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The light-dependent reactions, or light reactions, are the first stage of photosynthesis, the process by which plants capture and store energy from sunlight. In this process, light energy is converted into chemical energy, in the form of the energy-carrying molecules ATP and NADPH. In the light-independent reactions(also called dark reactions, by convention, as they are driven by products of light, ATP and NADPH-it should not be misunderstood that they occur in dark or they are independent of the need of light), the formedNADPH and ATP drive the reduction of CO2 to more useful organic compounds, such as glucose.
The light-dependent reactions take place on the thylakoid membrane inside a chloroplast. The inside of the thylakoid membrane is called the lumen, and outside the thylakoid membrane is the stroma, where the light-independent reactions take place. The thylakoid membrane contains some integral membrane protein complexes that catalyze the light reactions. There are four major protein complexes in the thylakoid membrane: Photosystem I (PSI), Photosystem II (PSII), Cytochrome c6f complex and ATP synthase. These four complexes work together to ultimately create the products ATP and NADPH.
The two photosystems absorb light energy through proteins containing pigments, such as chlorophyll. The light-dependent reactions begin in photosystem II. When a chlorophyll a molecule within the reaction center of PSII absorbs a photon, an electron in this molecule attains a higher energy level. Because this state of an electron is very unstable, the electron is transferred from one to another molecule creating a chain of redox reactions, called an electron transport chain (ETC). The electron flow goes from PSII to cytochrome b6f to PSI. In PSI the electron gets the energy from another photon. The final electron acceptor is NADP. In oxygenic photosynthesis, the first electron donor is water, creating oxygen as a waste product. Inanoxygenic photosynthesisvarious electron donors are used.
Cytochrome b6f and ATP synthase work together to create ATP. This process is called photophosphorylation, which occurs in two different ways. In non-cyclic photophosphorylation, cytochrome b6f uses the energy of electrons from PSII to pump protons from the stroma to the lumen. The proton gradient across the thylakoid membrane creates a proton-motive force, used by ATP synthase to form ATP. In cyclic photophosphorylation, cytochrome b6f uses the energy of electrons from not only PSII but also PSI to create more ATP and to stop the production of NADPH. Cyclic phosphorylation is important to create ATP and maintain NADPH in the right proportion for the light-independent reactions.
The net-reaction of all light-dependent reactions in oxygenic photosynthesis is:
2H2O + 2NADP+ + 3ADP + 3Piâ†’ O2 + 2NADPH + 3ATP
Light to chemical energy
The two photosystems are protein complexes that absorb photons and are able to use this energy to create an electron transport chain. Photosystem I and II are very similar in structure and function. They use special proteins, called light-harvesting complexes, to absorb the photons with very high effectiveness. If a special pigment molecule in a photosynthetic reaction center absorbs a photon, an electron in this pigment attains the excited state and then is transferred to another molecule in the reaction center. This reaction, called photoinduced charge separation, is the start of the electron flow and is unique because it transforms light energy into chemical forms.
The light-harvesting system
A common misconception is that photosynthesis relies only on chlorophyll pigments. The truth is that photosynthesis would be rather inefficient using only chlorophyll molecules. Chlorophyll molecules absorb light only at specific wavelengths (see image). A large gap is present in the middle of the visible regions between approximately 450 and 650 nm. This gap corresponds to the peak of the solar spectrum, so failure to collect this light would constitute a considerable lost opportunity. That's why photosynthesis organisms have developed a light-harvesti
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Answers:Heart of Darkness - Joseph Conrad Study Guides: http://www.shmoop.com/heart-of-darkness/ http://www.bookrags.com/notes/hod/ http://www.sparknotes.com/lit/heart/ http://www.schoolbytes.com/summary.php?id=378 http://summarycentral.tripod.com/heartofdarkness.htm
Answers:If an object absorbs light then the light that hits the object never leaves it. Any black object absorbs light. Reflected light is bounced off of the object at a similar angle to the normal (an imaginary line perpendicular to the surface.) Since light is energy, if it is absorbed, the object absorbing it also retains its energy. The result is that the object gets warmer.
Answers:Designers choose to use heavy-water in order to minimize the need for enriched uranium. If you use regular water as moderator and/or coolant, the water absorbs a significant amount of the neutrons needed to keep the reaction going. That means you need a big reactor, one with a lot of regular uranium, and lots of moderator and careful attention to saving every possible neutron. If you want to make a small reactor, or one that will run a long time, then you can't use regular uranium-- you have enrich the uranium, which is extremely expensive and requires huge investment in plants and energy. Plus the enriched uranium is excellent bomb-making material so you have to worry about that. An alternative is to design the reactor to use heavy water, deuterium, as a moderator and coolant. deuterium has all the good properties of water, including high heat conductivity, high heat capacitor, relatively non-corrosive, and low neutron absorption. If you use heavy water then you can get by using all natural uranium, which is very cheap. The Canadians designed a few reactors, the "CANDU" series, which work this way. The downside is you need a small boatload of heavy water, which is not cheap either, but a whole lot cheaper than enriched uranium. Heavy-water reactors have not really caught on very much, probably due to various factors, like competition and lack of heavy-water supplies.
Answers:The sacking of Rome in 410 AD by Alaric, leader of the Visigoths caused the gradual fall of the Western Roman Empire. By 476 AD the last Roman Emperor was deposed, beginning a new era, this being the Middle Ages. It is also referred to as "Medieval" in history, so the two terms are alike. The Dark ages is simply a word used to describe the early Middle Ages from 476 AD to approximately 1000 AD. It is named the Dark Ages because it was a specific period of time where there was very little technological development or advances in knowledge, where the quality of life for the people was moving backwards and they suffered as a result. There are several theories as to when the Middle Ages declined and the Renaissance period started. 1. The capture of Constantinople in 1453 AD by Mehmed II, a General of the Ottoman Empire effectively neutralizing the Eastern Roman Empire. 2. The discovery of "The Americas" in 1492 AD by Italian explorer and navigator Christopher Columbus. 3. The widespread use of gunpowder, making the icon of the Medieval era (castles) become obsolete. 4. The Black Death ravaged Europe in 1347 AD, killing millions and efficiently destroying the Feudal system, which provided the basis for society in Medieval times. Glad to help, if you need any additional information, just ask.