1. BIO 107 Lab # 4 Cell membranes, Osmosis & Diffusion
S. Badran

2. The plasma membrane
Is the boundary that separates the living cell from its nonliving surroundings
Is the minimal requirement for the formation of a cell
Was the first cell part to form during cellular evolution

3. Selective permeability
The plasma membrane allows some substances to cross it but not others
Figure 7.1

4. The fluid mosaic model
States that the cell membrane is a fluid structure with a “mosaic” of various proteins embedded in it
Fluidity results from the movement of phospholipids & proteins.

5. There are four primary types of membrane proteins, each of which performs a different function.
There are four primary types of membrane proteins, each of which performs a different function (Figure Plasma membrane molecules serve diverse roles).
Receptor proteins bind to chemicals in the cell’s external environment and, by doing so, regulate certain processes within the cell. Cells in the heart, for example, have receptor proteins that bind to adrenaline, a chemical released into the bloodstream in times of extreme stress or fright. When adrenaline binds to these heart cells, the cells increase the heart’s rate of contraction to pump blood through the body more quickly. You have experienced this reaction if you’ve ever been startled and felt your heart start to pound.
Recognition proteins give each cell a “fingerprint” that makes it possible for the body’s immune system (which fights off infections) to distinguish the cells that belong inside you from those that are invaders and need to be attacked. (Note that carbohydrates also play a role in recognition.) Recognition proteins also can help cells to bind to adhere to other cells or molecules.
Transport proteins are transmembrane proteins that help large and/or strongly charged molecules to pass through the plasma membrane. Transport proteins come in a variety of shapes and sizes, making it possible for a wide variety of molecules to be transported.
Enzymatic proteins accelerate chemical reactions on the cell membrane’s surface (a variety of different enzymatic proteins exist, with some accelerating reactions on the inside of the plasma membrane and other accelerating reactions on the outside of the plasma membrane). 5

6. The Permeability of the Lipid Bilayer
Small, hydrophobic molecules can pass through the membrane rapidly
Example: CO2, O2
Polar molecules do not cross the membrane rapidly
Example: H2O, sugars
Charged ions DO NOT cross the lipid bi-layer on their own

7. Facilitated Diffusion by Transport Proteins
Allows passage of hydrophilic substances and ions across the membrane and consist of:
Channels with ionic amino acids for passage of ions such as Na, K and Ca
Carriers that are specific for sugar, water and other hydrophilic molecules
Example: glucose carrier and aquaporins

8. Molecules such as oxygen and carbon dioxide, that are small and carry no charge, can pass directly through the lipid bilayer of the membrane without the assistance of any other molecules in a process called simple diffusion.
Each time you take a breath, for example, there is a high concentration of oxygen molecules in the air you pull into your lungs. That oxygen diffuses across membranes of the lung cells and into your bloodstream where red blood cells pick it up and deliver it to parts of your body where it is needed. Similarly, because carbon dioxide in your bloodstream is at a higher concentration than in the air in your lungs, it diffuses from your blood into your lungs and is released to the atmosphere when you exhale (Figure Simple and facilitated diffusion).  8

9. Passive transport Cell does not expend any extra energy for diffusion
Driving force for diffusion:
kinetic energy of particles diffusing
Difference in their concentration (concentration gradient)

10. Membrane (cross section)
Diffusion
tendency for molecules of any substance to spread out evenly into the available space
Activity: Diffusion
Figure 7.11 A
Diffusion of one solute. The membrane
has pores large enough for molecules
of dye to pass through. Random
movement of dye molecules will cause
some to pass through the pores; this
will happen more often on the side
with more molecules. The dye diffuses
from where it is more concentrated
to where it is less concentrated
(called diffusing down a concentration
gradient). This leads to a dynamic
equilibrium: The solute molecules
continue to cross the membrane,
but at equal rates in both directions.
Molecules of dye
Membrane (cross section)
Net diffusion
Equilibrium
(a)

11. Diffusion
Substances diffuse down their concentration gradient, from high to low concentration
Figure 7.11 B
Diffusion of two solutes. Solutions of
two different dyes are separated by a
membrane that is permeable to both.
Each dye diffuses down its own concen-
tration gradient. There will be a net
diffusion of the purple dye toward the
left, even though the total solute
concentration was initially greater on
the left side.
(b)
Net diffusion
Equilibrium

12. Effects of Osmosis on Water Balance
Osmosis is the diffusion of water across a semi-permeable membrane down its concentration gradient
Based on osmosis, solutions are classified as:
Hypotonic (hypo = less)
Isotonic (iso = same)
Hypertonic (hyper = more)
The prefix refers to the SOLUTE concentration NOT the water concentration

13. The solution with higher concentration of solutes is hypertonic.
Tonicity of a solution
The solution with higher concentration of solutes is hypertonic.
The solution with 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.

