Photosynthesis (Lab 4) Review

 

Some Key Review Concepts

In photosynthesis, plant cells convert light energy into chemical energy that is stored in sugars and other organic compounds. Critical to the process is chlorophyll, the primary photosynthetic pigment in chloroplasts.

Chromatography

Paper chromatography is a technique used to separate a mixture into its component molecules. The molecules migrate, or move up the paper, at different rates because of differences in solubility, molecular mass, and hydrogen bonding with the paper.

Photosynthesis

In the light reactions of photosynthesis, light energy excites electrons in plant pigments such as chlorophyll, and boosts them to a higher energy level. These high-energy electrons reduce compounds (electron acceptors) in the thylakoid membrane, and the energy is eventually captured in the chemical bonds of NADPH and ATP.

Lab Design

Chromatography

In paper chromatography the pigments are dissolved in a solvent that carries them up the paper. To separate the pigments of the chloroplasts, an organic solvent must be used.

In this experiment a drop of leaf extract is placed near the bottom of chromatography paper. This paper is then placed in a solvent, which moves up the paper by capillary action.

Photosynthesis

The rate of photosynthesis in chloroplasts in 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. 

Then a spectrophotometer is used to measure the color change, which gives an indication of the rate of the light reactions of photosynthesis under various conditions.

Analysis of Results

Chromatography

As the solvent touches the pigment extract, each pigment within the extract moves at a different rate. In the end there should be four spots on the paper, each representing one of the four pigments. (chlorophyll a, chlorophyll b, xanthrophylls, and carotenoids) 

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 can be calculated:

Photosynthesis

Three things can affect Photosynthesis. (1) Light Intensity (2) Light Wavelength (3) Temperature.

In experimental design, you can vary these three variables in order to determine their effects on photosynthesis.

Note: Only change one variable at a time.