Diffusion – Exchange and Transport Ep 2

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

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 lipid soluble, so it can dissolve in the phospholipid bilayer that makes up the cell membrane. Gas exchange in the lungs happens by simple diffusion.

Facilitated diffusion

But what about polar molecules that have partial positive and partial negative charges, or charged ions? These are water soluble so are repelled by the hydrophobic region of the phospholipid bilayer. And what about molecules that a too large to fit between the phospholipids? These substances 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 polar molecules, 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 transport protein is needed.

Firstly, let’s look at 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.

Rate of diffusion

The rate of diffusion across cell membranes can be affected by different factors:

1. The ‘steepness’ of the concentration gradient – a steeper concentration gradient means a faster rate of 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).
2. Temperature – the warmer it is, the more kinetic energy the particles have, so the rate of diffusion will be faster.
3. 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 across at a faster rate. Also, if there is more surface area for the particles to use, there will be a faster diffusion rate.
4. For facilitated diffusion, the number of proteins – more carrier or channel proteins means that the rate of diffusion can increase.

Some cells have adaptations to make them good for transporting substances rapidly. 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 rapid diffusion 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 higher concentration to an area of lower 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 by diffusion.

Read More

• 
  The Carbon Cycle – Nutrient Cycles Ep 3
• 
  The Phosphorus Cycle – Nutrient Cycles Ep 2
• 
  The Nitrogen Cycle – Nutrient Cycles Ep 1
• 
  Neuromuscular Junctions – Muscles Ep 5
• 
  Cardiac, Smooth and Skeletal Muscle – Muscles Ep 4
• 
  Phosphocreatine and ATP – Muscles Ep 3