Membrane Transport

1. Membrane Transport ECB Ch 12 (the passage of small water-soluble molecules through the cell membrane)

2. The interior of the lipid bilayer is hydrophobic, the plasma membrane tends to block the passage of almost all water-soluble molecules. • The membrane transport proteins: span the membrane, providing private passageways across the bilayer for select substance (small water-soluble molecules). 12_01_transport_prot.jpg • each type of transport protein in a cell membrane transfers a particular type of molecule, causing a selective set of solutes to end up inside the membrane-enclosed compartment.

3. The ion concentrations inside a cell are very different from those outside • Ions are crucial for a cell’s survival and function. • Ions in a cell’s environment and their movements across cell membranes play an essential part in many biological processes.

4. simple diffusion 12_02_diffusion_rate.jpg (nonpolar) (size, solubility properties) • The interior of the lipid bilayer is hydrophobic, the plasma membrane tends to block the passage of almost all water-soluble molecules. • Lipid bilayers are impermeable to solutes and ions.

5. Two main classes of membrane transport proteins • carrier proteins • which have moving parts, can shift small molecules from one side of the membrane to the other by changing their shape • transported solute: small organic molecules or inorganic ions • channel proteins • from tiny hydrophilic pores in the membrane through which solutes can pass by diffusion • most channel proteins let through inorganic ions only (therefore called ion channels) The membrane transport proteins that have been studied in detail shows the structure of multipass transmembrane proteins.

6. 12_03_carrier_channel.jpg A carrier protein allow passage only to solute molecules that fit into a binding site on the protein. A channel protein discriminates mainly on the basis of size and electric charge. • Channel proteins transport molecules at a much greater rate than carrier proteins. • Channel opening and closing is usually controlled by an external stimulus or by conditions within the cell.

7. 12_04_pass_act_transport.jpg (facilitate diffusion) (All channel proteins and many carrier proteins act such a way) (carrier proteins only)

8. Carrier proteins are required for transport of almost all small organic molecules across cell membranes. • Each cell membrane has its own characteristic set of carrier proteins. 12_05_carrier_proteins.jpg

9. To understand fully how a carrier protein transfers solutes across a membrane, we would need to know it’s 3-D structure in detail. • bacteriorhodopsin: a light-activated H+ pump • Ca2+ pump: moves Ca2+ from the cytosol into the sarcoplasmic reticulum sarcoplasmic reticulum: a specialized form of endoplasmic reticulum found in skeletal muscle cells

10. bacteriorhodopsin 11_28_Bacteriorhodop.jpg seven α helices polar a.a. side chain • generating a [H+]gradient (energy store) • driving ATP synthase (a membrane protein) generating ATP • retinal: • a light-absorbing non-protein molecule • deep purple color

11. recover from the contraction 12_06_Ca_pump.jpg stimulate cell contracts Ca2+ Ca2+ lead through the protein, allowing the ion to avoid contact with the lipid bilayer

12. passive transport of a carrier protein -by concentration gradient 12_07_conforma_change.jpg conformational change ex: glucose carrier Highly selective: bind only D-glucose

13. 12_08_electroch_gradient .jpg ex: Na+ ex: K+: The cytoplasmic side of the plasma membrane is usually at a negative potential. The [K+] is higher inside cells than out side. There is little net movement of K+ across the membrane.

14. 12_09_active_transport.jpg Cells carry out active transport in three main ways:

15. The Na+-K+ ATPase (The Na+-K+ pump) 12_10_Na_K_pump.jpg

16. 12_11_high_dam.jpg

17. phosphorylation 12_12_Na_K_cyclic.jpg (takes ~10 millisecond) dephosphorylation The Na+-K+ pump transports ions in a cyclic manner.

18. 12_13_Carrier_proteins.jpg the same direction opposite directions ex: glucose-Na+ symports, Na+-H+ exchanger: a Na+-driven antiport

19. glucose-Na+ symport 12_14_symport.jpg • using the electrochemical Na+ gradient to drive the import of glacose • binding of Na+ inducing a conformational change in the protein that greatly increasing the protein’s affinity for glucose

20. 12_15_glucose_gut.jpg uniport The two types of glucose carriers are kept segregated in their proper domains of the plasma membrane by a diffusion barrier formed by a tight junction.

21. Other Na+-transporters • Cells in the lining of the gut and in many other organs contain a variety of symports in their plasma membrane that are similarly driven by the electrochemical gradient of Na+ each of themspecifically imports sugars, amino acids, …… into the cell. • Na+-driven antiports are also important, such as Na+-H+ exchanger in the plasma membranes of many animal cells, is one of the main devices that cells use to control their pH in their cytosol.

