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From Wikipedia

Light-dependent reactions

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 + 3Pi→ 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

From Yahoo Answers

Question:There are two major products of photosynthesis water and organic substances such as glucose. How would the presence of DPIP (2,6-Dichlorophenol-Indophenol, electron accepting compound) affect the production of oxygen during the light reaction? How would the presence of DPIP affect the production of organic product during the dark reaction? What role did another artificial electron acceptor, ferricyanide, play in elucidating the process of photosynthesis?

Answers:By accepting electrons, DPIP will speed up electron flow, water splitting and oxygen production. DPIP ain't NADPH so I would imagine thed dark rxn would probably be inhibited.


Answers:water separates into its components of one oxygen and 2 hydrogen ions (H+). the 2 electrons released from the H+ go through the electron transport chain in the light dependent reactions.

Question:1All statesments of dark reaction are correct EXCEPT: a) this reaction begins soon after sundown and ends before sunrise b) the 5-carbon sugar is constantly being regenerated c)one of the end product is PGA 2Respiration and photosynthesis are complimentary reactions. WHich of the following applies to both processes? a)the raw materials of CO2 and O2 b)water molecules are slpit c)energy changes form d)oxygen is priduced 3Oxidation of water can be from: a) respiration b) photosynthesis c)both d)neither both In a graph, y-line is % absorption, x-line is wavelength with the colors left to right: violet(2,5) blue(4,2) green(6,7) orange (8,4) red(10,5) line starts at (0,3) The title is Action Spectrum of plant pigments ab) color of the plant pigment represent in the diagram is ac)which wavelength of light is most effective for photosynthesis the graph is another question

Answers:1) a -- calvin's cyle/ dark reaction is independent light reaction. it doesn't mean it doesn't occur when sunrise. ya 5-carbon is ribulose biphosphate. it's reproduced and regenerated. first ribulose biphophate combine with CO2 and form unstable 6-carbon compound.this reaction is catalysed by enzyme rubisco / ribulose biphosphate (RuBP) enzyme. as soon as 6-carbon is formed it splits into glycerate-3-phospahte. glycerate-3-phosphate is then phosphorylated by ATP (formed from light reaction) to form glycerate-1,3-phospahte. glycerate-1,3-phosphate is then reduced by NADPH(reduced nicotinamide dinucleotide phosphate). then glyceradehye-3-phosphate is formed and some of them is converted back to ribulose phosphate. this ribulose phosphate is then phosphorylated back to ribulose biphosphate. so 5-carbon is regenerated. it's correct. PGA is glyceraldehye-3-phosphate which is produced through calvin's cycle --- correct 2) the possible answers are a or c. b and d are out as b only and d occur in photosynthesis. for me i would choose a as the question said respiration and photosynthesis are complimentary reaction, to me, which means CO2 is produced from respiration and used by photosynthesis while O2 is produced by photosynthesis and used by respiration. energy , ATP are formed and used in both process, respiration and photosynthesis 3) b. formation of H2O in respiration but no oxidation for graph, i guess colour is dark blue. as the (0,3) is purple and blue(4,2) have the least % absorption . that means more reflection. so when two colors are mixed, it should be dark blue. the green colour is hte most effective as it's absorption is the highest

Question:What is the source of H+ in light-independent reaction? How is H+ used? I don't think H+ involves in dark reaction because H+ already combined with NADP+ in light dependent to form NADPH Thanks in advance

Answers:The source of H+ in light reactions is from water (H2O) During light reactions, water is split and oxygen is released as a by-product. Electrons and the H+ ions are carried from the water to carbon dioxide, reducing it to sugar. The electrons increase in potential energy as they move from water to carbon dioxide thus this reaction is endergonic. The source of energy comes from light (sun). Hope this helped! =)

From Youtube

Photosynthesis: Light Reactions 1 :Details on the light-dependent reactions of photosynthesis

Photosynthesis :Check us out at www.tutorvista.com Photosynthesis is a process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can create their own food. In plants, algae and cyanobacteria photosynthesis uses carbon dioxide and water, releasing oxygen as a waste product. Photosynthesis is vital for life on Earth. As well as maintaining the normal level of oxygen in the atmosphere, nearly all life either depends on it directly as a source of energy, or indirectly as the ultimate source of the energy in their food(the exceptions are chemoautotrophs that live in rocks or around deep sea hydrothermal vents). The amount of energy trapped by photosynthesis is immense, approximately 100 terawatts: which is about six times larger than the power consumption of human civilization.As well as energy, photosynthesis is also the source of the carbon in all the organic compounds within organisms' bodies. In all, photosynthetic organisms convert around 100000000000 tonnes of carbon into biomass per year. Although photosynthesis can happen in different ways in different species, some features are always the same. For example, the process always begins when energy from light is absorbed by proteins called photosynthetic reaction centers that contain chlorophylls. In plants, these proteins are held ...