Membrane transport 1. Summary

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2 Membrane transport 1. Summary A. Simple diffusion 1) Diffusion by electrochemical gradient no energy required 2) No channel or carrier (or transporter protein) is needed B. Passive transport (= Facilitated diffusion) 1) Transports solutes by electrochemical gradient (concentration gradient + electrical gradient) no energy required 2) Through channel or carrier protein C. Active transport 1) Transport solutes to cross membrane against their electrochemical gradient. Need ATP hydrolysis or an ion gradient 2) Through carrier protein D. Endocytosis & exocytosis : transport by memb-bounded vesicle (endosome)

3 2. Permeability of lipid bilayer by simple diffusion A. By electrochemical gradient : conc. + electrical gradient B. Diffusion rate - the size of molecule rate - H-bonds rate - solubility in oil rate, but exception: H 2 O (h-philic) highest rate very small

4 C. Osmosis & semipermeable memb.: Undiffusable molecule to semipermeable membrane affect H 2 O D. Osmotic pressure: strength of counting to H 2 O movement across semipermiable memb. Role: 1) intake of H 2 O by all cell. ex) - plant root intake water stem & leave (thru softer tissue) - To keep from bursting, wall-less organism pump out of H 2 O 2) balancing H 2 O in animal cell by in and out.

5 3. Channel proteins A. Need not bind the solute. Instead they form hydrophilic pores that extend across the lipid bilayer B. Transport solutes by passive transport only C. What kinds? 1) Aquaporin: water channel (through which water molecules cross the memb much more rapidly than they can diffuse through phospholipid bilayer) Structure of aquaporin

6 2) Ion channels a) Characteristics - transport is extremely rapid (1,000X faster than carrier-mediated transport) - not permanently open. The opening is regulated by gate - highly selective - K + cannot pass through Na + channel, vice versa. b) What kinds? - Voltage-gated channel (e.g., change of electric potential along the neuron) - Mechanically gated channel - Ligand-gated channel (e.g., neurotransmitter) Model of an ion channel

7 4. Carrier protein: in facilitated diffusion, carrier protein does not need energy for functioning - Bind specific solutes and change conformation to transfer the solute across the membrane - Transport solutes by either passive transport (or facilitated diffusion) or active transport - Uniport, symport, antiport

8 What kinds? a) Na + -K + pump (Na + -K + ATPase) (i) Anitiport (ii) Drives 3 Na + out, 2 K + in by hydrolysis of one ATP molecule (iii) Ouabain is inhibitory to the Na + - K + pump by competition for K + binding site (iv) Consumes ~25% of the ATP of the cell (v) Maintain osmotic balance and cell volume high [Na + & Cl - ] outside cell counterbalance the osmosis

9 b) Ca 2+ ATPase (i) Keep intracellular [Ca 2+ ] low makes the cell sensitive to small increase in intracellular Ca 2+ level (i) In the plasma membrane (ii) In the sarcoplasmic reticulum of muscle cells Stimulus release of Ca 2+ from sarcoplasmic reticulum into the cytosol muscle contraction Ca 2+ ATPase pump Ca 2+ from the cytosol into the sarcoplasmic reticulum Structural of the Ca 2+ pump

10 c) Na + -glucose symport : in epitherial cell of intestine Driving force of glucose uptake is concentration gradient of the Na+. This transport is referred to as secondary active transport. Glucose is moving against concentration gradient Low [Na + ] of the intestinal cell is maintained by this Na + -K + pump. Transport need ATP is called primary active transport. Glucose is moving from higher concentration to lower concentration.

11 d) H + ATPase - Pump H + into the organelle from the cytosol at the expense of ATP hydrolysis - Responsible for low ph in the lysosome. e) Na + -H + exchanger - Antiport - Couples an influx of Na + (generated by Na + -K + pump) to an efflux of H + - Prevents acidification of the cytosol f) Na + -Ca 2+ exchanger - Ca 2+ is exported from the cell not only by the Ca 2+ ATPase - Couples an influx of Na + (generated by Na + -K + pump) to an efflux of Ca 2+

12 g) ABC transporter superfamily (i) The tranporter protein has ATPase activity and pump in or out particular substances (ii) Over 100 ABC transporters have been described (iii) Each member contains highly conserved ATP-binding cassette (iv) Multidrug resistance (MDR) protein (or transporter) expressed in cancer cells - pump cancer cell-killing drug out of cell one of the biggest problems in cancer therapy Expressed in the brain - Protects brain from toxic chemicals CFTR (cystic fibrosis transmembrane conductance regulator) - Cl - channel of the epithelial cell - Defective cystic fibrosis Structure of an ABC transporter

13 Review Membrane transport - Simple diffusion - Passive transport - Active transport Preview Membrane transport - Membrane electrical potential - Endocytosis Wake up!