photosynthesis - Photosynthesis Bryant Miles The vast...

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Photosynthesis Bryant Miles The vast majority of energy consumed by living organisms stems from solar energy captured by phototrophic organisms. 1.5 X 10 22 kJ of energy produced by the sun reaches the earth every day. Photosynthetic organisms convert 1 % of the solar energy into chemical energy. This chemical energy is stored in the form of biomolecules, which are harvested by the organisms that eat those forming food chains. The basic equation of photosynthesis is deceptively simple. Water and carbon dioxide combine to form carbohydrates and molecular oxygen. 6CO 2 + 6H 2 O ± C 6 H 12 O 6 + 6O 2 Δ G o ’ = 2,798 kJ/mol Of course the process of photosynthesis is a complex process involving photoreceptors, reaction centers, protein complexes, electron carriers, ect. By this complex process 10 11 tons of carbon dioxide are fixed globally every year. A diverse group of organisms are capable of photosynthesis. From bacteria to the tallest trees, photosynthesis occurs in membranes. In photosynthetic bacteria the plasma membrane fills up the cells interior. In eukaryotes, the photosynthetic membranes are contained within an organelle called a chloroplast. The Chloroplast The chloroplast has many similarities to the mitochondrian. It has a porous outer membrane, an intermembrane space and an inner membrane that is impermeable to most molecules. The inner membrane encloses the stroma which is analogous to the matrix of the mitochondria. In the stroma are the soluble enzymes that utilize NADPH and ATP to convert CO 2 into carbohydrates. Also contained in the stroma is the DNA of the chloroplast and the machinery for replication, transcription and translation. Chloroplasts are not autonomous they require many proteins encoded by the nuclear DNA. Within the stroma are membraneous structures called thylakoids which are flattened discs stacked to form granum. Different grana are linked together by stroma lamellae. The thylakoid membranes are impermeable to most ions and molecules. The chloroplasts have three distinct membranes, outer, inner and thylakoid membranes, which enclose three separate spaces, intermembrane space, stroma, and the thylakoid space (also called the thylakoid lumen). The thylakoid membrane is the site of oxidation- reduction reactions that generate a proton motive force analogous to the cristae in the mitochondrian. The thylakoid membrane also contains the energy-transducing machinery that harvests the energy of the sun. The pigments that absorb light, the reaction centers, electron transport chains and ATP synthase are all contained within the thylakoid membrane.
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Light and Dark Reactions The light reactions are reactions that couple the energy produced by the absorption of a photon of light with redox reactions. These light reactions occur within the thylakoid membrane. The dark reactions are the chemical reactions involved in fixing CO 2 for the synthesis of carbohydrates. These reactions occur in the stroma. Dark reactions can occur in the dark or in the light.
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This note was uploaded on 04/11/2010 for the course BICH bich 411 taught by Professor Bryantmiles during the Spring '10 term at Texas A&M.

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photosynthesis - Photosynthesis Bryant Miles The vast...

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