Transport across cell membrane .pptx

physiology of fluids

1. Transport across cell membrane.

2. Pathways of transport can be passive or active. Small,non polar molecules(o2)and small uncharged polar molecules(co2) diffuse across the lipid membranes(passive simple diffusion) Membranes have limited permeability.other methods are endocytosis and exocytosis or by passage through highly specific transport proteins ion channels,ATP powered pumps or carrier proteins. Limited permeability even to water needs water channelsaquaporins

3. Ion channels Channel proteins transport water and specific ions and hydrophilic small molecules down their conc.or electric potential gradients Protein assisted transport is at times refered to as facilitated diffusion. Channel proteins from a hydrophilic passageway across the memb.through which multiple water molecules or ions move stimultaneously,single file at a very rapid rate Some ion channels are open much of the time,these are refered to as non gated channels Most ion channels hawever open only response in response to specific chemical or electrical signals gated channels Very fast rate of transport.

4. Pumps(ATP-powered pumps) Are ATPases that use the energy of ATP hydrolysi to move ions or small molecules across the membrane against a chemical conc.gradient or electric potential active transport. Examples: NA K ATPase H K ATPase gastric mucosa/renal tubules Ca ATPase

5. Carrier proteins(transporters) Bind ions and other molecules and then change their configuration,moving the bound molecule from one side of the cell membrane to the other. 2 types of transporters: 1) Uniporters 2) cotransporters

6. Co transporters: couple the movement of one type of ion or molecule against its conc gradient(the driven substrate) with the movement of one or more different ions down its concentration gradient(the driving substrate)  Secondary active transport. Direction of transmembrane movement of the driven substrate can either be same as( symport )or opposite to( antiport )to that of the driving substrate

7. Secreted proteins,lipids and other cellular products destined for export from the cell are transported to the plasma membrane in small vesicles from the trans face of golgi This pathway is either constitutive in which transport and secretion occurs more or less continuously or its regulated by external signals eg salivary secretion and the ANS.

8. EXOCYTOSIS: achieved by fusion of the secretory vesicular membrane with the plasma membrane and release of the vesicle content into the ECF ENDOCYTOSIS:involves internalization of vesicles derived from the plasma memb .==.PINOCYTOSIS/RECEPTOR MEDIATED ENDOCYTOSIS/PHAGOCYTOSIS.

9. Intercellular communication