Collin College. BIOL (Chapter 3) Membrane Transport. Facilitated diffusion via carriers

Transcription

1 Collin College BIOL (Chapter 3) Membrane Transport. Facilitated diffusion via carriers Refers to the movement of molecules down their concentration gradient across the cell membrane by means of an integral membrane protein or transporter (carrier) This is selective and highly specific as the molecule has to recognize and bind to the carrier molecule. This kind of diffusion occurs for those molecules that are too large and /or too lipid insoluble. ( such as amino acids, carbohydrates)(where would this happen quite often?) 1

2 Facilitated diffusion Low Low High High Concentration gradient Facilitated diffusion via carriers The magnitude of flux (movement) resulting from carrier mediated transport is depending on Extent of saturation of the carriers by the ligand (molecule) The number of specific carriers in a membrane The rate at which the carrier transports the ligand from one side to the other side of the membrane 2

3 Facilitated diffusion via carriers Let s look at Ficks equation for diffusion again Flux = A. D. ( C 2 - C 1 ) Assume we are dealing with simple diffusion from outside to inside which is being measured in cells with a constant shape. Also assume the inside concentration is zero to start with. In this equation, this means that A = constant, x is constant and C 1 = 0 and C 2 represents the extracellular concentration X Facilitated diffusion via carriers In the equation, the circled parameters are now constant Flux = A. D. ( C 2 - C 1 ) X The equation then becomes much easier since C 1 =0 Flux = K. C 2 Which mathematically represent a linear line through the origin. In other words, if we are dealing with simple diffusion, where no carriers are used, the rate of diffusion is purely and linearly dependent on the (extracellular) concentration gradient! 3

4 Simple versus Facilitated diffusion Linearly dependency of simple diffusion on (extracellular) concentration gradient! In facilitated diffusion, transport is dependent on protein carriers and concentration gradient. It is now not a linear relationship because when all carriers are busy, a maximal flux is reached. We cannot transport more than the available carriers allow. Simple versus Facilitated diffusion The fact that many transported molecules become metabolized once inside the cell (ex. glucose), keeps the internal concentrations low. This assures that diffusion will not be limited by a decreasing concentration gradient. In addition, the flux of Facilitated diffusion can be increased by increasing the numbers of transporters (insulin acts to up-regulate glucose transporters). 4

5 Some Definitions If a transporter only carries one molecule or ion, it is called a uni-porter If it transport two molecules it is called a coupled transporter. If both molecules are moved in the same direction is a cotransporter or symporter. If the molecules are moved in opposite directions it is referred to as an antitransporter ACTIVE Transport Some substances need to enter the cell but cannot cross the membrane passively due to the fact that they have to move against a concentration gradient In such a case, integral proteins and energy is needed to overcome those obstacles. The energy used is cellular energy in the form of ATP. Since the transport proteins are now moving molecules uphill ( against the thermodynamically favored way), they are usually referred to as PUMPS. 5

6 Active Transport Active Transport High Low concentration These pumps exhibit similar characteristics as facilitated diffusion such as saturation, specificity, competition, 6

7 Types of Active Transport Types of Active Transport Primary Active Transport uses ATP directly as a driving force to pump ions and small molecules across the membrane. Secondary Active Transport uses the energy in an ion concentration gradient to move another molecule against a concentration gradient the energy to create the ion concentration gradient came from ATP ; thus ATP is used indirectly Vesicular Transport Large particles are ferried across the plasma membrane via membrane vesicles. Primary Active Transport Remember from our earlier discussions that concentration values for Na + and K + are different inside versus outside the cell. Na + high outside the cell compared to inside, and K + high inside compared to outside. All cells however have leakage channels for these ions This means that Na + leaks into the cell and K + leaks out through their respective channels due to respective concentration gradients The Na/K pumps re-establish and maintain these important concentration gradients by pumping against concentration gradients. Without these pumps, cells will die. It requires input of cellular ATP to drive these pumps! 7

8 Primary Active Transport Most primary active transporters are recognizable by the fact that they are called pumps or ATPases. For example, if you know that intracellular concentration of Ca 2+ is very low compared to the outside of a cell, what is the function of a Calcium pump ( aka Ca 2+ ATPase)? Some cuboidal nephron cells have a Proton pump at their apical side. What does that imply? Primary Active Transport Example : Na + / K + pump or Na + / K + ATPase Na and K leak in or out down their respective concentration gradients. The Na/K pumps maintain the concentration gradients by pumping these ions against their concentration gradients. 8

9 Secondary Active Transport Secondary Active Transport Ion concentration gradients are like potential energy reservoirs Moving ions down a concentration gradient releases energy The unequal distribution of Na + generated by the Na-K pump across the membrane can be used as an energy source. Secondary active transport does not require ATP input directly ; it uses the energy input by another system, e.g. the Na/K pump. WHY? Because these pumps create the ion gradients for Na +. Most coupled transporters that involve sodium are secondary active transport system. Secondary Active Transport The energetically favored downhill flow of sodium into the cell can now be coupled via a carrier to drag a substance inward against its concentration gradient (via a symport ) or it can be used to move a molecule out of the cell against its concentration gradient (via an antiport) 9

10 Secondary Active Transport K + Na + Na + out ATP Ca ++ in In this diagram, the Na/K pumps creates and maintains a gradient for Na +. The energy in Na + gradient can now be used to drag another molecule against its concentration gradient! ( Calcium is always high outside compared to inside). Secondary Active Transport Not shown is the Na/K pump, but is assumed to be there. EXAMPLE : Na + /glucose co-transporter Moves glucose against a concentration gradient EXAMPLE : Na + /Ca 2+ anti-transporter Moves calcium against a concentration gradient 10

11 Vesicular Transport Vesicular transport is an active transport system, moving bulk material via membraneous structures. It includes RECEPTOR MEDIATED PHAGOCYTOSIS ENDOCYTOSIS PINOCYTOSIS EXOCYTOSIS EXOCYTOSIS SNARE proteins in vesicles and plasma membrane act as docking sites. They interact and promote the fusion of the vesicle with the cell membrane 11

12 TYPES OF ENDOCYTOSIS Clathrin-coated pits provide the main route for endocytosis of bulk solids and macro -molecules! Clathrins are proteins that are important in cargo selection and deforming the membrane to produce an internal vesicle. The system is used in transcytosis for example, to move particles in on one side of the cell, and out the other side! Clathrin ENDOCYTOSIS 12

13 Clathrin ENDOCYTOSIS Clathrin-coated pits are also used in the typical phagocytosis process, where large chinks of material are internalized! Phagocytosis is used by many white blood cells called macro-phages. The vesicles fuse with lysosomes where the content is digested. Clathrin ENDOCYTOSIS In receptor mediated endocytosis, the vesicle formation is triggered by the binding of a molecule to receptors in the Clathrin-coated pits! This process is thus quite selective in that it requires binding to a receptor. The process of Pinocytosis is also referred to as cell drinking. It is not very selective and allows the cell to take in dissolved substances from the outside. 13