DIFFUSION AND OSMOSIS - Laboratory - Cracking the AP Biology Exam

Cracking the AP Biology Exam



All AP biology courses have a laboratory component that gives students hands-on experience regarding some of the biology topics covered in class. Through these laboratory exercises you can learn the scientific method, lab techniques, and problem-solving skills. The folks at the College Board require that you complete 12 laboratory exercises during the school year. These labs are supposed to draw from some of the most important topics in biology.

How do these labs relate to the AP Biology Exam? Approximately 10 percent of the questions on the multiple-choice section and one essay question will refer to certain aspects of these 12 lab exercises. ETS incorporates questions concerning these labs to determine two things: (1) how well you understand the key concepts in biology, and (2) how well you think analytically. ETS wants to find out if you can design experiments, manipulate data, and draw conclusions from experiments.

There are basically two ways to approach a review of the laboratory exercises for this exam. We could review every single experiment, including all the procedures and results. This is what ETS expects you to do. Not only would this take up more than half of this book, it wouldn’t necessarily improve your score.

We at The Princeton Review have a better approach. Believe it or not, many schools do not cover all of the laboratory experiments. Why? Some schools don’t have the proper facilities to conduct the more complicated experiments. Other schools have opted to use their own exercises, which illustrate the same concepts. Given this, we’d rather not overwhelm you with all the boring details about each and every experiment. Instead, we’re going to tell you the objective of each lab. That way, if you conducted a different lab (or if you’ve never even seen the lab), it doesn’t matter. We’ll bring to light the important concepts and results related to each experiment.

In addition, some of these labs cover techniques or terms that are usually taught during labs. So pay particular attention to these exercises. How should you use this chapter to review for the AP Biology Exam? Read each summary and focus on the concepts stressed in each experiment. Now, let’s get cracking.


This lab investigated the process of diffusion and osmosis in a semipermeable membrane as well as the effect of solute concentration on water potential in plants.

What are the general concepts you really need to know?

  • Fortunately, this lab covered the same concepts about diffusion and osmosis that are discussed in this book. Just remember that osmosis is the movement of water across a semipermeable membrane from a region of high water concentration to one of low water concentration (from a region of low solute concentration to a region of high solute concentration).
  • Be familiar with the concept of osmotic potential. Osmotic potential is simply the free energy of water. It is a measure of the tendency of water to diffuse across a membrane. Water moves across a selectively permeable membrane from an area of higher water potential to an area of lower water potential.
  • Be familiar with the effects of water gain in animal and plant cells. In animals, the direction of osmosis depends on the concentration of solutes both inside and outside of the cell. In plants, osmosis is also influenced by turgor pressure—the pressure that develops as water presses against a plant’s cell wall.