- - AGRICULTURE CORE CURRICULUM - - (CLF6000) Advanced Core Cluster: ORNAMENTAL HORTICULTURE (CLF6150) Unit Title: PHOTOSYNTHESIS AND RESPIRATION ____________________________________________________________________________ (CLF6151) Topic: PHOTOSYNTHESIS Time Year(s) 2 hours 3 / 4 ____________________________________________________________________________ Topic Objectives: Upon completion of this lesson, the student will be able to: Learning Outcome #: (B-1) - Compare and contrast the processes of photosynthesis and respiration. (B-2) - Write the balanced formulas for both photosynthesis and respiration and discuss how energy is acquired and released through these reactions. (B-3) - List the requirements for photosynthesis and explain what function each performs (e.g., light, heat, CO2, chlorophyll, and water). Special Materials and Equipment: References: Hartmann, H. T., Flocker, W. J., & Kofranek, A. M. (1981). PLANT SCIENCE: GROWTH, DEVELOPMENT, AND UTILIZATION OF CULTIVATED PLANTS. Englewood Cliffs, NJ: Prentice-Hall. Evaluation: Unit Exam ===================================================================== *** INSTRUCTORS PLEASE NOTE *** The detail of this topic presentation goes beyond the scope necessary to meet the requirements of the Core Cluster in this area. It will take longer to teach than indicated above if covered entirely. It is included for local enrichment as appropriate to the class. ===================================================================== TOPIC PRESENTATION: PHOTOSYNTHESIS A. Photosynthesis as a Unique Function of Plants: The unique characteristic of plants, which sets them apart from other living creatures, is their ability to provide their "own" source of energy through photosynthesis. Through this process plants not only convert light energy into a storable form of chemical energy, but they fix the basic building blocks for their own growth and development from materials in the atmosphere. B. The Importance of Photosynthesis: It is often stated that without plants and their ability to photosynthesize, there would be no life on earth. Photosynthesis affects nearly all organisms, either directly or indirectly, and, of course, is central to horticultural practice. 1. Photosynthesis occurs in plants, some bacteria, and algae including primitive aquatic life forms such as kelps and green pond scum. 2. Animals, fungi, and most microbes cannot perform photosynthesis. However, they get their energy by feeding on materials that are produced by photosynthetic forms of life. 3. Thus, either directly or indirectly, all life on earth depends on photosynthesis for its energy supply. 4. Humans depend on plants which are cultivated for the complex molecules they synthesize such as carbohydrates, proteins, and fats which are the major constituents of food, fiber, and wood. 5. Photosynthesis, naturally, is the ultimate force which drives horticulture. To be effective in nursery production, landscape management, and the care of floricultural crops and interior plants, we must understand the requirements of photosynthesis and how it is affected by environmental conditions. C. Plant Adaptations for Photosynthesis: Plants have many morphological and physiological adaptations which enable them to photosynthesize efficiently. 1. The flat shape of the leaf helps to capture light and carbon dioxide by virtue of a large surface area compared to volume. 2. A waxy cuticle covers the leaf, and limits the loss of water to the air. 3. The leaf has many small pores (stomata) that allow carbon dioxide, oxygen and water vapor to move in and out of the leaf. This exchange of gasses is critical to the photosynthetic process. a. Each stomate can be opened and closed by a pair of guard cells. The guard cells keep the loss of water to a minimum. b. Sunlight heats the leaf, and the loss of water through stomata cool the leaf in the same way that sweating cools a human. 5. Stiff veins and other cells keep the leaf blade extended, like the ribs of an umbrella. 6. The veins also serve as a circulatory system, critical for photosynthesis. a. Some cells in the veins make up a tissue called xylem that carries water and nutrients, necessary components for the formation of cells and sustenance of the photosynthetic process. b. Other cells in the veins make up a tissue called phloem. This carries photosynthates, the sugary products of photosynthesis, out of the leaf. 7. Between the veins are many green cells that perform photosynthesis. Photosynthetic function is concentrated in the layer of palisade chlorenchyma cells in the upper portion of the leaf, as viewed in cross section. 8. Not all photosynthetic materials come from or go directly to the vascular system. The spongy parenchyma adjacent to the palisade layer acts to store the products needed for and resulting from photosynthesis. 9. Other plant parts (young stems and even aerial roots, exposed to light) may also perform photosynthesis. D. Cellular Adaptations for Photosynthesis 1. A leaf cell may contain as many as 200 football-shaped chloroplasts, the organelles in which photosynthesis takes place. 2. Each chloroplast has the molecular structure needed to trap light and produce sugar and oxygen. 