Chapter 5 Membrane Structure and Transport PDF

Membrane Structu re and transport  Learn i ng Objectives: 1. Describe cell membrane as a fluid mosaic model 2. Describe passive d i ffusion and osmosis 3. Describe faci litated transport 4. Discuss how energy-req ui ri ng transport protei ns move substances across the cell membrane 5. Dist...

Membrane Structu re and transport  Learn i ng Objectives: 1. Describe cell membrane as a fluid mosaic model 2. Describe passive d i ffusion and osmosis 3. Describe faci litated transport 4. Discuss how energy-req ui ri ng transport protei ns move substances across the cell membrane 5. Disti nguish between exocytosis and endocytosis and list si m i larities between the two  Plasma Membrane Boundary that separates the livi ng cell from it’s non-livi ng surround i ngs. Phospholi pid bi layer A m ph i path ic - havi ng both: hyd roph i lic heads hyd rophobic tai ls ~8 n m th ick Phospholi pid Is a dy nam ic structure  Phospholi pids Contai n Fou r Structu ral Elements Glycerol 2 Fatty acids Head g rou p Figure 04.01: Phospholipid structure. Flu id-Mosaic Model  Membrane Transport Protei ns Many molecu les m ust move back and forth from inside and outside of the cell Most can not pass through without the assistance of proteins in the membrane bi layer – Private passageways for select substances Each cell has membrane has a specific set of proteins depending on the cell Movement of Small Molecu les  Ion Concentrations The maintenance of solutes on both sides of the membrane is critical to the cell – Helps to keep the cell from ru pturing Concentration of ions on either side varies widely – Na+ and Cl- are higher outside the cell – K+ is higher i nside the cell – M ust balance the the n umber of positive and negative charges, both i nside and outside cell Im permeable Membranes Ions and hydrophi lic molecu les can not easi ly pass thru the hydrophobic membrane Small and hydrophobic molecu les can Must know the list to the left  2 Major Classes Carrier proteins – move the solute across the membrane by binding it on one side and transporting it to the other side – Req uires a conformation change Chan nel protein – small hydrophi lic pores that allow for solutes to pass through – Use diffusion to move across – A lso called ion chan nels when on ly ions moving Protei ns  Carrier vs Chan nel Chan nels, if open, wi ll let solutes pass if they have the right size and charge – Trapdoor-li ke Carriers req uire that the solute fit in the binding site – Turnsti le-li ke – Why carriers are specific li ke an enzy me and its substrate  Mechan isms of Transport Provided that there is a pathway, molecu les move from a higher to lower concentration – Doesn’t requi re energy – Passive transport or faci litated d i ffusion Movement against a concentration g radient req uires energy (low to high) – Active transport – Requi res the harnessi ng of some energy source by the carrier protei n Special ty pes of carriers Passive vs Active Transport  Carrier Protei ns Req uired for almost all small organ ic molecu les – Exception – fat-soluble molecules and small uncharged molecules that can pass by si m ple d i ffusion Usually on ly carry one ty pe of molecu le Carriers can also be in other membranes of the cell such as the m itochondria Carriers i n the Cell  Passive Transport by Glucose Carrier Glucose carrier consists of a protein chain that crosses the membrane about 12 times and has at least 2 conformations – switch back and forth One conformation exposes the binding site to the outside of the cell and the other to the inside of the cell  How it Works Glucose is high outside the cell so the conformation is open to take in g lucose and move it to the cytosol where the concentration is low When g lucose levels are low in the blood, g lucagon (hormone) triggers the breakdown of g lycogen (e.g., from the liver), g lucose levels are high in the cell and then the conformation moves the g lucose out of the cell to the blood stream Glucose moves according to the concentration g radient across the membrane Can move on ly D-g lucose, not m irror image L-g lucose  Active Transport 3 main methods to move solutes against an electrochem ical g radient – Cou pled transporters – 1 goes dow n gradient and 1 goes u p the gradient – ATP-d riven pu mps – cou pled to ATP hyd rolysi s – Light-d riven pu mps – uses light as energy, bacteri orhodopsi n  Exocytosis and endocytosis transport large molecu les across membranes ( active transport)  A cell moves large molecules across membranes by – Exocytosis to export bulky molecules – Endocytosis to i m port useful substances  In both cases, material to be transported is packaged w ith in a