Rate of Photosynthesis

Rate Of Photosynthesis

Planning

Aim of the experiment How does the rate of photosynthesis change with light intensity?

Hypothesis

If the temperature and concentration of CO2 are kept constant, then:  At low to medium light intensities the rate of photosynthesis is directly proportional to light intensity.  At high light intensities the rate of photosynthesis does not change anymore with the light intensity. [1]

Independent variables

 light intensity

Dependent variables

 time required for the disk to rise

Fixed variables

 temperature

 concentration of CO2

 leaf disks are prepared in the exactly the same way

Data Collection and Presentation

Accuracy of light meter was 5%. An experiment with two tracing papers below the beaker was made twice. Results are gathered together in table 1.

N 5 4 3 2 2 1 0 P [lux] 1400 2700 2800 5500 6000 8000 22500 t1 270 273 206 205 135 86 150

t2 370 274 230 220 145 121 160

t3 460 275 230 240 150 207 252

t4 495 285 280 270 155 222 270

t5 530 290 305 275 160 236 285

t6 690 338 330 295 170 275 290

t7 870 425 – 330 180 – 310

t8 – – – 340 200 – 360

t9 – – – 390 – – 405 t [s] 526  200 309  56 264  49 285  60 162  21 191  72 276  83

 t [%] 38 18 19 21 13 38 30 v [10-3 s-1] 1.9  0.7 3.2  0.6 3.8  0.7 3.5  0.7 6.2  0.8 5.2  2.0 3.6  1.1 Table 1: Results of an experiment (N – number of papers, P – light intensity, t - average time required

for the disk to rise,  t - relative error of time, v – rate of photosynthesis)

Rate of photosynthesis in dependency with light intensity was plotted in graph below (fig. 1).

Figure 1: Rate of photosynthesis Conclusion

Our results support our hypothesis. At medium light intensities is the rate of photosynthesis directly proportional to light intensity. But when illuminance reaches certain point, rate of photosynthesis does not change anymore with the light intensity. This is logical, because at that point all chloroplasts are already fully involved in photosynthesis. Results also to some way correspond with the known values. Plants are accommodated to the enviroment. From the graph (fig. 1) can be seen that the rate of photosynthesis reaches plateau at the illuminance between 8000 – 10000 lux. This value is also the average outdoor light intensity during the day [2].

Results in overall have big error. Relative errors vary from 13% to 38%. This shows that the experiment is relatively inaccurate. However, within some deviations, results show the right shape of the curve in the graph of rate of photosynthesis and light intensity (fig. 1). These are many reasons for these big uncertainties. The most important is heavy preparation of leaves. The experiment was done in several groups, each working on one light intensity. Each group cut and then compressed their own leaf’s disks. Because leaves were compressed differently, the amount of oxygen that is still present in leaves is different. Consequently, the leaves between the groups are not completely comparable. Secondly, leaves were slightly exposed to light during preparation of the experiment. Therefore the exact time, when the reaction of photosynthesis started to happen, cannot be determined. Despite all these problems, this experiment has also advantages. Because no special devices are needed, this experiment is easy and cheap.

First improvement of this experiment will be that the experiment should be done in complete dark. Secondly, all the leaves should be prepared in the same way. That means that all the leaves’ disks are compressed in one syringe. To assure that the temperature of the disks during the experiment is not changing, the disks have to rest in complete dark for a few minutes that the temperature of the room and disks is the same.

Resources

1. Allott, Andrew: Biology for the IB Diploma. Oxford University Press, 2001. 2. Kladnik, Rudolf: Osnove fizike II. del. Državna založba Slovenije, 1977.