Figure 5: The light and dark reactions in the chloroplast The chloroplast is involved in both stages of photosynthesis. The light reactions take place in the thylakoid. There, water H 2 O is oxidized, and oxygen O 2 is released. The dark reactions then occur outside the thylakoid. The products of this reaction are sugar molecules and various other organic molecules necessary for cell function and metabolism. Note that the dark reaction takes place in the stroma the aqueous fluid surrounding the stacks of thylakoids and in the cytoplasm.
The thylakoids, intake of water H 2 O , and release of oxygen O 2 occur on the yellow side of the cell to indicate that these are involved in the light reactions. The carbon fixation reactions, which involve the intake of carbon dioxide CO 2 , NADPH, and ATP, and the production of sugars, fatty acids, and amino acids, occur on the blue side of the cell to indicate that these are dark reactions. An arrow shows the movement of a water molecule from the outside to the thylakoid stack on the inside of the chloroplast.
Another arrow shows light energy from the sun entering the chloroplast and reaching the thylakoid stack. An arrow shows the release of an oxygen molecule O 2 from the thylakoid stack to the outside of the chloroplast. Once the light reactions have occurred, the light-independent or "dark" reactions take place in the chloroplast stroma. During this process, also known as carbon fixation, energy from the ATP and NADPH molecules generated by the light reactions drives a chemical pathway that uses the carbon in carbon dioxide from the atmosphere to build a three-carbon sugar called glyceraldehydephosphate G3P.
Cells then use G3P to build a wide variety of other sugars such as glucose and organic molecules. Many of these interconversions occur outside the chloroplast, following the transport of G3P from the stroma. The products of these reactions are then transported to other parts of the cell, including the mitochondria, where they are broken down to make more energy carrier molecules to satisfy the metabolic demands of the cell. In plants, some sugar molecules are stored as sucrose or starch.
This page appears in the following eBook. Aa Aa Aa. Photosynthetic Cells. What Is Photosynthesis? Why Is it Important? Figure 2. Figure 3: Structure of a chloroplast. Figure 4: Diagram of a chloroplast inside a cell, showing thylakoid stacks.
Shown here is a chloroplast inside a cell, with the outer membrane OE and inner membrane IE labeled. What Are the Steps of Photosynthesis? Figure 5: The light and dark reactions in the chloroplast. The chloroplast is involved in both stages of photosynthesis. Photosynthetic cells contain chlorophyll and other light-sensitive pigments that capture solar energy. In the presence of carbon dioxide, such cells are able to convert this solar energy into energy-rich organic molecules, such as glucose.
These cells not only drive the global carbon cycle, but they also produce much of the oxygen present in atmosphere of the Earth.
Essentially, nonphotosynthetic cells use the products of photosynthesis to do the opposite of photosynthesis: break down glucose and release carbon dioxide.
Cell Biology for Seminars, Unit 1. Topic rooms within Cell Biology Close. No topic rooms are there. Or Browse Visually. Student Voices. Creature Cast. Simply Science. Green Screen. Green Science. Bio 2. The Success Code. Why Science Matters. The Beyond. Plant ChemCast. Postcards from the Universe. Brain Metrics. Mind Read. Her work history includes working as a naturalist in Minnesota and Wisconsin and presenting interactive science programs to groups of all ages.
She enjoys writing online articles sharing information about science and education. Currently, Dr. Dowd is a dean of students at a mid-sized university. Chlorophyll b transmits green light and mainly absorbs blue and red light. Captured sun energy is handed over to chlorophyll a, which is a smaller but more plentiful molecule in the chloroplast. Carotenoids reflect orange, yellow and red light waves.
In a leaf, carotenoid pigments cluster next to chlorophyll a molecules to efficiently hand off absorbed photons. Carotenoids are fat soluble molecules, also believed to play a role in dissipating excessive amounts of radiant energy. Xanthophyll pigments pass along light energy to chlorophyll a and act as antioxidants. The molecular structure gives xanthophyll the ability to accept or donate electrons. Xanthophyll pigments produce the yellow color in fall leaves.
Anthocyanin pigments absorb blue-green light and aid chlorophyll a. Apples and autumn leaves owe their vibrancy to reddish, violet anthocyanin compounds. Anthocyanin is a water-soluble molecule that can be stored in the plant cell vacuole. What Is the Role of Carotenoids in Photosynthesis? In land plants, there are two classes of these photosynthetic pigments, the chlorophylls and the carotenoids. The ability of chlorophyll and carotenoid molecules to absorb the energy of light and use it effectively is related to their molecular structure and to their organization within the cell.
You learned in a previous lesson The Interaction of Light with Biological Molecules that pigments absorb the energy from photons through systems of conjugated double bonds.
Examine the molecular structure of representative chlorophyll and carotenoid molecules in Figures: Structure A and B. Notice the linear system of conjugated double bonds in the carotenoid lutein and the zig-zag of conjugated double bonds in the large ring structure of the chlorophyll.
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