Download Photosynthetic starch

Photosynthetic starch
(C)
Authors:
Niels Sievertsen, [email protected]
Stefan Zeyen, [email protected]
Connie Müller, [email protected]
Melanie Weis, [email protected]
Assistant:
Yvonne Steinbach, [email protected]
06. June 2013
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Objective
The aim of this experiment is to show that starch in geranium leaves can
only be formed in the presence of light, which is used in photosynthesis to
generate carbohydrates.
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Introduction
Photosynthesis, which is one of the most important biochemical process on
earth, is used by plants to convert light energy into chemical energy. This
resulting energy can be used by the plant for several processes. Other organisms profit from photosynthesis, as well. They eat plants and can use
products of photosynthesis for their metabolism. Hence, plants are the primary producer of useful energy. In the net reaction of photosynthesis water
and carbon dioxide are used to produce carbohydrates, and oxygen as a
byproduct. The chemical equation is given as:
6CO2 + 6H2 O −→ C6 H1 2O6 C + 6O2
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(1)
3 METHOD
Photosynthesis can be divided into two parts: the light reaction an the
dark reaction which is also known as Calvin cycle. Both parts occur in specialized organelles which are called the chloroplasts. In the light reaction
NADP is reduced to NADPH and ATP is produced by using the proton
gradient distributed across the thylakoid membrane. The proton gradient is
built up by the electron transport chain. Afterwards, these products can be
used in the Calvin cycle after fixation of a carbon dioxide molecule. In this
cycle, one molecule of glycerate-3-phosphate (G3P) is produced, which is a
three-carbon sugar called triose. G3P can be used to form soluble sucrose in
the cytosol which is transported to all parts of the plants and metabolized
during the day. Otherwise, G3P can be used to build up starch in chloroplasts
which is an immobile form of carbohydrates and used for storing. Starch is
stored in granules, in particular in amyloplasts, which are related to chloroplasts. In most starch granules, two forms of starch can be found: amylose
and amylopectin. The difference between these substances is the linkage of
the glucose subunits. Amlyose only has an α-1,4-glycosidically linkages and
is herefore linear. Glucose subunits of amylopectin are also connected using this type of linkage, but additionally this substance has α-1,6-glycosidic
linkages and is therefore branched. In order to use starch as a carbohydrate
source over night, the plant has to cleave these linkages and split starch into
maltose and glucose. These species can then be transported into the rest
of the plant. In our experiment, we wanted to show photosynthetic starch
production in geranium leaves and its dependence on a light source.
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Method
Generally, the methodology from the course material was followed. It can
be seen in figure 1. A geranium leaf (P elargoniumzonale) was wrapped
into a piece of aluminum foil. The bottom side of the leaf was covered
completely but on the upper side, the foil had a small cut out hole for light
to pass through. The diameter of this hole was measured at 0.8 cm. As a
consequence, the leaf was partly visible. The wrapped leaf was attached to a
glass tank filled with cold water. On the other side of the tank, a strong light
source was placed. The spacing and the water between the leaf and lamp was
used to prevent heat damage on the leaf. After two hours of illumination, the
geranium leaf was unwrapped again and was immersed in ethanol at 70◦ C
until it was bleached. Afterwards, we dived it into hot water of 70◦ C in
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4 RESULTS & DISCUSSION
Figure 1: Experimental setup for experiment C as seen in the course material.
A single geranium leaf was treated and its photosynthetic activity in terms
of starch synthesis was analyzed.
order to make the leaf more flexible. Finally, the leaf was put into an iodine
potassium solution also known as Lugol for staining the starch. Observations
regarding the location of starch in the leaf were made.
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Results & Discussion
We did not analyse the amount of starch quantitatively, but the qualitative
observations gave us clear results. Potassium iodide (Lugol) staining showed
the expected result. The part of the leaf, which was covered with aluminum
bleached out entirely and remained almost colorless. This means, that the
leaf did not produce any starch in the aluminum-covered part and the Lugol
solution did not have anywhere to bind to. In contrast, the light exposed part
of the leaf turned brown. This indicates, that there was starch in the cells
around that location and that the starch was produced by a light dependent
reaction such as photosynthesis.
The starch turns brown because of the Iodide. Starch is composed of branched
glucose (1,4 and some 1,6 glycosidic bonds) and forms helical structures.
The iodide can accumulate inside those helices and turns purple / brownred. That’s why only the starch rich cells become brown-red. Only the light
exposed part changes color in Lugol, because only this part of the leaf can
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4 RESULTS & DISCUSSION
make photosynthesis. The fixed CO2 becomes part of newly synthesized glucose which forms starch. In the darkened part of the leaf even consumes the
starch reserves to come up to the energy needs of the cells. The starch stays
in the cells, because starch is a much to big Molecule, to diffuse through
the membrane. So the starch test only shows the cells, that produce starch.
After 2 hours, most of the starch is used by the darkened cells, whereas the
lightened cells produce even more starch.
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