Реферат: Finding About Photosynthes Essay Research Paper

(b) placed in a flask with potassium hydroxide

(c) placed in a flask without potassium hydroxide

Sketch your results in a copy of table 2.5A

Results:

Treatment Sketch of leaf before testing for starch Sketch of leaf after testing for starch Observation

Leaf left in the dark 24 before experiment commenced.

Leaf left in the light for 24 hours

Leaf partly covered with aluminium foil

Leaf in the flask with

potassium hydroxide

Leaf in the flask without potassium hydroxide

Discussion:

If the starch test is performed, and the iodine remains yellow this indicates that there is no starch in the leaf. After the plant had been in the dark for 24 hours there was no starch present in the leaf. The plant was left in the dark for 24 hours prior to the class so that we could test if photosynthesis would occur without the presence of light. It was also a control for the plant that was tested for starch that had been in the light for 24 hours. Starch was present after the plant was exposed to the light for 24 hours, it was found in next to one of the veins, on a dark green section.

Without light plants can not photosynthesis, they have to rely on the starch the plant may have stored on previous occasions if no light is present. Therefore plants need light to photosynthesis. Photosynthesis does not occur on all parts of the leaf, but only the parts with chlorophyll (the green sections).

The differences in colour on the leaf are caused by adaptations made by the plant. The parts of the leaf that are primarily exposed to sunlight contain much more chlorophyll and are the dark green patches. This makes the plant more efficient at photosynthesis by only placing the chlorophyll pigments where the sun s rays will reach it. For example the small plants on the ground level of the rainforest do not always get maximum sunlight coverage because the taller trees above it block out some patches. The plant adjusts to this by having its chlorophyll pigments around the edges of the leaf where the sunlight will most probably hit it. In the experiment the only parts of the leaf that contained starch were the dark green sections.

The leaf that had a strip of aluminium foil covering a small part of it proved that light is necessary for photosynthesis. If the whole leaf was covered there would have been no control to prove that light was needed. The section of the leaf that was covered showed no trace of starch, but the uncovered section had a quite large amount of starch. Thus, light is an important factor of photosynthesis.

Carbon dioxide is needed for photosynthesis, to test this we set up a leaf in a flask with Potassium hydroxide, and one without. In the flask containing potassium hydroxide there was a little trace of starch in the leaf. This might have been there beforehand, or maybe some of the potassium hydroxide leaked out of the cotton wool. For this experiment to be more accurate we could have used something more reliable than the cotton wool to keep the potassium hydroxide trapped in the flask.

The flask, which did not contain Potassium hydroxide, showed that starch was present, much more than the leaf in the flask with the solution. The more carbon dioxide in the air increases the rate of photosynthesis. Because potassium hydroxide absorbed the carbon dioxide in the flask, photosynthesis could not be achieved at its full potential because one of its input factors (carbon dioxide) was lacking or not present at all.

Each of the experiments had a control; they were the part of the plant not covered in foil, which was to see if light was required. The plant left in the dark for 24 hours was also testing to see if light was essential, and the flask with no potassium hydroxide, was to see if carbon dioxide was essential.

Conclusion:

During this practical report I have learnt that there are three main factors that must be present for photosynthesis to occur. They are Carbon dioxide, light and chlorophyll. As a result of this experiment I have discovered that if a plant is deprived of light or carbon dioxide for a period of time it begins to use up its starch deposits to make food for its self.