Required Practical 7 - Investigation of the effect of a specific limiting factor such as light intensity on the rate of photosynthesis.

What kind of plant do you use for this investigation?

For this investigation, use an aquatic plant like pondweed.

What are the limiting factors and how do you control each one?

The factors that act as limiting factors are:

Light intensity - change the distance of a light source from the plant

CO2 Concentration- add different concentrations of Sodium hydrogencarbonate to the water, which dissolves to produce CO2.

Temperature- Place the boiling tube in water bath of different temperatures.

When investigating one of the factors, ensure the other two remain constant.

What is the apparatus needed for this reaction?

Apparatus:

Distilled water

Test tube

Beaker

Lamp

Aquatic plant, algae or algal beads

Ruler

Sodium hydrogen carbonate solution

Thermometer

Test tube plug

Syringe

Describe the method for this experiment

Method:

Ensure the water is well aerated before use by bubbling air through it

This will ensure oxygen gas given off by the plant during the investigation form bubbles and do not dissolve in the water

Ensure the plant has been well illuminated before use

This will ensure that the plant contains all the enzymes required for photosynthesis and that any changes of rate are due to the independent variable

Set up the apparatus in a darkened room

Ensure the pondweed is submerged in sodium hydrogen carbonate solution (1%) - this ensures the pondweed has a controlled supply of carbon dioxide (a reactant in photosynthesis)

Cut the stem of the pondweed cleanly just before placing into the boiling tube

Measure the volume of gas collected in the gas-syringe in a set period of time (eg. 5 minutes)

Change the independent variable (ie. change the light intensity, carbon dioxide concentration or temperature depending on which limiting factor you are investigating) and repeat step 5

Record the results in a table and plot a graph of volume of oxygen produced per minute against the distance from the lamp (if investigating light intensity), carbon dioxide concentration, or temperature

How do you calculate the rate of photoynthesis?

Rate of photosynthesis = volume of oxygen produced ÷ time elapsed

What does the light dependent stage involve?

Involves the release of high energy electrons, which is picked up by NADP. If a redox indicator is present, the indicator takes up the electrons instead of NADP.

What happens to DCPIP in the light-dependent stage?

Causes DCPIP to get reduced, going from blue to colourless. The rate at which the indicator changes colour can be used to measure the rate of the light-dependent stage.

What is the apparatus needed for this experiment?

Apparatus:

Leaves

Isolation medium

Pestel and mortar

Lamp

Test tubes

Stopwatch

Aluminium Foil

DCPIP or methylene blue indicator

Buffer solution

Describe the method used for this experiment

Leaves are crushed in a liquid known as an isolation medium

This produces a concentrated leaf extract that contains a suspension of intact and functional chloroplasts

The medium must have the same water potential as the leaf cells so the chloroplasts don't shrivel or burst (isotonic) and contain a buffer to keep the pH constant

The medium should also be ice-cold (to avoid damaging the chloroplasts and to maintain membrane structure)

The experiment should be set up in a dark room so that the light source and intensity can be controlled

The room should be at an adequate temperate for photosynthesis and maintained throughout, as should carbon dioxide concentration

Small tubes are set up with different intensities, or different colours (wavelengths) of light shining on them

If different intensities of light are used, they must all be of the same wavelength (same colour of light) - light intensity is altered by changing the distance between the lamp and the test tube

If different wavelengths of light are used, they must all be of the same light intensity - the lamp should be the same distance in all experiments

DCPIP or methylene blue indicator is added to each tube, as well as a small volume of the leaf extract

A control that is not exposed to light (wrapped in aluminium foil) should also be set up to ensure the affect on colour is due to the light

The time taken for the redox indicator to go colourless (or green, as the chlorophyll may also colour the solution) is recorded

This is a measure of the rate of photosynthesis

Describe the limitations of this experiment

This experiment is not measuring the rate of dehydrogenase activity directly (through measuring the rate of substrate use or product made) but is instead predicting what the rate would be by measuring the rate of electron transfer from the photosystems

The concentration of DCPIP will depend on the number of chloroplasts in a sample and therefore the number of light-dependent electron transport chains

It is therefore important to control the amount of leaf used to produce the chloroplast sample and also how much time is spent crushing the leaf to release the chloroplast

It is also a good idea to measure a specific wavelength absorption by each sample on the colorimeter before and after the experiment so you can get a more accurate change in oxidised DCPIP concentration

Results should also be repeated and the mean value calculated

The time taken to go colourless is subjective to each person observing and therefore one person should be assigned the task of deciding when this is