Photosynthesis is the process by which organisms that contain the pigment chlorophyll convert light energy into chemical energy which can be stored in the molecular bonds of organic molecules. Photosynthesis powers almost all trophic chains and food webs on the Earth.
The net process of photosynthesis is described by the following equation:
6CO2 + 6H2O + Light Energy = C6H12O6 + 6O2
This equation simply means that carbon dioxide from the air and water combine in the presence of sunlight to form sugars; oxygen is released as a by-product of this reaction.
Photosynthesis begins when light strikes Photosystem I pigments and excites their electrons. The energy passes rapidly from molecule to molecule until it reaches a special chlorophyll molecule called P700, so named because it absorbs light in the red region of the spectrum at wavelengths of 700 nanometers.
Until this point, only energy has moved from molecule to molecule; now electrons themselves transfer between molecules. P700 uses the energy of the excited electrons to boost its own electrons to an energy level that enables an adjoining electron acceptor molecule to capture them. The electrons are then passed down a chain of carrier molecules, called an electron transport chain. The electrons are passed from one carrier molecule to another in a downhill direction, like individuals in a bucket brigade passing water from the top of a hill to the bottom. Each electron carrier is at a lower energy level than the one before it, and the result is that electrons release energy as they move down the chain. At the end of the electron transport chain lies the molecule nicotine adenine dinucleotide (NADP+).
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Using the energy released by the flow of electrons, two electrons from the electron transport chain combine with a hydrogen ion and NADP+ to form NADPH.
When P700 transfers its electrons to the electron acceptor, it becomes deficient in electrons. Before it can function again, it must be replenished with new electrons. Photosystem II accomplishes this task. As in Photosystem I, light energy activates electrons of the Photosystem II pigments. These pigments transfer the energy of their excited electrons to a special Photosystem II chlorophyll molecule, P680, that absorbs light best in the red region at 680 nanometers. Just as in Photosystem I, energy is transferred among pigment molecules and is then directed to the P680 chlorophyll, where the energy is used to transfer electrons from P680 to its adjoining electron acceptor molecule.
From the Photosystem II electron acceptor, the electrons are passed through a different electron transport chain. As they pass along the cascade of electron carrier molecules, the electrons give up some of their energy to fuel the production of ATP, formed by the addition of one phosphorus atom to adenosine diphosphate (ADP).
Eventually, the electron transport carrier molecules deliver the Photosystem II electrons to Photosystem I, which uses them to maintain the flow of electrons to P700, thus restoring its function.
P680 in Photosystem II is now electron deficient because it has donated electrons to P700 in Photosystem I. P680 electrons are replenished by the water that has been absorbed by the plant roots and transported to the chloroplasts in the leaves. The movement of electrons in Photosystems I and II and the action of an enzyme split the water into oxygen, hydrogen ions, and electrons. The electrons from water flow to Photosystem II, replacing the electrons lost by P680. Some of the hydrogen ions may be used to produce NADPH at the end of the electron transport chain, and the oxygen from the water diffuses out of the chloroplast and is released into the atmosphere through pores in the leaf.
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The transfer of electrons in a step-by-step fashion in Photosystems I and II releases energy and heat slowly, thus protecting the chloroplast and cell from a harmful temperature increase. It also provides time for the plant to form NADPH and ATP.
The chemical energy required for dark reactions is supplied by the ATP and NADPH molecules produced in the light-dependent reaction. The dark reactions begin with a molecule that must be regenerated at the end of the reaction in order for the process to continue. In the Calvin cycle, the dark reactions use the electrons and hydrogen ions associated with NADPH and the phosphorus associated with ATP to produce glucose. These reactions occur in the fluid in the chloroplast and each step is controlled by a different enzyme.
The dark reactions requires the presence of carbon dioxide molecules, which enter the plant through pores in the leaf, diffuse through the cell to the chloroplast, and disperse in the fluid in the chloroplast. The dark reactions begin when these carbon dioxide molecules link to sugar molecules called ribulose bisphosphate in a process known as carbon fixation.
