1. The Movement of Water Across the Plasma Membrane

2. Objectives Understand how proteins and lipids are assembled to form a selectively permeable barrier known as the plasma membrane. Explain how the plasma membrane maintains an internal environment that differs significantly from the extracellular fluid. Explain the importance and characteristics of carrier-mediated transport systems. Understand how voltage-gated channels and ligand-gated channels are opened. Explain, using specific examples, the difference between primary and secondary active transport.

3. The Structure of the Plasma Membrane Lipid bilayer – two sheets of lipids (phospholipids)-Polar (water-soluble) heads face out and the non-polar fatty acids hang inside. Polar head non-polar tail

4. Embedded with proteins and strengthened with cholesterol molecules. -- Integral proteins (or intrinsic proteins) are embedded in the lipid bilayer, Include channels, pumps, carriers and receptors. -- Peripheral proteins (or extrinsic proteins) do not penetrate the lipid bilayer. They are in contact with the outer side of only one of the lipid layers either the layer facing the cytoplasm or the layer facing the extracellular fluid

5. Integral-Membrane Proteins Can Serve as Receptors

6. Integral-Membrane Proteins Can Serve as Adhesion Molecules, ex- integrins, Cadherins. Can Form a Submembranous Cytoskeleton, Ex- spectrin,ankyrin

7. Membrane Transport  Cell membrane is permeable to:  Non-polar molecules (0 2& C0 2 ).  Lipid soluble molecules (steroids).  H 2 0 (small size, lack charge).  Cell membrane impermeable to:  Large polar molecules (glucose).  Charged inorganic ions (Na +, K+ etc.,).

8. Passive transport Processes Includes: A) Diffusion & B) Osmosis The movement of large molecules is carried out by endocytosis and exocytosis, the transfer of substances into or out of the cell, respectively, by vesicle formation and vesicle fusion with the plasma membrane. Cells also have mechanisms for the rapid movement of ions and solute molecules across the plasma membrane. These mechanisms are of two general types: -- passive movement, which requires no direct expenditure of metabolic energy, and substances move across the membrane down their electrochemical gradient -- and active movement, which uses metabolic energy to drive solute transport against this gradient. Types of Membrane Transport

9. 9 Bulk Transport (Endocytosis and Excytosis)  Movement of many large molecules, that cannot be transported by carriers.  Exocytosis:  A process in which some large particles move from inside to outside of the cell by a specialized function of the cell membrane  Endocytosis:  Exocytosis in reverse.  Specific molecules can be taken into the cell because of the interaction of the molecule and protein receptor.

10. 10  Exocytosis  Vesicle containing the secretory protein fuses with plasma membrane, to remove contents from cell.

11. 11  Endocytosis  Material enters the cell through the plasma membrane within vesicles.

12. 12 Types of Endocytosis  Phagocytosis - (“cellular eating”) cell engulfs a particle and packages it with a food vacuole.  Pinocytosis – (“cellular drinking”) cell gulps droplets of fluid by forming tiny vesicles. (unspecific)  Receptor-Mediated – binding of external molecules to specific receptor proteins in the plasma membrane. (specific)

13. 13 Example of Receptor-Mediated Endocytosis in human cells

14. Diffusion By which molecules move from areas of high concentration to areas of low concentration and cations move to anions. Its of 2 types. 1. Simple 2. Facilitated Diffusion Simple Diffusion: means molecules move through a membrane without binding with carrier proteins. Facilitated: requires a carrier protein which aids in passage of molecules through the membrane by binding chemically and shuttling them through the membrane.

15. 15 1. Simple Diffusion  Molecules/ions are in constant state of random motion due to their thermal energy.  Simple diffusion occurs  whenever there is a concentration difference across the membrane  the membrane is permeable to the diffusing substance.

