AP Biology: Lab 1 - Diffusion and Osmosis

AP Biology Key Concepts

The processes of diffusion and osmosis account for much of the passive movement of molecules at the cellular level, which you will need to know for the AP Biology Exam.


Molecules are in constant motion and tend to move from regions where they are in higher concentration to regions where they are less concentrated. Diffusion is the net movement of molecules down their concentration gradient, which is also known as passive transport. Diffusion can occur in gases, in liquids, or through solids. An example of diffusion in gases occurs when a bottle of perfume is opened at the front of a room.


Osmosis is a special type of diffusion that involves the passive transport of water. In osmosis, water moves through a selectively permeable membrane from a region of high concentration to a region of low concentration. The membrane selectively allows passage of certain types of molecules while restricting the movement of others.

Water Potential

Water potential describes the tendency of water to leave one place in favor of another. Water always moves from an area of higher water potential to an area of lower water potential. Water potential is affected by two factors: pressure and the amount of solute.

Red blood cell – if a red blood cell is placed in distilled water, water will move into the red blood cell and cause the cell to expand. The pressure of the extra water will cause the cell to burst. However, this doesn’t happen to the red blood cells in our bloodstreams. Why?

Plant cell - if a plant cell is placed in distilled water, water will move into the cell enter the cell and the cell contents will expand. However, the elastic cell wall exerts a back pressure, which will limit the net gain of water. This prevents the cell from bursting.

Calculating Water Potential

Water potential is calculated using the following formula:

Water potential (Ѱ) = pressure potential (ѰP) + solute potential (ѰS)

Pressure potential (ѰP): In a plant cell, pressure exerted by the rigid cell wall that limits further water uptake.

Solute potential (ѰS):  The effect of solute concentration. Pure water at atmospheric pressure has a solute potential of zero. As solute is added, the value for solute potential becomes more negative. This also causes water potential to decrease. Therefore, as solute is added, the water potential of a solution decreases, and water will move into the solution. The water potential of pure water in an open container is zero because there is no solute and the pressure in the container is zero.

Analysis of Results

Once you know the solute concentration, you can calculate solute potential using the following formula:

Solute potential (ѰS) = –iCRT

i = The number of particles the molecule will make in water; for NaCl this would be 2; for sucrose or glucose, this number is 1

C= Molar concentration (from your experimental data)

R = Pressure constant = 0.0831 liter bar/mole K

T = Temperature in degrees Kelvin = 273 + °C of solution


Question: The molar concentration of a sugar solution in an open beaker has been determined to be 0.3M.

(a)   Calculate the solute potential at 27 degrees.

(b)  The pressure potential of a solution open to the air is zero. Calculate the water potential.



= -iCRT

= -(1) (0.3 mole/1) (0.0831 liter bar/mole K) (300 K)

= - 7.48



Water potential = -7.48 + 0

Water potential = -7.48


Key Words


Plasma membrane



Water Potential

Solute Potential 


Study Tips

- Draw labeled diagrams showing the movement of molecules in osmosis.

- Draw diagrams on red blood cells and plant cells in a hypertonic, isotonic, and hypotonic solution.

Good luck on the AP Biology Exam!

Image credit: SFU Public Affairs and Media Relations

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