Cell Membrane Structure and Cellular Transport

1. Cell Membrane Structure and Cellular Transport Accel Bio 2014

2. Biological Membranes are Phospholipid Bilayers

3. Phospholipid Structure • hydrophobic “tails” of fatty acids • hydrophilic “head” with a glycerol & a phosphate group (PO4) Hydrophobic = “afraid” of water Hydrophilic = “_______” of water friend

4. Biological membranes are selectively permeable barriers Lipid bilayer membranes are permeable to: • hydrophobic lipids • small, uncharged molecules such O2 and CO2 • small molecules such as waterLipid bilayers are notpermeable to: • ions (charged atoms) • large hydrophilic molecules (like glucose) These substances cannot pass through the lipid bilayer easily. They must find another (more favorable) way across…

5. A more complete picture of a Cell Membrane • Embedded proteins “float” in fluid phospholipid “sea” A model of the FLUID membrane: http://www.susanahalpine.com/anim/Life/memb.htm

6. Componentsof the Cell Membrane • Phospholipids: make up selectively-permeable barrier • Proteins:*many functions*, including transport channels - selectively allow some larger molecules and charged atoms (ions) through • Carbohydrates: chemical ID tags, found on outside of cell • Cholesterol: keeps membrane fluid

7. Cell Membrane Proteins: a slightly different representation http://www2.warwick.ac.uk/fac/sci/chemistry/research/dixon/dixongroup/members/msrhar/research/background/

8. Membrane Protein Functions are Varied

9. Transport Across the Cell Membrane Occurs Various Ways • Passive Processes 1. Diffusion 2. Osmosis 3. Facilitated Diffusion • Active Processes 4. Active Transport (protein pump) 5. Endocytosis • Phagocytosis • Pinocytosis • Receptor-Mediated Endocytosis 6. Exocytosis

10. Movement across a membrane can occur by diffusion Molecules are in constant motion. Due to their random movement, molecules have a natural tendency to exhibit net movement from areas of _______ concentration to areas of _____ concentration. This movement is called diffusion. high low

11. Molecular motion never ceases, but we can reach equilibrium Eventually, diffusion leads to a state where there are equal concentrations of molecules across a membrane. This state is called equilibrium. In equilibrium, the movement of molecules in a given direction (left or right) across a membrane is equally probable. Thus, unless disturbed, a system in equilibrium will tend to stay in equilibrium. DYNAMIC

12. Diffusion occurs down a concentration gradient A difference in concentrations of an atom/molecule across a membrane is called a concentration gradient. Diffusion of molecules to an area of low concentration is described as movement down the concentration gradient. (Active transport to an area of higher concentration is described as movement against the conc gradient)

13. Osmosis Osmosis is the diffusion of water across a selectively permeable membrane, from areas of ____ concentration to areas of ____ concentration. Cells contain and are bathed in solutions that contain dissolved substances like ions, sugars, proteins, and vitamins in water. A solution is a mixture of a liquid and at least one type of dissolved solid substance. The substance that is dissolved is called the solute. The liquid that it is dissolved in is called the solvent. low high

14. What drives osmosis? Setup: U-tube with sel. perm. membrane, pour two different concentration salt solutions on either side of tube. Membrane is permeable to H2O but NOT to salt. What happens??? WHY?

15. What drives osmosis? Setup: U-tube with sel. perm. membrane, pour two different concentration salt solutions on either side of tube. Membrane is permeable to H2O but NOT to salt. What happens??? WHY?

16. A little solution vocabulary below Hypo = Hyper = Iso = above / over same Hypotonic: refers to a solution that has a solute content than the solution you are comparing it to Hypertonic: refers to a solution that has a solute content than the solution you are comparing it to Isotonic: Guess what this means! lower higher

17. Movement of Water Across a Membrane: Which way net flow?

18. Movement of Water Across a Membrane: Which way net flow?

19. Red Blood Cells in Different Solutions Hypotonic Isotonic Hypertonic http://arbl.cvmbs.colostate.edu/hbooks/cmb/cells/pmemb/osmosis.html

20. Facilitated Diffusion: Movement across a membrane, from high to low concentration, using a transport protein

21. Water also crosses membrane through Aquaporin protein channels http://en.wikipedia.org/wiki/File:AQP-channel.png http://plantphys.info/plant_physiology/osmosis.shtml

22. Not all cell transport is passive! Passive Processes • Do not require the use of cellular energy. • Always involve net movement from areas of high concentration to areas of low concentration. Ex) Diffusion, Osmosis, & Facilitated Diffusion Active Processes •Require use of cellular energy. • Allow net movement from areas of low to areas of high concentration. Ex) Active Transport, Endocytosis, & Exocytosis

23. How do cells “use” energy? ATP (Adenosine Tri-phosphate) Energy “currency” of the cell. Transfers energy by transferring PO4- groups. The bonds between the phosphate (PO4-) groups are less stable and have high (potential) energy. Adenine http://scienceaid.co.uk/biology/biochemistry/images/atp.jpg

24. Hydrolysis of ATP • releases energy and produces ADP + Pi

25. Where does the energy in ATP come from? Cell Respiration C6H12O6 + O2 + ADP + PiATP + CO2 + H2O http://gleesonbiology.pbworks.com/f/1221960382/03_32_ATP_and_ADP_cycle.jpg

26. How does ATP transfer energy to do work? Phosphorylation: transfer of a phosphate group

27. Active Transport: Movement across a membrane from low to high concentration which requires the use of energy (ATP) and a protein “pump”

28. Another Active Process: Exocytosis • a transport vesicle fuses with cell membrane, releasing vesicle contents to the outside

29. Yet another Active Process: Endocytosis Again, motor proteins use ATP, as does the process by which the cell cytoskeleton proteins are re-arranged to re-shape the membrane. http://bioap.wikispaces.com/file/view/endocytosis_types.png/177216843/735x365/endocytosis_types.png

30. Receptor-Mediated Endocytosis Rec-Med Endocytosis of LDL particles (Low-Density Lipoproteins): http://www.susanahalpine.com/anim/Life/endo.htm

31. Specific Examples of Cell Transport Passive Processes • Diffusion: movement of oxygen into cells • Osmosis: movement of water into cells placed in pure water • Facilitated Diffusion: movement of glucose into cells through specialized glucose protein channels Active Processes • Active Transport: sodium-potassium (Na+/K+) pump • Endocytosis • Phagocytosis: ingestion of bacteria by white blood cells • Pinocytosis: “nursing” of human egg cells • Receptor-Mediated Endocytosis: uptake of LDL particles • Exocytosis:Ex1) secretion of mucus Ex2) secretion of insulin protein

32. Transport Animations Fluid Mosaic Model of the Membrane http://www.susanahalpine.com/anim/Life/memb.htm Sodium-Potassium Pump (narrated) http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html Endo/Exo-cytosis (narrated) http://local.brookings.k12.sd.us/biology/PHVideos/Chapter%2007E.mpg Receptor-Mediated Endocytosis of an LDL particle http://www.susanahalpine.com/anim/Life/endo.htm Overview of Membranes & Transport, including Action Potential http://www.wiley.com/legacy/college/boyer/0470003790/animations/membrane_transport/membrane_transport.htm Microscope Videos of various types of Cell Transport http://www.linkpublishing.com/video-transport.htm