Exercise 5 – The effect of the color of light on the rate of photosynthesis
Green leaves look green not because they absorb green light, but because they reflect green light to our eyes. A pigment’s absorption spectrum (Figure 3) quantifies graphically how much of each color (wavelength) the pigment absorbs. Chlorophyll a and chlorophyll b strongly absorb blue light (with wavelengths 400-500 nanometers) and red light (600-700 nm), but absorb very little green light (500- 600 nm). Figure 3: The absorption spectra of chlorophyll a and chlorophyll b
Before starting Exercise 5, make a prediction about how the color of the light might affect the rate of photosynthesis. Be prepared to explain your reasoning to your TA. Your prediction:
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Quantifying the effect of light color on the rate of photosynthesis: 1. To do the color experiment, use a cork borer to cut 100 leaf disks from a spinach leaf. Discard any disks that are bruised or torn. 2. You will need only one side-arm flask containing 80 ml of 0.1% sodium bicarbonate. Place 100 freshly cut leaf disks into the sodium bicarbonate solution and aspirate the disks following the prodecure outlined in steps 3-7 of Exercise 4 (see page 9). Asfter the disks sink, pour them into a 4” glass bowl and cover to protect them from bright light. 3. Obtain two (2) partitioned plastic boxes – one with a green/red top, the other with a blue/white top. Fill the 4 chambers with 0.1% sodium bicarbonate solution to a depth of about 1 centimeter. Now transfer 20 sunken leaf disks from the glass bowl into each of the 4 colored chambers. Use forceps to hold the disks by the edge so you don’t crush them! Set the boxes under the lights. Place a large, glass bowl of room temperature water on top of the boxes to act as a heat shield. Turn on the light and record the time. 4. Use Table 3 to record the number of disks re-floating in each chamber at 5 minute intervals for 45 minutes. Follow the counting steps listed in Exercise 3, step 9. Table 3: The number of leaf disks floating after successive 5 minute intervals under four different colors of light filters.
Time (min.) Red Green Blue Clear
0-
5-
10-
15-
20-
25-
30-
35-
40-
45-
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Analysis and Discussion 1. Based on your results, what influence does the color of light have on the rate of photosynthesis? 2. Are your results consistent with the predictions you made on page ? — If so, what can you conclude? If not, were there any problem with your methods, or does your hypothesis need to be revised? 3. Leaf chomotography (Part I) revealed that green leaves contain auxiliary pigments — the red, yellow and orange pigments that become visible every autumn. These pigments can absorb light energy and pass it over to chlorophyll. Why are auxillary pigments important?
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Lab 5 – DNA Fingerprinting 10
Living organisms are remarkably well adapted to their environments: every detail of their morphology, physiology and behavior seems tailored for surviving and reproducing in their particular environment. How did this close correspondence between organism and environment come about? Studies of taxonomy, compatative anatomy and the fossil record indicate that plant and animal species are not constant, but instead evolve, or change through time. Usually the changes make organisms better suited for surviving and/or reproducing in their ever-changing environments. Often, however, the changes in the environment occur too quickly and species go extinct. In fact, 99% of the species which have ever lived on this planet are now extinct. In 1859 Darwin proposed that evolution — the change through time evident in the fossil record — was due to natural selection. He called it natural because the direction of the change is determined by the environment (nature), not by a human or supernatural agent. He called it selection because the process is similar to the artificial selection used by humans for breeding more desirable animals and plants; some are allowed to reproduce, others aren’t. Darwin’s theory of evolution by natural secetion – along with its corollary, “descent from a common ancestor” — provides a major unifying theme in biology; it lets us see meaningful patterns in the otherwise disjointed facts of biology. Lab learning objectives You will have mastered the content of this minicourse when you are able to: Explain RFLPs and the procedure for making DNA fingerprints. Extract DNA from onion cells. Use gel electrophoresis to separate DNA samples into RFLP bands. Interpret the resulting DNA fingerprint patterns.