3. A chloroplast has an envelope composed of two thin membranes. This envelope encloses a fluid, wherein additional membranes occur, layered one on top of the other. These are termed lamellar membranes. In cross-section this would look something like a many-layered clubhouse sandwich. 4. The chloroplast's lamellar membranes are where light energy is captured for conversion to chemical energy (to be stored in the form of photosynthates). 5. The molecule chlorophyll, which occupies these membranes, is critical to the photosynthetic process. Central to every chlorophyll molecule are magnesium and nitrogen atoms. This is one of the reasons that plants suffering from nitrogen deficiency turn yellow--they don't have the raw materials to produce the green chlorophyll necessary for photosynthesis. 6. The fluid between the lamellar membranes, termed the stroma, is where the photosynthetic process is completed, as the energy captured in the lamellar membranes is used to produce sugar. E. Requirements and Products of Photosynthesis 1. Requirements of Photosynthesis: Beyond the presence of chlorophyll and the chloroplasts in which photosynthesis occurs, there are three basic constituents required for photosynthesis to take place: carbon dioxide, water, and light energy. a. Carbon dioxide, abbreviated CO2, is a by-product of respiration and is present in our atmosphere at a level averaging around 0.03%. Plants obtain CO2 from the atmosphere, and through photosynthesis "fix" carbon, incorporating it into carbohydrates. As plants use up carbon dioxide at the leaf surface, the level of CO2 can become the limiting ingredient of the photosynthetic process if there is insufficient air movement to replenish the supply. b. Water molecules, obtained from the roots, and to a small degree from water vapor, are broken down through photosynthesis and used in the creation of photosynthates. Besides its role as a direct ingredient in the photosynthetic reaction (which uses a relatively small amount), an abundant supply of water is necessary for transpiration in the plant keeping the leaf stomata open and maintaining the critical exchange of CO2 and O2 through the leaves. c. Sunlight (or artificial light) provides energy that is used to drive the reaction that makes sugar and oxygen from carbon dioxide and water. Thousands of calories of light energy are required for each gram of CO2 changed into carbohydrate. The kinds of light used by plants for photosynthesis are in the red and blue portions of the spectrum. Plants reflect (rather than absorb) the green part of the spectrum: This is why plants appear green to us. Normal white light from sunlight, which contains all parts of the spectrum, is ideal for photosynthesis. When using artificial light to maintain plant growth, care must be taken to provide the correct quality (wavelengths) of light. d. While not a direct ingredient of the photosynthetic reaction, an adequate ambient temperature is necessary for photosynthesis to proceed. Like other chemical reactions, the rate of photosynthesis speeds up as temperature increases, approximately doubling for each 18 degree (F) (or 10 degree Centigrade) increase in temperature. Too high a temperature, however, will actually slow photosynthesis due to closing stomata and reduced intake of carbon dioxide. 2. Products of Photosynthesis a. The primary product of photosynthesis is the glucose sugar molecule , abbreviated C6 H12 O6. From this point, other carbohydrates (sugars and starches) are produced. These, in turn, serve as building blocks for a wide variety of more complex materials, including polysaccharides, proteins, and fats. b. Oxygen (O2) is the waste product of photosynthesis. Originally taken from the water molecule, oxygen released in photosynthesis replenishes the oxygen supply used by all living creatures for respiration. F. The Photosynthetic Reaction: In actuality, the set of reactions required to complete photosynthesis is large and quite complex, beyond the scope of this discussion. Fortunately, we can break the photosynthetic process down into two main parts, which in turn can be summarized as one fairly easy-to-remember reaction. Those two main parts are the so-called "light" and "dark" reactions: 1. The Light Reaction: This reaction is the part of photosynthesis which captures light energy. a. It is called the light reaction because it occurs only in the presence of light, usually sunlight. b. The light reaction occurs within the lamellar membranes of the chloroplast. c. These membranes contain pigments that absorb the light, that is, they accept and hold on to the light energy. 1) "Chlorophyll a" and "chlorophyll b" are the chief pigments which give the leaf its green color. 2) Other pigments, such as carotenoids, are also present in many plants. (Carotenoids are the yellow, orange, and red pigments of autumn leaves and many fruits.) d. In a complex series of events, the trapped energy is used to split 12 water molecules. The result is: 1) The release of 6 oxygen (O2) molecules into the atmosphere. 2) The release of hydrogen in a form that can be used to build sugars from carbon dioxide. 