Lecture 7 Movement across membranes Dr. Angelika Stollewerk Chapters 5 & 35

Lecture 7

Movement across membranes

Dr. Angelika Stollewerk

Chapters 5 & 35


Aims:

• To understand the process of diffusion

• To understand the process of osmosis

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


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?

5.3 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


Diffusion: the process of random movement toward equilibrium

Equilibrium—particles continue to move, but there is no net change in distribution

Figure 5.8 Diffusion Leads to Uniform Distribution of Solutes


Net movement is directional until equilibrium is reached.

Diffusion is net movement from regions of greater concentration to regions of lesser concentration.


Diffusion rate depends on:

• Diameter of the molecules or ions

• Temperature of the solution

• Electric charges

• Concentration gradient


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.


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.


Osmosis: the diffusion of water

Osmosis depends on the number of solute particles present, not the type of particles.

Figure 5.9 Osmosis Can Modify the Shapes of Cells


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).


Isotonic solution: equal solute concentration (and equal water concentration)

Hypertonic solution: higher solute concentration

Hypotonic solution: lower solute concentration


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.

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.

Figure 35.1 The Pathways of Water and Solutes in the Plant


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.


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).


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.


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.

Figure 35.2 Water Potential, Solute Potential, and Pressure Potential


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).


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.


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).


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.


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.


Electric charge differences are also important.

Combination of electrical and concentration gradients is called an electrochemical gradient. Uptake against this gradient requires ATP.


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.


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.

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


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.


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.


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.


• Symplast: water passes through cells via the plasmodesmata.

Selectively permeable membranes of root hair cells control access to the symplast.

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,


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)


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Lecture 7 Movement across membranes Dr. Angelika Stollewerk Chapters 5 & 35