Leaf Disc Experiment

Photosynthesis is a process of converting light energy (trapped by chlorophyll) to chemical energy, and stores it in bonds of sugar. Photosynthesis occurs in two stages; the light (light dependant) stage and the dark (light independent) stage.

The light energy is used to split water molecules, and all the colours of while light are absorbed by pigments in the chloroplasts (chlorophyll-containing plastid found in algal and green plant cell).The breakup of water releases oxygen electrons, and protons: Electrons move to the chlorophyll (green pigment) while protons are stored in the chloroplast and the oxygen is used for aerobic respiration for the plant or ejected into the atmosphere. The high speed electrons and the stored protons combine with CO2 to form glucose. 6H2O + 6CO2 C6H12O6 + 6H2O + 6O2 (Water + Carbon Dioxide Glucose + Water + Oxygen) Photosynthesis takes place predominately in plant leaves; rarely in plant stems. The cells in these leafs store cyclical energy in the form of adenosine triphosphate (ATP) molecules by breaking the energy bonds to release glucose.This glucose is used as fuel for performing cellular processes, and Eucalypt trees even produce their own food through photosynthesis, by using energy trapped to use for its own life processes. Throughout this experiment, the buoyancy of the leaves was the main indicator. At first, the leaves in the syringe floated, as they were already carrying oxygen from photosynthesising in a natural environment.

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The syringe acted as a plunge to remove excess oxygen causing leaves to sink. Sodium bicarbonate will replace the naturally occurring carbonate ions for the purpose of the experiment. This speeds up the reaction and release of carbon dioxide gas.

0 discs of each sample of eucalyptus will be placed in different concentrations of sodium bicarbonate, and placed under an energy source which in this situation is a projector light, making it a light dependant reaction. Research Question: Is there a variation in rate of photosynthesis between different species for eucalyptus? Aim: To use the lead disk method to measure photosynthesis in 3 species of eucalyptus trees, Hypothesis: If the eucalyptus disk rises quickly then the rate of photosynthesis will be higher because photosynthesis produces oxygen, causing buoyancy Apparatus: * Sieve * Glass beaker X3 Eucalyptus leaves (3 types) * Syringe * Deionised Water * Sodium bicarbonate solution * Hole Puncher * Light projector * Stopwatch Methodology 1. Prepare 25Ml syringe with 20 ML of deionised water. 2. Select 3 different types of eucalyptus and using a hole punch create 10 leaf discs of each species avoiding veins and ensuring to keep species separated and labelled. 3. Place the first species 10 discs into the syringe.

4. Draw back plunger holding finger over syringe opening, holding with tip upwards, release oxygen and repeat until all leaf disks are settled on the rubber plug surface of syringe. 5.Empty syringe contents into sieve, disposing of water, but moving the leaf discs into the three beakers. 6. Fill three beakers with 30Ml of NaHCO3 in each and allow leaves to sink to the bottom of the solution.

7. Place all three beakers beneath the projector light (energy source) in synchronisation, starting the stopwatch. 8. Stop the stopwatch once all ten discs have reached the surface for a particular species, and repeat until all 3 species are floating. Record results Note well: All species must be placed beneath the same light at the same time so that heat and light variables are held constant.Variable| Type| Method| Light intensity| Controlled Variable| Putting all beakers under light at same time| Size of beaker| Controlled Variable| Used 3 40ml beakers| Circumference of leaf discs| Controlled Variable| Hole punch| Amount of NaHCO3| Controlled Variable| 30ml| Types of Leaves| Independent Variable| 3 different species, all discs from same leaf| Rate of photosynthesis(time)| Dependant Variable| Stopwatch| Results Disc #| 0%| 2%| 4%| 6%| 8%| 10%| 1| | 315| 207| 221| 174| 81| 2| | 327| 229| 321| 176| 83| 3| | 338| 242| 336| 206| 99| 4| | 353| 299| 348| 214| 154| | | 431| 330| 540| 241| 157| 6| | 382| 351| 373| 191| 171| Average| | 377| 276| 377| 200. 3| 144| Discussion The graph above shows huge range in results, clearly displaying how photosynthesis rates can differ greatly even within the same plant.

The time of Eucalyptus Globulus (21:00. 02) was almost double the time of the fastest species to photosynthesise: Eucalyptus Viminalis (11. 04. 30). However the cause for this range cannot be proven.

E. Globulus could have a higher density or thickness than E. Viminalis, therefore it takes longer for discs to float. It could also be that E.Viminalis contains more chlorophyll and therefore increasing the rate of oxygen uptake.

There are several issues with the way in which this experiment was carried out and subsequent variables overlooked: * Before leaves were introduced to heat (projector) the temperature of the deionised water and sodium bicarbonate were not controlled across all three beakers * Experiment was only undertaken once, therefore higher chance of error in result * All 10 discs were used in the results, also ensuing in a higher chance of error to occur * Timing was ineffective and too broad; ‘lap’s’ should have been taken each ime a disc rose. * To quicken the experiment, more sodium bicarbonate could’ve been used to make results more relative Should the experiment be repeated in future, possibly more leaves should be tested from each species; i. e.

3 species, 3 leaves used of each species, 10 discs out of each leaf. Conclusion In conclusion, the aim was correct as photosynthesis occurred in all leaf discs and was measured appropriately. In response to the hypothesis, it is possible that Eucalyptus Viminalis had the fastest rate of photosynthesis because all discs rose in the shortest amount of time because of higher levels of buoyancy.However this could be argued with more precise timing and repetition of experiment as mentioned in the discussion.

Author: Marcia Mcdaniel


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