1. Solutions The ICF and the ECF are homogeneous mixtures of substances including water, ions, amino acids, disaccharides, triglycerides… called solutions Solutions are divided into 2 parts –Solvent substance present in greatest amount the solvent of the body is water –Solute(s) substance(s) present in lesser amounts every other substance in the body is a solute –ions, carbohydrates, proteins, fats, nucleotides… Solutions are described in terms of their concentration –the amount of solutes in a given volume of solution Units include: molarity, %, weight per volume

2. ECF vs. ICF The concentration of individual solutes is different –the cell membrane transports and separates each solute between the ICF and ECF to maintain optimal working conditions within the ICF The total solute concentration of the ECF = ICF –prevents the cell from swelling or shrinking Concentration (mM) 150100500 Concentration (mM) 050100150 Na + K + Mg ++ Cl - HCO 3 - Protein Phosphates SO 4 -- Org. acids Major anions: Major cations: EXTRACELLULAR INTRACELLULAR

3. Modes of Membrane Transport Vesicular Transport –use vesicles to move substances into/out of the cell –transport of substances that are TOO LARGE to move through a membrane movement of few very large substances –cells, bacteria, viruses movement of very many large molecules –proteins Transmembrane Transport –movement of small molecules through a membrane plasma membrane, Golgi membrane, ER membrane, lysosome membrane… –ions, fatty acids, H 2 O, monosaccharides, steroids, amino acids…

4. Vesicles “Bubbles” of phospholipid bilayer membrane with substances inside –typically proteins –substances DO NOT contact the cytosol Created from a membranous organelle by the “pinching” or “budding” of the membrane Can “fuse” (merge) with a phospholipid bilayer membrane

5. Vesicular Transport - Exocytosis and Endocytosis Exocytosis –moves substance out of the cell Endocytosis –substances are moved into the cell by trapping them in a vesicle cell membrane creates pseudopods (“false feet”)

6. Exocytosis of Proteins

7. Exocytosis

8. Types of Endocytosis Phagocytosis –“cell eating” –endocytosis of few very large substances (bacteria, viruses, cell fragments) – vesicles containing cells fuse with lysosomes which digest the cells Pinocytosis –“cell sipping” –endocytosis of extracellular fluid Receptor-mediated endocytosis –endocytosis of a specific substance within the ECF –requires that the substance attaches to the extracellular portion of an integral membrane receptor protein

9. Endocytosis – Phagocytosis and Lysosomes

10. Pinocytosis

11. Receptor- Mediated Endocytosis

12. Transmembrane Transport of Nonpolar vs. Polar Substances Nonpolar substances cross a membrane through the phospholipid bilayer –ineffective barrier against the movement of nonpolar molecules across a membrane it is impossible to control the movement of nonpolar molecules through a membrane Polar substances cross a membrane by moving through integral membrane transporting proteins –Carriers –Pumps –Channels –Each carrier, pump and channel has a unique tertiary shape and therefore is designed to transport different substances across a membrane the cell can control the movement of polar molecules through a membrane by controlling the activity of the transporting proteins

13. Carriers and Pumps Integral membrane proteins that “carry” large polar molecules (monosaccharides, amino acids, nucleotides…) by changing their shape between 2 conformations –“flip-flop” between being open to the ECF and ICF –transport substances much more slowly across a membrane compared to channels the maximum rate at which these proteins can transport substances across a membrane is limited by how fast they can change shapes Pumps hydrolyze a molecule of ATP and use the energy to transport substances across the membrane The movement of only one substance across a membrane is called uniport The movement of more than one substance across a membrane is called cotransport

15. Types of Cotransport 2 or more substances are simultaneously carried across the cell membrane by a carrier Symport –2 substances are moved across a membrane in the same direction Antiport –2 substances are moved across a membrane in opposite directions

16. Channels Integral membrane proteins containing a water filled hole (pore) which allows the movement of small polar substances (ions and water) –transport substances very quickly across membrane there is virtually no limit to the rate at which substances can cross the membrane through channels –some channels have “gates” which can be: open –allows ion to cross the membrane closed –does not allow ion to cross the membrane

17. Transmembrane Transport Active –a transport process that uses an energy source produced by the cell for the movement of a polar substance through a membrane –substance(s) moves UP a concentration gradient –uses pumps for primary active transport –uses carriers for secondary active transport Passive –a transport process that DOES NOT use an energy source produced by the cell for the movement through a membrane –substance(s) moves DOWN a concentration gradient Diffusion –uses channels or carriers to transport polar substances through a membrane

