Photosynthesis
Students must be able to interpret specific examples of situations observed in lab and be able to answer questions demonstrating an understanding of the following concepts.
Go to the directory of raw images related to our activities on photosynthesis.
Photosynthesis: The cellular process where light energy is converted into the chemical energy of reduced carbon compounds. 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.
A. Absorption of light by chlorophyll - 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 students determine that the pigment extract absorbs light most strongly in both the blue and the red regions of the spectrum.
B. Fluorescence - Demonstration in lab. When chlorophyll in a solution absorbs light, this energy cannot be used in photosynthesis. The absorbed energy is stored as an excited electron boosted into a unstable orbital. The energy required to accomplish this is a constant. Photons of light exceeding this constant, expend energy in excess of this minimum as heat. When the electron returns to a stable orbital, a photon of light with a wave-length corresponding to the constant is emitted. These emitted photons are red.
The red light observed originated from the chlorophyll and not the light source. The green light observed originated from the light source and passed through the pigment extract.
C. Paper Chromatography - This process separates the different pigments in our pigment extract (derived from banana leaf) by their relative solubility in water. The pigment with the least affinity for water is carotene. Its band is at the boundary of the moving front of the chromotography solvent. The pigment with the greatest affinity for water is chlorophyll b at the very bottom. The sequence from the bottom to the top of the four bands is: chlorophyll b, chlorophyll a, xanthophyll (you may have observed two bands here), carotene.
D.Using Metabolic Starch to Determine Where the Calvin Cycle Occurs in Leaf Tissue - Typically, in a leaf, photosynthate is exported as it is produced. If a leaf is illuminated continuously, a condition can be created where the photosynthate can not be exported as rapidly as it is produced. When this happens, glucose is converted into starch in the leaf. We use this observation twice: to demonstrate the necessity of chlorophyll for photosynthesis in Coleus; and to determine which, of a series of plants, use C-4 photosynthesis. In C-4 plants, only the bundle sheath cells use the Calvin Cycle and produce photosynthate. Hence, the starch produced in the leaves of these plants is localized in the areas around the veins (as determined using I2KI - link to illustration) where the bundle sheath cells are located.