TitleTo Investigate The Effect Of Light Intensity On The Rate Of Photosynthesis.

Research QuestionDoes different light intensity affects the rate of photosynthesis?

HypothesisPhotosynthesis is the process used by plants and some other organisms top produce all their own organic substances using light energy and simple inorganic compound.1 One of the needed factors for the process to occur is the light energy. However, the process may occur at different rate because of different intensity of light. Thus, the hypothesis is different light intensity cause different rate of photosynthesis.

VariableIndependent variable : Light intensity Dependent variable : Rate of photosynthesisConstant variables : The length of pond weeds, volume of bicarbonate indicator

Apparatus/materialsRefer to hand out paged 135

MethodRefer to hand out paged 135

Data Collection

Table 1: Data for time taken for the bicarbonate indicator in each tube to turn from deep red to purple.

Test tube A B C D E F G HDistance from light source (cm)

10 20 30 40 50 60 (control) (control)

Time taken for the bicarbonate indicator to turn from deep red to purple (min)

13 15 30 45 52 66 76 0

Note: In all test tubes (excluding tube H), the red colour of brown bicarbonate indicator was slightly changed to purple colour.

1 Andrew Allott. IB Study Guide BIOLOGY for the IB Diploma STANDARD AND HIGHER LEVEL. New York: Oxford University Press, 2001. (p. 17)

Data Processing and PresentationThe rate of photosynthesis can be calculated using this formula;

Rate of photosynthesis = ____________________1____________________________ Taken for the bicarbonate indicator to change from deep red to dark purple (min)

Where rate of photosynthesis is 1 over time taken for the bicarbonate indicator to change from deep red to dark purple.

Table 2: Rate of photosynthesis of pond weed in every test tubeTest tube A B C D E F G HTime taken for the bicarbonate indicator to turn from deep red to purple (min)

13 15 30 45 52 66 76 0

Rate of photosynthesis(min-1)

0.07690.0667 0.0333

0.0222 0.0192 0.0151 0.0132 ∞

Discussion:

The red colour of brown bicarbonate indicator changed to dark purple because the oxygen gas which is produced by the pond weed underwent photosynthesis process. The oxygen gas oxidizes the red colour of bicarbonate indicator to dark purple.

In every test tube, the rate of colour change is different because of the difference in light intensity. The rate of photosynthesis is directly proportional to light intensity where at higher light intensity, the rate of photosynthesis is higher and at lower light intensity, the rate of photosynthesis is lower. At higher light intensity, more oxygen is produced and the time taken for the brown bicarbonate to change to dark purple is less than at lower light intensity.

However, from this experiment, the time taken for the brown bicarbonate indicator to change from deep red to dark purple at 10cm and 20 cm is not proportional with light intensity because the tubes are too near with the light source. The tubes receive almost equal amount of light intensity and thus the rate is also almost the same. Besides, other factor such as temperature and other limiting factor like the amount of carbon dioxide in each test tube (because all test tubes were closed with stopper) were also affecting the rate of photosynthesis which resulted in inaccurate data collection.

So, the rate of photosynthesis in each test tube is proportionate with time except test tube A, B and G. In test tube G, the rate of photosynthesis is infinite because the brown bicarbonate does not change to dark purple colour.

Evaluation1. There are several precaution manners that need the students attention:

Make sure that when placing all the test tubes according to their respective distance from the light source, any test tube will not be covered by previous test tube to receive light energy from the light source.

2. In the experiment, there is one test tube which was used as a control experiment. Test tube G (contains pond weed which sunk in brown bicarbonate indicator

and wrapped with black paper) where light energy is prevented from being absorb by pond weed. This control can be used to determine the important of the presence of light for photosynthesis.

The test tube is placed in the area of light source because to determine whether light energy is needed for photosynthesis process.

3. Weakness of the methodology. The distance of the test tubes from the light source especially the first two

test tubes were too close. This method had led to inaccurate result because of the effect of the heat released from the light source as it increased the rate of photosynthesis of pond weed.

Suggestion: The distant for each test tube should be added so that the effect of heat (temperature) could be minimized and better result could be obtained. The distant for the first test tube should be changed such as 60 cm from the light source, 70 cm for the second test tube, and 80 cm for the third test tube and so on.

ConclusionFrom the experiment, the rate of photosynthesis are proportionate except test tube A, B and Thus, the hypothesis is not accepted.