Question

Energy and Photosynthesis Pre-Lab Questions Describe how the functional units for beta carotene, xanthophyll, chlorophyll A, and chlorophyll B are different. Be sure to identify the subunits that adhere to paper during chromatography.

Describe a technique for measuring photosynthetic rate.

Many deciduous trees have leaves that turn yellow in the fall. What do you suppose is happening in the leaves at the cellular and molecular level?

Chloroplasts and mitochondria are both unusual in that they have double membranes and contain their own set of DNA. Can you think of any explanations for this observation?

Answer 1

1.

Carotenoids can be classified into Beta-carotene and Xanthophyll. Carotenes have no oxygen while xanthophylls are oxygen-containing compounds. Xanthophylls are yellow and carotenes are orange in color. Carotenes are terpenes formed from units of isoprene (C5H8). Chlorophyll A has a methyl group while Clh B has an aldehyde group attached to the porphyrin ring. In the paper chromatography, the chlorophyll b is at the lowermost point and it is a yellowish-green colored band while chlorophyll A is a dark green color band and it is the second last band. Carotene is represented by the topmost band of orange-yellow color while the second yellowish band represents xanthophylls.

2.

The rate of photosynthesis can be measured by measuring any of these parameters, measuring the increase in dry mass, production of carbohydrates, production of O2 or uptake of CO2. Measuring the rate of photosynthesis by measuring the increase in dry weight is done by harvesting and drying the plants at regular intervals. Plants are dried to constant weight before taking the weight. The measurement of the increase in weight after regular intervals illustrates the surplus of photosynthetic yields consumed after respiration. The rate of evolution of oxygen can also be measured to study the rate of photosynthesis. In this method, aquatic plants like Hydrilla or Cabomba furcata can be used for the study. The stem of the plant should be sliced only from one side and the rate of evolution of bubbles is the rate of evolution of oxygen. This experiment can be performed in different wavelengths or intensities of light to evaluate the variation in the rate of photosynthesis in those parameters. The gas evolved can be collected and measured.

The basic reaction for photosynthesis is:

CO₂ + H₂O                                         C₆ H₁₂O₆ + O₂

3. The yellowing of the leaves is called senescence. This is induced by the aging of the leaf or any stress condition. The chlorophyll content of the leaves degrade and form carotene which gives the characteristic yellow color of the dying leaf. see the fig.2

4. The origin of mitochondria and chloroplast can be explained by the endosymbiont theory. According to this theory, these organelles were once independent aerobic prokaryotic organisms. These organisms were engulfed by a eukaryotic cell in due course of time and they started an endosymbiotic relationship with the host and eventually became part of the cells and developed into the mitochondrion. According to the hypothesis, the mitochondria was aerobic prokaryote while chloroplast was photosynthetic prokaryote. They were independent organisms therefore they have their own set of DNA. Both of these organelles divide by binary fission and have similar to the size of prokaryotic cells.