Technical guide

Photosynthesis —A survival guide

Debbie Eldridge

Activity sheet

401 Technical guide – What sort of carbohydrates do plants make?

What sort ofcarbohydratesdo plants make?

Background informationA variety of plants and plant products canbe tested for glucose, starch and cellulose.Plants which give interesting results includeonion, apple, pear, celery, potato and grapes.It is also interesting to test materials such ascotton wool, paper towel, egg boxes andother materials made from recycled paper.These show up clearly the high proportionof cellulose fibres.

Benedict’s reagent will not give a colouredproduct with all simple sugars.All monosaccharide sugars such as glucose,fructose, and galactose will give a positiveresult as well as some disaccharide sugarssuch as maltose and lactose. Sucrose will notgive a positive result with Benedict’s reagent.

Equipment and materials required foreach working group• White tile• Knife/scalpel• 5 – 6 boiling tubes – one for each type of food• Small pestle and mortar• Small bottle of Benedict’s solution• Small bottle of iodine solution• Paper towels

Activity sheet

403 Technical guide – What sort of carbohydrates do plants make?

Activity sheet

402 Technical guide – What sort of carbohydrates do plants make?

Equipment and materialsrequired centrally• Access to water bath set at 85 – 90°C• Food samples containing carbohydratestores e.g. onion, apple, pear, celery,potato, grapes.• Optional extras: cotton wool, paper towels,egg boxes, flour, rice, powdered glucose.• Buckets/plastic containers for food remains

1 bottle of Schulze’s reagent (to beadministered by the teacher)

��Hazards:• Water bath at 85 – 90°C• Use of scalpel• Schulze’s reagent should be made up fresh,contained in a small stoppered bottle andadministered only by the teacher. Label thebottle CORROSIVE

Method/Preparationof materialsTo make the solutions:Benedict’s reagent –CLEAPSS recipe card No 8Iodine solution – CLEAPSSrecipe card No 39Schulze’s reagent –CLEAPSS recipe card No12 (G).

Ideally the samples fortesting should be choppedinto small pieces about1 cm3 maximum so thatstudents can easily selectthree pieces to carry outtheir tests.

Disposal of materials:In the laboratory have one ortwo large plastic containersin which students place theirwaste so that sinks do notget blocked.

Suppliers:All regular scientific suppliersprovide chemicals neededfor preparation of solutions.See suppliers list at end oftechnical guide.

Fig 1–Positive result Fig 2–Negative result

Fig 1–Egg carton stained with Schulze’s reagent Fig 2–Egg carton stained with Iodine

Fig 3–Cotton wool stained with iodine Fig 4–Cotton wool stained with Schulze’s reagent

Fig 5–Apple stained with Schulze’s reagent

Activity sheet

505 Technical guide – How can we show that plants use carbon dioxide?

Activity sheet

504 Technical guide – How can we show that plants use carbon dioxide?

Background informationStudents will use hydrogencarbonateindicator to detect changes in carbondioxide concentration. They will placean aquatic plant into a container ofhydrogencarbonate indicator andilluminate under different conditions.It is recommended to makehydrogencarbonate indicator fromthe pure dyes rather than buy it froman external supplier as the latter canbe very variable in quality.

Equipment and materialsrequired for each workinggroup:• 4 transparent containers1• 4 stoppers for containers orParafilm• Hydrogencarbonate indicator2• Cabomba.3 One long sprig• Light source4• Filters5• Flat sided transparent tank6• Scissors for cutting Cabomba intoequal sized pieces.

How canwe showthat plantsuse carbondioxide?

Notes1 Any clear container with a lid would besuitable. They can be boiling tubes (whichcan be sealed with Parafilm®) – or universalsor small squat chemical jars with lids. Theyneed to be wide enough for a small sprigof Cabomba.

2 The volume of indicator required dependson the containers used. There needs to beenough to cover the plant within the containersplus a little excess for rinsing out the containerprior to the activity.

