The building blocks of life Cells Chapter 3 Cell Transport -- Active Slide 162-184 The building blocks of life

Active Transport http://bcs.whfreeman.com/webpub/Ektron/pol1e/Animated%20Tutorials/at0502/at_0502_active_trans.html

The two main types of transport into and out of cells include passive transport and active transport. PASSIVE TRANSPORT involves the transport of molecules from regions of high concentration to regions of low concentration.  No energy is used in this process.  Three main types of passive transport: 1. SIMPLE DIFFUSION Diffusion is the tendency of molecules to spread from regions of high concentration to regions of low concentration.  This movement is due to a concentration gradient.  This is the difference in concentration between two different regions.  An example - when you spray perfume in the corner of a room and eventually you can smell it everywhere. 2. FACILITATED (HELPED) DIFFUSION Facilitated diffusion is similar to simple diffusion, but it involves molecules diffusion quickly across a cell membrane, through transport proteins.  Facilitated diffusion only works from high to low concentration.  A transport protein binds to the substance on one side of the cell membrane and then releases it on the other side. 3. OSMOSIS Osmosis is the diffusion of water across a selectively permeable membrane from a region of high water concentration to a region of low water concentration. ACTIVE TRANSPORT involves the transport of molecules from regions of low concentration to regions of high concentration.  Energy is needed for this type of transport.  There are two main types of active transport: 1. TRANSPORT PROTEINS - the same as facilitated diffusion, except a molecule goes from low concentration to high concentration across a cell membrane and it needs energy to take place. 2. MEMBRANE VESICLES - Substances entering the cell become enclosed by an inpocketing of the cell membrane to form a vesicle.  This process is called endocytosis. Endocytosis (two forms ): a) Pinocytosis -  a form of endocytosis that transports liquids into the cell. b) Phagocytosis - a form of endocytosis that transports solids into the cell. Exocytosis is the movement of a substance out of a cell via vesicles.  Exocytosis is the reverse of endocytosis.

Bulk transport across the plasma membrane occurs by exocytosis and endocytosis Small molecules and water enter or leave the cell through the lipid bilayer or via transport proteins Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles Bulk transport requires energy Larger materials are brought into the cell through endocytosis and moved out through exocytosis.

Exocytosis and Endocytosis Both mechanisms employ vesicles, the membrane-lined enclosures that alternately bud off from membranes or fuse with them.

Exocytosis In exocytosis, a transport vesicle moves from the interior of the cell to the plasma membrane and fuses with it, at which point the contents of the vesicle are released to the environment outside the cell.

Exocytosis Exocytosis is a cellular process where cells eject waste products or chemical transmitters (such as hormones) from the interior of the cell. In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents Many secretory cells use exocytosis to export their products Exocytosis is similar in function to endocytosis but working in the opposite direction. There are five steps to exocytosis:

Exocytosis Five Steps of Exocytosis: Vesicle Trafficking In this first step, the vesicle containing the waste product or chemical transmitter is transported through the cytoplasm towards the part of the cell from which it will be eliminated. Vesicle Tethering As the vesicle approaches the cell membrane, it is secured and pulled towards the part of the cell from which it will be eliminated. Vesicle Docking In this step, the vesicle comes in contact with the cell membrane, where it begins to chemical and physically merge with the proteins in the cell membrane. Vesicle Priming In those cells where chemical transmitters are being released, this step involves the chemical preparations for the last step of exocytosis. Vesicle Fusion In this last step, the proteins forming the walls of the vesicle merge with the cell membrane and breach, pushing the vesicle contents (waste products or chemical transmitters) out of the cell. This step is the primary mechanism for the increase in size of the cell’s plasma membrane.

Exocytosis Figure 5.9 (a) Exocytosis (b) Micrograph of exocytosis extracellular fluid protein transport vesicle cytosol Figure 5.9

Endocytosis Endocytosis is a cellular process where cells absorb molecules or substances from outside the cell by engulfing it with the cell membrane. This technique is critical to the survival of the cell, since most molecules important to the cell cannot normally pass through the cell’s hydrophobic plasma membrane. Endocytosis is the opposite of exocytosis. There are three types of endocytosis:

Endocytosis In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane Endocytosis is a reversal of exocytosis, involving different proteins There are three types of endocytosis Phagocytosis (“cellular eating”) Pinocytosis (“cellular drinking”) Receptor-mediated endocytosis

Endocytosis Three Types of Endocytosis: Phagocytosis Pinocytosis In this process, the cell membrane folds around the molecule or substance, isolating it for later use in a vacuole called a phagosome. (Phagocytosis is literally translated as “cell eating.”) Pinocytosis In this process, the cell membrane folds around the molecule or substance, and it is dissolved into the interior of the cell. Molecules such as proteins and other water-soluble substances are thus ingested this way, which could not otherwise breach the cell membrane. (Pinocytosis is literally translated as “cell drinking.”) Receptor-Mediated Endocytosis This process is similar to phagocytosis, except that the cell uses receptor proteins embedded within the cell membrane. These proteins target specific molecules or substances, attracting then seizing hold of them and pulling them into the cell within a fold of the cell membrane. Cholesterol is absorbed from our bloodstream by cells using receptor-mediated endocytosis.

The vacuole fuses with a lysosome to digest the particle In phagocytosis when certain cells use pseudopodia or “false feet” to surround and engulf whole cells, fragments of them, or other large organic materials. a cell engulfs a particle in a vacuole The vacuole fuses with a lysosome to digest the particle In pinocytosis is the movement of moderate-sized molecules into a cell by means of the creation of transport vesicles produced through an infolding or “invagination” of a portion of the plasma membrane. molecules are taken up when extracellular fluid is “gulped” into tiny vesicles that bud off from the plasma membrane. In receptor-mediated endocytosis, binding of ligands to receptors triggers vesicle formation A ligand is any molecule that binds specifically to a receptor site of another molecule For the Cell Biology Video Phagocytosis in Action, go to Animation and Video Files.

Phagocytosis http://www.stolaf.edu/people/giannini/flashanimat/cellstructures/phagocitosis.swf

Endocytosis (a) Pinocytosis receptors captured molecules coated pit vesicle Formation of a pinocytosis vesicle. (b) Phagocytosis bacterium (or food particles) pseudopodium vesicle A human macrophage (colored blue) uses phagocytosis to ingest an invading yeast cell.

Endocytosis http://www.youtube.com/watch?feature=player_embedded&v=4gLtk8Yc1Zc

Endocytosis and Exocytosis http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter5/endocytosis_and_exocytosis.html

Facilitated diffusion ATP Passive transport Active transport Figure 7.19 Review: passive and active transport. Diffusion Facilitated diffusion ATP 23

https://www.youtube.com/watch?v=SUyMRfuPQ_w