Talk:Photosynthesis

From Academic Kids

Template:WikiProjectNotice Template:Todo1 For photosynthesis, can the process of making organic food from inorganic substances occur in the absence of light? (Ref:1995 HKCEE Biology Paper 2 Question)

---

Someone needs to add more on the carbon cycle...something like this would be nice!

The cycle spends ATP as an energy source and consumes NADPH2 as reducing power for adding high energy electrons to make the sugar. There are three phases of the cycle. In phase 1 (Carbon Fixation), CO2 is incorporated into a five-carbon sugar named ribulose bisphosphate (RuBP). The enzyme which catalyzes this first step is RuBP carboxylase or rubisco. It is the most abundant protein in chloroplasts and probably the most abundant protein on Earth. The product of the reaction is a six-carbon intermediate which immediately splits in half to form two molecules of 3-phosphoglycerate. In phase 2 ( Reduction), ATP and NADPH2 from the light reactions are used to convert 3-phosphoglycerate to glyceraldehyde 3-phosphate, the three-carbon carbohydrate precursor to glucose and other sugars. In phase 3 (Regeneration), more ATP is used to convert some of the of the pool of glyceraldehyde 3-phosphate back to RuBP, the acceptor for CO2, thereby completing the cycle. For every three molecules of CO2 that enter the cycle, the net output is one molecule of glyceraldehyde 3-phosphate (G3P). For each G3P synthesized, the cycle spends nine molecules of ATP and six molecules of NADPH2. The light reactions sustain the Calvin cycle by regenerating the ATP and NADPH2.

Source: http://www.bio.umass.edu/biology/conn.river/calvin.html

The added introduction was an attempt to usefully incorporate material that was added to the Wikipedia based on the Household Cyclopedia.

Good, I was just going to do that - the original was inaccurate and out-dated. Thanks! -- Marj Tiefert

Anybody know what's up with the C3 thing in Photosynthesis? What does that refer to? -Jeshii 04:54, May 4, 2004 (UTC)

C3 refers to the number of carbon atoms in the product of the reaction. ~ FriedMilk 02:49, 24 May 2004 (UTC)

Contents

1 O2 comes from where? 2 Pending tasks 3 Rewrite 4 Question about blinking light 5 Ionisation of chlorophyll electrons?

O2 comes from where?

(Why did hitting return actually confirm my edit. The principle of least surprise was violated.)

The photosynthesis reaction is stated as:

6H₂O + 6CO₂ + light → C₆H₁₂O₆ (glucose) + 6O₂

Then, it is said that the oxygen for the O₂ comes from the water, not the carbon dioxide. But, looking at the reaction, there are 6 O atoms coming from water and 12 O atoms leaving in the form of O₂. Some of these oxygen atoms must come from the CO₂. --Snags 21:58, 3 Jun 2004 (UTC)

• Snags, this was what was thought for many, many years. It turns out that the O₂ is liberated when H₂O is reduced by photosystem I to turn NADP+ into NADPH. Alot of students I know ask about this too, so I was sure to include this information in my recent edits. ClockworkTroll 01:43, 2 Nov 2004 (UTC)
  • I have reverted the last edits because the equation was unbalanced and obviously wrong. But why have 12 H2O on one side and 6 H2O on the other? --hgrenbor 16:26, 14 Nov 2004 (UTC)
    • The H2O are broken up for the Hs and Os, then new ones are reformed at a different part of the process. --Whosyourjudas (talk) 22:53, 15 Nov 2004 (UTC)

Pending tasks

do we really need to "flesh out" the NADP+ reduction and light reactions more? I did some, but ther's already a light reaction article that's pretty in-depth, and the exact details of NADP+ reduction are non-specifically at reduction (chemistry) - all that's important is that it moves electrons. So is that step done? Whosyourjudas (talk) 04:55, 2 Nov 2004 (UTC)

• So there is - I had no idea. Light reaction article could use a little polishing and a nice diagram or two of the photosystems and electron transport chain, but its perfectly adequate to put the overview in photosynthesis and then just reference the "main article" (like we have it now). I also took a look at Calvin cycle. It needs a lot of work (more than light reactions), but I think that we can do pretty much the same thing there. ClockworkTroll 06:21, 2 Nov 2004 (UTC)
• Turns out I didn't actually answer you. Looks like that's a yes: light and dark reactions can be considered done, it looks like. We still need some good stuff on the carbon cycle for the middle school kids doing science reports, though. ClockworkTroll 06:27, 2 Nov 2004 (UTC)

