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

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

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

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

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

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

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

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

http://www.biosci.ohiou.edu/introbioslab/Bios170/diffusion/Diffusion.html http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_diffusion_works.html http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_facilitated_diffusion_works.html

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

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

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!

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

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

http://highered. mcgraw-hill http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html