Photosynthesis PAG PDF

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An experiment write up on photosynthesis rates are affected by the distance from a light source....

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PAG 12 – Investigating Oxygen Production (Rate of Photosynthesis) of Plants Photosynthesis – How is oxygen produced? Plants are autotrophs, producing their own food via photosynthesis with the by-product of oxygen. There are two stages to photosynthesis, the light dependent reaction (LDR) and light independent reaction (LIR). Oxygen is produced during the LDR in the thylakoid membranes, situated in chloroplasts. This begins with light energy exciting electrons in the chlorophyll of photosystem II (PSII). These electrons enter the electron transport chain towards PSI, creating an electrochemical gradient for chemiosmosis and finally binding with hydrogen ions and NADP to form NADPH, used in the LIR. However, these electrons must be replaced when they leave PSII. This is done by the photolysis of water into electrons, hydrogen atoms and oxygen. The oxygen can then be used in respiration or diffuse out of the leaf via stoma. Equipment -

100ml glass beaker 10cm3 syringe

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Hole punch Healthy plant

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Sodium bicarbonate Light source (lamp)

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Meter ruler Stopwatch

Method 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

Collect and set up apparatus as shown above. Add 50ml of sodium bicarbonate to a 100ml glass beaker and place by lamp. Using a hole punch, collect 4 discs from a healthy green leaf. (2) Add discs to the barrel of the syringe, replacing plunger afterwards and carefully draw 6-8cm3 of sodium bicarbonate into it. (2) Hold syringe with the tip facing up and gently push plunger to get all the air out of the syringe, the discs should be floating. (2) Place thumb over the tip and pull plunger down to create a vacuum, hold for a few seconds. (2) Release the plunger so it snaps back into the barrel and remove thumb from tip, this draws residual air out of the discs. (2) Continue steps 6-7 until discs sink to bottom of the sodium bicarbonate in the syringe, tapping on the plunger and barrel to help release bubbles from the discs. (2) Once all discs sink, carefully remove the plunger and pour discs into the beaker of sodium bicarbonate. Remove any that float. (2) Place beaker 0cm away from lamp, turn it on and start the timer. (1) Time how long it takes for one disc to float to surface and record in table. Repeat steps 4-11, four more times to calculate average. Repeat steps 3-12 for 5, 10, 20 and 40cm away from lamp. (1)

Results Table

Distance of light source from plant (cm) 0 5 10 20 40

Time taken for disc to float to surface (s)

Test 1 35 63 82 213 323

Test 2 45 72 91 193 341

Test 3 38 87 83 215 359

Test 4 41 58 84 187 401

Rate of photosynthesis (1/t) Test 5 31 65 89 215 382

Average 38.0 69.0 85.8 204.6 361.2

0.026 0.014 0.012 0.005 0.003

Conclusion As the graph shows, when the discs are closer to the light source, the rate of photosynthesis increases, with the fastest rate being 0.026 at 0cm and the lowest at 0.003, 40cm away from the light source. This is due to light intensity not being the limiting factor when the discs are closer to the light source. A higher light intensity (closer to the light source) increases the rate of the light dependent reaction, where photolysis occurs. Photolysis splits the water molecules into hydrogen ions, electrons and oxygen. This oxygen then diffuses out of the chloroplasts and will either be used in respiration or diffuse out of the disc. However, whilst it is trapped as a gas in the spongy mesophyll layer, it causes the disc to float to the surface. A lower light intensity would slow the rate of the LDR and therefore it takes longer for enough oxygen to be made and stored for the disc to float. References Source 1: Charlotte Burrows, Christopher Lindle, Christopher McGarry, Claire Plowman, Rachael Rogers, Hayley Thompson (eds) (2015), A Level Biology Exam Board: OCR A, Coordination Group Publications Ltd. (CGP) Source 2: Exploratorium (unknown), Photosynthetic Floatation [online], Last Accessed 29/11/2018: https://www.exploratorium.edu/snacks/photosynthetic-floatation...