16. Membrane Transport C1 C2 time particles ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● C1 C2

17. Diffusion Rate JxJx [X] Concentration Flux ↑P x ↓P x

18. Simple Diffusion Most molecules partition poorly – i.e. soluble in water but not lipid – therefore cannot cross lipid bilayer Need – pores, channels and transporters

19. The major property of biological membranes which makes them impermeable to most ions and polar molecules is A The presence of cholesterol B The absence of all proteins from the membrane C The structure of the lipid bilayer D The absence of charged groups on the membrane surface E The hydrophilic core of the membrane

20. Pores, Channels and Transporters Pore – transmembrane protein that is open, ex- aquaporins Channel – transmembrane protein with a pore that can open and close(gated) Transporter – transmembrane protein that undergoes a conformational change and facilitates the transport of a ‘packet’ of substrate across the membrane

21. Ion Channels. The cell membrane has ion channels that increase the permeability of the membrane for that ion species and allows the movement of those ions down their electrochemical gradient, channels can be opened or closed by gates These channels can show a high degree of specificity for a particular ion species, e.g. the epithelial sodium channel is 30 times more permeable to Na + than K +. Regulation of ion channels Ion channels gates may be open or closed and the time and frequency of opening may be regulated. There are three major factors that are involved in the regulation of the frequency and duration of channel opening. 1.Ligand-gated channels 2.Voltage-gated channels 3.Mechanosensitive channels

22. Ligand gated Ion Channel The channel is a channel/receptor complex Upon ligand binding there is a conformational change that opens the channel Selectivity is conferred by charged amino acids and size (selects for cations or anions and then selects for size e.g. K + ion much larger than Na + ion – hydrated form)

23. 23 Ion channel runs through receptor. Receptor has 5 polypeptide subunits that enclose ion channel. 2 subunits contain ACh binding sites. Ligand-Operated ACh Channels

24. Voltage Gated Ion Channel change in membrane potential( V m ) moves charged molecules within the channel changing channel conformation either opening or closing the channel. Charged amino acids inside the channel pore detect the electric field across the membrane – and conformational change can occur in response to a change in electric field -

26. Facilitated Diffusion via carrier Ex- glucose, amino acid transport. –Down concentration Gradient –Chemical Specificity: Carrier interact with specific molecule only, cysteinurea. –Competitive inhibition:  Molecules with similar chemical structures compete for carrier site. –Saturation:  V max (transport maximum):  Carrier sites have become saturated. glucose transporter 4 (GLUT4)  activated by insulin

27. Graph showing the relationship between net flux and concentration gradient of a substance moved across the membrane via facilitated diffusion. If the concentration gradient (and hence concentration) increases enough the transporters will become saturated and the net flux cannot be increased, this net flux value is called the transport maximum.

28. Active transport  When the cell membrane moves molecules or ions uphill against a concentration gradient  (or uphill against an electrical gradient),  the process is called active transport  Primary active transport  Secondary active transport:

29. Active transport  1 Primary active transport:  the energy used to cause the transport is derived directly from the breakdown of ATP or some other high-energy phosphate compound  2 Secondary active transport:  The energy is derived secondarily from energy  That has been stored in the form of ionic concentration differences between the two sides of the membrane  created by primarily active transport

30. Intracellular vs extracellular ion concentrations Ion Intracellular Extracellular Na + 5-15 mM 145 mM K + 140 mM 5 mM Mg 2+ 0.5 mM 1-2 mM Ca 2+ 10 -7 mM 1-2 mM H + 10 -7.2 M (pH 7.2) 10 -7.4 M (pH 7.4) Cl - 5-15 mM 110 mM

31. 3.1 Primary Active Transport  Hydrolysis of ATP directly required for the function of the carriers.  Molecule or ion binds to “recognition site” on one side of carrier protein.

32. 3.1 Primary Active Transport  Binding stimulates phosphorylation (breakdown of ATP) of carrier protein.  Carrier protein undergoes conformational change.  Hinge-like motion releases transported molecules to opposite side of membrane.

33. Mechanism of Acid Secretion The key player in acid secretion is a H+/K+ ATPase or "proton pump" located in the parietal cell membrane. Hydrogen ion is pumped out of the cell, into the lumen, in exchange for potassium through the action of the proton pump.