3) A supply of charged particles to drive the second part of the photosynthetic process, the "dark reaction." e. Diagrammatically, the light reaction can be summarized: (12) H2O --------> (6) O2 + (24) H+ + (24) e- light 2. The Dark Reaction: In this part of photosynthesis, sugar is produced. a. It is called the dark reaction because it does not require the presence of light to occur--it proceeds just as well in darkness. b. The dark reaction occurs within the stroma of the chloroplast, and requires complex enzymatic activity in order to be completed. c. The heart of the dark reaction is the reduction of atmospheric CO2 to glucose, by the joining of 6 carbon atoms to each other and the addition of hydrogen obtained from the light reaction. In short, 6 carbon dioxide molecules are converted into 1 molecule of sugar. Also, for each molecule of sugar produced in the dark reaction, there is a by-product of six molecules of water. d. Diagrammatically, the dark reaction can be summarized: (6) CO2 + (24) e- + (24) H+ ------> (1) C6 H12 O6 + (6) H2O 3. The Balanced Equation for Photosynthesis: If the light and dark reactions are balanced, one overall equation for photosynthesis is achieved. Notice how different sides of the equations cancel below, resulting in an equation which is easy to remember, and summarizes the net effect of photosynthesis. Light Reaction: (12) H2O ------> (24) H+ + (24) e- + (6) O2 {Net of (6) H2O} { Cancels } PLUS: Dark Reaction: (6) CO2 + (24) e- + (24) H+ ------> C6 H12 O6 + (6) H2O { Cancels } {Cancels} ========================================================================== YIELDS: Balanced (Net) Equation: (6) CO2 + (6) H2O ------> C6 H12 O6 + (6) O2 This balanced equation, with six molecules of carbon dioxide combining with six molecules of water to produce one molecule of glucose and a by-product of six molecules of oxygen, is the way photosynthesis is commonly represented as a chemical reaction. 3. Variations in the Photosynthetic Process: It should be noted that not all plants photosynthesize in the same way. While the details of these differences are beyond the scope of this lesson, a few points are noteworthy: a. There are three different types of plants, distinguished by the chemical pathway each employs to fix carbon in the dark reaction. The three groups are called C3, C4 and CAM plants. b. C4 plants are more efficient than C3 plants in their use of water and carbon dioxide. CAM plants are very water efficient. c. The final net products of the processes are the same in all three cases. G. Several factors influence the amount of photosynthesis a leaf will perform. 1. Vigorous photosynthesis requires red and blue light because these are the chief colors that chlorophyll absorbs. Sunlight contains all colors of light in adequate amounts, but artificial light and the shade cast by plants may be deficient in some colors. This can lead to spindly plants. 2. Within limits, the rate of photosynthesis can be increased by exposure to light of greater intensity (brightness). At very high intensities, however, additional light may give no further increases in photosynthesis because other factors (chiefly carbon dioxide) become limiting. 3. Photosynthetic production is proportional to the length of the day. 4. Temperature also affects photosynthesis. a. Photosynthesis increases with temperature, as noted above. b. This is true only if light is intense enough to be non-limiting. At lower light intensities, temperature may have little effect. 5. Adequate CO2 supplies are needed for rapid photosynthesis. a. CO2 makes up about 0.03% of the atmosphere. Thus, at high light intensities, photosynthesis may be limited by the low amount of CO2 available. b. Very high CO2 concentrations are damaging to plants, but moderate artificial increases (for instance, to 0.05%) can improve the rate of photosynthesis. This is only practical in greenhouses and other enclosed spaces. 6. Water stress reduces photosynthesis. a. When transpiration rates are higher than water intake rates, plants wilt, stomata close, and there is a dramatic drop in photosynthesis due to the restricted exchange of CO2 and O2. b. Excessive soil moisture around the roots of a plant may cause a lack of oxygen that reduces root respiration. This can decrease photosynthesis in the leaves due to poor transport of soil moisture from the soil. H. Relationship of Photosynthesis to Respiration: In terms of materials that are used and produced, photosynthesis is the opposite of respiration. When studied in detail, however, it is clear that the two processes proceed in very different ways. This will be explored in the next lesson (CLF6152) Respiration. __________________________________________________________ ACTIVITY: 1. Grow a number of plants: a. Remove different proportions of leaves. b. Completely remove leaves of some. c. Observe and discuss the differences in growth. 2. Cover all or a portion of the leaves of a fast-growing plant such as coleus. a. Students can make designs or draw their initials on leaves with lightproof tape or foil. b. Place plants in light for several days. c. Pick the individually treated leaves and run an iodine starch test on them. d. The dark stained areas will indicate photosynthetic activity (starch accumulation). __________________________________________________________ 12/13/90 MH/sg #%&C