3 Cabomba is an aquatic plant that is highlyresponsive to changes in lighting conditions.One long sprig of Cabomba will be cut into afew equal sized pieces, one for each container.

4 If this experiment is to work quickly then thelight source has to be very powerful. If it is abright day – sunlight on a windowsill is fine.150 W halogen lamps are very good forphotosynthesis work but they do get hot so aheat barrier is necessary to prevent the plantgetting scorched. The experiment is best setup in one lesson and the students collect theresults a couple of hours later to look at thecolour changes or take a few digital images.If this is not possible it may be useful to setone up earlier so that typical results can beseen. Slides on a PowerPoint show sometypical results.

5 Layers of thin cotton muslin can screen outsome of the light or neutral density filters canbe used. Neutral density filters are ‘grey’ filterswhich can be purchased in large sheets andcut down to cover the transparent containers.They reduce the intensity of light of allwavelengths equally.

6 A flat sided transparent container is needed toact as a heat screen. Chromatography tanksare useful but a cheaper alternative is topurchase a few flat sided glass vases or usemedicinal flats.

07 Technical guide – Measuring photosynthesis by oxygen evolution

Activity sheet

806 Technical guide – How can we show that plants use carbon dioxide?

Measuringphotosynthesisby oxygenevolution

Background information:In this practical students will placean aquatic plant into a container of1% sodium hydrogencarbonate andcount the bubbles of oxygenproduced at different distances froma light source. Background lightshould be kept to a minimum whilstthe experiment is carried out.Sometimes there are too manybubbles to count by eye and pupilscan be encouraged to think ofpossible ways of collecting the gasand measuring the volume.

Equipment and materialsrequired for each workinggroup:• 1 glass measuring cylinder (250cm3 )• A sprig of Cabomba1• Sodium hydrogencarbonatesolution (1%, 400 cm3)• A pair of scissors• A stopclock• A bright lamp (e.g. a 150 Whalogen lamp)2• A flat sided glass tank (to act asa heat screen)3• A one metre ruler

Activity sheet

5

�Hazards:150 W halogen lamps give out heat and shouldbe left to cool down before moving.

Method/preparation of materialsHydrogencarbonate indicator can be purchasedfrom all the main scientific suppliers but it doesvary a great deal. It is normally purchased inconcentrated form and has to be diluted x 10and then aerated before use. Some indicator isbarely red when purchased in a concentratedform and another bottle might contain a deeppurple liquid. The key thing is to use yourindicator from a single source and not to mixthem. If you can find a good source – the deeppurple colour seems to give the best depth ofcolour and most obvious colour changes.

The indicator can also be made quite easilyfrom cresol red and thymol blue (CLEAPSSrecipe card No 34):1. Dissolve 0.10 g of cresol red and 0.20 g ofthymol blue in 20 cm3 ethanol.

2. Dissolve 0.85 g sodium hydrogencarbonatein about 200 cm3 of freshly boiled distilledwater.

3. Add the ethanol solution and dilute to1000 cm3 with water.

4. For use, dilute the stock solution ten timeswith freshly boiled distilled water

5. Bubble air through the diluted solution toequilibrate it with atmospheric carbon dioxide.

The solution should be a deep cherry redwhen ready for use. Hydrogencarbonateindicator is a very sensitive pH indicator soit is important that all glassware used isrinsed out with a little of the indicator beforeuse. Avoid breathing over open vessels ofthe diluted indicator; the exhaled carbondioxide may alter the pH.

Suppliers:Cabomba is an aquatic plantavailable from most tropical fishsuppliers and Blades Biological,Cowden, Edenbridge, Kent, TN87DX (tel: 01342 850 242)email: sales@blades-bio.co.uk

Neutral density filters can bepurchased from LEE Filters(Central Way, Walworth IndustrialEstate, Andover, Hants SP10 5AN(tel 01264 366245);www.leefilters.com. Typically a sheetof filter (measuring 1.22 m x 0.55 m)costs £4.01 + VAT. A useful filter isnumber 209 which allows 50% ofavailable light to be transmitted. Thetransmission properties of each ofthe filters are available both from theLee Filters and SAPS websites(www.saps-plantsci.cam.ac.uk).

