Photosynthesis Experiments

Question 1

Describe and explain the PAG for investigating the pigments in leaves

Answer 1

• all plants contain several different photosynthetic pigments in their leaves.
• Each pigments absorbs a different wavelength of light - so having more pigments increases the range of wavelengths of light that a plant can absorb.
• in addition to photosynthetic pigments, some plants also have other photosynthetic pigments in their leaves, which play other essential roles, e.g. protecting the leaves from excessive UV radiation

A sample of pigments can be extracted from the leaves of a plant and separated using TLC (thin-layer chromatography).
You can then identify the pigments present in the sample by calculating their Rf values (the distance a substance has moved through the stationary phase in relation to the solvent)
Each pigment has a specific Rf value, which can be looked up in a database.

Question 2

Describe the steps of TLC for investigating the pigments in leaves

Answer 2

Mobile phase (liquid solvent), stationary phase (chromatography plate - solid plate of glass or plastic with a thin layer of silica gel, which the pigments can travel through).

1) grind up several leaves (e.g. spinach), with some anhydrous sodium sulfate, then ass a few drops of propanone
2) transfer the liquid to a test tube, add some petroleum ether and gently shake the tube. Two distinct layer will form in the liquid — the top layer is the pigments mixed in with the petroleum ether.

3) transfer some of the liquid inn the top layer into a second test tube with some anhydrous sodium sulfate

4) Draw a horizontal pencil line near the bottom of a chromatography plate.
Build up a concentrated spot of the liquid from step 3 on the line by applying everlasting drops, ensuring each one is dry before the next is added. This is the point of origin.

5) once point of origin is dry, put the plate into a glass beaker with some prepared solvent, enlighten so that the line is above the solvent.
Put a lid on the beaker and leave the plate to develop
As the solvent moves up the plate, the different pigments move with it, but as different rates - so they separate
6) once solvent nearly reaches the top, mark the solvent front, with a pencil and leave plate to dry
7) calculate Rf values of the diff spots (rf value = distance travelled by spot/ distances travelled by solvent)

Question 3

Describe PAG on investigating the rate of photosynthesis

Answer 3

• Elodea (pondweed) can be used to investigate the effect of light intensity, carbon dioxide concentration, ad temperature on the rate of photosynthesis.

For light intensity:
- light source, rule, test tube clamped attached to gas syringe, elodea in water

• plant is left to photosynthesis for a certain amount of time, and light source is at a measured distance away from plant
• gas syringe measures the oxygen produced, from plant
• variables are controlled
• repeat experiment with the light source being at another distance (e.g. intervals of 5cm, 10cm, 15cm, 20cm)