Diffusion, osmosis, and the cell membrane

Draw what you think a cell membrane looks like at the cellular level-

• A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer.

Membranes are mosaics of structure and function

Fig. 8.6

What do you think membranes do?

• The proteins in the plasma membrane may provide a variety of major cell functions.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 8.9

What is diffusion?

• Diffusion is a process where molecules move from greater molecule concentrations to areas of less molecule concentrations until an equal distribution of those molecules is reached.

• Movements of individual molecules are random.

• However, movement of a population of molecules may be directional.

Passive transport is diffusion across a membrane

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

What is the difference between permeable, semipermeable and

impermeable membranes?

Which way will the molecules travel?•The dye will cross the membrane until both solutions have equal concentrations of the dye.•At this dynamic equilibrium as many molecules pass one way as cross the other direction.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 8.10a

• In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated, down its concentration gradient.– This spontaneous process decreases free energy and

increases entropy by creating a randomized mixture.

• Each substance diffuses down its own concentration gradient, independent of the concentration gradients of other substances.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 8.10b

• The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen.– The concentration gradient represents potential energy

and drives diffusion.

• However, because membranes are selectively permeable, the interactions of the molecules with the membrane play a role in the diffusion rate.

• Diffusion of molecules with limited permeability through the lipid bilayer may be assisted by transport proteins.

• The plasma membrane functions as a selective barrier that allows passage of oxygen, nutrients, and wastes for the whole volume of the cell.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 7.6

Osmosis is defined as the diffusion of

water

across a selectively permeable membrane.

Practice problem- please write on notes

• Differences in the relative concentration of dissolved materials in two solutions can lead to the movement of ions from one to the other.– The solution with the higher concentration of solutes

is hypertonic.– The solution with the lower concentration of solutes is

hypotonic.– These are comparative terms.

• Tap water is hypertonic compared to distilled water but hypotonic when compared to sea water.

– Solutions with equal solute concentrations are isotonic.

Say what?• Let’s refer to the graph that you made for the

potato cores.

• What is causing the cores to lose their mass? Water is being lost from the potato core cells.

• What is causing them to gain in mass?

• Water is being gained by the potato cells.

• What is the only variable that is being manipulated?

• The solute concentration.

• Imagine that two sugar solutions differing in concentration are separated by a membrane that will allow water through, but not sugar.

• The hypertonic solution has a lower water concentration than the hypotonic solution.– More of the water molecules in the hypertonic

solution are bound up in hydration shells around the sugar molecules, leaving fewer unbound water molecules.

• Draw a diagram that represents this scenario-

• Unbound water molecules will move from the hypotonic solution where they are abundant to the hypertonic solution where they are rarer.

• This diffusion of water across a selectively permeable membrane is a special case of passive transport called osmosis.

• Osmosis continues until the solutions are isotonic.

Fig. 8.11

Back to your graph- • What is the solute concentration surrounding the

potato cores relative to the solute concentration inside the potato cells?

• they are the same- • When the average percent change in mass of the

potato cores increases how does the solute concentration compare between the outside solution and inside solution?

• The solute concentration in the cells is greater than solute concentrrtion in the surrounding solution

• When the average percent change in mass of the potato cores decreases how does the solute concentration compare between the outside solution and inside solution?

• The solute concentration in the cells is less than the solute concentration in the surrounding solution

• An animal cell immersed in an isotonic environment experiences no net movement of water across its plasma membrane.– Water flows across the membrane, but at the same

rate in both directions.– The volume of the cell is stable.

Cell survival depends on balancing water uptake and loss

• The same cell is a hypertonic environment will lose water, shrivel, and probably die.

• A cell in a hypotonic solution will gain water, swell, and burst.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 8.12

• Turgid cells contribute to the mechanical support of the plant.

• If a cell and its surroundings are isotonic, there is no movement of water into the cell and the cell is flaccid and the plant may wilt.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 8.12

• YouTube - Statkraft - Osmotic Power Plant