14. Osmosis
Is affected by the concentration gradient of dissolved substances
Activity: osmosis and water balance
Lower
concentration
of solute (sugar)
Higher
of sugar
Same concentration
Selectively
permeable mem-
brane: sugar mole-
cules cannot pass
through pores, but
water molecules can
More free water
molecules (higher
concentration)
Water molecules
cluster around
sugar molecules
Fewer free water
molecules (lower
Water moves from an area of higher
free water concentration to an area
of lower free water concentration 
Osmosis
Figure 7.12

15. Pressure due to osmosis

16. Cells in isotonic solutions
The concentration of solutes is the same as it is inside the cell
No net movement of water
Dynamic equilibrium: water moves but at the same rate in opposite directions

17. Cells in hypertonic solutions
The concentration of solutes is greater than it is inside the cell
The cell will lose water
Animal cells shrivel
Plasmolysis occurs in plant cells, when the cytoplasm shrivels and detaches from cell wall

18. Cells in hypotonic solutions
The concentration of solutes is less than it is inside the cell
The cell will gain water
Animal cells burst (or adapt by using contractile vacuoles)
Plant cells become turgid (preferred state)

19. Water Balance of Cells with Walls
Cell walls help maintain water balance

20. 3.9 Osmosis is the diffus the passive diffusion of water
Just as solutes will passively diffuse down their concentration gradients, water molecules will also move from areas of high concentration to low concentration to equalize the concentration of water inside and outside of the cell.
The diffusion of water across a membrane is a special type of passive transport called osmosis (Figure Osmosis overview).  
As molecules diffuse across a plasma membrane, molecules of water also move across the membrane, equalizing the concentration of water inside and outside of the cell. 20

21. Animal adaptations
Paramecium, a protist, is hypertonic when compared to pond water
Adaptations:
membrane is less permeable to water than usual
specialized organelle (contractile vacuole) pumps excess water out

22. Osmosis & Dialysis in today’s lab
Dialysis tubing: an artificial semi-permeable membrane
Molecules can pass if they are small enough to fit through the microscopic pores
Dialysis: diffusion of solute (not osmosis of water)
You will observe:
Osmosis and dialysis using dialysis tubing
Osmosis leading to plasmolysis of Elodea cells treated with high salt concentration

23. Elodea cells in isotonic solution
Osmosis leading to plasmolysis of
Elodea cells treated with high salt concentration

24. Question 1
A solution of 1 M glucose is separated by a selectively permeable membrane from a solution of 0.2 M fructose and 0.7 M sucrose. The membrane is not permeable to the sugar molecules. Which of the following statements is correct?
Side A is hypotonic relative to side B.
The net movement of water will be from side B to side A.
The net movement of water will be from side A to side B.
Side B is hypertonic relative to side A.
There will be no net movement of water.
Answer: D
Source: Taylor - Student Study Guide for Biology, Sixth Edition, Interactive Question #8.5

25. For Question 2
An artificial cell consisting of an aqueous solution enclosed in a selectively permeable membrane has just been immersed in a beaker containing a different solution. The membrane is permeable to water and to the simple sugars glucose and fructose but completely impermeable to the disaccharide sucrose.
Source: Campbell/Reece - Biology, Sixth Edition, EOC Self-Quiz Question #14-18

26. Which solute(s) will exhibit a net diffusion into the cell?
Question 3
Which solute(s) will exhibit a net diffusion into the cell?
sucrose
glucose
fructose
Answer: c
Source: Campbell/Reece - Biology, Sixth Edition, EOC Self-Quiz Question #14-18

27. Which solute(s) will exhibit a net diffusion out of the cell?
Question 4
Which solute(s) will exhibit a net diffusion out of the cell?
sucrose
glucose
fructose
Answer: b
Source: Campbell/Reece - Biology, Sixth Edition, EOC Self-Quiz Question #14-18

28. Which solution is hypertonic to the other?
Question 5
Which solution is hypertonic to the other?
the cell contents
the environment
Answer: a
Source: Campbell/Reece - Biology, Sixth Edition, EOC Self-Quiz Question #14-18