saps - photosynthesis with algal balls

Photosynthesis

with algal balls

Background

Debbie Eldridge

King Ecgbert School, Sheffield

‘A novel approach to photosynthesis practicals’

School Sci. Rev. (2004), 85, 37-45.

Photosynthesis KS4 - Problems

• understanding experimental procedures – controlled experiments with conditions or materials missing followed by iodine

test for starch.• with motivation

– pupils have ‘done it before’ or see plants as tedious.• obtaining data

– difficulty getting enough reliable data to illustrate change in rates – Elodea bubbling rates notoriously unreliable.

• keeping a range of photosynthetic organisms in school for use over the year.

New practical ideas• Photosynthetic organisms easy to grow

and culture.• The ability to be able to measure

– either oxygen evolution – or carbon dioxide uptake so that rates can be

determined.

• A novel species.

Algae

Scenedesmus and Chlorella -successfully cultured in the laboratory

Algae

• Immobilised algae to demonstrate rates of photosynthesis

• Algae ‘trapped’ in sodium alginate so that:• numbers can be regulated • they can be separated from the medium

Hydrogencarbonate indicator

Hydrogen carbonate indicator:

• routinely used to measure CO2 levels in aquatic systems.

• red in equilibrium with atmospheric air • becomes more orange/yellow with increased CO2

• changes from red through magenta to deep purple as CO2 is removed.

pH 7.6 9.2 (in 0.2 increments)

Hydrogencarbonate indicatorHydrogencarbonate indicator

Variables?• Colour of light• Light Intensity

• Distance from lamp• Neutral density experiment

• Number of balls• Ball size• Concentration of algae• Temperature • Starting [CO2] etc...

Filters

• LEE Filters – range of filters• Different colours available• Neutral density options• LEE Filters, Central Way,

Walworth Industrial Estate, Andover, Hants SP10 5AN. Tel 01264 366245; www.leefilters.com

FiltersSpectrum from www.leefilters.com/home.asp:

Transmission

FiltersData from www.saps.org.uk (coming soon…):

Wavelength

(nm)

Absorbance Transmittance Transmittance (%)

402 0.87 0.14 14.0

406 0.79 0.16 16.0

410 0.72 0.19 19.0

etc etc etc etc

730 0.41 0.39 39.0

FiltersSpectrum from www.saps.org.uk (coming soon…):

Filter 124, Dark Green

0.000.501.001.502.002.503.003.50

400 450 500 550 600 650 700 750

Wavelength / nm

Abs

orba

nce

FiltersSpectrum from www.saps.org.uk (coming soon…):

Filter 124, Dark Green

0

20

40

60

80

400 500 600 700

Wavelength / nm

Tra

ns

mis

sio

n /

%

FiltersSpectrum from www.saps.org.uk (coming soon…):

Filter 210, 0.6 Neutral Density

0

20

40

60

80

400 500 600 700

Wavelength / nm

Tra

ns

mis

sio

n /

%

Can we quantify the colour change?

Yes!• By measuring the absorbance using a

colorimeter

• By comparing the colours to a range of prepared buffer solutions.

Quantifying colour change

No access to a colorimeter?

• compare the colour change to a series of ready made buffer solutions.

• borax/boric acid buffers in pH range 7.6-9.2.

• 9 cm3 buffer + 1 cm3 concentrated indicator prepared freshly.