18. Concentration Gradients The difference in the amount of a substance between 2 locations –region of greater amount vs. region of lesser amount –the difference may be LARGE or very small –may exist across a physical barrier (membrane) –may exist across a distance without a barrier Form of mechanical energy –location of greater amount provides more collisions –collisions cause substances to move away from one another net movement from location of greater amount to locations of lesser amounts

19. Concentration Gradients

20. Concentration Gradients and Transmembrane Transport The movement of a substance from a region of lesser amount to a region of greater amount: –is an “uphill” movement –requires an energy source Active Transport –causes the gradient to increase The movement of a substance from a region of greater amount to a region of lesser amount: –is a “downhill” movement –releases energy during transport Passive Transport –causes the gradient to decrease

21. Transmembrane Concentration Gradients and Equilibrium Equilibrium –a condition that is met when substances move down a gradient until there is NO gradient equal concentrations of a substance between 2 locations no net movement of substances from one location to another –substances do continue to move due to heat energy –Equilibrium of substances across the various membranes in the cells of the body = DEATH your body is in a constant battle to ensure equilibrium of solutes across the membranes is never reached

22. Transmembrane Concentration Gradients and Steady State Steady State –a condition where a gradient is PRESENT and is MAINTAINED by the constant expenditure of energy by the cell –unequal concentrations of a substance between 2 locations –no net movement of substances from one location to another one substance moving passively down the gradient, is exactly balanced by one substance moving actively up the gradient –Life depends upon the ability of the organism to exist in a steady state

23. Passive Transmembrane Transport – Diffusion Movement a substance to move DOWN its gradient Releases energy as a result of the movement Does not occur if there is no gradient –Equilibrium Nonpolar molecules diffuse through the nonpolar phospholipid bilayer in a process called simple diffusion Polar molecules require the help (facilitation) of integral membrane proteins (channels or carriers) to cross the bilayer in a process called facilitated diffusion

24. Facilitated Diffusion through a Carrier

25. Factors Affecting the Rate of Diffusion The rate at which a substance moves by way of diffusion is influenced by 3 main factors –Concentration gradient a large concentration gradient, results in a high rate of diffusion –Temperature a high temperature, results in a high rate of diffusion –heat causes motion –Size of the substance a large substance, has a low rate of diffusion –larger objects move more slowly

26. Osmosis The diffusion of water (solvent) across a selectively permeable membrane –requires “water channels” called aquaporins Water diffuses toward the location with the greatest amount of solutes (region of less water) Solutes generate a force that “pulls” water molecules causing them to move toward the solutes –osmotic pressure the greater the osmotic pressure, the greater the amount of water movement Water in a container (such as a cell) exerts a pressure on the walls of the container –hydrostatic pressure the greater the amount of water in a container, the greater the hydrostatic pressure

27. Osmotic pressure is the amount of pressure which must be applied to the membrane in order to oppose osmosis.  due to the force generated by solutes which attracts water molecules towards them.

28. Tonicity The ability of the solutes in the ECF to cause osmosis across the plasma membrane Isotonic (same) –solute concentration in ECF = ICF – water does not move into or out of cell causing no change in the cell volume no change in intracellular hydrostatic pressure Hypertonic (more than) –solute concentration in ECF > ICF –water diffuses out of the cell causing the cell to shrink (crenate) intracellular hydrostatic pressure decreases Hypotonic (less than) –solute concentration in ECF < ICF –water diffuses into the cell causing the cell to swell intracellular hydrostatic pressure increases

29. Cell in a Hypotonic Solution. Cell swells Increases hydrostatic pressure

30. Osmosis and cell volume changes

31. Primary Active Transport Integral membrane pumps use the energy stored in a molecule of ATP to transport substances across a membrane UP a concentration gradient –The pump: hydrolyzes the high energy bond in a molecule of ATP (releasing energy) uses the energy of ATP hydrolysis to “flip-flop” between conformations while moving substances UP a concentration gradient This process converts chemical energy (ATP) to mechanical energy (gradient across the membrane) –the gradient can be used if necessary by the cell as a form of energy to do work

32. Examples of Primary Active Transport Na +,K + -ATPase (Na +,K + pump) –located in the plasma membrane –actively cotransports: 3 Na + from the ICF (lesser amount) to the ECF (greater amount) 2 K + from the ECF (lesser amount) to the ICF (greater amount) –creates/maintains a Na + gradient across the cell membrane –creates/maintains a K + gradient across the cell membrane

