Diffusion and Osmosis: Biology Lab

Lesson Transcript
Instructor: Jennifer Szymanski

Jen has taught biology and related fields to students from Kindergarten to University. She has a Master's Degree in Physiology.

Diffusion and osmosis occur when air or water molecules move from an area of high concentration to low concentration. Learn what diffusion and osmosis are, test their differential permeability in the biology lab, and explore the concepts of tonicity and water potential. Updated: 10/08/2021

Diffusion and Osmosis

Have you ever ridden in a crowded elevator? When the doors open, everyone hurries out of the car, eager to get away from each other as quickly as possible. When molecules are confined into a small area and are given a way to get away from each other, they do so. Not because they're hot and annoyed but because of diffusion.

Diffusion is the movement of molecules from an area of high concentration to low concentration due to molecular kinetic energy; that is, the endless and random movement of molecules. Osmosis is a specialized type of diffusion: the diffusion of water. In both diffusion and osmosis, materials move down a concentration gradient, the difference in the number of molecules between two areas. You can think of a concentration gradient as a hill, with the top of the hill being an area of high concentration, the bottom of the hill as an area of low concentration, and the angle of the hill being the concentration gradient. The steeper the hill, the faster objects roll down it; the steeper the concentration gradient, the faster molecules move from high to low concentration.

Diffusion and osmosis are directly affected by the ratio of a cell's volume to its surface area. An increase in this ratio means an increase in the rate of diffusion. Why? Let's use cubes to represent cells. A smaller cube will have higher surface area to volume ratio than a larger cube. This means that more of the cell's interior is exposed to molecules outside of the cell. The opposite is true in larger cells - less of the cell's interior is close to the cell's environment. These differences mean both more and faster diffusion occurs in smaller cells than in larger cells.

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  • 0:01 Diffusion and Osmosis
  • 1:59 Differential Permeability
  • 4:35 Tonicity and Water Potential
  • 7:00 Lesson Summary
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Differential Permeability

Diffusion and osmosis are important in helping cells to create homeostasis, a stable internal environment, inside of the cell membrane. There are several types of diffusion. Simple diffusion is defined as movement of molecules across a membrane by a concentration gradient, while facilitated diffusion occurs if molecules cross the membrane via a protein channel or carrier. Active transport happens when molecules are pushed against the concentration gradient. This requires energy. These different types of diffusion make the cell's membranes selectively permeable, which means the cell membrane helps control what materials enter and exit the cell.

We can demonstrate selective permeability by creating model cells using dialysis tubing, a material used in hospitals to mimic kidney function in patients that have kidney disease. Dialysis tubing is selectively permeable to common organic molecules based on size. Molecules (like glucose and water) and ions (like sodium) pass through readily, while large molecules (like sucrose) do not. Let's see what happens when we create artificial cells made of dialysis tubing.

We'll create four conditions. Condition one will be our control condition, since it's water both inside and outside of the cell. Condition two will be our first experimental condition, with a one molar sucrose solution inside the cell and distilled water outside the cell. Condition three will be another experimental condition, with a five molar sucrose solution inside the cell and distilled water outside of the cell. Condition four will use the same solutions as in condition three. This time, though, we'll put the distilled water inside of the cell and the sucrose solution outside of the cell. This condition will provide evidence of what happens when the solutions are reversed and assure us that any movement of solutions due to osmosis can happen in either direction.

Condition Number Cell contents Beaker contents
1 distilled water distilled water
2 1M sucrose distilled water
3 5M sucrose distilled water
4 distilled water 5M sucrose

If we observe the cells at the beginning of the experiment and weighed them at the end of an hour, what would you expect to see? Water will move from an area of high concentration to low concentration, but sucrose will not move because the artificial cell's membrane is selectively permeable. Furthermore, the greater the concentration gradient between the beaker and the cell, the greater the rate of osmosis. This is called water potential.

Tonicity and Water Potential

A cell's cytoplasm is mostly made of water, so we refer to a cell's contents as aqueous. Solid particles, like organic molecules and salts inside of the cell, are solutes. When water moves during osmosis in a cell, we use a special set of terms to clarify which way it's moving. These terms usually refer to how likely it is that water will move from one area to another, called water potential, represented by the Greek letter Psi. As stated at the beginning of the lesson, a high concentration of water molecules in one area means a high water potential.

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