Photosynthesis Experiment in Elodea in Direct Light, Distant Light, and Dark
In 1905 an English plant physiologist by the name of F.F. Blackmen found that increased light caused an increase in photosynthesis.
He broke photosynthesis into the light reactions, or a light-dependent phase, and the dark reactions, or carbon dioxide fixation.
The light dependent phase of photosynthesis is when a photon (a light energy unit) is needed to excite two electrons in chlorophyll that eventually breaks down a water molecule which creates ATP from ADP and forms NADPH2 from NADPH. This process is called photophoshphorylation.
Photophosphorylation is broken down into two types. One type is called cyclic photophosphorylation which refers to the excitation of electrons between the different pigments in the thylakoids.
The second type in non-cyclic photophosphorylation refers to the breaking down of a water molecule.
A collection of the different pigment types, mainly chlorophyll a, chlorophyll b, and carotenoids, used during photophosphorylation is called a photosystem.
There are three different photosystems used during photophosphorylation. The reactive pigment in each photosystem is called the reaction center while the others are called antenna pigments.
In photosystem II of non-cyclic photophosphorylation, the two electrons cause the water molecule to split forming two H+, and free oxygen that will combine with another oxygen to form an oxygen molecule. At this point 2ATP is created by the breakdown and NADPH grabs the free H+.
In 1938 Ruben and Kamen tagged both water and carbon dioxide molecules with oxygen-18. One plant was grown in the tagged water and another was fed the tagged carbon dioxide.
The plant that was grown in the tagged water produced a larger number of oxygen molecules with oxygen-18 than the plant fed the tagged carbon dioxide.
This experiment was able to show that the water molecule provided the oxygen molecules that are released into the atmosphere, thus carbon dioxide is primarily used in the formation of carbohydrates.
After the water molecule is broken down one H+ enters the respiratory chain and is reunited with oxygen during the final cytochrome oxidase step to recreate another water molecule. This process produces 3ATP.
The other H+ combines with the carbon dioxide to form carbohydrate. This process, called carbon dioxide fixation or the dark reaction, requires the ATP that was created during the respiratory chain.
There are many byproducts of photosynthesis including organic phosphates, amno acids, and organic acids. Some of the sugar phosphate compound byproducts are used to produce sucrose, starch, and cellulose.
Sugars are extremely soluble in water and can cause osmotic balance problems. To avoid this problem plants convert sugars into starch within the chloroplasts. Starch is then broken down again when glucose is needed for cellular metabolism.
In the Calvin-Benson pathway, carbon dioxide eventually anabolises into a 5-carbon sugar with 2 phosphates or "ribose-1.5-diphosphate." The 5-carbon sugar then splits into two 3-carbon compounds called phosphoglyceric acid (PGA) which, after the addition of H+, eventually becomes a starch.
The Hatch-Slack pathway is similar, yet a 4-carbon splits to form oxaloacetic acid.
The purpose of observing the change in oxygen production by Elodea when placed in direct white light, indirect white light, and darkness is to show how light is necessary to excite electrons and start photophosphorylation.
Materials and Methods
A YUS-190-M Photosynthesis Apparatus was clamped horizontally onto a stand. A leafy shoot of Elodea was cut and the cut end was placed inside a clear plastic tube located on the lower right corner of the apparatus.
The Elodea was then placed into a beaker with a 0.05% potassium bicarbonate solution. The solution was drawn through the apparatus by using syringes located on the upper left hand corner of the apparatus.
Once the Elodea formed a bubble of oxygen in the clear plastic tubing, the bubble was drawn into the measuring capillary located within the apparatus and measured. All the air bubbles produced were measured within one minute, and the sum of all the measurements within a minute was reported as mm/min.
These measurements were repeated, every five minutes with a close light source, a distant light source, and darkness.
Each situation was repeated, with different Elodea, three time and averaged.
There was a variance in the amount of oxygen produced in the averaged values of the three Elodea.
The first Elodea produced 1.46 mm/min of oxygen in the presence of close white light while the second Elodea produced 2.65 mm/min and the third produced 4 mm/min.
In distanced light the first Elodea produced 1.45 mm/min, the second 1.28 mm/min, and the third 1.5 mm/min.
In darkness, the first Elodea produced 0.052 mm/min, the second 0.128 mm/min, and the third 1.5 mm/min.
Elodea produced approximately 90% less oxygen in darkness then light, and approximately 45% less oxygen in distant light.
It is obvious that there is a significant difference in the percentage of oxygen produced in the Elodea between the dark and light environments.
This shows that the largest percentage of oxygen is produced in the light dependent phase of photosynthesis. Light is needed to excite the two electrons in cyclic photophosporylation in photosystem I to break down the water molecule in non-cyclic photophosphorylation and donate the oxygen necessary to create oxygen molecules that are released into our atmosphere.
This experiment also shows that a small percentage of oxygen is produced during the dark reaction, or the carbon fixation stage, when the carbon dioxide combines with H+ to form carbohydrates.
Please read for more information on similar Photosynthesis experiments:
- The Effect of Temperature on Production of Oxygen in Photosynthesis in Elodea
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