Movement across Membranes: Diffusion and Osmosis

1. Chapters 5 & 35 Lecture 7 Movement across membranesDr. Angelika Stollewerk

2. Movement across membranes Aims: • To understand the process of diffusion • To understand the process of osmosis

3. Movement across membranes Aims: • To understand the process of diffusion • To understand the process of osmosis These lecture aims form part of the knowledge required for learning outcome 1

4. Movement across membranes Essential reading • pages 105-107 • pages 764-768 • 5.3 What Are the Passive Processes of Membrane Transport? • 35.1 How Do Plant Cells Take Up Water and Solutes?

5. 5.3 What Are the Passive Processes of Membrane Transport? Membranes have selective permeability—some substances can pass through, but not others Passive transport—no outside energy required—diffusion Active transport—energy required

6. 5.3 What Are the Passive Processes of Membrane Transport? Diffusion: the process of random movement toward equilibrium Equilibrium—particles continue to move, but there is no net change in distribution

7. Figure 5.8 Diffusion Leads to Uniform Distribution of Solutes

8. 5.3 What Are the Passive Processes of Membrane Transport? Net movement is directional until equilibrium is reached. Diffusion is net movement from regions of greater concentration to regions of lesser concentration.

9. 5.3 What Are the Passive Processes of Membrane Transport? Diffusion rate depends on: Diameter of the molecules or ions Temperature of the solution Electric charges Concentration gradient

10. 5.3 What Are the Passive Processes of Membrane Transport? Diffusion works very well over short distances. Membrane properties affect the diffusion of solutes. The membrane is permeable to solutes that move easily across it; impermeable to those that can’t.

11. 5.3 What Are the Passive Processes of Membrane Transport? Simple diffusion: small molecules pass through the lipid bilayer. Lipid soluble molecules can diffuse across the membrane, as can water. Electrically charged and polar molecules can not pass through easily.

12. 5.3 What Are the Passive Processes of Membrane Transport? Osmosis: the diffusion of water Osmosis depends on the number of solute particles present, not the type of particles.

13. Figure 5.9 Osmosis Can Modify the Shapes of Cells

14. 5.3 What Are the Passive Processes of Membrane Transport? If two solutions are separated by a membrane that allows water, but not solutes to pass through, water will diffuse from the region of higher water concentration (lower solute concentration) to the region of lower water concentration (higher solute concentration).

15. 5.3 What Are the Passive Processes of Membrane Transport? Isotonic solution: equal solute concentration (and equal water concentration) Hypertonic solution: higher solute concentration Hypotonic solution: lower solute concentration

16. 5.3 What Are the Passive Processes of Membrane Transport? Water will diffuse (net movement) from a hypotonic solution across a membrane to a hypertonic solution. Animal cells may burst when placed in a hypotonic solution. Plant cells with rigid cell walls build up internal pressure that keeps more water from entering—turgor pressure.

17. 35.1 How Do Plant Cells Take Up Water and Solutes? Terrestrial plants obtain water and mineral nutrients from the soil. Water is needed for photosynthesis; it is essential for transporting solutes upward and downward, for cooling the plant, and for internal pressure that helps support the plant. Plants lose large quantities of water to evaporation, which must be replaced.

18. Figure 35.1 The Pathways of Water and Solutes in the Plant

19. 35.1 How Do Plant Cells Take Up Water and Solutes? Osmosis: movement of water through a membrane in accordance with the laws of diffusion. Osmosis is passive: no input of energy is required. Solute potential (osmotic potential): The greater the solute concentration of a solution, the more negative the solute potential, and the greater the tendency for water to move into it from another solution of lower solute concentration.

20. 35.1 How Do Plant Cells Take Up Water and Solutes? For osmosis to occur, two solutions must be separated by a selectively permeable membrane; permeable to water, but not to the solute. Plants have rigid cell walls. As water enters a cell due to its negative solute potential, entry of more water is resisted by an opposing pressure potential (turgor pressure).

21. 35.1 How Do Plant Cells Take Up Water and Solutes? Water enters plant cells until the pressure potential exactly balances the solute potential. At this point the cell is turgid: it has significant positive pressure potential.

22. 35.1 How Do Plant Cells Take Up Water and Solutes? The overall tendency of a solution to take up water from pure water, across a membrane, is called water potential (ψ). Water potential is the sum of its negative solute potential and positive pressure potential. ψ = ψs + ψp By definition the water potential of pure water is zero.

