Photosynthesis Pigments - Fundamental Biological Concepts - Lab Manuals, Study notes of Biology

Download Photosynthesis Pigments - Fundamental Biological Concepts - Lab Manuals and more Study notes Biology in PDF only on Docsity! Photosynthesis pigments a) Introduction Photosynthesis is the process by which plants (and also some types of bacteria and protista) make glucose from carbon dioxide and water. The exact chemical equation is: (sunlight) 6CO2 + 6H2O ---------------> C6H12O6 + 6O2 Making glucose from water and carbon dioxide is a non-spontaneous reaction, meaning that it requires energy. As you can see from the equation above, sunlight is the energy source. Today’s laboratory exercise will focus on that particular part of photosynthesis: Light and how plants capture light energy. b) Light and pigments Although sunlight appears white to our eye, in reality it is a mixture of several colors. These colors are sometimes called “the spectrum of light” or “the colors of the rainbow.” Traditionally, the colors of the spectrum are red, orange, yellow, green, blue, and violet. Light is made of tiny particles called photons. You can picture each photon as a tiny group of waves, like a few ripples on the surface of a pond. The wavelength of a photon is the distance between its waves. Interestingly, it is a difference in wavelength that makes photons different colors. For example, all photons with a wavelength of 450 nanometers are blue light. All photons with a wavelength of 700 nanometers are red light. (A nanometer is 10-9 meters). The table below lists the approximate wavelengths of each color of the spectrum. Violet Blue Green Yellow Orange Red 400 nm 450 nm 500 nm 580 nm 650 nm 700 nm Photons that are between two of these wavelengths are a color between the two colors. For example, a photon with a wavelength of 475 nm is blue- green in color. Docsity.com 2 Pigments are molecules that add color to an object. A leaf appears green, for instance, because it contains green pigments that capture light for photosynthesis. Blue ink is blue because it contains blue pigments. A red sweater contains red pigments. Pigment molecules get their colors by absorbing photons, but the relationship between what color a pigment is and what color photons it absorbs is slightly counterintuitive: A pigment is the color of the photons it doesn’t absorb. This seems backwards at first, but think about the red sweater. Its pigments absorb photons of all colors except red. Therefore red photons are the only ones left over when light falls on the sweater. All those red photons reaching your eye are what make the sweater appear red. If you have grasped this concept, the following statements should make sense: A white substance has no pigments and a black substance has a pigment (or a mixture of pigments) that absorb all colors. c) Separation and Identification of Plant Pigments by Paper Chromatography There are four pigment types in the chloroplast that absorb photons for photosynthesis: chlorophyll A, chlorophyll B, carotene, and xanthophyll. These four pigment molecules are shown on the next page. Chlorophyll A, which is light green in color, and Chlorophyll B, which is a darker olive green, absorb most of the light. Note that both chlorophyll types contain a magnesium ion. Carotene, which is orange-colored, and xanthophyll, which is yellow, are called accessory pigments because they assist photosynthesis by absorbing the photons that the chlorophylls don’t absorb. In this experiment, you will separate the 4 pigments using paper chromatography. These pigments have been extracted from spinach leaves. Recall from your previous lab on paper chromatography that the pigments are placed on a strip on paper (the stationary phase) near the bottom. The very bottom of the paper is placed in a solvent (the mobile phase) that wicks upward on the paper. As the solvent front passes the pigment spot, the pigments dissolve in the solvent and get carried upward with it. The pigments become separated from each other as they climb upward because they have different attractions to the stationary phase. The molecules that are greatly attracted to the stationary phase flow the slowest, the molecules that are somewhat attracted to it flow at a medium speed, and the molecules with little or no attraction to the stationary phase flow the fastest. Docsity.com 5 d) What colors of light are best for photosynthesis? The pigments in the chloroplasts of plant cells absorb photons. The energy of the captured light is what powers photosynthesis. But are some colors of light more effective than others? This experiment will answer that question. A few days before today’s lab, the biology technician put red, blue, and green filters on the leaves of a geranium plant. Filters are colored pieces of plastic that allow only one color to pass through. The red filter, for instance, only lets red photons reach the leaf. The technician also placed a piece of black construction paper on the leaf. The black paper blocked all light from reaching the leaf. The parts of the leaves that were uncovered (no filter or paper) received normal white light, containing all the colors of the spectrum. To determine which colors are best for photosynthesis, you will use iodine to stain the leaf for starch. The more starch a leaf contains, the more photosynthesis it was able to perform. Recall that iodine stains starch a dark purple color. To see the staining pattern clearly, you will first remove the green pigments in the leaf by boiling it in alcohol. 