vesicle that fuses w ith the membrane 20 Source: http://sli dep layer.com/sli de/4623234/ 21  Osmosis The movement of water from region of low solute concentration (high water concentration) to an area of high solute concentration (low water concentration) Driving force is the osmotic pressure caused by the difference in water pressure  Defi n itions Solution – m ixture of dissolved molecu les in a liq uid Solute – the substance that is dissolved Solvent – the liq uid  Osmotic Solutions – Ton icity (tonos = tension) Isotonic – eq ual solute on each side of the membrane Hy potonic – less solute outside cell, water rushes i nto cell and cell bu rsts Hy pertonic – more solute outside cell, water rushes out of cell and cell sh rivels  Osmotic Swelli ng A ni mal cells mai ntai n normal cell structu re with Na+-K+ pu m p (moves out Na+ and prevents Cl- from movi ng i n) Plants have cell walls – tu rgor pressu re is the effect of osmosis and active transport of ions i nto the cell – keeps leaves and stems u pright Protozoans have special water collecti ng vacuoles to remove excess water Hu man Red B lood Cells or Erythrocytes  Ton icity in Action A n i sotonic soluti on has an eq ual amou nt of di ssolved solute i n it compared to the th i ngs arou nd it. Ty pically i n h u mans and most other mammals, the i sotonic soluti on i s 0.9 weight percent (9 g/L) salt i n aq ueous soluti on, th i s i s also know n as sali ne, w h ich i s generally ad mi ni stered via an i ntra-venous d ri p. Red blood cells normally exi st i n a 0.9 percent salt soluti on (sali ne) w ith the same concentrati on of salt i n the outside soluti on. Sou rce: http://en.wikipedia.org/wiki/Isotonic.  Chan nel Protei ns Chan nel proteins create a hydrophi lic open ing in which small water-soluble molecu les can pass into or out of the cell – Gap junctions and pori ns make very large open i ngs Ion chan nels are very specific with regards to pore size and the charge on the molecu le to be moved – Move main ly Na, K, Cl and Ca  Ion Chan nels Have ion selectivity – allows some ions to pass and restricts others – Based on pore size and the charges on the i n ner ‘wall’ of the chan nel Ion chan nels are not always open – Have the abi lity to regulate the movement of ions so that control can mai ntai n the ion concentrations withi n the cell – Chan nels are gated – open or closed Speci fic sti muli triggers the change i n shape and open i ng or closi ng of chan nel Ion Chan nels Chan nels A re Either Open or Closed  Membrane Potential Basis of all electrical activity in cells Active transport can keep ion concentration far from eq ui librium in the cell Chan nels open and the ions rush in because of the g radient difference – changes the voltage across the membrane – As voltage changes, other ion chan nels open and other ions rush in A llows for the electrical activity to move across the membrane  Variety of Chan nels Ion chan nels vary with respect to – Ion selectivity – which ions can go thru – Gating – conditions that influence open ing and closing  Membrane Ion Chan nels Ty pes of plasma membrane ion chan nels Passive, or leakage, chan nels – always open Chem ically (or ligand)-gated chan nels – open with bi ndi ng of a speci fic neu rotransm itter (the ligand) Voltage-gated chan nels – open and close i n response to changes i n the membrane potential Mechan ically-gated chan nels – open and close i n response to physical deformation of receptors  Membrane flu idity is sensitive to ? 1. Ty pe of phospholi pids present ; Unsaturated fatty acids –more fluid; double bond causes kin ks Stacks poorly Shorter chains – stack poorly; More movement Length & saturation of hyd rocarbon tai ls affect packi ng & membrane fluidity  Fluid Viscous Unsatu rated hydroca rbon Satu rated hydro- ta i ls w i th ki nks Ca rbon ta i ls (b) Membra ne flu idi ty 2. Cholesterol; Cholesterol is com mon in animal cells Com pose entirely of carbon and hyd rogen and so very hyd rophobic  h igh concentration of it makes the membrane more rigid Low concentration of it makes the membrane more in motion and so more fluid cholesterol – At high tem perature has a loosen ing effect – At low tem perature has a stiffen ing effect Cholesterol (c) Cholesterol w i thi n the a ni ma l cell membra ne 3. Tem perature and A tmospheric pressure Raise in tem perature increase motion and so increasing fluidity Significant change in pressure w i ll h inder(make it difficult for) diffusion and decrease fluidity Membrane Com ponents Can Form Large Molecu lar Com plexes with Little/No Mobi lity Cell–cell junctions Cell–matrix junctions Figure 04.09: Plasma membrane proteins form clusters that permit cell ad hesion.