Environmental Studies: Photosynthesis Concept Report

Abstract

The rate at which photosynthesis is affected by various environmental parameters such as carbon dioxide concentration, temperature and light intensity was examined under conditions in which these parameters could be manipulated. The influence of temperature on the rate photosynthesis was measured at room and icy temperatures. Similarly, the effect of carbon dioxide and light intensity on the rate of photosynthesis was also measured by varying the concentration and distance of carbon dioxide (NaHCO₃) respectively.

When the hypothesis was tested, the concentration of NaHCO₃ solution to which the spinach disks are exposed increased, the rate of photosynthesis also increased. In addition, the hypothesis that as the temperature at which the spinach disks are exposed decreases, the rate of photosynthesis decreases.

The effect of these environmental on the rate of photosynthesis was measured concurrently. The rate of photosynthesis was relatively high at room temperature, high carbon dioxide concentration and high light intensity. The data obtained suggest that these environmental parameters are interlinked. That is, for the rate of photosynthesis to be at it optimum, these factors too must at their optima levels in the biological system where photosynthesis is taking place.

Introduction

Photosynthesis is an important process that supports aerobic life on Earth. This is because as the photoautotrophs including plants, algae and some species of bacteria converts carbon dioxide and water into sugars using sunlight energy, they release oxygen. This waste product is the major constituent of aerobic respiration.

The sugars produced are a source of food to all forms of life on earth (Smith 508) making photosynthesis a very vital component of food chains and food webs supporting all trophic levels as the primary source of energy. Photosynthesis takes place mainly in the chloroplasts which have chlorophyll, carotenes, xanthophylls and phycocyanin among other pigments depending on the organism. These are pigments that absorb light energy which is important during the process of photosynthesis.

Photosynthesis depends on both the light and dark reactions. Taking place in the thylakoid membranes of the chloroplasts, light-dependent reactions involve the capture and storage of light energy in the form NADPH and ATP which are chemical energy carrying molecules. These molecules are then used in the light-independent reactions which take place in the stroma. They are used in photolysis and carbon fixation leading to the formation of sugar compounds and release of oxygen as a waste product (Bidlack 367 – 372).

However, for the process of photosynthesis to place at optimum level capable of sustaining life on earth, the environmental conditions must be right. This will enable the various biological systems such as photosystems I and II involved in the process of photosynthesis to work at optimal levels, hence, higher rate of photosynthesis. However, when the levels surpass the optimal levels, the rate of conversion of carbon dioxide into sugars will either plateau or decline.

Carbon dioxide is an integral factor in this process as it is assimilated to produce carbohydrates. RuBisCO enzyme can fix both oxygen and carbon dioxide depending on their partial pressures, hence, it is very important for carbon dioxide concentrations to be high. This is because high oxygen concentrations will lead to photorespiration, a process that forms phosphoglycolate.

This two carbon compound is both toxic and requires higher energy to break down and, in the process, nitrogen is lost through the formation of ammonia (Blankenship, 452 – 470). Temperature plays a critical role in the photochemical reactions especially those involved in the absorption of light energy to be stored in the form of energy carrying molecules, ATP and NADPH (Reece and Campbell 402 – 415).

Moreover, temperature also affects the function of RuBisCO involved in carbon fixation. The rate of photosynthesis increases with increasing temperature up to an optimum then slows down. This is because high temperatures denature the RuBisCO enzyme and chlorophyll and other pigments involved in absorption of sunlight energy. Photolysis of water and carbon dioxide assimilation requires high light intensity and hence high photosynthesis rate will increase up to an optimum level. Past optimum, the rate of photosynthesis does not increase nor decrease, hence, the rate of photosynthesis levels off.

Hypotheses of the experiment

At the end of this experiment, it was hoped that the results would make us accept or reject the hypothesis that the distance between the lamps and the spinach disks increases the rate of photosynthesis. Moreover, this experiment was aimed at testing the hypothesis that as the concentration of NaHCO₃ solution to which the spinach disks are exposed increases, the rate of photosynthesis increases. In addition, the hypothesis that as the temperature at which the spinach disks are exposed decreases, the rate of photosynthesis decreases.

Materials, Apparatus and Equipments

• Two 50 ml beakers
• One 250 ml beaker
• Clamp for support stand
• Hole punch, 1
• One lamp with two 40W bulb
• 30 cm metric ruler
• Approximately 5 ml of liquid soap
• 1% solution of sodium bicarbonate (NaHCO₃) and 40 ml solution of 0.1% soap
• 5% solution of sodium bicarbonate (NaHCO₃) and 40 ml solution of 0.1% soap
• 150 ml of crushed ice as assigned
• 7.5% solution of sodium bicarbonate (NaHCO₃) and 40 ml solution of 0.1% soap
• 2 – 4 fresh spinach leaves
• Support stand
• 12 ml Luer lock tip syringe without needle
• Approximately 100 ml of cold tap water as assigned

Procedure

Using the hole punch, small 20 spinach leaf disks were cut to allow them fit into the syringe barrel and increasing the surface area for absorption of NaHC0₃.

