Bio_Chapter 6_Photosynthesis

Bio_Chapter 6_Photosynthesis - Chapter 6 Photosynthesis...

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Chapter 6: Photosynthesis Section 1: The Light Reactions Obtaining Energy All organisms get energy from the sun either directly or indirectly. Autotrophs get energy from the sun or chemical bonds in organic substances to make organic compounds. Heterotrophs get energy from food instead of directly from the sun. A biochemical pathway is when the products of one process are the reactants of another process and vice versa. Overview of Photosynthesis In photosynthesis autotrophs produce organic compounds from carbon dioxide (CO 2 ) and water. Photosynthesis also produces oxygen as well as organic compounds. The products of photosynthesis are then used by a process called cellular respiration which produces CO 2 and water. There are two stages of photosynthesis: light reactions and the Calvin Cycle (dark reactions). The photosynthesis equation is: CO 2 + H 2 O C 6 H 12 O 6 + O 2 Capturing Light Energy Light reactions require light to happen. It begins when light is absorbed by chloroplasts. There is a system of sacs inside the inner membrane called thylakoids. Stacks of thylakoids form grana which are surrounded by a solution called stroma. Light and Pigments Light from the sun appears white but is made up of many colors. The colors of white light can be separated when light passes through a prism. The array of colors is called a visible spectrum. When light strikes an object, it can be reflected, transmitted, or absorbed by the object. Many objects contain pigments which absorb light. Chloroplast Pigments The most important pigment is chlorophyll which is found in the membrane of thylakoids. The two most common types of chlorophylls are chlorophyll a and chlorophyll b . Only chlorophyll a is directly involved in the light reactions of photosynthesis. Chlorophyll b assists chlorophyll a in capturing light energy and is known as an accessory pigment. Carotenoids are also accessory pigments. Accessory pigments enable plants to capture more of the energy in light by absorbing the colors that chlorophyll a cannot. Chlorophylls mask the colors of other pigments. Leaves lose chlorophyll in the fall and take on the color of the carotenoids present.
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