23. Figure 35.2 Water Potential, Solute Potential, and Pressure Potential

24. 35.1 How Do Plant Cells Take Up Water and Solutes? Water always moves across a selectively permeable membrane toward a region of lower (more negative) water potential. Solute potential, pressure potential, and water potential can be measured in megapascals (MPa).

25. 35.1 How Do Plant Cells Take Up Water and Solutes? Osmosis is extremely important to plants. Physical structure is maintained by the positive pressure potential. If this is lost, the plant wilts. Over long distances in xylem and phloem, flow of water and dissolved solutes is driven by a gradient of pressure potential.

26. 35.1 How Do Plant Cells Take Up Water and Solutes? Bulk flow: movement of a solution due to difference in pressure potential. Bulk flow in xylem is between regions of different negative pressure potential (tension). Bulk flow in phloem is between regions of different positive pressure potential (turgidity).

27. 35.1 How Do Plant Cells Take Up Water and Solutes? Aquaporins - membrane channel proteins that water can pass through rapidly. Abundance in plasma membrane and tonoplast (vacuole membrane) depends on cell’s need to obtain or retain water. Rate of water movement can be regulated but direction of movement can not.

28. 35.1 How Do Plant Cells Take Up Water and Solutes? Mineral ions cannot pass membranes without transport proteins. Molecules and ions move with their concentration gradients as permitted by membrane characteristics. Concentration of most ions in the soil solution is lower than in the plant; uptake must be active transport, requiring energy.

29. 35.1 How Do Plant Cells Take Up Water and Solutes? Electric charge differences are also important. Combination of electrical and concentration gradients is called an electrochemical gradient. Uptake against this gradient requires ATP.

30. 35.1 How Do Plant Cells Take Up Water and Solutes? Plants have a proton pump that moves protons out of a cell against a gradient. Accumulation of H+ outside the cell results in an electric gradient and a concentration gradient of protons. Inside of cell is now more negative than outside, cations such as K+ can move in by facilitated diffusion.

31. 35.1 How Do Plant Cells Take Up Water and Solutes? The proton gradient can be harnessed to drive active transport of anions into the cell against a gradient; a symport couples movement of H+ and Cl–. The proton pump and other transport activities results in the interior of the cell being very negative; they build up a membrane potential of about –120 mV.

32. Figure 35.3 The Proton Pump in Active Transport of K+ and Cl–

33. 35.1 How Do Plant Cells Take Up Water and Solutes? Where water is moving by bulk flow, dissolved minerals are carried along. Water and minerals also move by diffusion, and minerals move by active transport (e.g., at root hairs). Ions must cross other membranes to reach the vessels and tracheids.

34. 35.1 How Do Plant Cells Take Up Water and Solutes? Movement of ions across membranes can result in movement of water. Water moves into a root because the root has a more negative water potential than the soil. Water moves from the cortex into the stele because the stele has a more negative water potential than the cortex.

35. 35.1 How Do Plant Cells Take Up Water and Solutes? Water and minerals can move into the stele by two pathways: The apoplast:cell walls and intercellular spaces form a continuous meshwork that water can move through, without crossing any membranes. Water movement is unregulated until it reaches the Casparian strips of the endodermis.

36. 35.1 How Do Plant Cells Take Up Water and Solutes? Symplast: water passes through cells via the plasmodesmata. Selectively permeable membranes of root hair cells control access to the symplast.

37. Movement across membranes Check out 5.3 Recap, page 110, first 2 questions only 35.1 Recap, page 752, first 2 questions only 5.3 Chapter summary, page 116, see WEB/CD Activity 5.1 35.1 Chapter summary, page 778 Self Quiz Page 117: Chapter 5, question 10 Page 778: Chapter 35, questions 1-5 For Discussion Page 779: Chapter 35,

38. Movement across membranes Key terms: active transport, apoplast, aquaporin, bulk flow, Casparian strip, diffusion,endodermis,equilibrium, facilitated transport, gated transport, hydrophilic, hydrophobic, hypertonic, hypotonic, isotonic, Megapascals (MPa), osmosis, pericycle, pressure potential (ψp), semipermeable membrane, solute potential (ψs), stele, symplast, turgid, water potential (ψ)