Passage II

Students performed 2 experiments to study plant pigments.

Experiment 1

The students crushed spinach leaves in a solvent, forming an extract that contained the pigments from the leaves. The extract was spread in a straight line 2 cm from the bottom of a piece of paper. After drying, the paper was placed in a tank containing a second solvent, such that the bottom of the paper was immersed in the solvent and the line of extract was above the surface of the solvent (see Figure 1).

Each pigment in the extract had a different degree of attraction to the solvent in the tank and a different degree of attraction to the paper. Consequently, as the solvent in the tank moved up the paper, the pigments moved up the paper at different rates. The extract separated into 4 bands each containing a different pigment (Pigments S, T, U, and V), as shown in Figure 2.

At 60 min, the paper was removed from the tank. Next, the distance traveled by the solvent was measured (63 mm), and the distance traveled by each pigment was measured (see Table 1). Then, the R_f, value for each pigment was calculated using the following expression:

The R_f values are also shown in Table 1.

Experiment 2

Each band was cut out from the paper and then soaked for 24 hr in a separate beaker containing 50 ml of a solvent. A sample of the solvent was then removed from each beaker. The absorption spectrum of the pigment (the relative absorption of light by the pigment over a range o wavelengths) in each sample was determined (see Figure 3).