I started a carbon cycle section. It's at least a start, but I'm not sure how much there is to put there. --Whosyourjudas (talk) 23:13, 3 Nov 2004 (UTC)

Dark reactions need light just as much as light reactions do. Dark reactions are called dark reactions because it was initialy thought that light wasn't directly required for the reactions. We now know that this is incorrect. The dark reactions DO depend on light to provide the ATP and NADPH necessary to form sugars from carbon dioxide, to control import and export of necessary molecules between the chloroplast and cytosol, and to directly activate many of the enzymes that catalyze the dark reactions. User:63.110.248.42

Rewrite

I have made some significant changes to this article. I agree with the general aim of keeping things simple, but I think we can do this without being inaccurate. For instance, I don't think there's any harm done in distinguishing phototrophs and autotrophs. By the same token, the older version focused on the reaction

12H₂O + 6CO₂ + light → C₆H₁₂O₆ (glucose) + 6O₂ + 6H₂O

I guess this is important to mention, because lots of textbooks use it, but really it's only one of many different reactions photosynthesis can power. Treating the light-independent reactions as part of photosynthesis, instead of something it drives, isn't really simpler and certainly makes it more confusing when groups like bacteria are mentioned. In fact, the article is considerably simpler now that it leaves the details of carbon fixation to related pages like Calvin cycle.

I've removed the bit about photosynthesis and the environment, because most of it is about carbon fixation. This includes the following text on the evolution of photosynthesis. I think it would be nice to save some of it. However, much of it talks about the evolution of life in general, which is a bad idea since our ideas about it change, and I'm not sure how much of it is current.

Life is generally believed to have evolved on Earth between 3.5 and 4.5 billion years ago. The primordial atmosphere is thought to have consisted of mostly methane, carbon dioxide, water vapor, hydrogen sulfide, and ammonia. Fossil evidence shows that most life prior to the aerobic extinction event probably used hydrogen sulfide fixation to synthesize Adenosine triphosphate (ATP). The original prokaryotic organisms were non-motile (couldn't move). Originally cells were dependent upon the environment to move them around to fresh sources of chemical energy.

The next step saw the formation of primitive flagella, organelles that could cause the cell to move under its own power. Originally these flagella were more or less autonomic (on all the time). This increased the cell's access to fresh sources of hydrogen sulfide. A cell that sits in one spot will eventually reduce the surrounding concentration of hydrogen sulfide to the point of stasis, at which point H₂S will diffuse into the cell only slowly. A mobile cell benefits from a continuously higher concentration, increasing not only the access to H₂S but also the rate at which the cell absorbs it in general.

Hydrogen sulfide is not the only resource needed for primitive life. The warm waters near the surface help to catalyze the reactions. Eventually photosensitive pigments evolved that allowed the flagella to move the cell towards the surface, and thus warmer regions. The region of the sun's spectrum that has the highest energy is in the yellow region; however, simple organic pigments have the largest bandwidth response in the red and infrared region. With infrared also being associated with heat, most likely the first photosensitive pigments responded to red and infrared light much as modern chlorophyll does. This would have given them a blue-green hue.

Please let me know if you have any concerns or suggestions. Josh 02:54, 14 Jan 2005 (UTC)

A common misconception is that glucose was the sugar end-product of photosynthesis. The fact is, glucose can not be produced by the Calvin cycle or after the Calvin cycle (it would be glucose phosphate). Sucrose is probably the major sugar end-product in most plants.

Question about blinking light

I have heard a rumour (I cannot find any source on the web), that NASA used blinking LEDs for growing things in space, that strongly blinked for a very brief period, and then after about a second again. Reportedly, this doesn't affects the photosynthesis (because it takes some time for the light accepting molecule to become prepared again), but saves a lot of energy. I heard that marijuana growers were interested in this. Is anything like this true? Samohyl Jan 21:26, 28 Jan 2005 (UTC)

Ionisation of chlorophyll electrons?

The article mentions something about sunlight ionising electrons in chlorophyll. What I would exactly like to know, is well, exactly how it happens, or at least a wikilink to the concept. Is it something like the photoelectric effect or much different? Is the reaction fundamental? Is it a charge thing? Electromagnetic fields, resonance frequency of chiorophyll? Anyone? -- Natalinasmpf 05:33, 23 Apr 2005 (UTC)