Solutions

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 INTRACELLULAR EXTRACELLULAR Major cations: Na + + K ++ Mg Major anions: Cl - - HCO 3 Protein Phosphates -- SO 4 Org. acids 150 100 50 0 0 50 100 150 Concentration (mM) Concentration (mM)

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, H2O, 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 (“falsefeet”)

6. Exocytosis of Proteins

7. Exocytosis

8. Types of Endocytosis • Phagocytosis • “celleating” • endocytosis of few very large substances (bacteria, viruses, cell fragments) • vesicles containing cells fuse with lysosomes which digest the cells • Pinocytosis • “cellsipping” • 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 primaryactive 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 transportpolarsubstances 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 “waterchannels” 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 (lessthan) • 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 mechanicalenergy (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 • activelycotransports: • 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

39. Na+, Glucose Cotransporter

40. Membrane Potential • Although the total solute concentration in the ICF and ECF are equal, there is an unevendistribution 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 highNa+concentration in the ECF • a lowNa+concentration in the ICF • a highK+concentration in the ICF • a lowK+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+ (ENa)

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+ (EK)