2 Main Types of Transport There are 2 main types of transport when it comes to molecules moving across the cell membrane. Passive transport: does not require energy. The movement of molecules across the membrane without the input of energy. Active transport: transport that requires the input of energy, usually from ATP (more on this to come).
#1: Diffusion Diffusion is the movement of molecules from an area of high concentration to an area of low concentration. Important Definitions Concentration: the number of molecules in a given volume of solution.
Inside: High Concentration Outside: Low Concentration
More Definitions Concentration gradient: the difference in concentration of a substance from one area to another. Molecules naturally diffuse down their concentration gradient; from high to low concentration.
Diffusion Experiment We are going to set up an experiment to observe diffusion over a period of days, starting in today’s class. Follow the instructions on the handout in order to complete the experiment. You will recognize some of the materials from the amylase lab.
#2: Facilitated Diffusion Facilitated diffusion is the movement of molecules across the cell membrane through transport proteins. Molecules are still moving DOWN a concentration gradient, they just need to go through a transport protein first.
#3: Osmosis Osmosis is very similar to diffusion, except that it refers to the movement of water. Osmosis is the diffusion of water molecules from an area of high concentration to an area of low concentration. This confuses a lot of people, see diagram on the next slide.
NOTE: The higher the particle concentration, the lower the water concentration.
Water Movement is Important for Cells Cells must not have too much or too little water in them at any time. Too much, and they could burst. Too little, and they shrivel up and die. There are three important words that we use to describe the amount of solute in a cell and the surrounding area.
Iso- Hyper- and Hypo- “tonic” Isotonic solution: The solution has the same number of solutes as the cell. Water will diffuse in and out of the cell at equal rates. Hypertonic solution: The solution has a higher concentration of solutes than the cell. Water will diffuse out at a higher rate than into the cell. Hypotonic solution The solution has a lower concentration of solutes than the cell. Water will diffuse into the cell at a higher rate than out of the cell.
Which picture represents hypertonic? Which represents hypotonic?
Active Transport Some molecules need to move against their concentration gradient. These molecules need to use transport proteins, known as pumps. Active transport moves molecules across the cell membrane from an area of low concentration to an area of high concentration of solutes. This is opposite to the way the particles naturally want to move, so it takes energy. The transport proteins are powered by chemical energy. Most use energy from molecules of ATP, which we will learn about soon in greater detail.
2 Other Types: Endocytosis and Exocytosis Endocytosis and exocytosis are two additional types of active transport; this means they require energy. Endocytosis Involves the cell membrane surrounding particles and taking them in. - Called phagocytosis if the particles are solid. - Called pinocytosis if the particles are liquid. Exocytosis The opposite of endocytosis; used to eject particles from the cell. http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120068/bio02.swf::Endocytosis%20and%20Exocytosis