Diffusion & Transport

1. Diffusion & Transport

2. Concentration gradient MEMBRANE DYNAMICS A concentration gradient exists where there is a difference in the number of molecules from one area to another. High Concentration Medium Concentration Low Concentration

3. Molecules tend to move from an area of high concentration to an area of low concentration. High Concentration Same concentration throughout This movement is known as diffusion. Net diffusion will stop when each area has the same concentration Low Concentration

4. Concentration gradients can also be found across cell membranes. High Concentration Small molecules like H2O, O2 and CO2 can easily diffuse across cell membranes. Equilibrium has been reached Net diffusion will stop when the concentration on each side of the membrane reaches equilibrium. Low Concentration

5. Even though the concentration gradient is now gone, molecules can still diffuse back and forth at the same rate in both directions.

6. Each molecule type will diffuse across its own concentration gradient. Net diffusion will stop when each molecule type reaches its own equilibrium.

7. These ions are repelled by the membrane’s hydrophobic region. Some small substances like ions cannot easily diffuse through the hydrophobic region of the membrane. This is because ions like sodium and potassium have an electrical charge.

8. Other molecules like sugars and proteins cannot diffuse through the membrane because of their large sizes. These molecules cannot diffuse across the membrane on their own. Molecules like these need a facilitator or helper to get across the membrane.

9. Special transmembrane proteins with internal channels are the helpers that will facilitate the diffusion of these substances. Each substance requires its own special protein. This type of transport is called facilitativediffusion.

10. ATP Transport protein P ADP Sometimes substances must be moved against their concentration gradient. Special transport proteins act as pumps to accomplish this. Energy supplied from a molecule called ATP changes the shape of the transport protein. This type of movement is called active transport.

11. Here is a sped-up model of active transport. What is the molecule that provides the energy needed to operate the pump? It is ATP.

12. ATP P ADP In review, there are three basic ways substances can be transported across a membrane. 1. Diffusion (Passive; no energy required) 2. Facilitative diffusion (protein assisted; no energy required) 3. Active transport (energy required)