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  Normal laboratory safety procedures should be followed.There is a slight risk of infection from pond water, so take sensible hygiene precautions, cover cuts and wash hands thoroughly after the work is complete.Read our standard health & safety guidance 1   Elodea  can be stored in a fish tank on a windowsill, in the laboratory or prep room. However it is probably a good idea to replace it every so often with a fresh supply from an aquarist centre or a pond. !t s worth finding out if any colleague has a pond.# $n the day of the e%periment, cut ' cm lengths of Elodea , put a paper(clip on one end to weigh them down and place in a boiling tube of water in a boiling tube rack, near a high intensity lamp, such as a halogen lamp or a fluorescent striplight. )heck the Elodea  to see if it is bubbling. *ometimes cutting +- mm off the end of the Elodea  will induce bubbling from the cut end or change the sie of the bubbles being produced. 2   Cabomba available from pet shops or suppliers of aquaria  used as an o%ygenator in tropical fishtanks# can be used as an alternative to Elodea , and some people find it produces more bubbles. !t does, though tend to break apart very easily, and fish may eat it very quickly. 3  !f possible, provide cardboard to allow students to shield their e%periment from other lights in the room./ook out for small aquatic invertebrates attached to the pond weed used, and remove them to a pond or aquarium. a  *et up the apparatus as shown in a darkened room. b  /ook for a stream of bubbles coming from the cut end of the pondweed.  c  )ount the number of bubbles produced in  minute. Repeat twice and calculate a mean bubble count  number of bubbles per minute. This is the mean rate of bubble production. d  )hange one factor that may affect the rate of photosynthesis, keeping all other factors the same asfar as possible. Refer to teaching notes.# e  /eave for + minutes and then take counts of the number of bubbles produced in one minute. Repeat the count and calculate the mean rate of bubble production as before. f   )hange the factor being varied to another value and leave the pond weed for + minutes. Take bubble counts and calculate the mean bubbles 0 minute for the new value. g  1lot data on a line graph. The values for the factor being varied should be on the %(a%is, and the mean bubble rate in bubbles 0 minute should be on the y a%is. 2n assumption being made in this e%periment is that the gas bubbles being counted are only o%ygen, and that production of o%ygen is proportional to the rate of photosynthesis. 2nother assumption is that all the bubbles are the same sie. 2 variable that is hard to control is the effect of temperature as the lights used may heat the water. /34 lamps produce less heat.5ou can develop more accurate measures by collecting the gas produced and measuring the volumeof gas produced each minute or over longer time periods#. $ne method of doing this is to cut the graduated portion from the tube of a plastic teat pipette, fill the tube with water, plug the smaller hole with plasticine or blu(tack and invert the tube over the bubbling stem.The students can be allocated to investigate a particular factor that affects the rate of photosynthesis, or they can choose from this list, or they can develop their own ideas. ã /ight intensity or distance of the Elodea from the lamp. /ight intensity is proportional to 0distance + . ã Temperature of the water  ã )arbon dio%ide concentration. 2dd '.'6 g of potassium hydrogencarbonate at a time. The available mass of carbon dio%ide increases with each addition. ã  2cidity7 use buffer solutions to maintain different pHs. ã )olour of light. )oloured filters can be used, but these will affect light intensity by different amounts depending on the colour.  *tudents should collect and set up all the other apparatus before collecting the boiling tube of pond weed. )heck the weed to see if it is bubbling. *ometimes cutting +- mm off the end of the weed willinduce bubbling from the cut end, or change the sie of the bubbles being produced.The students could develop this practical as an assessed investigation if they develop hypotheses and plan the e%perimental detail from the core protocol.


Jul 23, 2017
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