Nutrient Transport across M embranes

1. Nutrient Transport across Membranes

2. Membranes as barriers Except for H2O, most polar molecules do NOT move across the lipid bilayers

3. Relative speed of nutrient movement across bacterial membranes substance permeability water glycerol Tryptophan (色氨酸) glucose Cl- K+ Na+ 100 0.1 0.001 0.001 10-6 10-7 10-8 Note that many nutrients are polar

4. Membrane transport systems are the systems to move nutrients and waste products across membranes Passive Passive diffusion Channel proteins Facilitated diffusion Uniporter transport Active Antiport Symport ABC system group translocation

5. What is diffusion? • Molecules move along a concentration gradient (from region of higher concentration to lower concentration) • Movement is driven by random thermal action - no energy output by organism • Concentration reaches equilibrium • Water, gasses, lipids, small uncharged polar molecules • Not primary mode for hydrophilic ions

6. Passive 1 Passive diffusion Examples of gases that cross membranes by passive diffusion include N2, O2, and CO2; examples of small polar molecules include ethanol, H2O, and urea.

7. Passive2. Channel proteins Channel proteins蛋白通道 transport water or certain ions down either a concentration gradient, in the case of water, or an electric potential gradient, in the case of certain ions from an area of higher concentration to lower concentration. While water molecules can directly cross the membrane by simple diffusion, as mentioned above, their transport can be enhanced by channel proteins called aquaporins(运水蛋白).

8. Passive3. facilitated diffusion What is facilitated diffusion? • Diffusion aided by a carrier protein - permease - in cell membrane • Carrier provides specificity and increases rate to equilibrium • is powered by the potential energy of a concentration gradient and does not require the expenditure of metabolic energy. • Not highly important in prokaryotes

9. Diffusion facilitator protein Nutrient molec in high conc Passive 3Facilitated diffusion Specific binding of nutrient to facilitator protein Protein conformation(change shape) Release of nutrients into the cytoplasm of the cell

10. Important points about facilitated diffusion • Most transport proteins are specificfor a single nutrient. • Over time facilitated diffusion results in an equalconcentration ofnutrient inside and outside the cell

11. Passive 4Uniport Uniporters are transport proteins that transport a substance across a membrane down a concentration gradient from an area of greater concentration to lesser concentration. Uniporter transport is powered by the potential energy of a concentration gradient and does not require metabolic energy.

12. Passive 4Uniport Nutrient in high conc Transport protein outside inside e.g. potassium uniporterK+ Nutrient in low conc

13. Active transport What is active transport? • Movement against a concentration gradient. It can produce and intracellular nutrient concentration 1000x greater than that of the same nutrient outside the cell • Aided by a carrier protein • Requires energy from cell (ATP - primary or PMF-secondary/simple) • Many amino acids and sugars accumulated by this method

14. Trans-porters on mem-brane

15. Transport proteins include: 1). uniport单运体 2). Symport共运体 3). Antiport反运体 In prokaryotic environments, nutrientsare often scarce.

16. Active 1Antiport Antiporters are transport proteins that transport one substance across the membrane in one direction while simultaneously transporting a second substance across the membrane in the opposite direction. Antiporters in bacteria generally use the potential energy of electrochemical gradients from protons (H+), that is, proton motive force to co-transport ions, glucose, and amino acids against their concentration gradient. Sodium ions (Na+) and protons (H+), for example, are co-transported across bacterial membranes by antiporters.

17. H+ in low conc; glucose, and amino acids H+ H+ H+ H+ H+ compound 1 (nutrient) H+ compound 2 transport protein Na+, H+ as anti-anion outside inside H+ H+ 3. Antiport Nutrient in high conc

18. Alternative way to look at Antiport

19. ATP-Binding Cassette (ABC) transport ABC system is an example of an ATP-dependent active transport found in various gram-negative bacteria. It involves substrate-specific binding proteins located in the bacterial periplasm, the gel-like substance between the bacterial cell wall and cytoplasmic membrane. The periplasmic-binding protein picks up the substance to be transported and carries it to a membrane-spanning transport protein. Meanwhile, an ATP-hydrolyzing protein breaks ATP down into ADP, phosphate, and energy. It is this energy that powers the transport of the substrate, by way of the membrane-binding transporter, across the membrane and into the cytoplasm. Examples of active transport include the transport of certain sugars and amino acids. Over 200 different ABC transport systems have been found in bacteria.

