1. CELL TRANSPORT Passive and Active Transport Sources: Starr, C. 1997 Biology: concepts and applications. Wadsworth Publishing Company.

2. Diffusion Transport process facilitated by:  Selective permeability of the membrane: no ATP required –Ability of cell membranes to allow some substances but not others to cross them in certain ways, at certain times. –O 2, CO 2 and other small molecules with no net charge cross the the bilayer itself.  Following gradients –Molecules and ions have internal energy that keeps them in constant motion –Molecules move down following concentration gradients

3.  Factors that affect diffusion rate: –Each substance diffuses in the direction set by its own concentration gradient –Diffusion rates are faster when a gradient is steep –Temperature affects diffusion rates (heat causes molecules to move faster) –Small molecules move faster than large ones –An electric gradient (a difference in charge between two adjoining region) can modify the rate and direction of diffusion.

4.  Movement of H2O across a selectively permeable membrane  The number of molecules off all solutes on either side of the membrane affects the water concentration.  Tonicity is the name for the relative concentration of solutes in two fluids –isotonic fluids: equal solute concentration –hypotonic fluid: a fluid with fewer solute concentration –hypertonic fluid: a fluid with a high solute concentration. Osmosis

5. Passive Transport: Facilitated Diffusion  A solute moves through the interior of a protein ( facilitated diffusion)  It is a two-way transport  The net direction of movement at a given time depends on how many molecules or ions of the solute are making random contact with vacant binding sites in those proteins

7. Active Transport  This mechanism requires an energy boost (ATP). Mitochondria provides ATP that powers the molecular motors of cytoskeleton  Movement is against concentration gradient  This mechanism can continue until the solute becomes more concentrated on the side to which it is being pumped  When the transport protein interacts with a particular solute, it changes its shape  With the change, the bound solute becomes exposed to the fluid bathing the opposite side of the membrane.

9. Active Transport and the Cell Membrane  The cell membrane is not static. – It is renewed by the addition of new membrane vesicles from the Golgi, exocytosis. When the vesicle membrane and the plasma membrane come into contact, the lipid molecules of the bilayer rearrange themselves –Removal of membrane segments take place in the form of endocytosis:  Phagocytic vesicles: Cells engulf a particle by wrapping pseudopodia around it and packaging it into a membrane-enclosed sac, a vacuole. Particle is digested after it fuses with a lysosome  Pinocytic vesicles: the cell engulfs droplets of extracellular fluids in tiny vesicles (unspecific)

10. Receptor-mediated-endocytosis  Involves proteins with specific receptor sites exposed to extracellular fluids  Receptor proteins are usually clustered in regions called coated pits  Coated pits are lined up, on their cytoplasmic site by a fuzzy layer consisting on the protein clathrin  Extracellular substances that bind to the receptors are called ligands (general term for any molecule that binds specifically to a receptor site of another molecule)  When appropriate ligands bind to a receptor, they are carried into the cell by the inward budding of a coated pit, coated vesicle.

11. Receptor mediated endocytosis: An example  This process enable cells to take in large quantities of substrates  Animal cells use this process to take in cholesterol for use in the synthesis of membranes and steroids  In families with hypercholesterolemia, high levels of cholesterol in blood, the protein receptors are missing. What is the result of this condition? Early atherosclerosis Early atherosclerosis

12. Answer  Cholesterol is unable to enter cells and builds up in blood, contributing to early atherosclerosis (development of fat deposits on blood vessel linings)

13. How does the process work? An overview  Microtubules probably help guide secretory vesicles from Golgi Complex to plasma membrane. Below is a possible explanation: “Specialized proteins function as motor molecules moving vesicles and other organelles along microtubules. Motor molecules are proteins (such as kinesin) that convert chemical energy to movement. Kinesin attaches to specific receptors on the vesicles and its “feet”, movable extensions of the protein, walk along a microtubule. Kinesin powers it movement by hydrolyzing ATP” “Specialized proteins function as motor molecules moving vesicles and other organelles along microtubules. Motor molecules are proteins (such as kinesin) that convert chemical energy to movement. Kinesin attaches to specific receptors on the vesicles and its “feet”, movable extensions of the protein, walk along a microtubule. Kinesin powers it movement by hydrolyzing ATP” (Campbell, 2002) (Campbell, 2002)

14. Patching and Capping  “Receptors are brought to the plasma membrane by vesicles from the trans region of the Golgi complex. How does the Golgi complex maintain the fluidity of the plasma membrane, the receptors can move laterally in the membrane and collect in the specialized regions called clathrin coated pits. Golgi complex fluidity of the plasma membrane Golgi complex fluidity of the plasma membrane  When the ligand binds to its specific receptor, the ligand-receptor complex accumulates in the coated pits. In many cells, these pits and complexes begin to concentrate in one area of a cell. …this appears as patches of label on the cell surface (patching) Eventually, the patches coalesce to form a cap at one pole of the cell (capping). Not all cells form caps, but most do form patches. Why would this process be an advantage for the cells? Imagine the large amounts of extracellular fluid that would be taken up if the cells endocytosed the ligand receptor complex all over its surface. Thus, the pre-concentration process minimizes the amount of fluid that is taken up in the vesicle”. (http://cellbio.utmb.edu/cellbio/recend.htm#process )

15. What types of ligands enter by receptor mediated endocytosis? 1. Toxins and lectins Diptheria Toxin Pseudomonas toxin Cholera toxin Ricin Diptheria Toxin Pseudomonas toxin Cholera toxin Ricin 2. Serum transport proteins and antibodies Low density lipoprotein Yolk proteins IgE Polymeric IgA Maternal IgG IgG, via Fc receptors Low density lipoprotein Yolk proteins IgE Polymeric IgA Maternal IgG IgG, via Fc receptors

16. 3. Hormones and Growth Factors Insulin Epidermal Growth Factor Growth Hormone Thyroid stimulating hormone Nerve Growth Factor Calcitonin Glucagon Prolactin Luteinizing Hormone Thyroid hormone Platelet Derived Growth Factor Interferon Insulin Epidermal Growth Factor Growth Hormone Thyroid stimulating hormone Nerve Growth Factor Calcitonin Glucagon Prolactin Luteinizing Hormone Thyroid hormone Platelet Derived Growth Factor Interferon

17. 4. Viruses Rous sarcoma virus Semliki forest virus Vesicular stomatitis virus Adenovirus Rous sarcoma virus Semliki forest virus Vesicular stomatitis virus Adenovirus

18. Examples of Active Transport Pumps  Na-K pump is a major co-transport system  Ca pumps –This process helps keep the calcium concentration inside the cell at least a thousand times lower than outside

19. Putting it together Campbell, N. Biology Sixth Edition. Benjamin Cummnings Publishing 2002 1.“An experiment is designed to study the mechanism of sucrose uptake. Cells are immersed in a sucrose uptake by plants cells and the pH of the surrounding solution is monitored with a pH meter. The measurements show that sucrose uptake by plant cells raises the pH of the surrrounding solution. The magnitude of the pH change is proportional to the starting concentration of sucrose in the extracellular solution. A metabolic poison that blocks the ability of the cells to regenerate ATP also inhibits the pH change in the surrounding solution. Explain these results.”

20. 2. The rates of sucrose uptake from solutions of different sucrose concentrations are compared. Explain the shape of the curve in terms of what is happening at the membranes of plant cells? Rate of Sucrose up-take (umol/g.min) Sucrose concentration of surrounding solution (mM)