Counting bubbles

Recording the rate of photosynthesis by counting bubbles of oxygen being released from the cut stem of some upturned pondweed is a classic experiment (some would say a right of passage) for every biologist. How do we allow our students to enjoy the delights and disappointments of this investigation while teaching remotely? Here’s a possible lockdown solution.

There are some online simulations, but many of them rely on Flash animations and will no longer run in many browsers. This one, at is JavaScript/HTML5 and so will function in most browsers but like many simulations, lacks the magic of the real thing.

Luckily, I did the classic investigation of light intensity and rate of photosynthesis the other day with my year 10 biologists and we recorded videos of the results. Rather than keep these to myself, I’ve uploaded them to YouTube to share the bubble-counting joy.

Start by testing your skill with the practice video (10 seconds of bubbles), then you can try to keep up with the bubbles for an entire minute with the clips below.

How good are you at spotting the bubbles of oxygen and logging them accurately? The video will give you a 10-second clip to count, then it will give you a second chance at 50% speed.

Count the number of bubbles in these 60 second video clips recorded with the light at four different distances from the pondweed:

Here are the some clips of a 15 second sample at 50% with counters. You need to multiply by 4 to calculate the number of bubbles per minute:

Just to prove these are real results, you’ll see that they’re not perfect.

We’d expect the rate of photosynthesis to go down as the distance from the light increases but the 50cm sample really confuses things. What’s going on? I think that we’ve knocked the focus point slightly on this recording and we’re seeing lots of bubbles that haven’t come from the cut stem. I’ve not plotted this result on the graphs.

We didn’t get enough measurements with the light closer to the pondweed. Going from 5cm to 20cm was too big a jump. The significance of this becomes clear when you remember your physics and convert the distance between the samples into relative light intensity. The graph below really shows how we haven’t got enough sample measurements. If you compare this with the textbook you can see it’s certainly close to what we’d expect, but it needs more observations.

A partial success then, but it’s all part of the journey. Next time we do it, we’ll do it differently, try harder and fail better.

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