Cracking the AP Biology Exam




This lab has two parts.


In lab 4A, four plant pigments—chlorophyll a, chlorophyll b, xanthophylls, and carotenoids—were separated using paper chromatography.

What are the general concepts you really need to know?

  • Know how paper chromatography works. Paper chromatography is a lab technique used to separate the components in a mixture. In this experiment, a drop of a leaf extract was placed near the bottom of a paper called a chromatography paper. This paper was then placed in a solvent, which moves up the paper by capillary action.

As the solvent touches the pigment extract, each pigment within the extract moves at a different rate. In the end, four spots appear on the paper, each representing one of the pigments. The bottom line is this: Pigments move at different rates according to their ability to dissolve in the solvent. The pigment that dissolves the best moves up the paper the fastest.

  • The rate of migration of the pigments is calculated using an equation called the reference front ratio, Rf . The Rf value is the ratio of the pigment migration distance to the solvent migration distance.


In the second part of this lab, the rate of photosynthesis in chloroplasts was measured using a dye called DPIP. DPIP changes color when it accepts electrons that would normally be accepted by NADP+, the electron acceptor of the light-dependent reaction of photosynthesis. As DPIP accepts electrons, it changes from blue to clear. A spectrophotometer is used to measure the amount of light absorbed.

What are the general concepts you really need to know?

  • Three things can affect photosynthesis: light intensity, light wavelength, and temperature.
  • In an experimental design, you can vary each of the three variables in order to determine their effects on photosynthetic rates.