With the help of an enzyme, six molecules of carbon dioxide bond to six molecules of RuBP to create six new molecules. Several intermediate steps, which require ATP, NADPH, and additional enzymes, rearrange the position of the carbon, hydrogen, and oxygen atoms in these six molecules, and when the reactions are complete, one new molecule of glucose has been constructed and five molecules of RuBP have been reconstructed. This process occurs repeatedly in each chloroplast as long as carbon dioxide, ATP, and NADPH are available. The thousands of glucose molecules produced in this reaction are processed by the plant to produce energy in the process known as aerobic respiration, used as structural materials, or stored. The regenerated RuBP is used to start the Calvin cycle all over again.
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Light reactions in photosynthesis involve the absorption and use of light. The reactions take place in the thylakoid membrane where chlorophyll and other kinds of smaller organic molecules are present. There are two types of photosystems, photo system I and photo system II. The reaction center in photo system I is knon as P700 and the reaction center in Photosystem II is P680. The splitting of a ...
We have seen how plants convert sunlight into sugars. Now we need to understand how cells can use the products of photosynthesis to obtain energy. There are several possible metabolic pathways by which cells can obtain the energy stored in chemical bonds which include glycolysis, fermentation, and cellular respiration.
Glycolysis takes a six carbon sugar, splits it into two molecules of three carbon sugar and then rearranges the atoms. The rearrangement of atoms provides energy to make ATP which is the principle energy ‘currency’ in the cell.
Its two key functions are to generate some ATP from the free energy available from the rearrangement of the atoms and partially break down glucose to provide a starting point for the complete oxidation of glucose by another pathway to carbon dioxide and water with the generation of much ATP.
The starting point for glycolysis is normally glucose although other monosacharides may be brought into the pathway.
The end product, in anaerobic conditions, depends on the organism. In higher organisms the end product is lactate whereas in some microorganisms it is ethanol and carbon dioxide.
Several processes occur sequentially in the pathway. Step 1: the six carbon monosacharide is split into two molecules of three carbon monosacharide; Step 2: the triose is oxidized at one end; Step 3:the oxidized triose is reduced at the other end.
During step two ATP is synthesized from ADP. One mole of ATP is made for each mole of triose processed. Two moles of ATP for each hexose. Because the oxidation step is balanced with a reduction step a small amount of oxidizing agent can be recycled indefinitely.
During fermentation, the pyruvic acid produced during glycolysis is converted to either ethanol or lactic acid. This continued use of pyruvic acid during fermentation permits glycolysis to continue with its associated production of ATP.
Respiration is the general process by which organisms oxidize organic molecules and derive energy (ATP) from the molecular bonds that are broken.
Glucose: C6H12O6
Respiration is the opposite of photosynthesis, and is described by the equation:
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... + 6 oxygen → 6 Water + 6 Carbon Dioxide + ATP energy Visit the NASA website (http://data.giss.nasa.gov/gistemp/graphs/) and research global ... what are its three stages? 1. Glycolysis 2. Krebs Cycle (Citric Acid Cycle) 3. ... assertion is that the primary product of respiration is Carbon Dioxide which is one of ... photosynthesis life forms. It is the vital step in the beginning of the food chain ...
C6H12O6+6O2 ———-> 6CO2+6H2O+36ATP
Simply stated, this equation means that oxygen combines with sugars to break molecular bonds, releasing the energy (in the form of ATP) contained in those bonds. In addition to the energy released, the products of the reaction are carbon dioxide and water.
In eukaryotic cells, cellular respiration begins with the products of glycolysis being transported into the mitochondria. A series of metabolic pathways (the Krebs cycle and others) in the mitochondria result in the further breaking of chemical bonds and the liberation of ATP. CO2 and H2O are end products of these reactions. The theoretical maximum yield of cellular respiration is 36 ATP per molecule of glucose metabolized.