150 W halogen lamps areavailable from most DIY suppliersor Rapid Electronics. RapidElectronics Ltd, Severalls Lane,Colchester, Essex, CO4 5JS(tel: 01206 751166)email: sales@rapidelec.co.uk

Activity sheet

1009 Technical guide – What are chloroplasts?08 Technical guide – Measuring photosynthesis by oxygen evolution

Whatarechloroplasts?

Activity sheet

8Notes:1 Cabomba is an aquaticplant that is highlyresponsive to changesin lighting conditions.

2 If this experiment is towork well then the lightsource has to be verypowerful. 150 Whalogen lamps are verygood for photosynthesiswork but they do get hotso a heat barrier isnecessary to preventthe plant gettingscorched.

3 A flat sided transparentcontainer is needed toact as a heat screen.Chromatography tanksare useful but a cheaperalternative is topurchase a few flatsided glass vases oruse medicinal flats.

During the experimentit is important that thetemperature of the wateris controlled as far aspossible. It is alsoimportant to make surethe pupils give the planttime to equilibrate in thenew surroundings eachtime they change thedistance from the lamp.

�Hazards:150 W halogen lampsgive out heat and shouldbe left to cool downbefore moving.

Suppliers:Cabomba is an aquaticplant available from mosttropical fish suppliers andBlades Biological,Cowden, Edenbridge,Kent, TN8 7DX(tel: 01342 850 242)email: sales@blades-bio.co.uk150 W Halogen lampsare available from mostDIY suppliers or RapidElectronics, SeverallsLane, Colchester, Essex,CO4 5JS (tel:01206751166) email:sales@rapidelec.co.uk

Backgroundinformation:The aim of this practical isto let students observechloroplasts directly underthe microscope.

Following this they take athin section of potatotissue and stain it to showstarch grains.

Equipment andmaterials required foreach working group:• One microscope• One bench lamp• Microscope slides andcover slips• A small piece of potato• A small sprig of plantmaterial.1• Iodine solution• A scalpel or razor bladeStudents should select asingle leaf from the Elodeaplant and place it on amicroscope slide. Place adrop of water on the leafand then gently lower acover slip onto the slide

Notes:1A variety of plants aresuitable – most aquaticplants with fine leaves orthe edges of a leaf ofElodea. The more fleshymoss species e.g. Mniumspp. also give goodresults.

The digital image resourceon the SAPS website (seewww.plantscienceimages.org.uk/pages/intro.aspx) hassome chloroplast imageswhich can be used.

11 Technical guide – Can we show that only the green parts of the leaf produce starch in photosynthesis?

Activity sheet

1110 Technical guide – Can we show that only the green parts of the leaf produce starch in photosynthesis?

Can we show that only thegreen parts of the leaf producestarch in photosynthesis?

Background information:Students carry out a starch teston a variegated leaf to demonstratethat only the parts containingchloroplasts are able to synthesisestarch.

Equipment and materialsrequired for each workinggroup:• A leaf from a variegatedpelargonium1• One 250 cm3 beaker• One boiling tube• A Bunsen burner• Heatproof mat• Tripod and gauze• One white tile• Iodine solution in dropping bottle• A pair of forceps

12 Technical guide – Can we show that only the green parts of the leaf produce starch in photosynthesis?

Materials to be shared500cm3 ethanol (industrial alcohol)– Approximately 25 cm3 to be givenout to each group by the teacher.

�Hazard:Ethanol is highly flammable; harmfulif swallowed.

Do not allow students access toethanol whilst the Bunsen burner isstill lit.

Notes:1Place the variegated leaf underbright light for at least 24 hoursprior to the practical.