes shape to release the molecule on the other side of the membrane. Quite simple really.
Next, the charged particles. These include ions and polar molecules. This time, channel proteins are used. Channel proteins form a little passage for the charged particles to pass through so they don’t have to pass through hydrophobic regions. Just remember charged particles, channel proteins.
Rate of diffusion
The rate of diffusion across cell membranes can be affected by different factors:
- The ‘steepness’ of the concentration gradient – a bigger concentration gradient means faster diffusion, although the rate will gradually decrease as the substance evens out to equilibrium. (In facilitated diffusion sometimes the number of proteins can be limiting though).
- Temperature – the warmer it is, the more kinetic energy particles have so they can move and diffuse faster.
- For simple diffusion, the surface area and thickness of the exchange surface – if the particles have less distance to travel (a shorter diffusion pathway) then they can diffuse more quickly. Also, if there is more surface area for the particles to use, there will be a faster diffusion rate.
- For facilitated diffusion, the number of proteins – more carrier or channel proteins means that the rate of diffusion can increase.
(Note that even though water is a polar molecule, it is so small that it is able to pass between phospholipids and diffuse across the cell membrane. This is osmosis, covered in the next article.)
Some cells have adaptations to make them good for transporting substances quickly. The epithelial cells in the small intestine have lots of small projections called microvilli which increase their surface area by about 600 times, so absorption of nutrients by diffusion can be rapid. Some kidney cells have lots of channel proteins called aquaporins which allow reabsorption of water back into the blood by facilitated diffusion. If it wasn’t for aquaporins, we would not be able to retain enough water.
A great example of adaptation for substance transport is the adaptations of the lungs for gas exchange. We’ll look at this more closely in another article.
Summary
- Diffusion is the net movement of particles from an area of high concentration to an area of low concentration, and is a passive process.
- Simple diffusion across a cell membrane happens for small non-polar molecules.
- Facilitated diffusion across a cell membrane happens for large or polar molecules. Carrier proteins and channel proteins help transport them across (still down a concentration gradient).
- Surface area, temperature, concentration gradient, and the number of carrier/channel proteins can all affect the rate of diffusion.
- Some cells are adapted for rapid transport of substances across the cell membrane.
