Photosynthesis an Overview

9.2 Photosynthesis: An Overview 4. KEY QUESTIONS What role-do pigments play a the process of photosynthesis? What are electron carrier molecules? What are the reactants and products of photosynthesis? photos chem HS-LS2-SD a made teftite 10 VOCABULARY pigment chlorophy!! thylakoid stroma NADP light-dependent reactions light-independent reactions READING TOOL Fill in the concept map in your Biology Foundations Workbook to show the organization of a chloroplast. BUILD VOCABULARY Prefixes The prefix chloro-refers to the color green. Chlorophyll and chloroplasts are green in color How would you design a system to capture the energy of sunlight? How could you convert the energy into a stable, useful, chemical form? Solving such problems may well be the key to making solar power a more useful energy alternative. Plants, however, have already solved all these issues on their own terms-and maybe we can laam a trick or two from them Chlorophyll and Chloroplasts Our lives, and the lives of nearly every living thing on the surface of Earth, are made possible by the sun and the process of photosyn thesis. In order for photosynthesis to occur, light energy from the un must somehow be captured Light Energy from the sun travels to Earth in the form of light Sunlight, which our eyes perceive as "white" light, is actually a mi ture of different wavelengths. Many of these wavelengths are visible to our eyes and make up what is known as the visible spectrum Our eyes see the different wavelengths of the visible spectrum a different colors: shades of red, orange, yellow, green, blue, indigo and violet. Pigments Light-absorbing compounds, whether they are found on a painter's brush or in a living cell, are known as pigments Photosynthetic organisms use pigments to capture the energy in sunlight. The principal pigment of green plants is known as chlorophyll (kuwe uh fil). The two types of chlorophyll found in plants, chlorophyll a and chlorophyll b, absorb light very well in the blue-violet and red regions of the visible spectrum. However, chicro phyll does not absorb light well in the green region of the spectrum but instead reflects it. That's why most leaves are green 286 Chapter 9 Photosynthasis CASE STUDY Figure 9-4 The Chloroplast Thylakoid In plants, photosynthesis takes place inside chloroplasts Each chloroplast is filled with grana, which are stacks of thylakoid membranes. Plants also contain red and orange pigments such as carotene that absorb light in other regions of the spectrum. Most of the time, the intense green color of chlorophyll overwhelms these accessory pigments, so we don't notice them. As temperatures drop late in the year, however, chlorophyll molecules break down first, leaving the reds and oranges of the accessory pigments for all to see. The beautiful colors of fall in some parts of the country are the result of this process. Chloroplasts Recall from Chapter 8 that in plants and other pho tosynthetic eukaryotes, photosynthesis takes place inside organelles called chloroplasts. Chloroplasts are surrounded by two envelope membranes, and they are filled with saclike chlorophyll-containing membranes called thylakoids (THY luh koydz). These thylakoids are interconnected and arranged in stacks known as grana (singular: granum). The fluid portion of the chloroplast, outside of the thyla-koids, is known as the stroma. The structure of a typical chloroplast is shown in Figure 9-4 Energy Collection What's so special about chlorophyll that makes it essential for photosynthesis? Chlorophyll absorbs light very efficiently, transferring light energy to its own electrons. These high-energy electrons are then available to do chemical work, such as the building of sugar molecules from low-energy compounds like carbon dioxide and water. This is what makes photosynthesis work. READING CHECK Cause and Effect How does photosynthe- sis depend on chlorophyll? INTERACTIVITY Learn more about the inputs and outputs of matter and energy in photosynthesis with an interactive macel High-Energy Electrons In a chemical sense, the high-energy electrons produced by chlorophyll are highly reactive and require a special "carrier Think of a high-energy electron as being similar to a red-fint coal from a fireplace or campfire. If you wanted to move the coal tom one place to another, you wouldn't pick it up in your hands. You would use a pan or bucket a carrier to tramport it, as show in Figure 9-5 Plant cells treat these high-energy electrons in much the same way, instead of a pan, however, they use electron carmens to tran port high-energy electrons from chlorophyll to other moleculos An electron carrier is a compound that can accept a pair of high-energy electrons and transfor them, along with most of their energy, to another molecule. One of these carrier molecules is a compound known as NADP (nicotinamide adenine dinucleotide phosphate). The name is com cated, but the job that NADP does is simple. NADP accepts and holds two high-energy electrons, along with a hydrogen ion (H This converts the NADP into NADPH. The conversion of NADP Into NADPH is one way in which some of the energy of sunight can be trapped in chemical form. The NADPH can carry the high-energy electrons that were prociuced by light absorption in chlorophyll ta chemical reactions elsewhere in the chloroplast. Those high-energy electrons can then be used to help build sugars like glucose from nothing more than carbon dioxide and water READING CHECK Define What is NADP? Visual Analogy Figure 9-5 Carrying Electrons Like a pan being used to carry hot coals, NADP carnes pairs of electrons and an Ht ion from place to place. CASE STUDY Develop a Solution Lab HS-151-5 Open-ended Inquiry Plant Pigments and Photosynthesis Problem Which wavelengths of light support plant growth? in this lab, you will investigate the wavelengths of light that are used in photosynthesis, and construct an explanation for how light wavelength affects plant growth. You will use Elodea, a type of aquatic vegetation, as a test subject. You can find this lab in your digital course. An Overview of Photosynthesis Many steps are involved in the process of photosynthesis. However, the overall process of photosynthesis can be summarized in one sentence. Photosynthesis uses the energy of sunlight to con vert water and carbon dioxide (low-energy reactants) into high-energy sugars and oxygen (products). Plants use these sugars to produce complex carbohydrates such as starches, and to provide energy for the synthesis of other compounds, including proteins and lipids. Because photosynthesis usually produces 6-carbon sugars (CHO) as the final product, the overall reaction for photosynthesis can be shown as follows: In Symbols: 600, +6H₂O CHO+60 In Words: Carbon dioxide + Water Light Sugars + Oxygen Light-Dependent Reactions Although the equation for pho tosynthesis looks simple, there are many steps to get from the reac tants to the final products, in fact, photosynthesis actually involves two sets of reactions. The first set of reactions is known as the light-dependent reactions because they require the direct involvement of light and light-absorbing pigments. The light-dependent reactions take place in thylakoid membraries and use energy from sunlight to add a third phosphate to ADP to make ATP. The light-dependent reactions also do something truly remark-able, which is to take low-energy electrons from water molecules, and use solar energy to raise them to a much higher energy level. These high-energy electrons are then transferred to the electron car rier, NADP, which is converted to NADPH as shown in Figure 9-5. NADPH is used in other chemical reactions. What happens to water after a pair of electrons is taken away from it? That's pretty remark-able, too. The oxygen atoms left over are releaseci to the atmo-sphere as the oxygen gas we and all other animals breathe. READING TOOL Create a two-column table. to describe and compare the light-dependent and light-independent reactions of photosynthesis. INTERACTIVITY Conduct a virtual exper ment to examine how. the distance from a light source can affect the rate of photosynthesis 9.2. Photosynthesis: An Overview 289