The effect of light quantity on photosynthesis of green land plants Introduction
Photosynthesis is the process of converting light energy into chemical energy (Hoober 1984). Pigments within chloroplasts, primarily chlorophyll, absorb the incoming solar energy which excites their electrons (Hoober 1984). These pigments exist in photosystems in the thylakoid membrane of the chloroplast (Ladiges et al.2010). As the electrons return to ground level, they are captured by the electron acceptor in the reaction centre of the photosystem (Ladiges et al. 2010). The reaction centre than transfers captured electrons to the electron transport chain (Hoober 1984). The electrons are carried in the form of NADPH, which is then reduced (Hoober 1984). The hydrogen ion produced from this reduction reaction then passes through ATP synthase, generating ATP (Hoober 1984). The chemical DCPIP acts as an electron acceptor and is used to measure the rate of electron transport in the thylakoid membrane of chloroplasts (Dean and Miskiewicz 2006). Initially DCPIP is a blue colour. Although, when it gains electrons from the transport chain it is reduced and turns colourless (Dean and Miskiewicz 2006). A high photosynthetic rate can be interpreted by a fast rate of change from blue to colourless of the DCPIP as more electrons are flowing through the transport chain and reducing the DCPIP. The colour of DCPIP is measured using a spectrophotometer at 605nm. The rate of photosynthesis is dependent on many factors, in particular light quantity (Johkan et al. 2012). A greater quantity of light received by the chloroplasts equates to a greater amount of solar energy potentially converted into ATP. In turn this causes a higher flow of electrons in the transport chain. The importance of light quantity for photosynthesis is relevant to the growth and harnessing of energy by plants. This can be useful for understanding ecosystems, such as rainforests where the amount of light received by plants is greatly reduced further down the canopy (Lee 1987), and for agriculture as optimum growth conditions can increase production of crops. The hypothesis was that the greater the quantity of light the higher the rate of photosynthesis and hence the faster reduction of DCPIP, as more electrons travel through the transport chain. The effect of light quantity was answered using isolated chloroplasts exposed to differing intensities of light, with photosynthetic activity measured using DCPIP and a spectrophotometer at 605nm. Method
Isolating chloroplast: Approximately 4g of spinach leaf was torn into pieces, removing major veins. The leaf pieces were then placed into a cooled mortar and pestle. 15mL of cold isolation medium was then dispensed into the same mortar and pestle, and the leaf pieces were ground into a smooth paste. Using a funnel and two pieces of gauze, the ground leaf paste was then filtered into a cold 15mL plastic centrifuge sitting on ice. The plant material was then squeezed to recover as much filtrate as possible. While the filtrate was being centrifuged the mortar and pestle were washed, whilst the funnel and gauze were placed on the bench to dry. Tables were then set up for record keeping. The supernatant was carefully decanted by pouring it into a new, cold 15mL centrifuge tube and stored on ice, being careful not to lose the dark green pellet at the bottom. A sufficient amount of ice-cold isolation medium was added to the tube containing the pellet so that the final volume was approximately 1mL. If 1mL isolate was already present, no more isolation medium was added. Very carefully, the pellet was re-suspended by swirling and gently flicking the tube. The chloroplast isolate was then stored on ice. Measuring absorbance: The spectrophotometer was blanked at 605nm using a phosphate buffer. Three cuvettes were labeled and 5mL DCPIP was dispensed into each cuvette. Using a pipette, 20uL chloroplast isolate was added to each cuvette, which were then...
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