Movement in and out of cells

Movement in and out of cells

1. Diffusion

   • Explanation
     • Diffusion is passive transport, which refers to the natural, inbuilt motion of particles.
       • All particles are constantly in motion due to kinetic energy they possess.
       • The motion is random
       • Particles are constantly bouncing off one another and other objects
         • If all particles are at one side, they will bounce and move to the other side. There will be no movement in the reverse direction initially, so there will be a net overall movement from an area of high concentration to an area of lower concentration
   • Definition
     • The net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower
   • Rate of diffusion
     • Concentration gradient
       • Greater the difference in concentration of molecules or ions on either side of an exchange surface, the faster the rate of diffusion
       • The path from an area of higher concentration to an area of lower concentration
         • Particles diffuse down the concentration gradient
     • Area over which diffusion takes place
       • The larger the area of an exchange surface, the faster the rate of diffusion
     • Thickness of exchange surface
       • The thinner the exchange surface, the faster the rate of diffusion
     • Surface area x difference in concentration/length of diffusion path

2. Facilitated Diffusion

   • Explanation
     • Like diffusion, it occurs down a concentration gradient, but it occurs at specificpoints on the plasma membrane where there are special protein molecules.
       • These proteins form water-filled channels (protein channels) across the membrane.
         • The channels are selective, each openin only in the presence of a specific ion.
         • These channels allow water soluable ions to pass through. Such molecules would usually diffuse only very slowly through the phospholipid bilayer of the plasma membrane.
     • An alternative form of facilitated diffusion involves carrier proteins that span the plasma membrane. When a molecule, eg. glucose, that is specific to the protein present, it binds with the protein. This causes it to change shape in such a way that the molecule is released to the inside of the membrane. No external energy is required for this. The molecules move from a region where they are highly concentrated to one of lower concentration, using only kinetic energy

3. Osmosis

   • Definition
     • The passage of water from a region where it has a higher water potential to a region where it has a lower water potential
   • Solutes and water potential
     • Water potential
       • The pressure created by water molecules. Pure water is said to have a water potential of 0
     • The addition of a solute to pure water will lower its WP
     • WP of a solute must always be <0
     • The more solute added, the lower its WP
     • Water will move by osmosis from a region of higher (less negative) WP to one of lower (more negative) WP

4. Active Transport

   • Definition
     • The movement of molecules or ions into or out of a cell from a region of lower concentration to an area od higher concentration
   • Explanation
     • ATP is needed
       • Used in 1 of 2 ways
         • To directly move molecules
         • Using a concentration gradient that has already been set up by direct active transport. Also known as co-transport
     • Against the concentration gradient
     • Carrier protein molecules are involved
     • Process is selective, specific substances are transported
   • Process
     • Carrier proteins span cell surface membrance and accept molecules or ions to be transported
       • Molecules or ions bind to receptors on the channels of the carrier protein
         • Inside the cell, ATP binds to the protein, causing it to split into ADP and a phosphate molecule, casuing the protein molecule to change shape and open to the opposite side of the membrane
           • The molecules or ions are then released to the other side of the membrane
             • The phosphate molecule is released from the protein and recombines with the ADP to form ATP during respiration
               • The protein reverts to its original shape