Variables affecting the rate of Photosynthesis

The variables that could change the rate of photosynthesis are: Temperature, light intensity, Co2 concentration and amount and type of weed.

The variable I will change is Co2 concentration. The variable that we have changed, Co2 concentration will change the variable we measure in that the higher the concentration of Co2 is the higher the rate of photosynthesis. But at a certain point the rate of photosynthesis will level out and no longer increase.


It slows due to the law of limiting factors; cause we are increasing the amount of Co2, but the temperature is not going up so this law comes into play eg: If one factor slows all the others slow down after it, its like a factory production line if the person putting the icing on the cake slows the person putting on the cherries has to slow down cause the person putting the icing on is not giving her the cakes so she can put the cherries on.

But in our case it is that we are putting more Co2 in but the temperature is not going up to help the Co2 increase the rate of Photosynthesis.

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References for my explanation came from earlier work in class on the law of limiting factors and a whole module on photosynthesis and information out of Biology for life book.

Co2 + 2H2A + Light Energy ( CH2 ) + H2O + H2A

Experimental Plan:

What I will measure is how many bubbles come out of the cut end of our pondweed.

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The number of bubbles coming from the cut end of our pondweed is directly proportional to the rate of photosynthesis.

What I will keep constant is the temperature, we will not change temperature cause it is to hard to change and easy to make mistakes, but with the Co2 concentration we can just use a syringe with a scale on the side to put in this much easier to do and is less prone to an unfair test happening.

In our trial experiment we discovered that increments of 5ml Co2 solution added was too much and it slowed down far to soon so we changed it to 2ml of Co2 solution added and this made our results much better. Also we found that timing for one minute to get our results was too much as well so we changed that to 30 secs and the temperature stayed constant at 25?c room temperature all the time.


Thermometer, 60w lamp, flat-bottomed beaker, syringe and Co2 solution, stopwatch our books for recording results in and pond weed with paper clips on it to weigh it down.


First we placed a jug full of water on our workbench then we put our thermometer in the water to find our temperature. Then we got our lamp and shone it on the jug and then we put some pond weed in there to weight it down we placed two paper clips on the pond weed and waited for ten minutes for the pond weed to get acclimatized to the water. After this we timed the pond weed for 30 seconds and then put this result down in our table as 0ml of Co2 so many bubbles, then we took this result again and again until we had 4 results for 0ml Co2 and then we used a calculator to find out the average of the whole set of results. We did this for 2ml of Co2; 4ml of Co2 all the way up to 8ml of Co2 all the way through we kept the same pondweed. To add the Co2 we used a syringe to add it to the water this was the easiest way to do it.

Co2 in ml

Result 1

Result 2

Result 3

Result 4

Average for Results 1,2,3 and 4 in ml.













2 2


















Analysing Evidence: At the start the graph was not very steep cause the reaction was going slowly but then it sped up and the line got steeper then it slowed down and the gradient of the line got shallower.

The variable we change makes the variable we measure go up in that the more Co2 we put in the more bubbles came out and how many bubbles come out is directly proportional to the rate of photosynthesis. Also this made the graph change, if we put in more Co2 the gradient of the line gets steeper. The Co2 and the sunlight are reacting with the chlorophyll to make glucose which gives the plant energy to respire therefore the bubbles are coming out of the end of the pond weed. But when it gets to a certain point the plants photosynthesis rate drops due to the law of limiting factors. Yes my results did match my prediction in that when the plant got to a certain point its rate of photosynthesis actually dropped due to the law of limiting factors which I said would happen in my prediction.


We had some irregularities but they were due to fluctuations in temperature which unfortunately we cannot do anything about this as we do not have the Wright equipment. These odd results were due to fluctuations in temperature and this is very hold to control due to the changing climate so these cannot be helped.

Our results are not that accurate and this is due to the equipment that we are using and the conditions we are doing the tests under. We could improve in many different ways but the main one is we should do the test in a blacked out room accept for the lamp shining on the plant this would stop fluctuations in The Light Intensity. The results we collected were reliable enough to support my conclusion but if you wanted to work this out for scientific uses these results would be very unreliable and would be far from the results you would get under controlled conditions. To start of with we could be given more time this would give us the abilitie to black out the room to stop the problems with light intensity fluctuations and also we could use another test tube collect the Oxygen given of by the pond weed then we could measure the amount given of more accurately and thus calculate the rate of photosynthesis easier.

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Variables affecting the rate of Photosynthesis. (2020, Jun 02). Retrieved from

Variables affecting the rate of Photosynthesis

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