Problem Based Learning: Putting It All Together with the Science of Photosynthesis

What is problem-based learning?

Problem-based learning in the science lab means hands-on, happening-right-in-front-of me discovery of key science principles. Role changes in problem-based learning mean the student takes the reigns of self-discovery as the teacher guides the student or group toward a learning outcome.

If you were to ask a teacher down the hall, would they be able to explain problem-based learning? The good news is that problem based learning isn’t one silver-plated answer. It is as old as Socrates but is being rediscovered, attached to our digitally based, technology driven learners. It is research based with results that show higher performing life skills in problem solving and real world application in transition to their paycheck days after class is over.

Problem Based Learning and STEM

As the state of our educational system changes in search of solutions to low Science, Technology, Engineering, and Math scores nationwide, teachers are in pursuit of learning strategies that meet the goal we’ve always had with our students (learning gains) with the statewide goals (student learning gains). Problem based learning is one of many methods in our teaching tool box we use to make this happen.

In a middle school science lab, how many students would you predict would get tongue tied when asked to describe photosynthesis? As a working example of learning gains in the classroom, the science concept of photosynthesis will take the stage as an example of how problem based discovery engages an unmotivated student.

Photosynthesis is one of those need-to-know science concepts that causes young STEM science students to draw a blank when needing to describe it. Many students have a problem when it comes to understanding photosynthesis. Combining the strength of problem-based learning (self-discovery versus memorization) with challenging science concepts generates a win-win outcome for you and for your struggling students. Problem-based learning can re-engage a student bored with the pages and pictures in the text book.

The beauty of problem-based learning in the science lab for a teacher is that you can witness that often invisible process of learning begin to take shape in the form of verbal feedback and ease of ability of your students to proudly explain how they “get it” now. They saw it, discovered it, and got to “touch” it.

Finding hands-on, budget friendly discovery labs is part of my weekend hunt for ideas as I set the stage for future units in our problem-based learning lab. Here are a few of the ideas I found along my search for hands-on know-how for a photosynthesis unit:

Click to see Photosynthesis Lab details

New STEM Discovery Begins with Student Learning Goals

I frame a new unit of discovery with the student’s own learning goals. It can be as basic a process as having each student write “my learning goal for photosynthesis is…” on a small note or logging it into their STEM Learning Journal.

Another quick method to begin that initial inquiry process is to post categories around the room. For discovery of photosynthesis, categories could be “Food? Chemicals? Light? Word Origin?”,  to start the curiosity process that leads to long term learning. Have the student select a category, stand near the posted category, and describe their own learning expectation that connects them to the key concept word they chose.

This corralling technique helps any learner. Why? Because we “don’t know what we don’t know.” But the categories suggested by the teacher can hook them into something that matters to them. As a teacher, I recognize that learning only occurs when the information is useful to the person. Long term learning occurs when it is used in their life on a regular basis. Motivating a student to connect themselves with the science of photosynthesis can be as simple as using the term “food” to generate curiosity in a hungry teenager.

Origins and Putting it All Together

What does photosynthesis mean? I pulled up the word origin on

  • Word origin: from the Greek photo-, “light”, and synthesis, “putting together”.

An interesting connection between the meaning of photosynthesis and problem based learning cannot go unnoticed. Problem based learning embraces a spectrum of learning modes: science literacy, technology, text books, teams, self-discovery, and communicating the results of discovery. Photosynthesis is a process that uses light energy spectrum and the biological system inside the leaf surface to generate energy for life. Putting it all together, in both processes, an energy is generated to ignite something necessary.

Sample Discovery Labs: Photosynthesis

Pulling together labs from my internet search, you can create your own problem-based learning lab that fits your students and bell schedule.

The photosynthesis lab photo above was from Michael Gregory’s post and includes details for you and your students to follow.

I am including Karen Adam’s (Burnside Scholastic Academy) Photosynthesis Lab. This lab is a budget friendly, visual discovery that builds another layer of understanding for our budding STEM scientists. Using elodea (a common aquarium plant) and baking soda, students can get hooked on the idea that photosynthesis is more than just a chemical equation.


