Plasma Membrane Structure and Function

1. Plasma Membrane Structure and Function Image from: http://www.biologie.uni-hamburg.de/b-online/ge22/03.gif

2. Cell Size Limitations • The size of cells is limited by the surface area to volume ratio of cells • As cells grow the volume inside of the cell grows faster than the surface area available to diffuse nutrients into the cell and expel wastes out of the cell • Surface area determines rate molecules enter and leave cell • Volume determines the demand for resources needed by the cell (increase in volume requires more resources to keep the cell functioning properly)

3. Smaller cells have more favorable surface area to volume ratios for exchange with the environment Villi: Small Intestines Cells of alveoli: Lungs Root hairs

4. You should be able to solve basic surface area and volume equations given to you on the AP Biology exam.

5. In order for cells to grow, reproduce and maintain dynamic homeostasis, cells must create and maintain an internal environment that is different from the external environment • Cell membranes are selectively permeable to accomplish this goal • Cell membranes separate the internal environment from the external environment

6. FLUID MOSAIC MODEL Animation from: http://www.sp.uconn.edu/~terry/images/anim/fluidmem.gif 1972- S.J. Singer and G. Nicolson propose membrane is a “mosaic” of proteins and phospholipids that are constantly moving and changing Click here to See Fluidity Click here to SeeFLUIDITY

7. More than just a barrier… • The membranes selective permeability is a direct consequence of the membranes structure consisting of phospholipids, embedded proteins, cholesterol, glycoproteins and glycolipids

8. A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer

9. Molecules need to move across membranes in cells OUT waste ammonia salts CO2 H2O products IN food carbohydrates sugars, proteins amino acids lipids salts, O2, H2O Image modiified from: http://www.accessexcellence.org/AB/GG/importProt.html

10. Membranes provide a variety of cell functions

11. Cell Membranes are made of PHOSPHOLIPIDS & PROTEINS Amphipathic – Molecules with both hydrophilic and hydrophobic regions phosphate hydrophilic lipid hydrophobic

12. Membrane Proteins • Proteins determine most of membrane’s specific functions • cell membrane & organelle membranes each have unique collections of proteins • Membrane proteins: • peripheral proteins = loosely bound to surface of membrane • integral proteins = penetrate into lipid bilayer, often completely spanning the membrane = transmembrane protein

13. HYDROPHILIC/HYDROPHOBIC areas determine positions of molecules in cell membranes hydrophobic amino acids • Those with nonpolar side groups • stick in the lipid membrane • anchors the protein in membrane hydrophilic amino acids • Amino acids with charge and polar side groups • stick out in the watery fluid in or out of cell

14. Membrane Carbohydrates • Attached to proteins (glycoproteins) or lipids (glycolipids) • Play a key role in cell-cell recognition • ability of a cell to distinguish neighboring cells from another • important in organ & tissue development • basis for rejection of foreign cells by immune system http://faculty.southwest.tn.edu/rburkett/GB1-osmosis.htm

15. Semi-permeable membrane • Need to allow passage through the membrane • But need to control what gets in or out • membrane needs to be semi-permeable sugar aa lipid H2O salt NH3 • So what makes a membrane semi permeable? See a movie

16. PHOBIC TAILS in center determine what can pass through

17. inside cell outside cell Small non-polar (uncharged) molecules (O2, N2, & CO2) and hydrophobic molecules (fats & other lipids) can slip directly through the phospholipid cell membrane, but… What molecules can get through directly? lipid O2 salt What about other stuff? NH3 sugar aa H2O

18. Types of Cell Transport

19. Diffusion • 2nd Law of Thermodynamicsgoverns biological systems • Universe tends towards disorder Animatioin from: http://www.biologycorner.com/resources/diffusion-animated.gif • Diffusion • movement from [higher][lower] concentration

20. Diffusion will also happen across a cell membrane as long as there is a difference in concentration and the membrane will let the molecule pass through.

21. Example: DIFFUSION IN CELLS http://facstaff.bloomu.edu/gdavis/links%20100.htm O2 automatically moves from HIGHER concentration (in lungs) to LOWER concentration (in blood) CO2 automatically moves from HIGHER concentration (in blood) to LOWER concentration (in lungs) http://www.le.ac.uk/pa/teach/va/anatomy/case2/2_2.html

22. Diffusion of 2 solutes • Each substance diffuses down its own concentration gradient, independent of concentration gradients of other substances

23. What if cell needs to move a molecule _________ the CONCENTRATION GRADIENT?_______________ AGAINST Cell example: Want to put MORE glucose into mitochondria when there is already glucose in there (LOWER HIGHER) Image from: http://www.biologyclass.net/mitochondria.jpg

24. What if a cell needs to move _____ or ______ moleculesthat can’t getthrough themembrane? LARGE POLAR http://www.d.umn.edu/~sdowning/Membranes/membraneImages/jpegimages/diffusionmedium.jpg

