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Photosynthesis

Energy can not be destroyed or created, but can be transfered.  Light energy is converted into chemical energy (energy contained in the bonds between elements).

Light (photons) striking an object can be reflected, transmitted, or absorbed. Only light that is absorbed can have an effect on the object. Light is absorbed by complex, organic molecules called pigments, which appear coloured because they also reflect light. Green pigments absorb all colours except green, which is reflected back into our eyes. Chlorophyll absorbs light maximally at wavelengths 400-500 nm(violet-blue) and 600-700 nm(orange-red).

There are several kinds of chlorophyll, the most important is chlorophyll a, a grass-green pigment whose structure includes an atom of magnesium (Mg). It occurs in all photosynthetic organisms except photosynthetic bacteria and absorbs light maximally at wavelengths 430 and 662 nm.

Chlorophyll b is a bluish green pigment that absorbs light maximally at 453 and 642 nm. It occurs in all plants, green algae, and some prokaryotes. Plants usually contain about half as much chlorophyll b as chlorophyll a.


Plants also contain a rainbow of other pigments that are often called accessory pigments. They extend the range of light useful for photosynthesis by absorbing photons in wavelengths not absorbed by chlorophyll a and transmitting that energy to chlorophyll a. The most common of these pigments in plants are the carotenoids, which occur in all photosynthetic organisms. Carotenoids absorb maximally between 460-550 nm; therefore these pigments are red, orange and yellow. The most common is beta-carotene, which when split becomes two molecules of vitamin A. Oxidising carotenes produces xanthophylls, which are red and yellow pigments in tomatoes, carrots, leaves, algae, and photosynthetic bacteria, but are less efficient at energy transfer than carotenoids.

 

Put simply, in plants photosynthesis occurs in chloroplasts where a molecule of water is split, an electron is transferred from the H atom using light energy and is incorporated into a sugar molecule.

6CO2 + 12H2O + Light Energy ------>  C6H12O6 + 6O2 + 6H2O
Carbon    Water      Photon                       Sugar     Oxygen  Water

A pigment goes from its ground state to an excited state when when a photon boosts one of its electrons to a higher energy shell.  The electrons are then trapped in a phosphorus chemical bond before they can release their energy as heat and light. This stored energy is then put to use and H2O is split.  An enzyme, Rubisco, combines CO2 with a 5-Carbon sugar (C5, RuBP).  Then, using excited electrons, some 3-Carbon sugars are made, most of which cycle back to the 5-Carbon sugar form, but some escape and are used to synthesise essential organic molecules.


C4 plants are adapted to hot, dry conditions (sugarcane, corn, members of the grass family).  They avert photorespiration and dehydration by incorporating the CO2 into 4-Carbon compounds in specialised mesophyll cells.  These C4 compounds are exported to photosynthetic bundle-sheath cells, where they release their CO2 for use.

CAM plants are also adapted to hot, dry conditions (pineapple, succulents), open their stomata during the cooler night and incorporate the CO2 that enters the leaf into organic acids, which are stored in vacuoles of mesophyll cells.  During the day, stomata close, conserving water, and the CO2 is released for use.

Veins export the sucrose made in green cells to non-photosynthetic parts of the plant.  Cellular respiration in the mitochondria degrade about 50% of the carbohydrate made in photosynthesis.  Much of the remaining carbohydrate is converted to other molecules, such as cellulose.  Excess organic material is stored as starches, oils/lipids and proteins in leaves, roots, tubers, seeds and fruits.

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