(Adaptable to grade levels 6-9)

The student will:
1.  Observe evidence of photosynthesis in a water plant.
2.  Assemble the equipment needed to measure the rate of photosynthesis in
elodea (water plant).
3.  Count bubbles of oxygen gas given off by elodea to determine the rate of
4.  Change the conditions of photosynthesis by altering light intensity and
carbon dioxide amount, and determine the effects on the photosynthesis rate.
5.  Prepare a graph of the collected data and analyze it.

Materials Needed:

(For each group of four students)

elodea (water plant)                                                                                       lamp (40 watt)
test tube                                                                                                              razor blade (single-edge)
dechlorinated water (room temperature)                                           tape
sodium bicarbonate powder (baking soda)                                         clock or timer
metal stand with rod or test tube rack                                                  metric ruler


PART A. Setting Up the Experiment
1.  Obtain a sprig of elodea.  Remove several leaves from around the cut end of
the stem.  Slice off a portion of the stem at an angle and lightly crush the
cut end of the stem.
2.  Place the plant into the test tube, stem end up, filled with water.
3.  Secure the test tube to a metal stand with tape or place the test tube in a
test tube rack.
PART B. Running the Experiment
1.  Place a 40 watt lamp 5 cm from the plant.  After one minute, count and
record the number of oxygen bubbles rising from the cut end of the stem.
Count bubbles for five minutes.  If bubbles fail to appear, cut off more of
the stem and recrush.
2.  Run a second five-minute trial.  Record and average your results.
3.  Move the lamp so it is 20 cm from the plant.  After one minute count and
record bubbles for two five-minutes trials.  Again, average and record your
4.  Add a pinch of sodium bicarbonate powder to the test tube.  Place the lamp
5 cm from the test tube.  After one minute, record bubbles for two five-
minute trials.  Average and record your results.
5.  Prepare a graph of your results.  Use the average number of bubbles for the
vertical axis.  Use the type of environmental condition for the horizontal

Performance Assessment:

The students will answer these questions using specific values from the
investigation.  Diagrams should included.

1.  How does this investigation demonstrate that plants give off oxygen during
photosynthesis?  Explain and illustrate your answer based on your observations.
2.  How does the rate of photosynthesis change when the light source is moved
from a distance of 5 cm to 20 cm?
3.  How does the rate of photosynthesis change when sodium bicarbonate is added
to the water?


Plants use green pigments called chlorophylls to trap light energy.  The
chlorophylls give a plant its green color.  Inside the cells that have
chloroplasts, the light energy is used to make a simple sugar called glucose.
The process by which plants use light energy to make glucose is called

During this process of sugar production, carbon dioxide combines with water to
form glucose and oxygen is released.  Oxygen that is produced in photosynthesis
is given off as a gas.  If a lot of oxygen is being given off, photosynthesis is
occurring rapidly.  If little oxygen is being given off, photosynthesis is
occurring slowly.  The amount of trapped light energy and the amount of carbon
dioxide available affects the rate of photosynthesis.

The purpose of adding sodium bicarbonate powder to the water increases the
amount of carbon dioxide in the water.

This investigation can be performed with water plants grown in many parts of
the world, except regions that have permanent ice.


How do you, as a learning facilitator, know that learning occurred for the student?

If the student is able to explain and connect the process, purpose, and outcome of photosynthesis and report it to you and their peers, then initial learning took place. But, intuitively, you also recognize that learning is not a one time event. To embed that learning event into a long term experience, frequent opportunities to review and expand on that one photosynthesis experience will need to be provided for the student.

As a guide on the path to discovery, your students will engage in the exciting world of science. Problem based learning is an intuitive process that some educators have forgotten on the path to “teaching” their students. Problem based learning is nothing new in the academic arena. It’s just time it took center stage for our students.

It’s a great time to be an educator.

~Kay Borglum, MS

Biotechnology and Space Lab Science Teacher (FL)

Bonus blog source for PBL classroom ideas

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