22. osmosis • osmotic pressure • the Na+-K+ pump helps maintain the osmotic balance of animal cells

23. 12_16_osmosis.jpg

24. 12_17_osmotic_swelling.jpg Na+-K+ pump cell wall contractile vacuoles turgor pressure

25. Ca2+ pump • drive by ATP (an ATPase) • maintain the low concentration of Ca2+ in the cytosol • Ca2+ is often used as a signal to trigger other intracellular events • the lower the background concentration of free Ca2+ the more sensitive the cell is to an increase in cytosolic Ca2+ • the eucaryotic cells in general matain very low concentrations of free Ca2+ in their cytosol (~10-4 mM) in the face of very much higher extracellular Ca2+ concentrations (~1-2 mM) • it is archived mainly by means of Ca2+ pump in both the plasma membrane and the ER membrane, which actively pump Ca2+ out of the cytosol

26. ATP-driven pumps (Na+-K+ pumps, Ca2+ pumps,……) have similar amino acid sequences and structures • with about 10membrane-spanning α helices in each subunit • it is likely that they have a common evolutionary origion

27. The H+ pumps • Plant cells, fungi (including yeasts), and bacteria do not have Na+-K+ pumps in their plasma membrane (instead of an electrochemical gradient of Na+) • They rely on the electrochemical gradient of H+ to drive the transport of solute into the cell • The H+ gradient is created by H+ pumps in the plasma membrane • The H+ pump also creates an acid pH in the medium surrounding the cell • In some photosynthetic bacteria the H+ gradient is created by the light-driven H+ pumps such as bacteriorhodopsin • Plant, fungi and many other bacteria, their H+ pumps are driven by ATP (as the ATPase)

28. A different type of H+ ATPase is found in in the membranes of some intracellular organells such as lysomes (animal cells) and central vacuole (plant and fungal cells). • Their function is to pump H+ out of the cytosol into the organelle, helping to keep the pH of the cytosol neutral and the pH of the interior of organelle acidic.

29. 12_18_solute_transport.jpg

31. Channel proteins • A few channel proteins form relatively large pores • gap junctions (channels between two adjacent cells) • porins (channels in the outer membrane of mitochondria and some bacteria)

32. 21_28_Gap_junctions.jpg The gap junctions

33. 11_25_Porin.proteins.jpg The porin protein

34. Ion channels • Almost all of the channel proteins in the plasma membrane of animal and plant cella are ion channels. • narrow, highly selective pores concerned exclusively with the transport of inorganic ions, mainly Na+, K+, Cl-, Ca2+ • Distinguish from simple aqueous pore: • ion selectivity • depends on the diameter, shape of the ion channel and on the distribution of charged a. a. in its lining • gated (not continuously open) • Advantages over carrier proteins • With a transport rate (more than a million ions can pass through one channel each second) 1000 times greater than the fast known carrier protein

35. K+ channel 12_19_selectivity_filter.jpg bear a partial negative charge and form transient binding site for the K+ that have shed their watery shells

36. 12_20_ion channel.jpg Most ion channels are not continuously open, they are gated.

37. membrane potential: the voltage across the membrane • When a ion channel opens, ions rush through it that changes the membrane potential, thus forcing other membrane potential sensitive ion channels to open or close in a matter of milliseconds. • The resulting flurry of electrical activity can spread rapidly from one region of the cell membrane to another conveying an electrical signal.

38. 12_21_Venus _flytrap.jpg Membrane potential: the voltage across the membrane The membrane potential is the basis of all electrical activity in cells

39. Patch-clamp recording 12_22_patch_clamp_record .jpg

40. Measurement of the current through a single ion channel of muscle by the patch-clamp technique 12_23_current _measured .jpg When open, fully open; when closed, fully closed

41. Ion channels differ from one another primarily with respect to their • ion selectivity: the types of ions they allow to pass • gating: the conditions that influence their opening and closing • voltage-gated channel (nerve cell and others) • ligand-gated channel • stress-activated channel (ex: auditory hair cells)

42. 12_24_Gated _ion_chan .jpg

43. stress-activated ion channel of auditory hair cells 12_25_hair.cells.jpg

44. 12_26_mimosa.jpg • voltage-gated ion channels underlie the leaf-closing response • voltage-gated ion channels have specialized charged protein domains called voltage sensors

45. 12_27_membr.potential.jpg

46. membrane potential • resting membrane potential • the membrane potential in a steady-state conditions, in which the flow of positive and negative ions across the membrane is precisely balanced • the resting membrane potential in animal cells: -20~-200 mv (interior of the cell is negative with respect to the exterior) • the K+ leak channels plays a major role in generating the membrane potential across the plasma membrane

47. 12_28_K_leak_chan.jpg

48. 12_29_Nernst_equation .jpg

49. 12_30_neuron .jpg (to receive signals) (to conduct signals) nerve cell (neuron): to receive, conduct and transmit signals

50. action potential (nerve impulse): (p412) • action potentials are usually mediated by voltage-gated Na+ channels • an action potential in a neuron is typically triggered by a sudden local depolarization of plasma membrane (membrane potential shift to a less negative value) caused by neurotransmitters of another neurons