Biology, Answering the Big Questions of Life/Photosynthesis/Photosynthesis2

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The overall equation for Photosynthesis is

6 CO2+ 12 H2O + light-->C6H12O6 + 6 O2+ 6 H2O The equation can then be simplified down to: 6 CO2+ 6 H2O + light-->C6H12O6 + 6 O2 after balancing the chemical reaction.

The equation in words says that six molecules of carbon Dioxide plus twelve molecules of water in the presence of light will make one molecule of glucose, six molecules of oxygen gas, and six molecules of water.

Photosynthesis is actually divided into two parts. The Light reactions which convert light into chemical energy, and the Dark reactions (also called the Light-independent reactions) which capture CO2 from the air and use it to make sugar.

The Light Dependent Reactions of Photosynthesis

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The structure of a chloroplast

The light dependent reactions occur primarily in the thylakoid membrane of the chloroplast. They take the energy of light and use it to make the high energy molecules NADPH and ATP.

Chlorophyll absorbs light, which then causes it to lose an electron. The electron is taken by the primary electron acceptor, which distributes it into a complex of proteins that pass it from molecule to molecule like a game of hot potato. This is called the electron transport chain. As the molecules pass the electron, they pump hydrogen ions from one side of the membrane to the other. The high concentration of hydrogen ions pumped into the inner thylakoid space (the thylakoid lumen) can then be used to make ATP by a process called chemiosmosis. The electron finally gets dumped on the molecule NADP (Nicotinamide adenine dinucleotide phosphate) changing that molecule into NADPH. The chlorophyll molecule left without an electron can take that electron from water splitting the water into Hydrogen ions and oxygen gas. This is why photosynthesis releases oxygen into the air.

The point of the Light reactions is to make large quantities of NADPH and ATP. These molecules store chemical energy that is used to power the dark reactions. The cell needs this energy because making sugar is hard work.


The Light Independent Reactions of Photosynthesis

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Light Independent reactions take Carbon Dioxide (CO2) and water and use the energy to create sugar.

CO2 is a gas. It has a low energy. Only one enzyme can grab CO2 and stick it to a large molecule to make sugar. This enzyme is Ribulose Bis phosphate carboxylase, but most people just call it Rubisco.

This amazing enzyme takes CO2 out of the air and sticks it on to the five-carbon molecule Ribulose bis-phosphate. This makes a six-carbon molecule that breaks apart immediately into the two three-carbon molecules (PGA and 3-phosphoglycerate).

If that sounds complicated, be assured that it gets worse. This process is called the Calvin cycle and it is a pretty complex process. Because one molecule of CO2 enters at each turn of the wheel, this pathway takes six cycles to make one new glucose molecule.

The Calvin cycle occurs in the stroma of the chloroplast, and the 3PGA molecules made are transported out of the chloroplast and into the cytoplasm where they can be used to build glucose.


Accessory Pigments

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"In every wood, in every spring, there is a different green." J.R.R. Tolkien

Plants contain other pigments besides chlorophylls including carotenoids (yellow orange) and anthocyanins (mostly red). These other accessory pigments sit in the leaf along with the chlorophyll and they are the reason that leaves are different colors in the fall is when chlorophyll can no longer be created.

Many of them actually assist in photosynthesis by absorbing light in wavelengths that chlorophyll cannot and reemitting the light at a lower wavelength so it can be absorbed again by chlorophyll. The process of absorbing and then releasing particles of light is called photosynthesis.

See Also

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Detail Level

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  • Why are plants green?
  • Why do leaves change color in the fall?


  • Excited states
  • Photosystems
  • C4 plants

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