The Effect of Light Intensity On Photosynthesis of Green Algea



The Effect of Light Intensity On Photosynthesis of Green Algae

Aim

Identify the effect of light intensity of green algae on the rate of photosynthesis.

Hypothesis

At low to medium light intensities the rate of photosynthesis is proportional to light intensities, at high light intensities the rate reaches a plateau.

Planning

Photosynthesis is the process used by plants and some other organisms to produce all their own organic substances (food), using only light energy and simple inorganic substances [1]. The simplified chemical equation for photosynthesis is:

6 CO2 + 12 H2O C6H12O6 + 6 O2 + 6 H2O

The rate of photosynthesis can be determined if the concentration of dissolved oxygen in the solution is measured.

Independent variable:

- light intensity

Dependent variable:

- concentration of dissolved oxygen in water sample

Controlled variable:

- water temperature

- room temperature

- humidity

The experiment bottles are exposed to several different light intensities. This will be done using the reflector at five different distances (range from 20 – 100 cm). During the whole experiment the temperature of water is observed. In case water temperature rises too much (more than 5°C), the experiment is aborted and the water is changed with the new one that has the right temperature (standard temperature for the whole experiment).

In each experiment two bottles are used, one with 2 grams of algae and another as a control sample. Bottles are exposed to the light for fifteen minutes. Then water samples are taken from the bottles and the concentration of the dissolved oxygen is measured using Azide – Winkler method:

1. A 300 cm3 glass stoppered bottle is filled with sample water without any air bubbles.

2. Immediately 2 cm3 of manganese sulphate is added to the collection bottle by inserting the calibrated pipette just below the surface of the liquid. (If the reagent is added above the sample surface, you will introduce oxygen into the sample.) Squeeze the pipette slowly so no bubbles are introduced via the pipette.

3. Add 2 cm3 of alkali-iodide-azide reagent in the same manner.

4. Stopper the bottle with care to be sure no air is introduced. Mix the sample by inverting several times. Check for air bubbles; discard the sample and start over if any are seen. If oxygen is present, a brownish-orange cloud of precipitate or floc will appear. When this floc has settle to the bottom, mix the sample by turning it upside down several times and let it settle again.

5. Add 2 cm3 of concentrated sulphuric acid via a pipette held just above the surface of the sample. Carefully stopper and invert several times to dissolve the floc. At this point, the sample is "fixed" and can be stored for up to 8 hours if kept in a cool, dark place. As an added precaution, squirt distilled water along the stopper, and cap the bottle with aluminium foil and a rubber band during the storage period.

6. In a glass flask, titrate 201 cm3 of the sample with sodium thiosulfate to a pale straw colour. Titrate by slowly dropping titrant solution from a calibrated pipette into the flask and continually stirring or swirling the sample water.

7. Add 2 cm3 of starch solution so a blue colour forms.

8. Continue slowly titrating until the sample turns clear. As this experiment reaches the endpoint, it will take only one drop of the tritrant to eliminate the blue colour. Be especially careful that each drop is fully mixed into the sample before adding the next. It is sometimes helpful to hold the flask up to a white sheet of paper to check for absence of the blue colour.

9. The concentration of dissolved oxygen in the sample is equivalent to the number of millilitres of titrant used. Each millilitre of sodium thiosulfate added in steps 6 and 8 equals 1 mg/cm3 dissolved oxygen.

[pic]

Figure 1: Schematic procedure for Azide – Winkler Method of measuring concentration of dissolved oxygen in water sample

Accessories:

1. Chemicals

• manganese (II) chloride (MnCl2) – 45 g

• potassium hydroxide (KOH) – 70 g

• potassium iodide (KI) – 15 g

• concentrated (50%) sulphuric acid H2SO4 – 100 cm3

• sodium thiosulphate (IV) (Na2S2O3) – 10 g

• starch – 100 cm3

• sodium chloride (NaCl)

• distilled water

2. Apparatus

• balance

• thermometer

• test tubes, corks (10x)

• rack for tubes

• 250 cm3 bottles (2x)

• pipette – 2 ml (2x)

• birette

• measuring cylinder (100 cm3)

• reflector

• lux meter

• transparent 1 l jar (2x)

Literature

1. Andrew Allot: Biology for the IB Diploma; Oxford University Press, 2001.

2. Gareth Williams: Techniques And Fieldwork In Ecology; Collins Educational, 1987.

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