Plasma Membrane

1. Plasma Membrane mhhe.com LECTURE #7

2. Function • Plasma membrane  boundary • Separates cell contents from the environment • Regulate movement of molecules to and from the cell • EX: Chloride (Cl-)

3. Characteristics • Thin • Fatty • Fluid • Flexible • Stable • Dynamic • Active blog.pegasuscom.com

4. Components • Phospholipidbilayer (major component) • Cholesterol • Proteins • Glycocalyx

5. Phospholipidbilayer • In water, phospholipids will form bilayers • Amphipathic: Hydrophobic tails point in; hydrophilic heads point out • Contacts water on intracellular and extracellular surfaces watery extracellular fluid – hydrophilic hydrophobic hydrophilic watery cytosol

6. Phospholipidbilayer • This chemistry regulates passage of molecules (no covalent bonds hold it together!) • Bulk of thickness is hydrophobic • Hydrophobic molecules pass through readily • e.g., fatty acids, steroid hormones, etc. • Movement of hydrophilic molecules is restricted • e.g., ions, amino acids, sugars, etc.

7. Membrane fluidity

8. Factors affecting fluidity Degree of saturation

10. Cholesterol • Nestled between phospholipid molecules • Restricts movement through membrane • Maintains membrane fluidity • Not too rigid in cold temps • Not too fluid in high temps

11. Cholesterol Cholesterol can “patch” weak spots in the lipid bilayer

12. Proteins • Some embedded, some on surface • Roles • Structural support • Recognition • Communication • Transport • Etc.

13. Proteins • Structural support These proteins areattached to cytoskeleton • Stabilize the cell • Give animal cells characteristic shapes

14. Proteins • Recognition Cells in the immune systemcan discern between self and foreign molecules • Cell surface proteins

15. Proteins • Communication Various ways of communicating • Signals sent to other cells • Cell ejects hormones or proteins • Signals received by receptor proteins • Binding site “fits” a specifictype of signaling molecule

16. Insulin is a signaling hormone Insulin’s message: “Glucose levels are rising in the bloodstream; please construct some protein channels so you can take some of this glucose in!”

17. Proteins • Transport • Many materials cannot pass through plasma membranes • Transport proteins move specific molecules across

18. Glycocalyx • Short, branched carbohydrate (sugar) chains • Attach to membrane lipids and proteins • Functions • Binding sites for signaling molecules • Lubricate cells • Adhesion to adjacent cells

20. A semi-fluid “sea”

21. Diffusion & Osmosis

22. Movement • All molecules and ions are in a constant state of random motion • Temperature is a measure of the degree of motion • There is no motion below absolute zero • −273 oC (−459.7°F) chemistryland.com

23. Diffusion • Molecules will tend to move from a region of high concentration to a region of lower concentration. • Net movement is down aconcentration gradient = difference between highestand lowest concentration • Movement of molecules ordered  disordered state • Spontaneous movement • Can move up concentration gradient, but it takes energy! pictureninja.com

25. Semi-permeable membranes • If a membrane is permeable to water and a solute, then both will diffuse to equilibrium… • But what if the membrane is permeable to water but not the solute?

26. Osmosis Net movement of water is down its concentration gradient Osmosis is the diffusion of fluid

27. Semi-permeable membrane The plasma membrane is semi-permeable • Somewhat permeable to water and lipids • Impermeable to large or charged substances watery extracellular fluid – hydrophilic hydrophobic hydrophilic watery cytosol

28. Plasma membrane • Osmosis takes place across the plasma membrane all the time • e.g., Water uptake by plants • e.g., Return of fluid to blood vessels

29. Question Why shouldn’t you drink salt water? (digestive system cells) hairremovalarea.com

30. Cell environments Types of solutions - Isotonic = Solute and solvent conditions are identical - Hypertonic = A solution with excess solute - Hypotonic = A solution with little solute

31. Isotonic Animal  Plant (OK)

32. Hypertonic Animal  Plant 

33. Hypotonic Animal  Plant 

34. Moving stuff in & out • Some molecules can pass through the plasma membrane by simple diffusion • - No proteins needed • CO2 out of cell • Steroids in and out

35. Moving stuff in & out • Protein-mediated movement • Facilitated diffusion = movement down the concentration gradient requiring a transport protein • sugars, amino acids, ions

36. Moving stuff in & out • Protein-mediated movement • Active transport = movement up the concentration gradient requiring a transport protein and energy (ATP) • Important to maintain osmolarityof the cell. (Macromolecules in cell are charged (-) and attract counter-ions that need to bepumped out of cell.)

37. Moving big stuff in and out • Sometimes it is necessary to move large molecules across membranes • Large molecules cannot be moved by the same mechanisms • Channels and pumps are too small • Movement employs vesicles

38. Moving big stuff out • Exocytosis =Movement of materials out of a cell by fusing a vesicle with the plasma membrane • Vesicle’s contents released into extracellular fluid • e.g., Vesicles budding from Golgi complex fuse with plasma membrane to export proteins • e.g., When waste products released by cell

40. Moving big stuff in Endocytosis = Movement of large material into cell • e.g., Ingestion of an entire bacterial cell • Accomplished by enclosing them within vesicles derived from the plasma membrane (exocytosis in reverse). • Three forms • Pinocytosis • Receptor-mediate endocytosis • Phagocytosis

41. Moving big stuff in • Pinocytosis (“cell drinking”) = cell folds inward

42. Moving big stuff in • Receptor-mediated endocytosis= Groups of receptors congregate in depression in cell membrane • Receptors specific to molecule • e.g., Cholesterol

43. Moving big stuff in • Phagocytosis= (“cell eating”) Large material engulfed • e.g., Amoeba and white blood cells • Send out “pseudopodia” to engulf, form vesicle, fuse vesiclewith lysosome.

45. Plasma membrane • The plasma membrane is constantly being “remade” • Membrane is lost via endocytosis • Membrane is gained via exocytosis

46. Cell death crmagazine.org

47. Cell death • There are 2 ways cells die… • 1. They are killed by injurious agents • Mechanical damage • Exposure to toxic chemicals • 2. They are induced to commit suicide = apoptosis

48. Cell death • Death by injury leads to… • Disruption of plasma membrane’s ability to regulate passage of ions in/out of cell • Swelling of the cell  bursting or leaking technologyreview.com

49. Cell death • Induced suicide (PCD) steps… • Cell shrinks itself, packages into “bites” • Develops blisters on surface • Chromatin and mitochondria degrade • ATP is released • Dying cell releases a “find me” signal to phagocytes • Phagocytes secrete proteins that inhibit inflammation • Cell is engulfed by phagocyte and contents recycled • The ultimate sacrifice!

50. Why would a cell commit suicide? 1. For proper development • Formation of fingers/toes in human fetus  • Sloughing of inner lining of uterus (menstruation) • Tadpole loses its tail as it matures octopusmom.com courses.washington.edu