The topic of gas exchange in plants is often tested in exams because it can be a good discriminator between A grade and A* grade candidates.  If you can master the understanding needed for these questions, important marks can be gained towards your top grade.

Firstly you must completely remove from your answers any indication that you think that plants photosynthesise in the day and respire at night.  Even typing this makes me feel nauseous….  Yuk?  Respiration as you all know happens in all living cells all the time and so while the first half of the statement is true (photosynthesis only happens in daytime), respiration happens at a steady rate throughout the 24 hour period.

Although the equations above make it look like these two processes are mirror images of each other, this is far from the truth.

How can gas exchange in plants be measured?

The standard set up involves using hydrogen carbonate indicator to measure changes in pH in a sealed tube.  In this experiment an aquatic plant like Elodea is put into a boiling tube containing hydrogen carbonate indicator.  The indicator changes colour depending on the pH as shown below:

• acidic pH:  indicator goes yellow
• neutral pH: indicator is orange
• alkaline pH: indicator goes purple

a) If the tube with the plant is kept in the dark (perhaps by wrapping silver foil round the boiling tube), what colour do you think the indicator will turn?  Explain why you think this.

b) If the tube with the plant is kept in bright light, what colour do you think the indicator will turn and why?

c) If a control tube is set up with no plant in at all but left for two days and no colour change is observed, what does this show?

In order to score all the marks on these kind of questions, there are two pieces of information/knowledge you need to demonstrate.  You need to show the examiner that you understand that carbon dioxide is an acidic gas (it reacts with water to form carbonic acid) and so the more carbon dioxide there is in a tube, the more acidic will be the pH.  As oxygen concentrations change in a solution, there will be no change to the indicator as oxygen does not alter the pH of a solution.

Secondly you need to show that you understand it is the balance between the rates of photosynthesis and respiration that alters the carbon dioxide concentration.  If rate of respiration is greater than the rate of photosynthesis, there will be a net release of carbon dioxide so the pH will fall (become more acidic).  If the rate of photosynthesis in the tube is greater than the rate of respiration, there will be a net uptake of carbon dioxide (more will be used in photosynthesis than is produced in respiration) and so the solution will become more alkaline.

So to answer the three questions above I would write:

a) The indicator will turn yellow in these conditions.  This is because there is no light so the plant cannot photosynthesise but it continues to respire.  Respiration releases carbon dioxide as a waste product so because the rate of respiration is greater than the rate of photosynthesis, there will be a net release of carbon dioxide from the plant.  Carbon dioxide is an acidic gas so the pH in the solution will fall, hence the yellow colour of the solution.

b) The indicator will turn purple in these conditions.  This is because the bright light means the plant photosynthesises at a fast rate.  Photosynthesis uses up carbon dioxide from the water.  The plant continues to respire as well and respiration releases carbon dioxide as a waste product.  As the rate of photosynthesis is greater than the rate of respiration in these conditions there will be a net uptake of carbon dioxide.  Carbon dioxide is an acidic gas so if more is taken from the solution than released into it, the pH in the solution will rise as it becomes more alkaline, hence the purple colour of the solution.

c) This shows that without a living plant in the tube there is nothing else that can alter the pH of the solution.  It provides evidence that my explanations above about the cause of the colour change is correct.