Light Independent Reactions and the Calvin Cycle

[ender7x77]

Can anyone confirm this as being right? I'm reviewing for my photosynthesis test and these are among few that I have been dubious about.

 

In the dark, or light independent reactions, carbon dioxide is fixed to ribulose biphosphate in the Calvin Cycle. The primary end product of this cycle is G3P, which may be converted to glucose and polymerized into starch.

 

The molecule glycolate underoges subsequent metabolism such that it results in the release of a molecule of G3P.

 

Light energy, captured by chlorophyll a, is transferred to a series of primary electron acceptors.

 

The process whereby a plant uses a oxygen in light is photorespiration.

 

Is green light the most effective for photosynthesis in the spectrum of white light?

 

If anyone could anwser just one that would help me out greatly.

[Airmid]

In the dark, or light independent reactions, carbon dioxide is fixed to ribulose biphosphate in the Calvin Cycle. The primary end product of this cycle is G3P, which may be converted to glucose and polymerized into starch.

Correct. (This is the mechanism used in green plants. There are other mechanisms, but I don't think you should let that worry you now.) I like this picture of the Calvin cycle.

 

The molecule glycolate underoges subsequent metabolism such that it results in the release of a molecule of G3P.

I think you're referring to photorespiration here. Wiki has a pretty clear description of the process:

"As carbon dioxide concentrations rise, the rate at which sugars are made by the light-independent reactions increases until limited by other factors. RuBisCO, the enzyme that captures carbon dioxide in the light-independent reactions, has a binding affinity for both carbon dioxide and oxygen. When the concentration of carbon dioxide is high, RuBisCO will fix carbon dioxide. However, if the oxygen concentration is high, RuBisCO will bind oxygen instead of carbon dioxide. This process, called photorespiration, uses energy, but does not make sugar

 

RuBisCO oxygenase activity is disadvantageous to plants for several reasons:

 

1. One product of oxygenase activity is phosphoglycolate (2 carbon) instead of 3-phosphoglycerate (3 carbon). Phosphoglycolate cannot be metabolized by the Calvin-Benson cycle and represents carbon lost from the cycle. A high oxygenase activity, therefore, drains the sugars that are required to recycle ribulose 5-bisphosphate and for the continuation of the Calvin-Benson cycle.

2. Phosphoglycolate is quickly metabolized to glycolate that is toxic to a plant at a high concentration; it inhibits photosynthesis.

3. Salvaging glycolate is an energetically expensive process that uses the glycolate pathway and only 75% of the carbon is returned to the Calvin-Benson cycle as 3-phosphoglycerate.

 

A highly simplified summary is:

 

2 glycolate + ATP → 3-phophoglycerate + carbon dioxide + ADP +NH3

 

The salvaging pathway for the products of RuBisCO oxygenase activity is more commonly known as photorespiration since it is characterized by light dependent oxygen consumption and the release of carbon dioxide."

 

Light energy, captured by chlorophyll a, is transferred to a series of primary electron acceptors.

Correct. If this wasn't done, the result would be a large amount of free energy, which the plant can't use and only would cause damage. By transferring the energy through the acceptor chain, smaller amount of energy are freed in each step, which can be used by the plant for instance to produce ATP. (I am aware that this is a simplified view.)

 

The process whereby a plant uses a oxygen in light is photorespiration.

Correct, see above.

 

Is green light the most effective for photosynthesis in the spectrum of white light?

No. The chlorophylls and other light harvesting pigments mostly absorb light in the red and violet parts of the spectrum, and hardly use green light at all. The green light instead is reflected back, and that is why plants have a green color.

 

Airmid.

[ender7x77]

Thanks for the great help! I didn't think anyone would respond because of the length of it. The pigment correction helped me out a lot. I'm going to have to change my anwser for my lab. I thought because leaves appeared green because chlorophyll is green, but now i see that it is reflected back to us. I still need to figure out though if viewed under red light, what would a chlorophyll extract to appear in colour.

 

Lastly, I got two more questions.

 

Am I correct to assume that air temperature, amount to water in sorl, brigtness of ambient light, turgor pressure of the gurad cells, and humidity will affect the opening and closing of the stomata? I know the last 3 are. The amount of water will create the turgor pressure or relieving of the pressure so I'd like to think it does too.

 

A high rf value in chromatography paper implies that the solute is structually similar to the the solvent? I have no idea about this one. I have been looking all night for it with no luck.

 

Anyways, thanks again.

[ender7x77]

In regards to chlorophyll extract being viewed under red light...would it not appear to be black in pigment due to chlorophyll reflecting green light. When green and red combine they will create black like pigment.