Understanding Membrane Transport Processes

1. Membrane Transport

2. Membrane Transport • method by which substances move through the cell membrane • cells bathed by interstitial fluid • cells must take nutrients from this fluid to remain healthy • reject unnecessary substances

3. Selective Permeability • the ability of the plasma membrane to regulate entry and exit of materials • Movement of materials through plasma membrane can be: - passive - active

4. Passive Processes • simple diffusion • osmosis • filtration • facilitated diffusion

5. Passive Processes - Simple Diffusion • random movement of particles (ions and molecules) • particles move along a concentration gradient - a region of higher concentration to a region of lower concentration.

6. Passive Processes - Osmosis • diffusion of solvent -water diffuses from a region of higher solvent concentration toward a region of lower solvent concentration through a selectively permeable membrane • Example: - the small intestines absorb water from digested food by osmosis

7. Osmotic Pressure • the pressure required to prevent the net movement of water from one solution to another when the solutions are separated by a membrane permeable only to water • the pressure needed to stop the flow of water across the membrane

8. Tonicity • normal firmness or functional readiness in body tissues or organs • maintain the shape of red blood cells; they must be bathed in an isotonic solution. - normal saline solution is isotonic for red blood cells

9. Isotonic Solution(iso = same) • same concentration of water and particles (solute and solutes) as in the cells • when RBC’s are in 0.9% NaCl solution water molecules enter and exit the cells at equal rates (5% glucose) • RBC’s maintain their normal shape and volume

10. Hypotonic Solution (hypo = less than) • lower concentration of solutes than in cells • higher concentration of water • when RBC’s are placed in distilled H2O, water molecules enter the cell faster than they leave • RBC’s will swell and burst (hemolysis)

11. Hypertonic Solution(hyper = greater than) • higher concentration of solutes • lower concentration of water than in the cells • if RBC’s are placed in hypertonic solution more water leaves the cells than enters - cells will shrivel and die (crenation)

12. Passive Processes - Filtration • solvent and solute move across a membrane due to gravity or water pressure (hydrostatic) • Example: - in kidneys blood pressure forces water and small creatinine (a waste product) molecules through a plasma membrane

13. Filtration (cont.) • filtered liquid enters kidneys • large molecules such as proteins remain in blood • smaller harmful molecules can then be eliminated in the urine

14. Passive Processes -Facilitated Diffusion • lipid-insoluble molecules and molecules too large to diffuse through membrane or ion channels are carried through membrane by transporter (carrier) proteins • Example: - glucose

15. Facilitated Diffusion (cont.) • after glucose bonds to transporter on the outside of the membrane, the transporter changes shape • glucose passes through membrane and is released inside the cell

16. Active Transport • requires the use of energy from ATP to move substances across the membrane • substances move from areas of lesser concentration to areas of greater concentration • opposite direction of diffusion • movement occurs against a concentration gradient

17. Na+ (Sodium) K+ (Potassium) H+ (Hydrogen) Ca + 2 (Calcium) I- (Iodine) Cl- (Chlorine) amino acids monosaccharides Actively Transported Substances:

18. Active Transport (cont.) • Two Types: - primary active transport - secondary active transport

19. Primary Active Transport: The Sodium Pump • ATP energy directly moves substances across membrane • sodium pump - most prevalent - maintains a low concentration of (Na+) ions in the cytosol - also moves (K+) ions into cells • in nerve and muscle cells

20. The Sodium Pump • Na+ ions are more abundant outside the cell • they are constantly diffusing into the cell, (their area of lesser concentration) • not needed in such abundance • sodium pumps must return Na+ ions to outside (their presence will bring about unwanted nerve impulses or muscle contractions)

21. Secondary Active Transport • energy stored in ion gradients moves substances across the membrane • indirectly uses energy from splitting ATP

22. Endocytosis • Processes that bring large substances into cells: - phagocytosis - pinocytosis - receptor-mediated endocytosis

23. Phagocytosis • projections on outside of plasma membrane, called pseudopods, flow around large particles (bacteria, debris) engulfing and bringing them into cell • once surrounded, membrane sac is formed (phagocytic vesicle) • vesicle enters cytoplasm • material in vesicle is digested by enzyme produced by lysosome

24. Pinocytosis • membrane folds or sinks beneath liquid to be digested forming pinocytic vesicle • pinocytic vesicle allows liquid to flow inward trapping the liquid • vesicle pinches off or detaches from membrane into cell interior

25. Receptor-Mediated Endocytosis • similar process to pinocytosis • substances bind to receptors on the plasma membrane causing a folding of the membrane and the formation of an endocytic vesicle • highly selective • cells take up specific molecules or particles

26. Exocytosis • membrane enclosed sacs (secretory vesicles) form inside cell - fuse with plasma membrane - release contents into extracellular fluid - important in nerve cells - secrete digestive enzymes or protein hormones such as insulin • requires ATP