1) In the space below, record your hypothesis about which of the 3 colors of light (red, blue, and green) will be the best for photosynthesis and which will be the worst. It might be helpful to review section B on pigments before you make your predictions. Best: ________ Medium: ________ Worst: ________ 2) Get a leaf with filters from the plant. At the bottom of page 10, draw a diagram of the leaf with labels showing exactly where each color filter is located. 3) Your instructor has set up the boiling alcohol bath. Remove the paper and filters from your leaf and give it to your instructor for boiling. Return the plastic filters to the front desk. 4) The leaf will be boiled until it is almost white. When your instructor returns your leaf, place it in a Petri dish, and rinse it gently with distilled water. Pour off the water. Docsity.com 6 5) Add I2KI (iodine solution) to the leaf. Add enough so that all parts of the leaf are covered. Let the leaf soak in the iodine for at least 5 minutes. If starch is present, the iodine will react with it and a purple-black color will form at the site of the starch. 6) Dump the iodine down the sink and wash the leaf in water again to remove the last of the iodine. 7) Make another sketch the leaf in the results section, page 10. Be sure to use shading in your sketch to show exactly the amounts starch in each area of the leaf. Show your instructor your results before continuing. e) Pigment color and photosynthesis in the coleus leaf Most leaves are a uniform shade of green because each cell has green cholorphyll pigments in its chloroplast organelles. Recall that the chlorophyll pigments capture the photons that drive photosynthesis. The leaves of certain plants, such as the coleus, have non-green areas that contain other pigments instead of chlorophyll. Do non-green pigments also capture light for photosynthesis? Some do and some don’t. The pigments that do drive photosynthesis are always located in the inner membrane of the chloroplast and are always hydrophobic molecules. (They must be hydrophobic to remain in the membrane). These pigments are usually green, yellow, or orange. Pigments that do not play any role in photosynthesis are usually hydrophilic and are located in the central vacuole of the plant cell. These pigments are called anthocyanins and are usually blue, violet, and red. In this experiment, you will test a coleus leaf to see which pigmented areas are performing photosynthesis and which are not. Just as in the previous experiment, you will use iodine staining to determine starch content as a way to judge how much photosynthesis is occurring. 1) Remove a multicolored leaf from a Coleus plant that has been in strong light for several hours. Docsity.com 7 2) In data table 3, sketch the leaf and color distribution. Include in your sketch the colors of the leaf. In the space below, predict what colors will have the most photosynthesis and which will have the least: Colors with most photosynthesis: Colors with least: 3) Give the coleus leaf to your instructor for alcohol boiling. After boiling, stain the leaf using the same procedure described in procedure E. 4) In the data table 3, sketch the leaf after the starch test. Fill in data table 3 with the amount of photosynthesis in each pigment region. f) Determining the absorption spectrum for the four photosynthesis pigments The absorption spectrum for a pigment is a graph that shows which wavelengths (colors) of photons it absorbs and to what degree it absorbs them. A sample absorption spectrum for a violet pigment is given below. 0.4 Absorbance 0.3 0.2 0.1 0 400 450 500 550 600 650 700 750 Wavelength (nm) Notice that the violet pigment absorbs most wavelengths well except those in its own color (the 400 nm area of the spectrum). Review section B of this handout if you would not have predicted this. Docsity.com 10 G) Data Tables Data Table 1 Sketch your four chromatography paper bands in the square below. On the right of each pigment band, write the following: (1) The pigment’s name, (2) The pigment’s color, (3) The pigment’s Rf. Data Table 2 Sketch of leaf Sketch of leaf leaf before boiling leaf after iodine staining Red Blue Green Black Best and worst color for photo- synthesis: ______ Docsity.com 11 Data Table 3 Sketch of leaf Sketch of leaf before boiling (label all colors) after iodine staining List all colors on leaf where photosynthesis took place: List all colors on leaf where photosynthesis did not take place: Data Table 4 Your group’s pigment: ___________ Absorbance: 400 425 450 475 500 525 550 575 600 625 650 675 700 In the rectangle on the right, graph The absorbance spectrum of all four pigments. Use the graphs on the blackboard. Docsity.com 12 Review questions 1) Write the complete photosynthesis equation: 2) What is a photon? What is the range of photon wavelengths that is visible light to our eye? 3) What color on the spectrum has the lowest wavelength? What color has the highest? 4) What is a pigment? 5) What color photons would you expect a blue pigment to absorb the least well? 6) Make a generalization about what color a pigment appears and the colors it absorbs and the colors it does not absorb. 7) List the four photosynthetic pigments in plants. After one, list its color. Circle the one(s) that absorb the most photons. 8) Why couldn’t you use water as the mobile phase when you separated the spinach leaf pigments by paper chromatography? 9) Why must all photosynthetic pigments be hydrophobic? 10) If you had been given a mixture of photosynthesis pigments and anthocyanins (non-photosynthesis pigments), which group do you predict would have the higher Rf in the chromatography procedure? Why? 11) In the experiment where geranium leaves were covered with colored filters, what wavelengths pass through the blue filter? The red filter? The green filter? Docsity.com