Floating Discs

Saturday 22 September 2012

Measuring the rate of photosynthesis

Just before all the leaves die, I am teaching the topic of photosynthesis now.  I wanted to expose my students to the various ways of measuring the rate of photosynthesis before they write a Design practical. With an aquarium of Elodea canadensis in the lab, the students could do the bubble production and gas collection method.  But to give them a choice of protocols, I gave my class 4 types of leaves to determine which would work well with the floating disc method.

For a monocot, maize (or corn, Zea mays)

For dicots, broccoli (Brassica sp), a variegated leaf (Acer negundo), English ivy (Hedera helix)

The students used hole punchers to get a number of discs from each type of leaf.   These were then put into 10 ml plastic syringes. With a little water drawn in, the students covered the tip with their thumbs and pulled the syringe plunger back to pull out the air from the mesophyll air spaces. Repeated efforts were needed to ensure that the air was removed so that the discs would sink.

The maize, broccoli and English ivy leaves worked very well.  

The variegated leaf much less so perhaps because the leaves are much thinner.

The students will now have to choose which method they want to use to design their practical for the rate of photosynthesis

The sets of discs will eventually be put into different beakers of water under various conditions of colour of light, light intensity, temperature, pH, salinity, carbon dioxide levels and so on.  The time for the discs to rise to the surface will be recorded for each condtion.  Obviously, the longer the time taken for the discs to rise, the slower the rate of photosynthesis at that condition.

One important controlled variable which students tend to forget is allowing the tissues to acclimate to the conditions of the independent variable.  As I repeatedly say in my classes, "Life is not MTV."  Now maybe I am showing my age with this but they tend to get the idea that they need to consider that living things are slower than we think.


Tags: LDR, photosynthesis, chloroplast oxygen