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Light Intensity's Effect on Photosynthesis

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Light Intensity's Effect on Photosynthesis

Planning

Photosynthesis is the building up of sugars using carbon dioxide and
water as the raw materials. The energy for the process comes from
light and a green pigment called chlorophyll allows the plant to
transfer the energy from light to sugar. The chemical equation for
this process is:

6CO2 + 6H2O → C6H12O6 + 6O2 + energy

↑

(Light & Chlorophyll needed)

Plants can only photosynthesise during the day, as light is needed.
The rate at which a plant photosynthesises depends on the conditions
in which it is kept, in particular the light intensity, the amount of
carbon dioxide available, the temperature and the amount of water
available. In general the best conditions for photosynthesis are found
in areas with long hours of sunshine, warmth and high rainfall.

In this investigation, I will assess how light intensity affects the
rate at which photosynthesis occurs. The rate of photosynthesis can be
calculated by measuring the amount of oxygen produced by the plant
over a set period of time, the greater the volume of oxygen produced
the greater the rate of photosynthesis. An experiment can be set up to
measure the amount of oxygen as follows:

[IMAGE]

Apparatus: - lamp, ruler, calibrated syringe, capillary tube, test
tube, pondweed.

Method: -

1. Set up the apparatus and fill the syringe, capillary tube and
test tube with water.

2. Cut the stem of the pondweed so that the bubbles of oxygen
produced during photosynthesis can escape the plant and place it,
upside-down, in the test tube.

3. Starting with the maximum value of x, allow the plant to
photosynthesise for 5 minutes noting the amount of oxygen that has
been produced every minute.

4. When 5 minutes have passed, note the final reading then increase
x by 5cm. Leave the plant to adjust to the new light conditions
for 5 minutes.

5. Reset the apparatus and repeat the experiment for the new value
of x.

6. If a result appears to be anomalous, repeat the experiment for
that value of x.

It is vital that the only variable in this experiment is x. All other
possible variables must be kept the same for each test. Below is a
list of possible variables and how they will be controlled:

· The pH of the water and the minerals present in the water. The pH of
the water must be the same for each test. This can be done by using an
indicator or litmus paper. Also the presence of some minerals can
inhibit photosynthesis and therefore, de-ionised water which does not
contain minerals must be used in order to perform an accurate
experiment.

· The water temperature. This must be constant for every test. It can
be checked before each experiment commences using a thermometer.

· The pondweed. The same piece of pondweed must be used for each test.
This is because a different piece may have more or less leaves and as
the leaf is the organ of photosynthesis, it will photosynthesise at a
different rate, thus introducing another variable.

· The time for the plant to adjust to the new light conditions. This
will be set at 5 minutes. This must be constant because if in one
test, the plant is only given 2.5 minutes to adjust and in another it
is given 5 minutes the one given 2.5 minutes will not be as well
adjusted and thus the results will not as accurate.

· The room temperature. This must be constant for each test as a
change in room temperature could cause a change in water temperature
from one test to the next. It can be checked before each experiment
commences using a thermometer.

· The ambient light. The ambient light in the room must remain
constant for each test. If it fluctuates, this may cause inaccuracies.
It is therefore necessary to perform the experiment under constant
artificial light or in the dark (the only source of light being the
lamp). Natural light cannot be used as its intensity is prone to
fluctuations (e.g. If the sun goes behind a cloud).

· The apparatus. The same apparatus must be used for each test. This
eliminates another source of error. For example if a syringe was
inaccurately calibrated but it was used for all the tests it would not
matter. If however it were replaced after the first test with a more
accurate syringe, a fair test would not have been achieved.

· The time each experiment is allowed to run for. This must be kept
the same for all the tests as if it is not, the whole experiment will
be fundamentally flawed. Each test will run for 5 minutes, as this
will provide a fair test.

· The orientation of the pondweed within the test tube. The pondweed
must be upside-down within the test tube for each test. This is so
that the bubbles of oxygen that are released can pass into the
capillary tube without obstruction.

· The point from which x is measured. This must be kept the same in
order to use accurate x values. If one test uses an x value measured
from the left hand side of the test tube and another uses an x value
measured from the right, a fair test will not have taken place.

The above variables which must be controlled became apparent as a
result of the preliminary work. This entailed a crude version of the
above experiment which was set up as shown below.

[IMAGE]

The rate of photosynthesis was not measured by the amount of oxygen
released over a set period of time but by the relative size of the
bubbles of oxygen released by the plant. This was found to be a very
inaccurate method and thus helped to create the better, more accurate
method that will be used in the actual experiment.