25. What if cell needs to move molecules really _______? (can’t wait for it to diffuse) FAST Cell example: Movement of Na + & K+ ions required to send nerve signals http://www.steve.gb.com/images/science/neuron.png

26. Cells need a ____ to ____ molecules across cell membranes that _______ across by ___________ WAY HELP can’t go themselves

27. Facilitated diffusion • Move from HIGH to LOW concentration with aid of membrane transport proteins • passive transport • no energy needed • facilitated = with help • Examples: • Way glucose enters cells • Also how charged and polar molecules enter the cell

28. Facilitated Diffusion Animation from: http://bio.winona.edu/berg/ANIMTNS/facdifan.gif Carrier Proteins Grab molecule, undergo conformational change, flip to other side Channel Proteins Create passageway forsubstances to pass through May be gated or not:open in response to chemical or electrical signals Animation from: http://www2.uic.edu/~myilma1/ionchannel.gif

29. Gated channels • open only in presence of stimulus (signal) • stimulus usually different from transported molecule • ex:ion-gated channelswhen neurotransmitters bind to a specific gated channels on a neuron, these channels open = allows Na+ ions to enter nerve cell • ex:voltage-gated channelschange in electrical charge across nerve cell membrane opens Na+ & K+ channels

30. Carriers and Channels are specific inside cell aa sugar H2O NH3 salt outside cell . . . BUT STILL MOVES FROM [HIGHER] to [LOWER]

31. Active transport Uses energy from ATP to move molecules against concentration gradient • Moves from [lower] → [higher] • Uses protein pumps OR vesicles Animations from:http://academic.brooklyn.cuny.edu/biology/bio4fv/page/cell-movement.html http://www.cat.cc.md.us/courses/bio141/lecguide/unit1/eustruct/images/sppump.gif

32. PROTON PUMP Moves Protons (H+) across membrane EXAMPLES: - Creates acidic condition inside lysosomes - Photosynthesis/respiration use H+ gradients to generate ATP See a movie proton pump

33. Active transport SODIUM-POTASSIUM PUMP Sets up difference in charge across membranes Pumps 3 Na+ outand 2 K+ in Makes cells more + outside more - inside See a movieabout Na+ - K+ pump Animation from: http://www.cat.cc.md.us/courses/bio141/lecguide/unit1/eustruct/images/sppump.gif

34. All cells have voltages across their membranes • = membrane potential • Cytoplasm inside cell is more negative • than extracellular fluid outside • TWO FORCES drive diffusion • Concentration gradient • Electrical force • Ions move DOWN the electrochemical gradient Favors the passage of cations (+) into cells

35. MEMBRANE POTENTIAL created by electrogenic pumps (proteins that generate voltage by pumping ions) PROTON PUMPMain electrogenic pump in plants, fungi, and bacteria Na+- K+ PUMPMain electrogenic pump in animal cells http://fig.cox.miami.edu/~cmallery/150/memb/electrogenic.jpg Animation from: http://www.lionden.com/cell_animations.htm

36. Electrogenic pumps can be coupled to actively transport other substances = COTRANSPORT Ex: Cells pump H+out of cell then use the diffusion of H+back into cell downits gradient to drivethe uptake of sucrose into cells http://faculty.southwest.tn.edu/rburkett/GB1-os29.jpg

37. Na+ - K+ pump sets up MEMBRANE POTENTIAL http://hyperphysics.phy-astr.gsu.edu/hbase/biology/nervecell.html#c2

41. BULK TRANSPORT Watch two video clips aboutendo/exocytosis endo/exocytosis • ACTIVE transport • Requires energy (ATP) • Uses vesicles Animation from: http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBooktransp.html

42. Endocytosis “Cell eating” large molecules; whole cells phagocytosis “Cell drinking” Fluids; Small molecules pinocytosis triggered byligand signal receptor-mediated endocytosis

43. Example in cells:WHITE BLOOD CELL ENGULFING BACTERIA SEE PHAGOCYTOSIS MOVIE http://fig.cox.miami.edu/~cmallery/255/255ion/fig14x28.jpg

44. EXOCYTOSIS • Active transport (requires ATP) • Uses vesicles • Releases substances to outside INSULIN being released by pancreas cells using exocytosis

45. Video: http://www.southtexascollege.edu/tdehne/BC_ShockwaveAnimations/07SWF-TourOfTheCell/07-16-EndomembraneSystem.swf GOLGI BODIES USE EXOCYTOSIS Animation from: http://www.franklincollege.edu/bioweb/A&Pfiles/week04.html See a Golgi movie

46. http://fig.cox.miami.edu/~cmallery/255/255ion/fig14x26.jpg

47. Transport summary

48. Water Transport The Special Case of Water

49. The Special Case of WaterMovement of water across the cell membrane = OSMOSIS

50. Osmosis is diffusion of water • Water is very important, so we talk about water separately • Diffusion of water from high concentration of water to low concentration of water • across a semi-permeable membrane • Passive • Uses no energy