1 Review of eukaryotic cells science/cell_biology.html

2 Eukaryotic cell reminders Eukaryotic cells have a variety of compartments –Membrane-bound organelles, carry out functions –DNA in nucleus. NO NUCLEUS in Prokaryotes! Mitochondrion is an enslaved bacterium –Inner and outer membrane like a Gram – bacterium –Mitochondrion has its own DNA and ribosomes –It is the same size as a bacterium. Lysozome is an organelle –Contains various digestive enzymes –Important part of WBC’s defenses against bacteria

3 How things get in (and out) of cells Eukaryotic cells –Have transport proteins in membrane –Have a cytoskeleton made of microtubules Allows for receptor mediated endocytosis, phagotcytosis, etc. Cell membrane pinches in, creates vesicle Prokaryotic cells –Have a stiff cell wall –Can NOT carry out endocytosis –Entry of materials into cell by diffusion or transport processes ONLY.

4 Ann. Review of Biophysics and Biomolecular Structure Vol. 33: January 7, 2004 MOLECULES OF THE BACTERIAL CYTOSKELETON Jan Löwe, Fusinita van den Ent, and Linda A. Amos The structural elucidation of clear but distant homologs of actin and tubulin in bacteria and GFP labeling of these proteins promises to reinvigorate the field of prokaryotic cell biology. Prokaryotic origin of the actin cytoskeleton FUSINITA VAN DEN ENT, LINDA A. AMOS & JAN LÖWE Nature 413, (6 September 2001)

5 Illustrations: entry into cells Both prokaryotes and eukaryotes. Only eukaryotes.

6 Type of molecule affects transport Small molecules can pass through a lipid bilayer –Water; otherwise, no osmosis –Gases such as O 2 and CO 2 Lipid molecules can –Dissolve in lipid bilayer, pass through membrane –Many antibiotics, drugs are lipid soluble Larger, hydrophilic molecules cannot –Ions, sugars, amino acids cannot pass through lipids –Transport proteins required

7 Transport through membranes Simple diffusion –Molecules travel down concentration gradient –Membrane is not a barrier to their passage Facilitated diffusion –Molecules travel down concentration gradient –Cannot pass through lipid bilayer; their passage is facilitated by protein transporters Active transport –Molecules travel against concentration gradient –Requires input of metabolic energy (ATP), transporter

8 How molecules get through the membrane

9 ABC transport systems Include a periplasmic binding protein, a transmembrane channel, and an ATP-hydrolyzing enzyme. High affinity binding system. Family of related proteins. Example of Active Transport –Requires transport protein –Requires metabolic energy

10 Group translocation As molecule passes through the membrane, it is chemically changed. Requires energy in the form of PEP. Requires series of proteins Prevents substrate from “backing out of cell” Use energy that would have been spent anyway; prepares substrate for use.

11 Permeases Transport proteins are often called permeases (-ase = enzyme) because they have the same properties –Instead of changing a chemical, they change its location –Permeases have an “active site” –Permeases are specific –Permeases are saturable lib/media_portfolio/text_images/FG0 9_32.JPG

12 ATP is not always used directly in active transport An electrochemical gradient exists across the cell membrane –Positive just outside the membrane, negative within –Gradient in the form of H+ ions –Maintained by the hydrolysis of ATP or by the same metabolic reactions that make ATP –Powers uniports, symports and antiports

13 Uniport Transport of a single substance Example: transport of K+ into the cell –Against its chemical gradient, but down its electrical gradient. –(red ball = K+) –Doesn’t require energy DIRECTLY, but making the electrical gradient DOES require energy.

14 Antiport and Symport Molecules (red balls) transported against a gradient. Coupling to flow of H+ into the cell powers this. prostruct/u1fig6e1.html