Cell Membrane and Diffusion


Cell Membrane and Diffusion

Diffusion across cell membranes is just like simple diffusion in air, PARTICLES flow through cell membrane from a region of HIGH CONCENTRATION (many of them) to a region of LOW CONCENTRATION (only a few of them).  This is constantly happening in your lungs.



Cell membranes hold everything inside a living cell.  However, for a cell to function properly they let stuff in and out as well.  These membranes are quite choosey.  They only allow very small molecules to diffuse through them - stuff like glucose and/or amino acids.

Large molecules like PROTEINS and/or STARCH can't diffuse through cell membranes.


Exchange of gases in the Lungs


Breathing is a gas exchange mechanism.  Your Lungs help to do just that!  They contain millions of  tiny air pockets called ALVEOLI (air sacs)  which are adapted to maximise the diffusion of carbon dioxide and oxygen (see diagram below).  Alveoli  are good at gas exchange because they have:

  • a large surface area.  If all the air sacs were flattened out, they would cover an area of 100m2
  • moist surface which speed up diffusion.
  • very thin walls which makes diffusion quicker.
  • lots of blood capillaries to carry the gasses.
  • the blood capillaries help the diffusion O2 and CO2 as their walls are only one cell thick.

In the alveolus (single air sac) oxygen passes from a high concentration (loads of oxygen) through the thin wall into the blood capillary where there is a low concentration (very little oxygen) .  This is called diffusion, because oxygen is moving from high concentration to low concentration.  When blood returns to lungs it has high concentration of carbon dioxide, so CO2 diffuses out of the blood into the alveolus.



Active Transport Uptake


Tags:Lungs, Cell membrane, Diffusion, Proteins, Starch, Gas exchange, Cells in living things



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