The effect of different radiations in the photosynthetic
rate of Elodea sp.
Inês Vaz, João Correia, João Fernandes, Maria Soares, Pedro Teixeira, Pedro Guedes, Sofia Santos, Vítor Freitas
Escola Secundária da Gafanha da Nazaré
1.Introduction
Elodea is the generic name given to some underwater plants that
belong to Hydrocharitaceae family, Angiosperms class,
monocotyledon sub-class such as Egeria densa and Egeria
brasiliensis.
It is an aquatic macrophyte plant important for establishing the
equilibrium of ecosystems in which they operate because they not
only produce oxygen but also feed various species of animals and
provide shelter to living beings like phyto and zooplankton [1].
Photosynthesis is the process used by green plants to synthisize
organic compounds using light, water and CO2.
It occurs in the cloroplasts which contain the pigments which
capture the light, the electron carriers and the enzymes. The
pigments and electron carriers are contained in a specialized type
of membrane known as thylakoid.
All photosynthetic plants contain two chemically distinct types
of photosynthetic pigments: the chlorophylls and the carotenoids [2]. The ability of a pigment
to absorb light is determined by its wavelength-dependent optical absorption cross-section [3].
The rate of photosynthesis is dependent on environmental factors such as light intensity,
availability of carbon dioxide, availability of water, nutrients and temperature [4].
The photosynthetic process can be divided into two parts: the
light and the dark reactions. Light reactions take place in the
thylakoid membrane system: hydrogen is withdrawn from
water and passed along a series of hydrogen carriers to NADP,
so that NADPH2 is formed and oxygen is released. In
association, ADP and inorganic phosphate is converted to ATP.
Dark reactions take place in the stroma of the chloroplast:
NADPH2 produced is used to reduce CO2 to the level of
carbohydrate [2].
Therefore, the overall photosynthetic process can be represented by
6H2O + 6CO2 -> 6O2 + C6H12O6
2.Aims
The aim of this investigation was to increase knowledge on
aquatic plants photosynthesis and determine the effect of
different wavelengths on the rate of photosynthesis. This study
also allowed to learn research methodology in science
(scientific method), acquire laboratorial skills and explore
different ways of treatment and communication of the results.
3.Materials and Methods
• Label the flasks (P- containing the aquatic plant and V- without aquatic plant). To each
treatment corresponds a set of 3 replicates.
• Fill the flasks with 1L of water.
• Submerge 20g of Elodea sp. previously weighted in flasks (p1, P2, P3).
• Submit the samples to several light conditions (white, purple, green, dark).
• Measure oxygen concentration every 2 hours during a period of 6 hours.
4.Results
4.Results (cont.)
Figure 1B – Oxygen concentration curve-lines produced by Elodea sp., after 6h of
experiment, under different light conditions (W- white, P- purple, G - green, Ddark).
Figure 2 – Oxygen concentration, in the absence of Elodea sp., after 6h of
experiment, under different light conditions (white, purple, green, dark). Data are
the means of 3 replicates and error bars represent the standard deviation.
5.Discussion & Conclusion
In order to minimize the mask factors that could affect our results, water from
different containers was mixed into one before filling the flasks and samples were
randomly placed under the light source.
The results for oxygen concentration were different between trial flasks and control
flasks, being higher when the aquatic plant was present. Thus, we conclude that
photosynthesis only occurred when Elodea sp. was present.
When natural (white) light was provided, a gradual and significant increase in
oxygen concentration was registered between the first (T0) and third (T2)
measurements, followed by a slight decrease on the fourth (T3) measurement as a
result of a reduction on the light provided to the plant associated with the daylenght
and solar elevation of the season [2].
When Elodea sp. was subjected to purple radiation the photosynthetic rate was high
although not as high as when it received natural light. This result confirms the high
absorption capacity of chlorophyll (mainly a) when low wavelengths radiations are
provided.
In the presence of green light, the results were almost invariable and quite close to
the ones obtained in the beginning of the experiment (corresponding to the oxygen
widespread in water). The photosynthetic efficiency was very low, but not zero, as
this radiation is reflected instead of absorbed [2].
Under no light circumstances, Elodea sp. is unable to perform photosynthesis. In
this situation the plant stops the light-dependent reactions started before collecting,
failing to accomplish water photolysis. However, the plant may trigger the chemical
phase of photosynthesis for some time. From this moment on, a continuous decrease
of oxygen concentration would be expected as the aquatic plant initiates cellular
respiration [2].
In conclusion, our study supports that:
• The photosynthetic efficiency depends on the presence of the
light, which is responsible for the process.
• Oxygen concentration results are optimized when Elodea sp.,
in different treatments, performs photosynthesis under natural
light, receiving all wavelengths.
• Photosynthesis efficiency is influenced by the radiation
wavelength provided to Elodea sp., being enhanced when
radiation provided is more energetic.
• The low wavelength radiation – purple, is efficiently
absorbed by photosynthetic pigments.
• Green radiations are not absorbed by photosynthetic
pigments, because they’re being reflected.
• After a certain period of time in the absence of light
the photosynthetic process ceases.
References
Figure 1A – Oxygen concentration produced by Elodea sp., after 6h of
experiment, under different light conditions (white, purple, green, dark). Data are
the means of 3 replicates and error bars represent the standard deviation.
[1]
http://ead.hemocentro.fmrp.usp.br/english/index.php/publication/science-infocus/210-elodea-algae-no-aquatic-plant
[2] Kirk, J.T.O. (1994), Light & Photosynthesis in Aquatic Ecosystems. 2nd edition.
Cambridge University Press.
[3] Baker, N.R. (1996), Photosynthesis and the Environment. Kluwer Academic
Publishers.
[4] http://www.passmyexams.co.uk/GCSE/biology/gas-exchange-plants.html
[5] Zar, J.H. (1996) Biostatistical Analysis. 3rd edition. Prentice-Hall International,
Inc., New Jersey, pp1-662.