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Photosynthesis and Respiration

Photosynthesis and respiration are processes occurring in plants for the purpose of making energy available to be used for metabolic needs. Photosynthesis involves the conversion of light energy from the sun into chemical energy stored in glucose molecules. It requires carbon dioxide and water as reactants in glucose synthesis with oxygen as a by-product. On the other hand, respiration is the process of breaking down glucose to release ATP – the form of energy that cells can use. Glucose has to combine with oxygen in order to produce ATP with by-products of carbon dioxide and water (Farabee, 2007).

These by-products are again used in photosynthesis. This experiment entails the study of the effects of different variables on the occurrence and rate of photosynthesis and respiration. These variables are: the presence or absence of light, the intensity of light, wavelength and temperature. Rate is measured as the time it takes for the processes to occur. Oxygen is a common gas molecule used in both processes. During photosynthesis, it diffuses out of the cell into the air spaces of the leaves and out into the atmosphere (Farabee, 2007).

In the event that the leaf is submerged in water, diffusion into the atmosphere cannot proceed because of the pressure of water molecules on the surface of the leaf. Like an inflated balloon, the trapped oxygen causes the leaf to rise to the surface. Conversely, as respiration utilizes the oxygen produced, buoyancy decreases and the leaf sinks. These two events are used as indicators to determine if photosynthesis or respiration is taking place in the leaf and allow the comparison of their rate under different variables. Materials and Methods

In this experiment, the following materials are needed: fresh and healthy leaves, small straw, syringe, baking soda, film canister, baking soda, detergent, timer, water bath with ice, and light source. To investigate photosynthesis, create eight leaf discs by punching out the leaves using the straw. Be sure not to include veins and stems. In a syringe, put four disks. Place a small amount of baking soda into the film canister so that the bottom is barely covered then fill with water and a drop of detergent. Shake to form a solution and using the syringe, obtain 4ml from the canister.

With the leaf discs floating, as is normal, carefully push the plunger to force out surplus air from the barrel. Remove the needle and covering the hole of the syringe with a finger, repeatedly push and then pull the plunger until the leaf disks descend to the bottom. Pushing compresses the leaf disks while pulling builds a vacuum and liberates the air contained in the air spaces. Facilitated by the detergent, the solution enters the stomata and the air spaces, increases the weight of the leaf disks and causes them to sink (Williamson, 2008).

Meanwhile, the dissolved baking soda functions as a reactant in place of carbon dioxide. In examining respiration, prepare the syringes using the same procedure except this last step. To observe the effects of light intensity on photosynthesis, three syringes are placed at the same time – one near a strong light source, one farther away and one in the dark. To determine if respiration occurs in the absence of light, place a syringe near a light source and one in the dark. In both procedures, temperature should be controlled.

To determine the effect of temperature, prepare two pairs of syringes with each pair consisting of one photosynthesis and one respiration syringe. A pair is set in an ice bath and the other pair at room temperature. Because they are dependent on light, photosynthesis syringes should be near a light source. The respiration syringes should be in the dark. In all experiment procedures, the time it takes for the leaf disks to rise if the syringe is for photosynthesis is recorded.

Similarly, the time it takes for the leaf disks to sink is noted for the respiration syringes. Results and Discussion In the photosynthesis syringes, it took an average 825 seconds for the disks to float in room temperature. On the other hand, it took 1,590 seconds on the average for the disks to rise in ice cold temperature. Therefore, very low temperatures decrease the rate of photosynthesis. Concerning the respiration syringes, no leaf disks sank at ice cold temperature while one leaf disk sank after 420 seconds at room temperature.

The data demonstrates that although light does not affect respiration, temperature does. Very low temperatures inhibit respiration. Why this occurred may be because both photosynthesis and respiration are catalyzed by enzymes. At an optimum temperature (around 25oC), the rate of reaction of these proteins is at its peak (Todd, 2000). Above and below this temperature, the rate of reaction decreases or comes to a halt so that no processes occur or if they do, take longer time. List of References Farabee, M. J. (2007). Photosynthesis.

Retrieved April 6, 2009 from http://www. emc. maricopa. edu/faculty/farabee/BIOBK/BioBookPS. html. Photosynthesis Experiment Procedures Handout Todd, G. W. (2000). Photosynthesis and Respiration of Vegetative Reproductive Parts of Wheat and Barley Plants in Response to Increasing Temperature. http://digital. library. okstate. edu/OAS/oas_pdf/v62/p57_62. pdf. Williamson, B. (2008). The Floating Leaf Disk Assay for Investigating Photosynthesis. Retrieved April 6, 2009 from http://www. elbiology. com/labtools/Leafdisk. html.

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