20. ATP-Binding Cassette (ABC) transport The transporters are a family  of periplasmic binding proteins that have high affinity for substrate only in Gram negative prokaryotes Transport channel Supplying energy

21. Active Transport, The "ABC" System of Transport Step 1 This form of active transport involves both transporter proteins and the energy provided by the hydrolysis of ATP. A specific periplasmic-binding protein carries the substance to be transported to a membrane-spanning Step 2 The molecule to be transported across the membrane enters the transporter protein system and a molecule of ATP enters the ATP binding site of the ATP-hydrolyzing protein.

22. Step 3 Energy provided by the hydrolysis of ATP into ADP, phosphate, and energy moves the molecule across the membrane. Step 4 The carrier protein releases the molecule being transported and the transporter system is ready to be used again.

23. 4. Symport Symporters are transport proteins that simultaneously transport two substances across the membrane in the same direction. Symporters use the potential energy of electrochemical gradients from protons (H+), that is, proton motive force to co-transport ions, glucose, and amino acids against their concentration gradient. Sulfate (HSO4-) and protons (H+) as well as phosphate (HPO4-) and protons (H+) are co-transported across bacterial membranes by symporters.

24. lac permease 4. Symport compound 1 (nutrient) compound 2 transport protein outside inside e.g. lactose H+

25. Alternative way to look at symport

26. C. group translocation The phosphotransferase system (PTS) The nutrient is chemically altered during transport. Phosphoenolpyruvate (磷酸烯醇丙酮酸PEP) (a "high energy compound") supplies the energy. Phosphate is transferred to the nutrient by a series of phosphorylationdephosphorylation reactions Materials such glucose; mannose; fructose; NAG; purines; pyrimidines are transported through this mechanism

27. The phosphotransferase system (PTS) 1 Phosphoenolpyruvate undergoes a series of phosphorylation and dephosphorylation reactions till EnzIIc receives the phosphate 2 Glucose is phosphorylated on EnzIIc and is transported into the inside of the cell.

28. P P PEP IIb IIa Hpr I pyruvate glucose phosphate Glucose uptake by the PTS Glucose Enzyme IIc P Phosphoenolpyruvate

29. VI. Proton motive force (PMF): an energy source for active transport A. proton pumping B. proton gradient C. charge gradient D. symport with H+ E. symport with Na+ Note: PMF is also a general energy source

30. A. proton pumping Most cells "pump" protons out. H+ H+ H+ H+ H+ H+ H+ H+ H+ cytoplasmic membrane This creates two sources of energy: a proton gradient and a charge gradient.

31. B. proton (pH) gradient The cell membrane is a barrier that holds the protons back the way a dam holds back water. H+ H+ H+ H+ H+ H+ H+ H+ H+ cytoplasmic membrane The controlled movement of protons back into the cell can be used as energy for nutrient concentration.

32. - - - - - - - - C. charge gradient When the cell membrane holds back protons italso holds back a charge. + + + + + cytoplasmic membrane + + + + The controlled movement of charge across the cell membrane can also provide energy for nutrient concentration.

33. Thus, proton pumping provides two sources of energy: 1) a proton (pH) gradient 2) a charge gradient This dual energy source is called proton motive force (PMF).

34. D. Symport with H+ allows the concentration of nutrients using PMF compound 1(nutrient) H+ Transportprotein outside inside H+ Usually, one proton is used per nutrient molecule

35. E. Symport with Na+ also allowsthe concentration of nutrients. compound 1 (nutrient) Na+ transport protein outside inside Na+ Usually, one sodium is used per nutrient molecule

36. Symport with H+ uses both the proton gradient and the charge gradient as energy, while symport with Na+ uses only the charge gradient. During symport, uptake of the nutrient and the second molecule is "coupled", that is they must be taken up together.

37. Membrane transport proteins