Biology Photosynthesis Autotrophs use inorganic material: carbon dioxide from the air, and minerals and water from the soil to make their own organic materials, including carbohydrates, lipids, proteins, and nucleic acids. On the other hand, heterotrophs, meaning “other feeders”, cannot make organic materials from inorganic materials. That is why we must eat fruits, vegetables, meats, and grains. Heterotrophs depend on autotrophs for nutrients. Autotrophs depend on a process called photosynthesis for nutrition. We will examine the process of photosynthesis in the following Learning Guide slides. Photosynthesis Photosynthesis is the process in which light energy is used to create organic material. Photosynthesis cannot occur without the help of chloroplasts. Chloroplasts are disc shaped organelles found in certain plant cells that carry out the process of photosynthesis. Chloroplasts are located within the mesophyll of the leaf. The mesophyll, is the green tissue found inside the interior of the leaf. Also an important part to the photosynthetic processes are tiny pores located on the leaf surface called stomata. The stomata allow carbon dioxide to enter the leaf and allows oxygen to exit the leaf. Found throughout the leaves and body of the plant are vital components called xylem and phloem. Xylem consists of dead cells designed for transporting water and minerals from roots to leaves. Phloem cells consist of living cells that distribute sugars from the leaves to roots and other parts of the plant that are not photosynthetic. Another important component of the photosynthesis process is the ability of plants to receive water through precipitation. As seen in the diagram to the right, the water cycle plays a vital role in delivering a vital resource to all plants. The chemical equation below provides a look at the reactants and products of photosynthesis. You will notice that the reactants of photosynthesis, carbon dioxide and water, are also the waste products of cellular respiration. Photosynthesis is a very complex process. The process of photosynthesis has two stages. The two stages include the Light Reactions and the Dark Reactions/Calvin Cycle. Sunlight, a type of energy produces an assortment of wavelengths. The full range of the electromagnetic spectrum is shown to the right. Note: plants only use the band of energy referred to as visible light. Visible light, is only a small portion of this spectrum. It consists of those wavelengths that our eyes see as different colors. Electromagnetic Spectrum In the first step called the Light Reaction, sunlight shines on pigmented materials called chlorophyll a and chlorophyll b found within the chloroplasts. Chlorophyll a - absorbs the blueviolet and red wavelengths produced by sunlight. Whereas, Chlorophyll b primarily absorbs the blue and orange wave lengths and reflects the yellowgreen wavelengths. This is the reason why most plants are green in color. When a pigment molecule, like that of chlorophyll, gains energy, it will not only absorb this energy it will also emit light and heat. The light reaction generates ATP, oxygen and hydrogen by splitting water. This step creates the necessary energy and hydrogen that is required for the second step of photosynthesis. Energized electrons from the watersplitting photosystem pass down an electron transport chain to the NADPHproducing photosystem (2) . The chloroplast uses the energy released by this electron “fall” to make ATP. * Light reaction process taken from Campbell and Reece’s Essential Biology textbook. 2001 The NADPHproducing photosystem transfers its photo-excited electrons to NADP , reducing it to NADPH (3) . The electron transport chain replaces the electrons lost from the photosystem’s chlorophyll.* Calvin Cycle Plants then make sugar molecules during the second stage of photosynthesis using carbon dioxide during a process known as the Calvin cycle or the Dark Reaction. In the first step of the Calvin cycle, Carbon atoms (three gray CO2 balls enter the cycles (1) . An enzyme adds the CO2 to RuBP (a ribulose bisposphate, a five-carbon sugar already present in chloroplast. The product created is now called 3PGA or 3phosphoglyceric acid) In step two of the Calvin cycle, ATP and NADPH from the light reactions provide energy and electrons (2) . Enzymes use the ATP energy and highenergy electrons from NADPH to convert the 3-PGA to threecarbon sugar, G3P (glyceraldehyde-3phosphate). In step three of the Calvin cycle, Carbon exits the cycle as sugar (Glucose) (3) . Therefore, three CO2 molecules have been converted to one molecule of sugar G3P. This is the direct product of photosynthesis but plant cells can use the G3P to make glucose and other organic compounds for growth and fuel. In the fourth and final step of the Calvin cycle, the cycle regenerates its starting material. Note that of the six G3P molecules produced in step 3 only one of them represents net sugar output (4) . That’s because we started with a total of 15 sugar carbons in the three RuBP molecules that accepted CO2 back in step one. Enzymes now regenerate the RuBP by rear-ranging the five G3P molecules that are left after one of those sugars exits the cycle.* Throughout the evolution of plants natural selection has forced plants to adapt their photosynthesis capabilities. Plants that use CO2 directly from the air are called C3 plants because the first organic compound produced, by these plants, is a three-carbon compound called 3-PGA. Another type of plant, a C4 plant has special adaptations that save water without shutting down the process of photosynthesis. These C4 plants keep their stomata/pores closed during the daytime to conserve water, yet at the same time continue to make sugar. Examples of these plants include - Cacti, ice plants, and jade plants are classified as CAM plants. CAM plants conserve water by opening its stomata and taking in CO2 at night. The O2 is released during the day through the Calvin cycle. Pineapple plant Works Cited http://www.chagres.com/ae-rainforest-1.jpg http://pantransit.reptiles.org/images/1996-07-28/washington-rain-forest.png http://www.free-desktop.net/wallpapers/wa/tall-forest.jpg http://wuarchive.wustl.edu/aminet/pix/views/forest.jpg http://www.bio.umass.edu/biology/conn.river/plant_images/forest2.jpg After completing the test, move onto Unit #8.
