This document discusses transport of substances through the cell membrane. There are molecular gradients that drive diffusion across the membrane. Channel and carrier proteins selectively transport ions and molecules through simple diffusion, facilitated diffusion, or active transport. Primary active transport directly uses ATP to pump molecules against their gradients, while secondary active transport uses the gradient of another molecule like Na+ indirectly.

1. Transport of Substances Throughthe Cell Membrane

2. Molecular Gradientsinside(in mM)141402010-4(pH 7.2)10527540outside(in mM)14241-21-2(pH 7.4)28110145Na+K+Mg2+Ca2+H+HCO3-Cl-SO42-PO3-protein

3. CO2O2N2H2OureaLipid Bilayer: barrier to water and water-soluble substancesglucoseions

4. Proteins:Channel proteins allow free movement of water and selected ions

5. Carrier proteins bind with molecules or ions that are to be transported

6. Both channel proteins and the carrier proteins are highly selective in the types of moleculesChannel proteincarrier proteinsK+

7. DiffusionActive Transport occurs against a concn. gradient involves a “carrier”

8. requires ENERGY

9. occurs down a concn. gradient no mediator or involves a “channel” or “carrier” no additional energySimple diffusion can occur through the cell membrane by two pathways Through lipid bilayer if the diffusing substance is lipid solubleWater-soluble molecules cross via channels or pores(b)(a)Simple Diffusion

10. Ion ChannelsCharacteristics:ungateddetermined by size, shape, distribution of charge, etc.gated voltage (e.g. voltage-dependent Na+ channels)

11. chemically (e.g. acetylcholine receptor channels)inoutNa+ and other ionsNa+

12. Facilitated Diffusion(also called carrier mediated diffusion)Rate of diffusion is limited by Vmax of the carrier protein

13. the density of carrier proteins in the membrane

14. Simple vs. Facilitatedsimple diffusionrate of diffusionVmaxTmfacilitated diffusionConcn of substanceWhat limits maximum rate of facilitated diffusion?

15. Factors that affect the net rate of diffusion:1. Concentration difference (Co- Ci)net diffusion (Co- Ci)

16. +------- ----- -- --- -- ----- -- --- --- --------------------+--2. Electrical potential (EMF)When will the negatively charged molecules stop entering the cell?The Nernst potential (equilibrium potential) is the theoretical intracellular electrical potential that would be equal in magnitude but opposite in direction to the concentration force.EMF (mV) = ±61 log (Co / Ci)

17. 3. Pressure difference Higher pressure results in increased energy available to cause net movement from high to low pressure.Osmosis:- Net diffusion of water -Osmosis occurs from pure water toward a water/salt solution. Water moves down its concn gradient.

18. Osmotic Pressure:the amount of pressure required to counter osmosisOsmotic pressure is attributed to the osmolarity of a solution

19. Relation between osmolarity and molaritymOsm (millisomolar) = index of the concn or mOsm/L of particles per liter solutionmM (millimolar) = index of concn of or mM /L molecules per liter solution150 mM NaCl =300 mM glucose =300 mOsm300 mOsm

20. Estimating Plasma OsmolarityDominated by [Na+]and the associated anionsUnder normal conditions, ECF osmolarity can be roughly estimated as:POSM = 2.1 x [Na+]p270-290 mOSM

21. Active TransportPrimary Active Transport molecules are “pumped” against a concentration gradient at the expense of energy (ATP) – direct use of energySecondary Active Transport transport is driven by the energy stored in the concentration gradient of another molecule (Na+)– indirect use of energy

22. Primary Active Transport1. Na+/K+ ATPasecarrier proteinlocated on the plasma membrane of all cells

23. plays an important role in regulating osmotic balance by maintaining Na+ and K+balance

24. requires one to two thirds of cells energy2. Ca2+ ATPase present on the cell membrane and the sarcoplasmic reticulum maintains a low cytosolic Ca2+ concentration 3. H+ ATPase found in parietal cells of gastric glands (HCl secretion) and intercalated cells of renal tubules (controls blood pH)

25. Na+gluc2 HCO3-Na+Na+AASecondary Active Transport- co-transport and counter-transport -Co-transport (co-porters): substance is transported in the same direction as the “driver” ion (Na+)Examples:outsideinside

26. Na+/HCO3-Na+Na+Cl-/H+H+Ca2+2. Counter-transport (anti-porters):substance is transported in the opposite direction as the “driver” ion (Na+)Examples:outsideinside