Photosynthesis

Photosynthesis is the cellular process where light energy is converted into the chemical energy of reduced carbon compounds. In plants, it consists of two parts.

1. The light dependent reaction where light energy is converted into chemical energy in ATP and NADPH+. In this process water is broken apart to generate O2 .

2. The light independent reaction where ATP and NADPH+ react with CO2 to produce glucose.

View electron micrograph to consider where each part of photosynthesis occurs.

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Testing the hypothesis, "Chlorophyll is necessary for Photosynthesis."

• Link to each step in processesing a Coleus leaf
• Link to view of live leaf and processed leaf stained with iodine

• Link to results from lab.

The Absorption of Light by the Chloroplast Pigments

Before light energy can be converted into chemical energy it must first be absorbed. In lab we both directly observe what colors are absorbed by a pigment extract (using a spectrum viewer), and measure (using a spectrophotometer) the transmittance of different colors through a pigment extract. In both cases.

• View transmitted spectrum through a pigment extract from banana leaf
• Sample of the transmittance data through a pigment extract of banana leaf collected in lab

Fluorescence:

When chlorophyll in a solution absorbs light, this energy cannot be used in photosynthesis. It is only when chlorophyll is part of an intact photosystem, embedded in a tylakoid membrane, that light can be usefully harnessed in photosynthesis. When in solution, the energy absorbed by chlorophyll boosts electrons into an unstable orbital. The minimum energy required to accomplish this is a constant. When these electrons return to a stable orbital, a photon of light with a wave-length corresponding to the constant is emitted. These emitted photons are red.

• View example of fluorescence vs. transmittance of chlorophyll as seen in lab

Paper Chromatography:

This process separates the different pigments in our pigment extract (derived from banana leaf) by their relative solubility in water. Water is associated with the cellulose fibers of the paper and does not move. The chromotography solvent is a mixtue of non-polar solvents that do move up the paper. As this solvent moves up the paper, it carries the piments along with it at various rates. The stationary acts as a brake. Those molecules with the least affinity for water (that are less polar) move faster and higher. Those with the greatest affinity move slower and remain lower down on the paper.

• View chromatogram of banana leaf extract from lab

Using Metabolic Starch to Determine Where the Calvin Cycle Occurs in a C-4 Plant:

If a leaf is illuminated continuously, typically, starch builds up in the tissues associated with the Calvin Cycle. We used this observation to evaluate the hypothesis that chlorophyll is necessary for photosynthesis using variegated Coleus. Coleus is a C-3 plant, and in this case we did distinguish between the light and the light independent reactions as all the photosynthetic cells wee undergoing the Calvin Cycle.

In the lab on photosynthesis, we used this tool specifically to determine which photosynthetic cells in a C-4 plant are undergoing the Calvin cycle.

• View results from lab

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