Lec-4 Membrane Transport 2 Lecturer: Dr. Twana A. Mustafa

OSMOSIS

Definition: The diffusion of water down its concentration gradient (that is, an area of higher water concentration to an area of lower water concentration) thru a semi-permeable membrane is called Osmosis. Concept: Because solutions are always referred to in terms of concentration of solute, water moves by osmosis to the area of higher solute concentration. Despite the impression that the solutes are “pulling,” or attracting, water, osmosis is nothing more than diffusion of water down its own concentration gradient across the membrane.

Osmotic pressure: is the pressure that is required to stop osmosis. It is the pressure necessary to prevent osmosis into a given solution when the solution is separated from the pure solvent by a semipermeable membrane. The greater the solute conc. of a solution, the greater its osmotic pressure. (HYDROSTATIC PRESSURE = OSMOTIC PRESSURE) An osmole is one mole of dissolved particles in a solution. E.g. glucose when dissolved in solution does not dissociate, so 1 mole of glucose is also 1 osmole of glucose. On the other hand, NaCl dissociates into 2 ions (Na and Cl) so is taken as 2 moles. Osmolarity is the number of osmoles of solute per liter of solution. Simply put, osmolarity is a measure of total solute conc. given in terms of number of particles of the solute in 1 liter of solution. The osmolarity of body fluids is usually expressed in milliosmoles per liter (mOsm/L). (The normal osmolarity of body fluid is 300 mOsm.) It is usually employed in clinical settings. Osmolality is the number of milliosmoles of solute per kg of solvent. It is usually calculated in laboratories using an osmometer.

Hyper = above tonic refers to the Iso = same shape of the cell Hypo = below

8 Isotonic Solution NO NET MOVEMENT OF H 2 O (equal amounts entering & leaving) Hypotonic Solution CYTOLYSIS Hypertonic Solution PLASMOLYSIS copyright cmassengale

Active transport Primary active transport ： the transporter itself is an ATPase that cause the breakdown of ATP and phosphorylate itself. Therefore, change the affinity of the transporters solute binding site. Secondary active transport ： use of an ion concentration gradient across a membrane as the energy source. The flow of the ion provides energy for the uphill movement of the actively transported solutes.

10 Na + /K + Pump

Ca-ATPase In the plasma membrane, the active transport of Ca is from cytosol into extracellular fluid. In the organelle membranes, the active transport of Ca is from cytosol into organelle. The extracellular [Ca] is 1 mM, while the intracellular [Ca] is 0.1 uM.

H + Pumps

Figure 4-15 Diverse examples of carrier-mediated transport.

Secondary Active Transport  Coupled 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.

Secondary active transport uses the energy in an ion gradient to move a second solute. Figure 4-13

Cotransport: the ion and the second solute cross the membrane in the same direction. Countertransport: the ion and the second solute move in opposite directions. Figure 4-14

Secondary active transport Na + glucose Na + H+H+ 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. H +, in the opposite direction to the Na +.

2 Methods of Glucose Transport 2 mechanisms are separate –Passive transport at the basal surface –Active transport at the apical surface Caused by the tight junctions

Na + -Driven Transport Na + driven symport –Used to move other sugars and amino acids Na + driven antiport –Also very important in cells –Na + -H + exchanger is used to move Na + into the cell and then moves the H + out of the cell Regulates the pH of the cytosol

Figure 4-22

Figure 4-24

24 4. 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.

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

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

27 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)

28 Example of Receptor-Mediated Endocytosis in human cells

Alternative functions of endocytosis: 1.Transcellular transport 2.Endosomal processing 3.Recycling the membrane 4.Destroying engulfed materials Figure 4-21