1. Introduction – Membrane Transport
One of the most important functions of a cell is to keep its internal environment relatively constant
This is done by regulating the movement of solutes and water from of side of the membrane to the other

2. How do substances move into and out of cell?
Two major ways:
Passive transport
Examples of passive transport:
Simple diffusion
Facilitated Diffusion
Osmosis
Active transport
Examples of active transport
Protein pumps
Endocytosis
Exocytosis

3. Solute Movement Across Membranes
Cellular cytoplasm consists of many different substances dissolved in water (these substances are called solutes)
Sometimes there is a higher concentration of solutes on one side of the membrane than the other
This is called a concentration gradient: where there is difference in concentration of a substance on the inside and outside of the cell

4. Movement of Substances & the Concentration Gradient
Solutes are constantly in motion & collide with one another and spread out randomly until they are spread out evenly, or are at equilibrium
Solutes naturally move from areas where they are more concentrated to areas where they are less concentrated until equilibrium is reached
This is called moving with or down the concentration gradient

5. Diffusion
The process by which solutes move from areas of higher concentration to lower concentration is called diffusion

6. One type of Diffusion: Simple diffusion
Simple diffusion means that solutes move directly through the lipid bilayer cell membrane
Because diffusion works by random solute movement down the concentration gradient, solutes diffuse across the membrane without requiring energy

7. Another type of diffusion: Facilitated diffusion
Solutes which easily pass through the lipid bilayer membrane are generally small and/or uncharged
O2 and CO2
But many ions, like Ca2+, and larger molecules, like glucose, also pass through easily and quickly…how is this possible?
Proteins in the cell membrane act as channels making it easy for certain solutes to diffuse easily
Each type of channel lets only a specific solute through

8. Facilitated Diffusion
The process by which molecules which can not diffuse directly through the membrane pass through special protein channels is called facilitated diffusion
Facilitated diffusion does not require energy: solutes still move from areas of high concentration (more concentrated) to areas of low concentration (less concentrated), like simple diffusion, it’s just that they move through protein channels instead of directly through the membrane

11. Factors that Affect How Fast Diffusion Occurs
What environments would result in diffusion occurring faster or slower?

12. Due Monday: Complete osmosis worksheet Quiz TUESDAY 11/16 on:
Homework 11/12
Due Monday: Complete osmosis worksheet
Quiz TUESDAY 11/16 on:
(1) Membrane structure and (2) Simple and facilitated diffusion

13. Factors Which Affect The Rate of Diffusion (how fast diffusion occurs)
Surface area to volume ratio (size of cell)
Smaller is better!
Temperature
Diffusion occurs faster at higher temperatures!
Concentration gradient
The more concentrated the solutes, the faster diffusion occurs!

16. Osmosis – An Example of Facilitated Diffusion
Is the lipid bilayer hydrophobic or hydrophillic?
How then… does water pass through?
Water passes through the cell membrane through specialized proteins channels called aquaporins

17. Osmosis is similar but different than other types of diffusion
Molecules still move from [high] to [low] (called “down” or “with” the concentration gradient)
This means that osmosis is still passive transport (like simple and facilitated diffusion): does not require energy
Different: in the case of osmosis, it is water which moves across the lipid bilayer, not solutes

18. http://highered. mcgraw-hill