34. Na + /K + Pump

35. 35 A Model of the Pumping Cycle of the Na + /K + ATPase

36. Importance of the Na + -K + Pump  Control cell volume  Develop and Maintain Na + and K + concentration gradients across the membrane  Electrogenic action influences membrane potential  Provides energy for secondary active transport

37. 37 2 Secondary Active Transport  Energy needed for “uphill” movement obtained from “downhill” transport of Na +.  Hydrolysis of ATP by Na + /K + pump required indirectly to maintain [Na + ] gradient.

38. 38 Secondary active transport Na + glucose Na + ca 2 + out in co-transport counter-transport (symport) (antiport) Co-transporters will move one moiety, e.g. glucose, in the same direction as the Na +. Counter-transporters will move one moiety, e.g.ca2 +, in the opposite direction to the Na +.

40. Insulin Secretion and Membrane Transport Processes Glycolysis and citric acid cycle ATP Ca 2+ signal triggers exocytosis, and insulin is secreted. Ca 2+ (b) Beta cell secreting insulin. Closure of the K ATP channel depolarizes the cell, triggering exocytosis of insulin. High glucose levels in blood Metabolism increases. ATP increases. Glucose Cell depolarizes and calcium channels open. K ATP channels close. Ca 2+ entry acts as an intracellular signal. GLUT transporter 23145 6 7

41. Epithelial Transport

42. The Movement of Water Across the Plasma Membrane Water can move in and out of cells. But the partition coefficient of water into lipids is low meaning the permeability of the membrane lipid bilayer for water is low. Specific membrane proteins that function as water channels explain the rapid movement of water across the plasma membrane These water channels are small integral membrane proteins known as aquaporins

43. Water Movement NaCl 0 mOsmNaCl 100 mOsm [water] HIGH[water] LOW If membrane impermeable to NaCl Aquaporin

44. Osmosis - Osmosis is the flow of water across a semipermeable membrane from a solution with low solute concentration to a solution with high solute concen­tration.

45. Osmotic pressure of a solution is defined as the pressure necessary to stop the net movement of water across a selectively permeable membrane When a membrane separates two solutions of different osmotic pressure, water will move from – the solution with low osmotic pressure (high water and low solute concentrations) to the solution of high osmotic pressure (low water and high solute concentrations). The Movement of Water Across the Plasma Membrane Is Driven by Differences in Osmotic Pressure

46. O smolarity is an “inherent” property of the solution. It depends strictly on the number of particles in solution (not the number of molecules, since some molecules (e.g. NaCl) dissociate into ions when in solution). Osmolarity is therefore, the number of particles per liter of solution and is expressed in osmol/L or OsM or in the case of dilute solutions as milliosmol/L. Ex- A solution of 1 M CaC1 2 has a higher osmotic pressure than a solution of 1 M KCl because the concentration of particles is higher. The higher the osmotic pressure of a solution, the greater the water flow into it.

47. The osmotic pressure of a solution can be calculated by - Van't Hoffs law, which states that osmotic pressure depends on the concentration of osmotically active particles. The concentration of particles is converted to pressure according to the following equation: where: 7T = osmotic pressure (mm Hg or atm) g = number of particles in solution(osm/mol) R = gas constant (0.082 L-atm/mol-K) σ = Reflection coefficient (varies from 0 to 1) T = absolute temperature (K) C = concentration (mol/L)

48. The Concept of Tonicity Tonicity describes a solution, and how that solution affects cell volume. The tonicity of a solution depends not just on the osmolarity of the solution but also on whether the solutes (particles) in the solution are penetrating or not. Two solutions having the same effective osmotic pressure are isotonic because no water flows across a semipermeable membrane separating them. If two solutions separated by a semipermeable membrane have different effective osmotic pressures, the solution with the higher effec­tive osmotic pressure is hypertonic and the solution with the lower effective osmotic pressure is hypotonic. Water flows from the hypo­tonic to the hypertonic solution.

49. RBC hypotonic solution SWELL Rules for predicting tonicity If the cell has a higher concentration of non-penetrating solutes than the solution, there will be net movement of water into the cell. The cell swells, and by definition that solution is hypotonic.

50. RBC isotonic solution NO VOLUME CHANGE

51. RBC hypertonic solution SHRINK

52. Tonicity Tonicity describes the volume change of a cell placed in a solution