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In plants the photosynthetic process occurs inside chloroplasts, which are organelles found in certain cells. Chloroplasts provide the energy and reduced carbon needed for plant growth and development, while the plant provides the chloroplast with CO2, water, nitrogen, organic molecules and minerals necessary for the chloroplast biogenesis.
Most chloroplasts are located in specialized leaf cells, which often contain 50 or more chloroplasts per cell. Each chloroplast is defined by an inner and an outer envelope membrane and is shaped like a meniscus convex lens that is microns in diameter Fig.
For details of chloroplast structure, see Staehlin The inner envelope membrane acts as a barrier, controlling the flux of organic and charged molecules in and out of the chloroplast. Water passes freely through the envelope membranes, as do other small neutral molecules like CO2 and O2. There is evidence that chloroplasts were once free living bacteria that invaded a non-photosynthetic cell long ago.
They have retained some of the DNA necessary for their assembly, but much of the DNA necessary for their biosynthesis is located in the cell nucleus. This enables a cell to control the biosynthesis of chloroplasts within its domain.
Inside the chloroplast is a complicated membrane system, known as the photosynthetic membrane or thylakoid membranethat contains most of the proteins required for the light reactions. The proteins required for the fixation and reduction of CO2 are located outside the photosynthetic membrane in the surrounding aqueous phase.
The photosynthetic membrane is composed mainly of glycerol lipids and protein. The glycerol lipids are a family of molecules characterized by a polar head group that is hydrophilic and two fatty acid side chains that are hydrophobic.
In membranes, the lipid molecules arrange themselves in a bilayer, with the polar head toward the water phase and the fatty acid chains aligned inside the membrane forming a hydrophobic core Fig. The photosynthetic membrane is vesicular, defining a closed space with an outer water space stromal phase and an inner water space lumen.
The organization of the photosynthetic membrane can be described as groups of stacked membranes like stacks of pita or chapati bread with the inner pocket representing the inner aqueous spaceinterconnected by non-stacked membranes that protrude from the edges of the stacks Fig.
Experiments indicate that the inner aqueous space of the photosynthetic membrane is likely continuous inside of the chloroplast. It is not known why the photosynthetic membrane forms such a convoluted structure. To understand the energetics of photosynthesis the complicated structure can be ignored and the photosynthetic membrane can be viewed as a simple vesicle.
The chemical structure of chlorophyll a molecule is shown in Fig. Plants appear green because of chlorophyll, which is so plentiful that regions of the earth appear green from space.
The absorption spectrum of chloroplast chlorophyll a and b and carotenoids along with the action spectrum of photosynthesis of a chloroplast is shown in Fig. Light is collected by pigment molecules that are bound to light- harvesting protein complexes located in the photosynthetic membrane.
The light-harvesting complexes surround the reaction centers that serve as an antenna.
Photosynthesis is initiated by the absorption of a photon by an antenna molecule, which occurs in about a femtosecond s and causes a transition from the electronic ground state to an excited state.
Within s the excited state decays by vibrational relaxation to the first excited singlet state. The fate of the excited state energy is guided by the structure of the protein.out-of-order i.e., they look ahead and schedule subse-quent operations to idle execution units of the core.
How-ever, such operations often have unwanted side-effects. Environmental Research Letters covers all of environmental science, providing a coherent and integrated approach including research articles, perspectives and review articles.. All content is published on an open access basis under a CC BY licence and is free to readers, funded by an article publication charge..
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