33. Sodium, Potassium ATPase

35. Secondary Active Transport Integral membrane carriers move at least 2 different substances across a membrane –One substance moves across a membrane UP a concentration gradient –One substance moves across a membrane DOWN a concentration gradient –The movements of the substance DOWN a concentration gradient releases energy which the carrier uses to “flip- flop” between conformations and move a second substance UP a concentration gradient “piggyback” transport This type of transport is called secondary because this process is driven by a gradient that is created by a previous occurring primary active transport process

36. Example of Secondary Active Transport Na +, glucose cotransporter –located in the plasma membrane –cotransports: Na + DOWN its concentration gradient –from the ECF (greater amount) into the ICF (lesser amount) gradient created by the Na +,K + -ATPase –releasing energy that is used by the cotransporter to transport: glucose UP its concentration gradient –from the ECF (lesser amount) into the ICF (greater amount)

37. Example of Secondary Active Transport Na +, glucose cotransporter –located in the plasma membrane –cotransports: glucose UP its concentration gradient from the ECF into the ICF driven by the energy released from the movement of: Na + DOWN its concentration gradient (created by the Na +,K + -ATPase) from the ECF into the ICF

38. Na +, Glucose Cotransporter

40. Membrane Potential Although the total solute concentration in the ICF and ECF are equal, there is an uneven distribution of charged substances across the cell membrane of every cell in the body –creates an electrical potential (energy) between the ICF and ECF –measured as a voltage in millivolts (mV) –causes the cell membrane to be polarized a measurable charge difference between the ICF and ECF The ICF is negatively charged compared to the ECF –a typical membrane potential is –70 mV In an UNSTIMULATED (resting) cell this potential remains constant and is referred to as the resting membrane potential (RMP)

41. Resting Membrane Potential

42. Basis of the Resting Membrane Potential Due to the permeability characteristics of the plasma membrane to charged (polar) substances –permeability is the ease in which one substance can move through another substance Permeable charged substances –K + –Na +

43. Basis of the Resting Membrane Potential In a resting cell, Na + and K + are constantly pumped across the cell membrane by the Na +,K + -ATPase maintaining: –a high Na + concentration in the ECF –a low Na + concentration in the ICF –a high K + concentration in the ICF –a low K + concentration in the ECF

44. Basis of the Resting Membrane Potential

47. Diffusion of Na + and K + There is a constant diffusion of Na + into the cell by: Na + channels that are always open (leaky) There is a constant diffusion of K + out of the cell by: open K + channels that are always open (leaky) The permeability of the cell membrane in a resting cell to potassium is approximately 40 times greater than the permeability to sodium –due to a much larger number of potassium leak channels compared to sodium leak channels When a cell is at rest, the pumping of the Na +,K + -ATPase, exactly equals the diffusion of Na + and K + –results in a steady state condition

49. Contribution of Na + to the RMP If the cell membrane were permeable only to sodium then sodium would diffuse into the cell –as sodium diffuses into the cell it causes the inside of the cell to become positively charged (it only takes a few ions because each ion has a large charge) which begins to reduce additional sodium ion entry (due to repulsion) Sodium diffusion stops when the inside of the cell has 58 more mV of charge compared to outside (membrane potential = +58 mV) –at this potential, the concentration gradient moving Na + into the cell exactly balances the positive electric charge repelling Na + out of the cell –Equilibrium potential for Na + (E Na )

50. Contribution of K + to the RMP If the cell membrane were permeable only to potassium then potassium would diffuse out of the cell –as potassium diffuses out of the cell it causes the inside of the cell to become negatively charged (it only takes a few ions because each ion has a large charge) which begins to reduce additional potassium ion exit (due to attraction) Potassium diffusion stops when the inside of the cell has 90 less mV of charge compared to outside (membrane potential = -90 mV) –at this potential, the concentration gradient moving K + out of the cell exactly balances the negative electric charge attracting K + into the cell –Equilibrium potential for K + (E K )

51. RMP Note that the RMP is neither equal to E Na or E K, but is somewhere between these 2 values If the permeability of these 2 ions through the cell membrane were exactly the same, the RMP would be exactly between the values of E Na and E K, or -16 mV. However, the permeability of the cell membrane to potassium is approximately 40 times greater than that of sodium due to a much greater number of potassium leak channels. This causes potassium to have a much greater influence on the RMP compared to sodium, which is why at -70 mV the RMP is closer to -90 mV than +58 mV.