C2006/F2402 '11 -- Outline For Lecture #6 Show (c) 2011 Dr. Deborah Mowshowitz , Columbia University, New York, NY. Last update 02/06/2011 02:55 PMHandouts:
6A-- Transport of glucose through body (gif) 6A-- pdf I. Putting all the Methods of Transport of Small Molecules Together or What Good is All This? A. How glucose gets from lumen of intestine → muscle and adipose cells. An example of how the various types of transport are used. (Handout 6A) Steps in the process:
For additional examples of the uses of the various types of transport processes, see Becker fig. 8-1 & 8-2. For pictures of steps 1-3, see http://www.biology.arizona.edu/cell_bio/problem_sets/membranes/graphics/cotransport_sys.gif or http://www.biochem.arizona.edu/classes/bioc462/462a/NOTES/LIPIDS/Fig12_36GlcNaSymport.GIF Note both of these come from classes with extensive on line notes. The biochem course includes several animations of transport proteins. B. How Glucose Reaches Body Cells -- Another look at handout 6-A. The steps in the process are described above in the order in which they occur. Here is a summary with the focus on the various types of transport involved.
Try problem 2-9 & 2-12. A. Pinocytosis= bulk phase endocytosis; no receptor.Cells take in random samples of surrounding fluid containing a random selection of extracellular substances. B. Phagocytosis -- in specialized cells only -- extensions of cells (pseudopods) reach out and engulf solids. See Becker fig. 12-14. Vesicle that is formed is called a phagocytic vesicle (or vacuole) or phagosome. Requires MF. C. RME = receptor mediated endocytosis. Cells take in specific substances from surrounding fluid using a receptor. See Becker fig. 12-15 (diagram) & 12-16 (micrograph). Different cell types have different combinations of receptors.
A. General and/or important Features. 1. Receptors -- Need specific receptor for each substance (or class of closely related substances) to be transported.2. Concentrates substances transported -- usually moves them up their gradient.3. Requires energy -- multiple stages in process use ATP or GTP. Energy must be required because substances move against their gradients. Energy is required to form the vesicles and to process and/or transport the vesicles inside the cell.4. Role of clathrin -- A peripheral membrane protein is needed to deform membrane and allow vesicles to form -- provides a coat. (See Becker figs. 12-15 to 2-18 and/or Sadava fig. 6.19 (5.17)
5. It's a cycle -- Exocytosis balances endocytosis so cell surface area stays the same. See Sadava fig. 6.18 (5.16) or Becker fig. 12-15. For LDL receptor, it takes about 10-20 minutes for one "round trip." 6. Topology -- material can enter and/or exit cell without being in contact with cytoplasm. Material can remain inside a vesicle or outside cell at all times. (See Transcytosis.) 7. Possible fates of endocytosed material -- Where does vesicle go? Where do receptor &/or ligand end up?
d. Transcytosis -- vesicle crosses cell and fuses with opposite cell surface. For examples see handout 6C or diagram of transcytosis (shows how antibodies enter lumen)
B. Stages of Cycle (Numbers match steps on handout 6B.) Click here for animation.
Try Problem 2-6. C. Some Specific Examples 1. LDL (Low density lipoprotein) -- receptor recycled, but ligand (including protein part) degraded. See Becker, Box 12B or text of Sadava Ch. 51.4 (50.4). Many of LDL details may have been included in general case, but are summarized below. Click here for a picture of LDL.a. What is LDL? A lipoprotein particle containing cholesterol esters + some other lipids + a protein. Particle contains esterified cholesterol covered by monolayer of amphipathic lipid (phospholipid plus some unesterified cholesterol) + one molecule of protein (apoprotein B or apoB). 2. EGF (Epidermal Growth Factor) -- all the protein involved (ligand + receptor) is degraded a. No separate protein ligand required; EGF is a protein -- unlike cholesterol, or Fe (see case below). EGF itself binds to receptor = ligand for cell surface receptor & substance that will be transported into the cell. b. Function of uptake -- to regulate signaling. EGF is a signaling molecule. Uptake turns off signal and down regulates receptors (reduces # of cell surface receptors). c. Receptor not recycled -- Ligand (signal molecule) and receptor degraded together. d. Need lysosomes (to degrade both receptor and ligand).
a. What is transferrin? Fe needs a protein (like cholesterol needs apoB) for transport and binding to receptor; protein (= ligand for cell receptor) is called transferrin. b. Both apotransferrin & receptor are recycled. c. No lysosomes needed -- iron is transported out of endosome (using transporter protein or channel in membrane); no protein is degraded. d. Transferrin and receptor separate outside cell after recycled (1). Fe/transferrin binds to receptor at neutral pH and enters cell by RME.(2). Inside cell, Fe transported out of vesicle into cytoplasm, leaving apo-transferrin stuck to receptor ("apo" means without ligand, cofactor, etc.). D. For Reference: Compare & Contrast for the examples described above for transport of X
A. Types of Labeling (using added tracers) B. Detection -- How do you find where the radioactivity (or whatever tracer/label you used) is?
To review labeling and RME, try problems 2-8 & 2-11; by now you should be able to do all the problems in problem set 2 & 2R. Next Time: How do proteins get sorted to their proper place? How do molecules get in and out of the nucleus? Why is it necessary for glucose to be pumped into the cell rather than diffusion in?Although glucose can be more concentrated outside of a cell, it cannot cross the lipid bilayer via simple diffusion because it is both large and polar, and therefore, repelled by the phospholipid membrane.
Why is glucose pumped into the cell?Glucose is a primary energy source for most cells and an important substrate for many biochemical reactions. As glucose is a need of each and every cell of the body, so are the glucose transporters. Consequently, all cells express these important proteins on their surface.
Why glucose does not diffuse out of the cell?Explanation: Glucose cannot move across a cell membrane via simple diffusion because it is simple large and is directly rejected by the hydrophobic tails. Instead it passes across via facilitated diffusion which involves molecules moving through the membrane by passing through channel proteins.
Why is a transporter necessary to get the polar glucose into a cell?Since polar molecules cannot be transported across the plasma membrane, carrier proteins called glucose transporters are needed for cellular uptake. Glucose transporters are found in the plasma membrane where they bind to glucose and enable its transport across the lipid bilayer.
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