The preliminary work also helped to decide the range of values for x.
The lamp was moved back until the bubbles of oxygen released were very
small and barely visible. This (80cm) was taken to be the maximum
value of x. When deciding upon the minimum value of x it was important
to consider the fact that enzymes are vital for photosynthesis and as
the lamp generates heat it cannot be so close to the plant that the
enzymes are denatured. Thus the minimum value for x was set at 15cm.
In addition, the preliminary work showed that it took longer for the
bubbles to get smaller when the light intensity decreased than it did
for them to get bigger when it increased. Thus it was decided to start
with the maximum value of x so that the plant does not need too long
to adjust to new light conditions.

The results for this experiment will be collected in the table below.

Amount of oxygen gas released (cm³)

x (cm)

0 mins

1 min

2 mins

3 mins

4 mins

5mins

15

20

25

30

35

40

45

50

55

60

65

70

75

80

Prediction: -

I predict that because plants photosynthesise at a greater rate in
greater light intensity, if a fair test is performed, the increase in
light intensity (decrease in x) will be directly proportional to
increase in the volume of oxygen gas released. If a graph of x vs.
volume of gas released is plotted, I predict it will have the
following shape:

[IMAGE]

If this is the case, and the graph produced is a straight line, it
will be possible to calculate a formula for the volume of gas produced
in terms of x. This can be done by using the general case of y=mx+c
(m=gradient of straight line, c=intercept on the y axis). In this
case, y is the volume of gas produced.

The above may be true for my range of values (15-80cm) however, if a
much wider range of values were used the graph would probably have a
different shape:

[IMAGE]

The graph becomes a straight line in which the gradient = 0 towards
the end because there is a limiting factor. This means that the plant
will not photosynthesise at a greater rate even if the light intensity
is increased further. This is because there is not enough of one of
the other factors which affect photosynthesis (heat, water, carbon
dioxide) to sustain a higher rate of photosynthesis. It is unlikely to
be heat or water as the pondweed is in water and is next to a lamp
which provides heat. Therefore, the limiting factor is probably the
level of carbon dioxide and this could be tested by raising the
concentration of carbon dioxide in the atmosphere surrounding the
plant.



Obtaining
=========

The experiment was carried out and the following results were
obtained:

Light Intensity

Size of oxygen bubble (mm) (1st attempt)

(2nd attempt)

Average

10

5

5

5

20

10

10

10

30

15.5

15.5

15.5

40

20.5

20.5

20.5

50

26.5

26.5

26.5

60

31.5

31.5

31.5

70

36.5

36.5

36.5

80

41

41

41

90

47

46.5

46.75

100

51

52

51.5



Analysing
=========

The results of the experiment clearly show that as the light source
moves closer to the plant, the size of the oxygen bubble increases.

Light Intensity

Size of oxygen bubble (mm) (1st attempt)

(2nd attempt)

Average

10

5

5

5

20

10

10

10

30

15.5

15.5

15.5

40

20.5

20.5

20.5

50

26.5

26.5

26.5

60

31.5

31.5

31.5

70

36.5

36.5

36.5

80

41

41

41

90

47

46.5

46.75

100

51

52

51.5

As the light source moves closer to the plant, the light intensity
increases. As this happens, the oxygen bubble increases in size
meaning that more oxygen gas is being produced. This shows that the
plant is photosynthesising at a greater rate.

A graph (graph 1) can be plotted of light intensity Vs average size of
oxygen bubble. The graph produced is a straight line and therefore the
general case of Y = MX + C can be used to find a relationship between
light intensity and the size of the oxygen bubble:

Y = size of oxygen bubble,

X = light intensity,

M = gradient of straight line on graph of light intensity Vs average
size of oxygen bubble = 0.51

C = the value at which the straight line crosses the vertical (y) axis
= 0.

Thus:

Y = 0.51X

[IMAGE]

This means that, in general, the length of the oxygen bubble (in mm)
is equal to the light intensity multiplied by 0.51. For example if the
light intensity were equal to 32, the length of the bubble would be
equal to (0.51 x 32) which is equal to 16.32mm. This can be checked by
drawing a line on the graph from the point on the x axis where light
intensity = 32 and then drawing another to a point on the y axis from
where the first line touches the straight line.

As the only variable for each experiment was the distance between the
light source and the plant (light intensity), all other possible
variables were kept constant. This means that the time each experiment
ran for was the same (10 mins). Thus a value for the rate at which the
plant photosynthesises at a certain light intensity can be calculated.
By dividing the size of the oxygen bubble after 10 mins by 10, a value
for the amount of oxygen produced per minute can be calculated. This
is shown in the following table.