The leaf disks were placed in the barrel of the syringe.

The plunger was then used to create pressure in the barrel by pushing it to the 2 ml mark. Since the structure of the leaf disks is important for this experiment, the depression was done carefully.

After measuring and pouring approximately 10 ml of 1% NaHCO₃ into the 50 ml beaker, approximately 3 ml of the solution was drawn into the syringe by pulling back the plunger using the tip of the syringe. The same was repeated for 1% (three more separate tests), 5% and 7.5% NaHCO₃. The NaHCO₃ provided the CO₂ required for photosynthesis. The varying concentrations were used to measure photosynthesis at varying concentrations of CO₂ and under varying light intensity and different temperatures.

Using a tip cover, finger or thumb, the syringe tip was covered tightly and swirled to suspend the spinach leaf disks in the syringe. After creating a resistance by pulling back plunger, the position was maintained for 10 seconds while simultaneously swirling the syringe. This was meant to increase the density of the disks by filling their airspaces with liquid thus making them sink.

After releasing the plunger and the disks floated, the step above was repeated three times. A half drop of liquid soap was added if after the three trials the disks failed to sink and the previous step repeated. This was necessary to increase the density of the disks by filling their airspaces with the liquid and consequently their sinking. However, when the disks floated after the initial step above, the experiment proceeded to the next step.

Approximately, 40 ml cold tap water and 1% freezer-stored sodium bicarbonate were poured in two separate 50 ml beakers and 20 leaf disks added to each beaker. The same was repeated for beakers of 40 ml 1% (three separate beakers), 5% and 7.5% NaHCO₃.

The beakers in the above procedure were all left under room temperature with the exception of the one containing 40 ml of freezer-stored 1% NaHCO₃ which was placed deep down into approximately 100 ml of crushed ice in a 250 ml beaker.

After fixing the 40 watts bulb on the clamp and attaching it to the support, the lamp was switched on and its position, relative to the top of the 50 ml beakers containing the disks. The beaker with the 20 disks in 1% NaHCO₃ at room temperature was placed at 5 cm, and the beakers with disks in 7.5% and 5% NaHCO₃ solutions at room temperature were placed at 10 cm.

The beaker with 1% NaHCO₃ at room temperature was placed at 20 cm while the remaining two beakers with disks in 1% NaHCO₃ solution in ice and room temperatures were placed at 10 cm. This was done to measure photosynthesis rate at room and ice temperatures respectively, at different CO₂ concentrations and varying light intensity.

The rate of photosynthesis was estimated by the number of floating disks after every one minute for 15 minutes. The data were then tabulated for analysis.

Results

Table 1: Showing number of spinach disks floating at 1 minute intervals for 15 minutes for 6 combinations of three environmental parameters

Table 2: Showing rate of photosynthesis as number of spinach leaf disks floating per minute for 6 combinations of three environmental parameters

When the influence of temperature on the rate photosynthesis was measured at room and icy temperatures, the rate of photosynthesis was relatively high at room temperature, high carbon dioxide concentration and high light intensity. The data obtained suggest that these environmental parameters are interlinked.

Discussion

When the disks were placed into the solution and the syringe used to force the liquid into the airspaces of the leaf disks, they became heavier and sank. However, as photosynthesis took place at various light intensity, temperature and carbon dioxide concentrations, they began to float. This is because oxygen, which is produced as a waste product during photosynthesis, fills in the airspaces of the leaf disks making them less dense than the water and 1% NaHCO₃ they are placed in.

The carbon dioxide needed by the process of photosynthesis is supplied by NaHCO₃ solution. As the concentration of NaHCO₃ increases, the rate of photosynthesis increases as recorded by the indirect proportionality between the number of leaf disks floating. Higher concentration of NaHCO3 releases more carbon dioxide and as more oxygen is released, the rate of cellular respiration also increases.

The optimum concentration for photosynthesis in this experiment was 7.5% NaHCO₃ while the optimum temperature is at room temperature. Carbon dioxide assimilation is at optimum when the light intensity is at 10 cm from the beaker containing the liquids and the leaf disks.

In conclusion, Environmental parameters do not act as single entities to influence the rate of photosynthesis. Even if the light intensity and carbon dioxide concentration are at their optima, the rate of photosynthesis will be relatively low if the temperature is low and vice versa. Thus, for the rate of photosynthesis to reach its optimum level, the concentration of carbon dioxide, temperature as well as the light intensity must be at their optima.

Works Cited

Blankenship, Richard. Molecular Mechanisms of Photosynthesis (2^nd ed.). New York: John Wiley & Sons Inc, 2008. Print.

Bidlack, James, et al. Introductory plant biology. New York: McGraw-Hill, 2003. Print.

Reece, Julie and Campbell, Nick. Biology. San Francisco: Pearson, Benjamin Cummings, 2005. Print.

Smith, Allen. Oxford dictionary of biochemistry and molecular biology. Oxford: Oxford University Press, 1997. Print.