1. 3. Membrane Transport - Passive Transport
Plasma Membranes
3. Membrane Transport
- Passive Transport

2. Moving Through Membranes
Molecules such as water, CO2, O2 and other small molecules can pass easily through
Larger molecules and various ions need to go through transport proteins
Membranes are differentially permeable – select what passes through & what does not
Substances pass in and out of the cell by 4 main processes:

3. Moving Through Membranes
Diffusion (passive transport)
Osmosis (passive transport)
Active transport
Endocytosis and exocytosis

4. 1. Diffusion (Passive Transport)

5. Passive Transport - Diffusion
Passive transport – movement of materials across a membrane with no energy need
Diffusion: The passive movement of materials from regions of high to regions of low concentration
Particles in the crystals are in continual random motion, can move in any direction. To start with, there are far more of them near the crystal. This increases probability that they will move away from the crystal – net movement of particles away from the crystal - - diffusion.

6. Diffusion
Difference in concentration called concentration gradient or diffusion gradient
Always takes place when gradient exists
Continues until particles distributed evenly – equilibrium reached
Takes place in non-living and living systems
At equilibrium, particles continue to move randomly, but move at equal rates in all directions

7. Activity - Diffusion Add water to a beaker
What do you think will happen to the crystals of Potassium Permanganate if you add them to one section of the water (don’t scatter the crystals)?
Predict, Observe, Explain
When you have observed what has occurred, think about what the molecules are then doing
Write up a practical report explaining what you predicted, what you saw, what happened, and any differences between what was predicted and what occurred

8. Factors Increasing Diffusion
Diffusion rate is higher when:
The concentration gradient is great
Heat is applied (particles move faster)
Molecules are smaller
Movement occurs through a gaseous medium (particles further apart)
Water, O2, CO2 and other small uncharged particles pass easily through the membrane by diffusion

9. Plasma membranes and Diffusion
So how do cells manipulate diffusion for their own benefit?

10. Facilitated Diffusion
Still requires a concentration gradient
For charged particles and relatively large particles
Plasma membrane proteins assist this diffusion
Two types of proteins involved:
- Channel proteins
- Carrier proteins

11. Carrier Proteins Open and close areas along the plasma membrane
When open, ions or molecules can pass through
They bind to specific molecules or ions on one side of the membrane, change shape and release the molecule or ion on the other

12. Channel Proteins May be open or closed
Form narrow passageways through which small ions can diffuse rapidly from areas of high to low concentrations
Only ions of specific size and shape can pass through
Channels therefore select which molecules the cell does or doesn’t need

13. Questions Complete review questions (passive diffusion/diffusion)
What is moving when diffusion occurs?
What do you think will happen if the particles are too large or otherwise unable to pass through the membrane?

14. 2. Osmosis (Passive Transport)

15. Osmosis Special type of diffusion No energy input
Net movement of solvent (usually H2O) across a differentially permeable membrane
Solvent moves from the side of the membrane with a dilute solute (eg sugar) concentration, to the side of the membrane with a high solute concentration
Water moves down a concentration gradient

16. Osmosis
Osmotic pressure of a solution relates to the liklihood solvent (usually water) will diffuse into it
Solvent moves from an area of low osmotic pressure (low concentration solute) to an area of high osmotic pressure (high concentration solute)
When concentrations of solutions separated by a selectively permeable membrane is the same, water still moves, but there is equal movement – net movement is zero, and no volume changes
The result of osmosis is an evening out of solute concentrations on either side of a differentially permeable membrane, by movement of water molecules

17. Tonicity and Osmosis
The environment surrounding cells may contain concentration of dissolved solutes that are equal to, less than or greater than those found within the cell.
The relative concentration of solutes is described in terms of tonicity.
If a cell is in a surrounding environment that is:
isotonic - there is no net movement of water between cell and environment.
hypertonic - this is the area with a higher concentration of solute.
hypotonic - this is the area with a lower concentration of solute.

18. Tonicity and Osmosis
These terms are dependent on the relationship between the environments on either side of the membrane, and apply to the environment inside the cell or the environment outside the cell.
 The key to understanding osmosis and tonicity is to remember that water will always move toward a hypertonic environment!

19. Activity - Osmosis Predict, observe, explain
Three workstations have been prepared, with slides which show red blood cells that have been placed in water, normal saline, and a high salt solution
State what you will think will happen, observe what has happened, and explain any differences from what you predicted to what you observed.
If your predictions were correct, explain what happened in terms of movement of water.
What do you think would happen if these three solutions were injected into your bloodstream?

20. Osmosis and Animal Cells
Ideally cells are surrounded by a solution that has an equal solute concentration as the inside of the cell (isotonic)
Fluids given to patients eg IV drip must be at a concentration equal to blood plasma so osmosis doesn’t occur into or out of blood cells
If the plasma becomes more dilute (hypotonic), water will move into blood cells and burst them (haemolysis)
If the plasma becomes more concentrated (hypertonic), water will move out of blood cells into plasma (crenation)
Isotonic – water diffuses equally in both directions, no net movement of water
Haemolysis – too many cells burst, O2 transport to tissues reduced, may cause death
Crenation – crenated cells stick together, clogging small veins and arteries, preventing O2 reaching body tissues

21. Osmosis and Plant Cells
Turgor of cells supports plants and maintains shape and form
Vacuoles in their cells can rapidly gain or lose water
Cell wall prevents cell lysis during turgor
Plasma membrane comes away from the cell wall during water loss
When this happens, cells become flaccid and wilting occurs, causing plant to droop

22. Activity 2 - Osmosis
Design your own simple experiment to perform at home which you think shows osmosis occurring
Write up a practical report, with labelled diagrams, your aim, method, and discussion of the results
Explain why you set up the experiment the way you did, what has occurred, and how you have judged whether the experiment was a success or not

23. Questions
What applications are there for this information in real life?
What is moving in osmosis?
Review questions (osmosis)
BioZone questions (osmosis)

24. Reverse Osmosis
Separation process that uses pressure to force solvent through a semipermeable membrane
Solute remains on one side, and pure solvent is filtered through
Used most often to produce pure water

25. Activity – Passive Transport
Draw a Venn Diagram to compare and contrast the two types of passive transport - diffusion and osmosis
Ensure you include concentration gradients, what is moving, the direction of movement, and anything else you think is relevant