Light intensity

Total size of oxygen bubble

Size of oxygen bubble produced per min (mm)

(RATE OF OXYGEN PRODUCTION/min)

10

5

0.5

20

10

1

30

15.5

1.55

40

20.5

2.05

50

26.5

2.65

60

31.5

3.15

70

36.5

3.65

80

41

4.1

90

46.75

4.675

100

51.5

5.15

A graph of the above can be plotted (light intensity Vs size of bubble
produced per min) (graph 2). The straight line produced proves that
the increase in the rate of photosynthesis is directly proportional to
the increase in light intensity. Also, it is found by again using the
general case of Y = MX + C and the same method as before that the size
of oxygen bubble produced per minute is equal to the light intensity
multiplied by 0.051.

[IMAGE]

The above is slightly deceptive. If the light intensity were increased
further, there would come a point at which the increase in the rate of
photosynthesis would no longer be directly proportional to the
increase in light intensity. This is the point where a limiting factor
would be introduced. This is because there is not enough of one of the
other factors which affect photosynthesis (heat, water, carbon
dioxide) to sustain a higher rate of photosynthesis. It is unlikely to
be heat or water as the pondweed is in water and is next to a lamp
which provides heat. Therefore, the limiting factor is probably the
level of carbon dioxide. The rate of increase in the rate of
photosynthesis would decrease and would eventually become 0 as there
would not be sufficient carbon dioxide for the rate of photosynthesis
to increase any further.

It has been found that as the light intensity increases, the rate of
photosynthesis increases. This is because light intensity is one of
the four factors which govern the rate at which photosynthesis occurs.
The other factors are the abundance of the raw materials carbon
dioxide and water and the temperature. The light provides the energy
for the process. Thus, if there is more light there is more energy
available and photosynthesis can occur at a greater rate.

6CO2 + 6H2O → C6H12O6 + 6O2 + energy

↑

(Light & Chlorophyll needed)

As the chemical equation for photosynthesis shows, a green pigment
called chlorophyll is needed for the process. This substance allows
the energy from light to be converted into sugar. The chlorophyll only
uses two wavelengths (colours) from the spectrum of white light, red
and blue. An increase in light intensity causes an increase in the
levels of the two required light wavelengths and thus photosynthesis
can occur at a greater rate.

In my prediction, I stated that I thought that an increase in the rate
of photosynthesis would be directly proportional to the increase in
light intensity. Although this may not be entirely correct due to
limiting factors (as mentioned above), for the range of light
intensities featured in my experiment it was the case and my
prediction was correct. The limiting factor did not affect the
experiment, as the light intensity did not increase enough to make the
plant require more carbon dioxide than was available. The shape of my
predicted graph of light intensity Vs amount of oxygen produced was
correct. In my opinion, my conclusions show that my prediction was
accurate.



Evaluating
==========

The experiment performed to asses how light intensity affects the rate
at which plants photosynthesise was adequate to allow accurate
conclusions to be drawn. The results taken can be considered accurate.
None of my results were particularly anomalous showing that the
experiment had been performed to a good degree of accuracy. The
results which were slightly anomalous (i.e. line of best fit on
results graph did not pass through them) were very close to the
best-fit line and basically within the general trend and thus did not
affect the accuracy of the conclusions. These anomalies are evident on
the rate of photosynthesis graph which has been reproduced with the
slight anomalies circled.

The computer simulation of the experiment has its limitations. The
only variables that it allows to be regulated are light intensity,
temperature and level of carbon dioxide. It does not allow other
possible variables to be controlled. Possible variables such as
ambient light, ambient temperature and water pH level cannot be
controlled. It is not known whether these variables were controlled
when the actual experiment was carried out to get the data for the
computer simulation and therefore it can be said that the
discrepancies in the experiment may have caused the slightly anomalous
results. For example, a change in ambient temperature from one
experiment to another would cause inaccurate results.

Further experiments could be performed to asses whether all plants
photosynthesise at the same rate. This could be achieved by repeating
the experiment with plants other than the Canadian pondweed used in
this experiment. Also, different light sources could be used to
investigate how different types of light affect the rate of
photosynthesis. This could be achieved by using sources of light other
than the lamp used in this experiment such as coloured lights. The
limiting factors such as level of carbon dioxide and temperature could
also be investigated to see how they affect the rate of
photosynthesis. If these factors were investigated, light intensity
would become a constant and 2 experiments would be performed, one with
level of carbon dioxide as the variable and one with temperature as
the variable.

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MLA Citation:
"Light Intensity's Effect on Photosynthesis." 123HelpMe.com. 23 Apr 2014
    <http://www.123HelpMe.com/view.asp?id=122660>.




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