Biology-Online • View topic - photosynthesis

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Postby dranseth » Mon Mar 31, 2008 2:27 am

Can somebody ready my description of photosynthesis and tell me if it is accurate. Note that this is very basic and probably simplified to an extreme (especially the calvin cycle as no enzymes are mentioned.)

As visible spectra comes into contact with the light harvesting units of the chloroplast’s thylakoid membrane (PSI, PSII,) quantum’s of energy are absorbed and electrons on ground state are thus excited. Instead of returning to ground state, electron transport molecules made of lipids or proteins that are adjacent to the unit of pigment molecules attract the electron. It is important to note that the flow of electrons is non-cyclic; meaning they flow in one direction and do not return to where they originated from. This is because each carrier molecule has a greater affinity for electrons than its neighboring molecule; meaning electrons travel down the transport chain starting with the oxidation of H2O and ending with the reduction of NADH+.

In order for the plant to continuously supply NADPH (reduced NADP+) for the light independent reactions, there must be a steady supply of electrons for the reaction to occur. Each molecule can only donate one e-1, therefore it must be replaced before another electron can be excited (if electrons were continuously donated without being replaced, the covalent bonds would merely break since they depend on electrons for sharing.) Photosystem two reverts to its reduced form after it has been oxidized because of an enzyme named oxygen-evolving apparatus. The oxidation of water leaves two electrons for every split:
H2O => 2H+ + .5O +2e-1
Note that the oxygen diffuses through the thylakoid membrane and is one of the worlds biggest supplies of oxygen. The hydrogen protons build up inside of the thykakois sac and will later be put to use, and of course, the electrons are donated to PSII.
The travel of electrons go as follow.
Oxidation of water => PSII =>transport molecules =>PSI => transport molecules => NADP reductase which is an enzyme who catalyzes the reduction of NADP+. For every oxidation of water, one NADP is reduced.

Meanwhile, as hydrogen protons continue to build up inside the thylakoid sac, they create a strong concentration gradient called an electrochemical gradient. When an enzyme named ATPase that is embedded in the thylakoids membrane allows the hydrogen protons to follow their concentration gradient, a flow of energy is created. The enzyme harvests this energy and uses it to put together an ATP molecule. This is called photophosphorolation. The important products of the light dependent reactions are oxygen, NADPH, and ATP. NADPH and ATP are used in the light independent reactions in a set of reactions known as the Calvin cycle.
These light independent reactions occur in the stroma of the chloroplast.

6 CO2 combine with 6 ribulosebisphosphates to create 6 very unstable and thus reactive 6-carbon molecules. Shortly after these molecule splits into 2 3-carbon molecules named phosphoglycerate (12 after all have split.) Another phosphate group is added to the phosphoglycerate by the dephosphorylation of 12 ATP (products are 12 diphosphoglycerate and 12 ADP.) We now have 12 diphosphoglycerate molecyles which are reduced by the reduction of 12 NADPH, the products are 12 NADP and 12 glyceraldehyde 3-P. At this point, 2 glyceraldehyde 3-P molecule are outputted while the other 10 continue cycling through. Note that it takes 2 glyceraldehyde 3-P molecules for the cell to make a fructose and later a glucose (both are isomers of a hexose.) A series of reactions uses the energy from 6ATP to rearrange the atoms of the 10 glyceraldehyde 3-P to forming another 6 ribulosebisphosphate molecules to cycle through again.
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Re: photosynthesis

Postby dranseth » Tue Apr 01, 2008 1:36 am

Can someone please read over this? I have to be able to present this information soon. I'm teaching myself this course online and just want to know if all information is valid. Thanks.
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Postby MrMistery » Fri Apr 11, 2008 3:48 pm

Well my first remark after reading that was "What are you talking about?" I know photosynthesis and thus was able to make sense of your description. However, i doubt a student that has no idea about the mechanisms of photosynthesis would.
More minor problems include:
1. You mentioned NADH at the beginning. It should be NADPH.
2. You said "a concentration gradient called an electrochemical gradient". An electrochemical gradient consists of a concentration gradient and an electrical gradient, hence its name.
3. While the term ATPase is correct, you should use the term ATP Synthase. Yeah, the enzyme can work in both directions, but you need to emphasis the physiological role.
4. Glucose and fructose are not isomers of a hexose. They are isomers of one another, and both of them are hexoses.
5. I would mention the Rubisco enzyme. It's important.
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