Investigating Photosynthesis

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  Investigating photosynthesis using immobilised algae Class practical This procedure offers a method for measuring the rate of photosynthesis  which depends directly on the rate of uptake of carbon dioxide  by the photosynthetic organism. Hydrogencarbonate indicator   changes colour with pH, which is determined by the concentration of carbon dioxide in solution.The photosynthetic organism is a fast-growing green alga  – such as Scenedesmus quadricauda  – immobilised  in alginate beads. These algal balls make it easy to standardise the amount of photosynthetic tissue in any investigation.There is scope for students to develop the protocol to investigate a range of factors.This protocol is adapted with permission from information on the Science and lants for Schools !S S# website !see $%& #. Lesson organisation 'ou can make up the algal balls in one lesson, discuss how to set up the main procedure, and carry it through in the next lesson. preliminary investigation by teacher( technician will allow you to estimate the amount of algal material and indicator to use to get a result with your e)uipment in the time available. Apparatus and Chemicals For each group of students: Transparent containers !glass bottles are ideal# with sealable lids, around %* cm + , –%lamp stand, boss and clampSyringe barrel, %* cm + /eaker, %** cm + , ocktail stick to stir alginate%0* 1 lamp ! Note 5 #ontainer of water as heat filter ! Note 6 #  2uler( tape measure For the class – set up by technician/ teacher: olorimeter ! Note  #  lgal suspension, .0 cm +  concentrated for each group ! Note ! #Sodium alginate, –+3, %** cm +  for a class of +* students ! Note #alcium chloride solution, 3, 0* cm +  per groupHydrogencarbonate indicator, 0*–%** cm +  per group ! Note # #Hydrogencarbonate indicator, standard colour scale, if colorimeter not available ! Note # #4ight meters, if available Health $ %afety and &echnical notes 5o not look directly into the lamps.5o not touch the lamps while hot.6eep flammable material away from the lamps in use.2ead our standard health 7 safety guidance   olorimeter. There is a linear relationship between absorbance of the indicator !at 00* nm – bright green filter# and pH over the range studied in this procedure. !  8rowing your alga. repare a culture of green alga such as unicellular Scenedesmus quadricauda .9ake up a solution of algal enrichment medium, and subculture the alga into this. erate gently and keep at temperatures between %:– ;. onstant illumination ensures faster growth of the alga.   fter +–< weeks, the culture should have a green =pea soup> colour. Subculture the alga again to maintain a healthy culture. 'ou could use other algae, but Scenedesmus  should produce  to + litresof dark green =soup> in about < weeks from 0* cm +  of srcinal culture. !5etails from S S Sheet +#.  reparing solutions to make alginate beads !2efer to 2ecipe card #? ã 5issolve + g of sodium alginate in %** cm +  of cold, pure water. Stir with a spatula every half hour or so. 4eave overnight and stir in the morning. ã 5issolve < g of calcium chloride--water in ** cm +  of pure water in a 0* cm +  beaker. #  Hydrogencarbonate indicator. 2efer to 2ecipe card +< and Ha@card +. 4ow ha@ard once madeA must be made fresh by )ualified staff using fume cupboard. The indicator is very sensitive to changes in pH, so rinse all apparatus with the indicator before use. void exhaling over open containers of the indicator. 9ake up a =standard colour scale> of reaction bottles containing buffers  from pH B.- pH C. with hydrogencarbonate indicator if students will not have access to a colorimeter. 5  4amps. 'ou need a brighter light than a standard <* 1 or * 1 bench light. 4ow energy bulbs produce too limited a spectrum of light for full activity. %0* 1 tungsten or halogen lamps are best. %0* 1 portable halogen lamps have a stand and handle separate from the body of the lamp which makes them safer to handle. /ut they do produce heat, so you will need a heat filter for the investigation ! Note 6 #. 6  Heat filter. Dse a large flat-sided glass vase !available from Ekea or Homebase or other domestic suppliers# or a medical =flat> filled with water. 1ith a high power lamp, the small volume in a medical =flat> may get too hot for comfort. '  9aking alginate beads? ã 1hen making up the alginate or diluting the algal culture it is essential to use pure waterA otherwise calcium ions in the water will cause the alginate to FsetF prematurely. ã Ef your beads are not the si@e and texture you want, try different mixes with your active material, or different concentrations of sodium alginate !around –+3#, or make the syringe no@@le narrower !with glass capillary tubing# or wider !by sawing off and adding a plastic tube#. 5ifferent brands of alginate have different consistencies. 'ou need a viscous mixture that will drip steadily through the syringe. 6eep a note of what worked for this supply and keep your syringe barrels with no@@les for next time. (  Geutral density filters? can be sourced as a film from photographic suppliers !for example, 4ee filters  $+& . neutral density filter reduces the amount of light transmitted by the same amount at all wavelengths. )thical issues There are no ethical issues associated with this procedure. *rocedure S IT'? Take precautions to avoid burns, fires and da@@le caused by hot, bright lamps.5o not leave the apparatus unattended overnight as the lamps are so hot. *reparation of algal beads  a  oncentrate your active algal culture ! Note ! #. 5o this by leaving 0* cm +  to settle for +* minutes ina measuring cylinder until you have a darker green sediment. arefully pour off the pale green suspension to leave Just 0 cm +  of concentrated culture. b  9ake up the solutions you need to prepare alginate beads ! Note #. c  9ix 0 cm +  of the algal culture with 0 cm +  of the +3 sodium alginate solution ! Note # in a very small beaker.


Jul 23, 2017

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