Plasma Membrane Structure and Function

1. Plasma Membrane Structure and Function • Separates the internal environment of the cell from its surroundings. • Regulates what materials enter and leave a cell • Made of 2 layers of phospholipids(phospholipid bilayer)with proteins embedded throughout. • Fluid consistency (like a light oil) and a mosaic pattern of proteins. 0

2. Because of consistency and pattern of components, referred to as fluid-mosaic modelof membrane structure 0 http://www.tamu.edu/classes/bich/mullins/bich410/olprg/10-2a_FluidMosaic/FluidMosaic.htm http://www.stolaf.edu/people/giannini/flashanimat/lipids/membrane fluidity.swf

5. 0 • Cells live in fluid environments, with water inside and outside the cell. • Components of plasma membrane: • 2 layers of phosphlipids • Polar head and nonpolar tail • Integral proteins • Peripheral proteins • Cholesterol • Carbohydrates

6. Phospholipid bilayer: • Hydrophilic(water-loving) polar heads • Face outside and inside of cell. • Hydrophobic(water-fearing) nonpolar tails • Extend to the interior of the plasma membrane.

7. 0 • Proteins: • Peripheral proteins • On inside surface • Held in place by cytoskeletal filaments • Integral proteins • Embedded in membrane, can move laterally • Most span the membrane, but some protrude only from one side

8. 0 • Cholesterol - strengthens the plasma membrane. • Carbohydrates: • Glycoproteins– proteins with carbohydrates attached • Glycolipids – phospholipids with carbohydrates attached

9. Functions of membrane proteins 0 • Some help to transport materials across the membrane. • Channel Protein – allows certain molecule or ion to cross membrane freely • Carrier Protein – interacts with certain molecule or ion to help move it across membrane

10. 0 • Others receive specific molecules, such as hormones. • Receptor Protein – has certain shape so that specific molecule, such as a hormone or some other signaling molecule, can attach • Attachment can cause protein to change shape & cause cell to perform certain action

11. 0 • Still other membrane proteins function as enzymes. • Enzymatic Protein - Can carry outmetabolic reactions • Peripheral proteins – stabilize and shape the membrane

12. 0 • Some proteins aid incell recognition • Glycocalyx - In animal cells, the carbohydrate chains of cell recognition proteins are collectively called this • Can function in cell-to-cell recognition, adhesion between cells, and reception of signal molecules. • The diversity of carbohydrate chains is enormous, providing each individual with a unique cellular “fingerprint”.

13. 0 Cell recognition protein • Foreign carbohydrate chains are why transplanted tissue is often rejected by the body

14. 0 The Permeability of the Plasma Membrane • Differentially permeable (sometimes called selectively permeable) – meaning only certain materials can cross the membrane. • Two mechanisms of transport: • Active– requires ATP (chemical energy) • Passive– does not require chemical energy

15. 0 • Examples of passive transport: • Diffusion (no carrier protein needed) • Facilitated transport (which usually requires a carrier protein) • Active transport: • Requires a carrier protein • May sometimes involve the formation of vesicles to take in or get rid of materials (for example endocytosis and exocytosis). • **More on these types of transport later in chapter**

16. 0 • How do materials get across the plasma membrane? • Small, uncharged molecules pass through the membrane, followingtheirconcentration gradient (gradual change in chemical concentration from one area to another – molecules tend to move from area of high to low concentration). • Larger macromolecules or tiny charged molecules rely on proteins to help them to get across.

17. 0 How molecules cross the plasma membrane

18. 0 Diffusion and Osmosis • Diffusion is the passive movement of molecules from a higher to a lower concentration untilequilibriumis reached. • Equilibrium– state in which all materials are evenly concentrated • Movement of molecules still occurs, but there is no NET movement of molecules • Gases move through plasma membrane by diffusion.

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20. 0 Osmosis • Defined as the diffusion of water across a differentially permeable membrane due to concentration differences. • Molecules always move from higher to lower concentration. • Water enters cells due toosmotic pressure (pressure that develops in a system due to osmosis) within cells.

21. 0 Osmosis demonstration

22. 0 Osmosis in cells • A solution contains a solute (solid) and a solvent (liquid). • Cells are normally isotonic to their surroundings, and the solute concentration is the same inside and out of the cell. • Cell is in equilibrium; there is no net movement of water across the cell membrane. http://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/Osmosis.htm http://www.linkpublishing.com/video-transport.htm

23. Isotonic = same No net movement of water X = Solute X = water

24. 0 Cells in isotonic solutions http://www.linkpublishing.com/Videos/transport/blood_isotonic.wmv http://www.linkpublishing.com/Videos/transport/elodea_normal.wmv

25. 0 • Hypotonic solutions cause cells to swell & possibly burst; “hypo” means less than. • Lower concentration of solute, higher concentration of water outside than inside cell. • Net movement of water from outside to inside the cell. • Animal cells - may undergolysis(cell is disrupted – may even burst) • Plant cells - increased turgor pressure (makes plant cell rigid); plant cells do not burst because they have a cell wall.

