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The transparent epidermis layer allows light to pass through to the palisade mesophyll cells where most of the photosynthesis takes place.
The first photosynthetic organisms probably evolved early in the evolutionary history of life and most likely used reducing agents such as hydrogen or hydrogen sulfide, rather than water, as sources of electrons.
Photosynthetic organisms are photoautotrophs, which means that they are able to synthesize food directly from carbon dioxide and water using energy from light.
In the first stage, light-dependent reactions or light reactions capture the energy of light and use it to make the energy-storage molecules ATP and NADPH. Embedded in the thylakoid membrane are integral and peripheral membrane protein complexes of the photosynthetic system.
During the second stage, the light-independent reactions use these products to capture and reduce carbon dioxide. Plants absorb light primarily using the pigment chlorophyll.
Photosynthesis and cellular respiration are distinct processes, as they take place through different sequences of chemical reactions and in different cellular compartments. Enclosed by the membrane is an aqueous fluid called the stroma.
The general equation for photosynthesis as first proposed by Cornelis van Niel is therefore: This equation emphasizes that water is both a reactant in the light-dependent reaction and a product of the light-independent reaction, but canceling n water molecules from each side gives the net equation: Photosynthesis occurs in two stages. Embedded within the stroma are stacks of thylakoids (grana), which are the site of photosynthesis. The thylakoid itself is enclosed by the thylakoid membrane, and within the enclosed volume is a lumen or thylakoid space.
This produces a proton gradient more directly, which is then converted to chemical energy. In such proteins, the pigments are arranged to work together.
The process does not involve carbon dioxide fixation and does not release oxygen, and seems to have evolved separately from the more common types of photosynthesis. Such a combination of proteins is also called a light-harvesting complex.
However, not all organisms use carbon dioxide as a source of carbon atoms to carry out photosynthesis; photoheterotrophs use organic compounds, rather than carbon dioxide, as a source of carbon.
In plants, algae, and cyanobacteria, photosynthesis releases oxygen.