Photosynthesis
Photosynthesis is the cellular process where light energy is converted into the chemical energy of reduced carbon compounds. Photosynthesis includes two processes working in tandem.
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 by some molecule. One can directly observe what colors are absorbed by a plant pigment extract using a spectrum viewer, or measure which are absorbed using a spectrophotometer.
B. Fluorescence - When chlorophyll in an alcohol 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 minimum 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 originates 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 separates the different pigments in our pigment extract (in this example 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, and carotene.
D. The liberation of oxygen - In photosynthesis a mole of CO2 is consumed for every mole of O2 generated. However, because CO2 is much more soluble in water than O2 the net effect of photosynthesis in an aquatic environment is often the generation of oxygen bubbles. This phenomenon can be used to study the rate of photosynthesis in a simple lab exercise.
E. 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. In C-4 plants, only the bundle sheath cells produce photosynthate. Hence, the starch produced in the leaves of these plants is ususally localized in the areas around the veins (as determined using I2KI - link to illustration).
E. Microcosm Experiment: To better integrate an understanding of the complementary nature of respiration and photosynthesis, an ecology activity to explore the concept of stability can be developed for your lab on photosynthesis.