26. Hypotonic = less than Water moves in (hypo/hippo – swells like a hippo) X = Solute X = water

27. Cells in a hypotonic solution 0 http://www.linkpublishing.com/Videos/transport/blood_hypo.wmv http://www.linkpublishing.com/Videos/transport/elodea_hypo.wmv

28. Hypertonic solutions cause cells to lose water; “hyper” means more than • Higher concentration of solute, lower concentration of water outside than inside cell. • Net movement of water from inside to outside of cell. • Animal cells - undergo crenation (shrivel) • Plant cells - undergo plasmolysis, the shrinking of the cytoplasm. • Turgor pressure is lost as plant cells shrink. 0

29. Hypertonic = more than Water moves out X = Solute X = water

30. Cells in a hypertonic solution 0 http://www.linkpublishing.com/Videos/transport/blood_hyper.wmv http://www.linkpublishing.com/Videos/transport/elodea_hyper.wmv

31. 0 • Transport by Carrier Proteins • Some materials cannot enter or leave cell due to theirsize and/or nature. • Some of those molecules use the channel proteins and carrier proteins that span the membrane in order to enter or leave the cell. • Carrier proteins are specific and combine with only a certain type of molecule. • Facilitated transport and active transport both require carrier proteins.

32. 0 Facilitated transport • Substances pass through a carrier protein following their concentration gradients (high  low concentration). • Does not requireenergy. • Brings in materials such as glucose & amino acids; proteins are specific to molecules taken in. • Some molecules can be taken in faster than others – differential permeability. http://www.coolschool.ca/lor/BI12/unit4/U04L03/facilitated transport.swf http://www.northland.cc.mn.us/biology/Biology1111/animations/passive1.swf

33. 0 Active transport • Ions or molecules are moved across the membrane against the concentration gradient – from an area of lower to higher concentration. • Energy in the form of ATP is required for the carrier protein to combine with the transported molecule. • Occurs in cells such as kidney cells (taking sodium from urine), thyroid gland cells (taking in iodine), and cells in digestive tract (absorbing nutrients).

34. 0 Active transport

35. 0 • Carrier proteins involved in active transport are calledpumps. • The sodium-potassium pump is active in all animal cells (particularly nerve & muscle cells), and moves sodium ions to the outside of the cell and potassium ions to the inside. • The sodium-potassium pump carrier protein exists in two conformations; one that moves sodium to the outside, and the other that moves potassium into the cell.

36. 0 The sodium-potassium pump http://www.brookscole.com/chemistry_d/templates/student_resources/shared_resources/animations/ion_pump/ionpump.html

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39. Exocytosis and Endocytosis • For molecules that are too large to be transported with carrier proteins – transported using vesicle formation – requires energy. • Exocytosis- vesicles fuse with the plasma membrane forsecretion. • Causes cell membrane to enlarge – process occurs during growth. • Molecules released become part of cell membrane, part of matrix surrounding cell, or nourish nearby cells. 0

40. 0 Exocytosis

41. 0 • Some cells are specialized to produce and release specific molecules. • In some cases, release of molecules may occur only in the presence of signals received by the plasma membrane. • Examples include release of digestive enzymes from cells of the pancreas, or secretion of the hormone insulin in response to rising blood glucose levels.

42. Endocytosis - cells take in substances when a portion of the plasma membrane folds in, and forms a vesicle around the substance. • When vesicle fuses with lysosome, digestion occurs. • Endocytosis occurs as: • Phagocytosis – large particles(food or other cells) • Pinocytosis – small particlesor liquids • Receptor-mediated endocytosis – form of pinocytosis for specific particles such as vitamins, hormones, or lipoproteins 0 http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter6/animations.html

43. 0 Phagocytosis • Seen in unicellular organisms like amoebas. • White blood cells can use this process to take in bacteria & worn out red blood cells

44. 0 Pinocytosis • Used by root cells of plants. • Seen in blood cells, cells that line kidneys and intestines.

45. Receptor-mediated endocytosis 0 • Seen during exchange of maternal & fetal blood in placenta; & intake of cholesterol in body cells (failure to do so results in high blood pressure, blocked arteries, and heart attacks).