Photosynthetic Pigments

There are various methods by which you can investigate aspects of photosynthesis. These methods are used in research by biologists to shed light on the many different reactions that occur during photosynthesis.

One method of investigating photosynthesis involves chromatography, which analyses the range of photosynthetic pigments in leaves. Photosynthetic pigments are small molecules that are capable of absorbing energy from light.

Chloroplasts are an essential part of the light-dependent reaction during photosynthesis. Without chloroplasts and the chlorophyll contained within them, the plant wouldn't absorb light energy, and photoionization could not occur. If you're curious about how light energy is converted into chemical energy via photosynthesis, check out our articles on light-dependent and light-independent reactions.

Chromatography is a laboratory technique used by biologists and biochemists. It allows for the separation of a mixture, which is achieved by passing it in a solution or suspension in which the mixture's components can move at different rates. It is often used to separate a product from any unreacted solution.

There are two primary types of chromatography: thin-layer chromatography (sometimes shortened to TLC) and column chromatography.

The function of both methods are the same, but each uses different equipment to separate a mixture into its various components. You are most likely to use thin layer chromatography during your studies; however, at university-level or a professional laboratory, you will make use of both. The differences between the two are illustrated in Figure 1 below.

How do you investigate photosynthesis using chromatography?

For your A-level course, you will be asked to use chromatography to investigate the pigments isolated from the leaves of different plants. You might, for example, be asked to compare the leaves of shade-tolerant vs shade-intolerant plants or leaves that are different colours from the same plant. To do this, you will need to carry out the experiment as detailed below.

Equipment:

Method:

Fig. 1 - Example of Thin Layer Chromatography

Although this won't look exactly like your experiment, it will be set up similarly.

How do you calculate an Rf value?

Once you have collected your filter paper with the different pigments separated, you will calculate the Rf value of each spot. Rf stands for retention factor, and this formula is used to help identify the pigments present on your chromatography sample.

The equation for the Rf value is as follows:

$Rf=\frac{dis\tan cetravelledbythecompound}{dis\tan cetravelledbythesolventfront}$

When you have your set of Rf values, you can then compare them to the known Rf values (provided by a teacher) to identify which pigments are present in your leaf. If you need to find your own database, make sure that they are specifically for paper chromatography. Use the same solvent as you since these variables will affect the distance travelled by certain pigments!

Which photosynthetic pigments are in chloroplasts?

You are likely to find the following pigments in a leaf during chromatography:

Why are photosynthetic pigments important in photosynthesis?

Photosynthetic pigments are crucial to photosynthesis as they absorb photons (waves of light) at certain wavelengths. As you will see in the diagram below, different pigments can absorb and reflect different wavelengths of light, which allows a plant to get as much energy as possible from a single light source.

After absorbing photons, the electrons contained in the pigments become excited and increase their energy level. Accessory pigments such as carotenoids, anthocyanins, and xanthophylls can then pass on this energy to primary pigments like chlorophyll a, which can oxidize and donate an electron to the electron transport chain in the light-dependent reaction.